UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. C. Mendenhall, Director

Bulletin 914

MICROSCOPIC DETERMINATION 

OF THE ORE MINERALS

M. N. SHORT

SECOND EDITION

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1940

For sale by the Superintendent of Documents, Washington, D. C. - - - - - Price $1.00 (Paper)

CONTENTS

Page

Preface_ ________________________________________________________ VII

PART 1. Microscopic technique..--___________._____..._______-___--- 1

Introduction........______...___________._._____._____._._._.__ 1

The metallographic microscope.__-____-____________-_..__-_-_--. 4

Mounting the specimen________________________________________ 8

Mounting in modeling wax. ____________.._..._._...-___---- 8

Mounting in brass boxes--..---------.---------------------- 8

Warming oven___________-___-___-__^___-_-__-_.__________ 10

Mounting in bakelite_.___-----_-----_---_-______-__----_-_- 11

Grinding and polishing.____.____________---_______-_.___-___--- 13

Sawing the specimen,___------__-_-___-______..______-_---,.- 13

Diamond saw_________________-__-____--_____-___---_- 13

Corundum and silicon carbide saws._____________________ 15

Patt'on lathe feed.._---_---____---________-_____-_-__-- 16

Grinding and polishing machinery___________________--_-_--. 18

Eardley-Hatton polishing table_______-_______--.-----_.. 18

Suggestions for construction of grinding table.------------ 21

Abrasives and polishing powders_____________--_._-_-_-_--- 22

Sizing of fine abrasives._________________________________ 22

Rodda method_____-_____-_---____. --------------- 23

Vanderwilt classifier._____-_-_-___________.------_- 23

Sizes of some well-known abrasives______________________ 26

Polishing process at the Geological Survey,___.______._-____-_ 27

Procedure,____________---_-_-_---_____-_.___._--_---- 27

Geneial remarks.-.-------------------------..___. ------ 30

Impregnating the specimen_____________________________ 30

Harvard polishing process.______-_____________-_.___._-_--- 32

Summary. __..__-_-._________._-____-__.____-.__--__-. 32

Principles of polishing__--_,__-_---____-__--_-___--.---. 33

Graton-Vanderwilt polishing machine.__.___________._--. 34

Details of process as used at Harvard-___________________ 36

Details of process used at Canada Department of Mines and

Resources._____.._______._________._._._____-_----. 38

General remarks______________________-_,____--_-_-_- 38

Polishing process at Columbia University____-_____..------- 4.0

Polishing process at the University of Minnesota.______--_---_ 42

Metallographic polishing at the Watertown Arsenal.__..-....-. 42

Photomicrography of polished sections.___---_________-__.------.' 44

Photomicrographic camera____ _______._____-___._____..-_-.. 44

rilms_______-_.-...---._-_._----.-...------....--..----.. 51

Light filters.-,___-______-____.___-____.__-___-_._._------. 51

Exposure formula.______._________._____________._._-__-.-. 55

Developing and printing..__________-__-_______._.____-----. 57

References...______-___________._________.___._._____-_-.- 58

in
IV CONTENTS

Pago

PART 2. Physical properties and measurements._______________________ 59

Color.____ 59

Hardness-___----------___---______-_______.___---_-_---_-_-__ 63

Examination of opaque minerals in polarized light__.-___-__--__-_. 66

Summary.. _ _______________________________________________ 66

References.._ _____________________________________________ 67

Apparatus.-______.____________-__.__-_____-------__--_-_. 67

Details of the method..__--___-______--_____--____-----__-_ 69

Methods of observation._______________________________ ... 73

Adjustment of the instrument-__ _ ____________--._-___-___-_"- 74

Sources of error.___._________________.______ _____________. 75

Electroconductivity__ _-______--__--_______--____---_-_-_--_-___ 76

Measurement of reflectivity...--___-_________._..___________ _ ___ 76

The slit microphotometer of Berek_____j___________________ 78

Measurement of reflectivity with photoelectric cells......______ 83

Alkali cells___________________________:>-_____________ 84

Rectifying cells___-_-_____-_________._-______-_-_--__- 86

Spectroscopic methods.______________^____________________.. ____ 92

\J PARTS. Etch reactions and determinative table_-__-_-_-_- ____________ 95

Etch reactions.________-_-_-___________________-_-_-_-_-_.--_- 95

Determinative table.____----___-_______________---_____-_.-_-- 103

Introduction _______________________________________________ 103

Outline of table.__-_-__--____-_-_______________---_-_-_-_- 105

Soft minerals___-___--____-_______________------_--_-____ 112

Hard minerals__-__--_____-_-____--_______-_-_-_-__-__-_- 159

Binnenthal minerals.______________________ __-.__-_-____-_ 168

Discredited or doubtful minerals_________-_-_-_-_-_-___-_- 169

Staining tests__________________________________________________ 170

PART 4. Microchemical methods___________________________________ 173

Introduction. _ ________________________________________________ 173

Procedure and apparatus.__---____-_-_-_-_______----____-_-_-_- 174

Collecting the material,____________________________________ 176

Glass slides.______________________________________________ 178

Microburner. _---__-_-_________-_____._____-_-____---...__- 178

Reagents-__-_---___----_-_-__--_-.__._______-___._-_._-.. 179

Capillary tubes______ _____________________._______.______ 179

Methods of applying tests._____________________________________ 180

Sensitivity of microchemical tests.---.-._________________________ 183

Reagents-_-_________-----_-_____--____-_._____-_-_._-_--._--- 184

Tests made directly on a glass slide.__-___-_______-___-_-_-___-_- 187

Zinc___-_____----_..___..___..._:.-.._._._-_-_._._._._-.- 187

Cadmium_____ _._-_____________-_________._____-___--___-- 190

Copper_-__-----__-__-_______-___-_____-_-_-_-_-_-___--- 190

Nickel . .I 192

Cobalt________-_--.--_______.___-_-_-_____--_____--_._--- 193

Iron.__--____-----.__ __ _.._ 198

Silver_________-___._____._____.___.___.___-_______---__-- 199

Selenium _________________________________________________ 204

Tellurium. __________.___._____._.____._._-__..___.._.-. 206

Arsenic_________________________________________________ 208

Antimony and bismuth________-_-_---__-______-_-__-------- 212

Tin..__.__---_-------__-__-__---___._-_____--_____________ 218

Lead... __________________________________________________ 220

Gold .- .. _ __-__-__ _______---_-_-__-__.-- 223
CONTENTS V

PART 4. Microchemical methods Continued.

; Tests made directly on a glass slide Continued. Page

Mercury__. ________________-_-_-______--..------__-----.. 225

Manganese __>_________.___________________________.___.- 226

Sulphur_____..._.___.__-__.__---___--_-------------- - 227

Tests after fusion in loop of platinum wire.________________________ 231

Molybdenum. ____-____-----------------_-----_----------. 233

Tungsten_._...__.__._..._._._._..._...___.._.._._......_ 235

Vanadium._ ___-_______-_---_-_-_-___-___-----_-_--_---___ 237

Titanium. __.__... .._.....___. .______._._..._........... 239

Chromium.._ _____..-___-____---______-___-_-----_-_----__- 240

Tests requiring special apparatus.".__--_______-_--__._-_______-_- 240

Germanium. ______________________________________________ 240

The platinum metals..._______.._-_________-..____-_____-____ 243

Palla d i u m _.__._._._..._.._._........__..-.._......_.. 244

Platinum......._..... _.__...___.._.._..._........... 245

Iridium _______________________________________________ 247

Osmium._____________--_-------___-_-_.---_-__-_-_--- 248

Rhodium. ______ .....__..._............._._......_.._... 250

Ruthenium _________________--____..____-_-_______-_--- 250

A systematic scheme of microchemical analysis.__-_-__--_.__.-___- 252

Hard minerals______-___-_-__-----________._.-_-____..-_-_- 252

Soft minerals_________________-__-__-___-_-._-_-__-__-_--_ 254

Outline of systematic analysis-______________________________ 257

Outline of microchemical tests.__-__-_-----.-_---______________ 259

Potassium mercuric thiocyanate___-_--_-___--__-________---. 259

Cesium chloride____________---_-__--_.._-___---.-_._-_-_--- 200

Potassium iodide___________.___-_---_____-_______..___-_.._- 201

Cesium chloride and potassium iodide...______________________ 262

Potassium bichromate____-_-_--_---___-____--_-_-_.______ 263

Thiourea__.___......_.______..._._....._...___....._. 264

Specific tests for minerals______-_____-_---_.___-_-_-_____ ._-____ 265

Index of minerals, grouped by elements.____-_-_-_-_---------___----- 297

Index of minerals and elements, alphabetical._________________________ 307

General index.____________.______-------___-----_-------------._-.. 313

ILLUSTRATIONS

Page

PLATE 1. Detail of Patton lathe feed__________________________________ 16

2. Photomicrographs of ores polished by Harvard process...______ 296

3. Photomicrographs of ores polished by Watertown process____ 296

4. Photomicrographs of polished section of specimen from Magma

mine, Superior, Ariz____--__---_--_-_____---_______.____ 296

5. Microchemical tests for zinc and sulphur.___.__._._____.____ 296

6. Microchemical tests for copper.__-__-_____._._._________.__ 296

7. Microchemical tests for cobalt and nickel..--.-----__-__-___. 296

8. Microchemical tests for silver._____________________________ 296

9. Microchemical tests for arsenic and tellurium ________________ 296

10. Microchemical tests for antimony._________________________ 296

tl..Microchemical tests for bismuth and tin_____________________ 296

12. Microchemical tests for lead and germanium.________________ 296

13. Microchemical tests for gold._..__--__--____------_-_._---_- 296

14. Microchemical tests for mercury____________-___---___--.. 297
VI CONTENTS

	Pag-e 

FIGURE 1. Metallographic microscope with equipment for. investigating

polished sections.______-_._______-____-______..____--__ 4

2. a, Prism illuminator; b, Glass-disk illuminator. _____._-___-_- 5

3. Bakelite press__^_________________________._.._____._._. 12

4. Diamond saw and lathe feed..____.___._______ ._____.______ 16

5. Eardley-Hatton polishing table and detail of mounting._____ . 19

6. Siphon for abrasiveg___---_-______-_-----_------_--___--_ 23

7. Vauderwilt abrasive classifier.____________________________ 24

8. Graton-Vanderwilt polishing machine_____________._.___;__ 35

9. Mortar and pestle__-__--_---------_---_------.---_------- 39

10. Improvised microscope camera_____ ____-__-._________---_- 45

11. Leitz "Makam" camera attachment-..-......--..-_----_-_-.. 48

12. Vertical camera with observation eyepiece.__..___________._. 49

13. Bausch & Lomb large metallographic camera.___--__-_--___ 50

14. Comparison microscope.____-_---_-_-_-_--_-'-_-._--_--___ 60

15. Talmage hardness machine__----__-______----------_----_ 65

16. Microscope arranged for use with polarized reflected light.... 68

17. Photomicrographs of ores taken in polarized reflected light.... 75

18. Microscope with Berek slit micro photometer attachment.--.. 78

19. Sectional view of microscope and Berek slit microphotometer.- 79

20. Alkali photoelectric cell._________________________________ 84

21. Simple amplifying circuit for alkali celL__-____-_--------_-- 85

22. Thermionic amplifying circuit for alkali cell..._________._-_. 85

23. Construction of rectifying cell________________ .___._____._- 87

24. Cross section through rectifying cell-__ ____________________ 87

25. Platinum wire for transferring drop of reagent to polished surface_______-___.__-___-___--__-_------------_-----_- 97

26. Enlarged view of needle for scraping polished surface- _ ______ 177

27. Improvised alcohol lamp.________________________________ 179

28. Capillary tube______._.-:___--.-_-_______________-_----- 180

29. Diagram showing Chamot's method I_______.___-.._______. 182

30. Glass-stoppered dropping bottle.-_____-____-____-_-.._-- . _. 187

31. Zinc and copper mercuric thiocyanate crystals.___-.___--__- 189

32. Apparatus for germanium test.___--_------_------..------- 241

33. Apparatus for decomposition of platinum minerals...... .____ 244
PREFACE

The development of improved methods for the determination of the 

ore minerals and the collection of data on these minerals occupied the 

attention of the writer for more than 4 years prior to the publication 

in 1931 of Bulletin 825, the first report on this study. That work was 

done mostly in the laboratories of the United States Geological Survey, 

except from September 1927 to June 1928, while the writer occupied 

the position of lecturer in economic geology in Harvard University 

and carried forward the work with the aid afforded by the resources of 

the Harvard Laboratory of Mining Geology.

In the 8 years since the publication of the first edition, the study of 

ore minerals has been widely extended. It is safe to say that the 

number of articles published annually dealing partly or wholly with 

microscopic studies of ores has doubled in this interval. Many new 

methods of identification have been devised or adapted, among which 

the most important are spectroscopy and reflectivity of metallic surfaces. In Germany the study of the theory of light reflected from 

minerals has been carried on intensively. The results have been 

encouraging, but much yet remains to be done.

Improvements in the technique of polishing have proceeded apace 

in the last few years, as shown by comparison of photomicrographs in 

current periodicals with, those in older publications. The importance 

of getting a good polish on specimens is taken into consideration in this 

report, and much more space is devoted to this feature than in the 

first edition.

For the 8 years 1931-39 the writer has taught the subject of polished-section study to advanced students of geology at the University 

of Arizona. A new collection of reference minerals has been studied 

by them, resulting in most of the corrections made in part 3 of this 

report. Experience in teaching microchemistry has led to most of the 

changes in part 4. Because tests must be simple enough for students 

of average ability to apply, some microchemical tests have been 

eliminated and others added. Tests involving preliminary fusion 

with sodium carbonate have been added for molybdenum and some 

other elements.

In a work of this nature much of the material is a compilation of the 

published researches of others. The writer takes this opportunity of 

thanking those who have generously given permission to take material 

from their articles. Among these are Drs. Hans Schneiderhohn and. 

Paul Ramdohr, of Germany; J. Orcel, of France; J. A. Dunn, of the
VIII PBEFACE

Geological Survey of India; Maurice Haycock, of the Canada Department of Mines and Resources; J. H. Moses, of the Cerro de Pasco 

Copper Corporation, Peru; H. J. Fraser, of the California Institute 

of Technology; and A. M. Gaudin, of the Montana School of Mines. 

Prof. L. C. Graton, of Harvard University, gave the writer full use of 

his laboratory and supplied most of the information regarding the 

Harvard polishing process. Miss M. R. Norton, of the Watertown 

Arsenal, prepared for the writer a description of the metallographic 

polishing process used there.

The writer had the full cooperation of the United States Geological 

Survey. . In working out the microchemical tests for molybdenum, 

tungsten, and vanadium, he is especially indebted to J. J. Fahey and 

E. T. Erickson, of the Section of Chemistry and Physics, for assistance 

and helpful suggestions. K. E. Lohman, of the Section of Paleontology and Stratigraphy, revised the section on photomicrography.
MICROSCOPIC DETERMINATION OF THE ORE

MINERALS

By M. N. SHOET

PART 1. MICROSCOPIC TECHNIQUE 

INTRODUCTION

The metallographic microscope is the most valuable instrument 

for identifying opaque minerals and for studying their mutual relationships. It plays the same part for the opaque minerals that the 

petrographic microscope does for transparent minerals. The metallographic microscope differs from the petrographic microscope in that 

it contains a reflector placed in the barrel of the microscope directly 

above the objective, and light is reflected vertically downward through 

the objective onto the specimen to be studied. The specimen or 

mineral is prepared for study by grinding and polishing a flat surface. 

It is then mounted so that the polished surface is parallel to the microscope stage, and the microscope is focused on the surface to be studied.

The identification of the opaque ore minerals in polished sections 

has made much progress since 1916, when Murdoch's pioneer researches were first published. 1 The textbooks of Davy and Farnham, 2 

Schneiderhohn,3 and Van der Veen 4 have all contributed new ideas 

and methods of attack. Farnham's text, 5 published in 1931, is in part 

a revision of the earlier text of Davy and Farnham, but it contains 

many new data.

The most comprehensive text on the ore minerals is that by 

Schneiderhohn and Ramdohr. 6 It comprises two volumes and an 

appendix. The second volume, which was the first to be published, 

contains a detailed description of every ore mineral known to the 

authors. This description comprises the usual mineralogic properties, 

many new physical properties, such as reflective power and polariza1 Murdoch, Joseph, Microscopical determination of the opaque minerals, New York, 1916.

3 Davy, W. M., and Farnham, C. M., Microscopic examination of the ore minerals, New York, 1920.

3 Schneiderhohn, Hans, Mikroskopischen Bestimmung und Untersuchung von Erzen, Berlin, 1922.

4 Van der Veen, R. W., Mineragraphy and ore deposition. The Hague, 1925. 

8 Farnham, O. M., Determination of the ore minerals, New York, 1931.

«Schnelderhohn, Hans, and Ramdohr, Paul, Lehrbuch der Erzmikroskopie, Berlin, Qebriider Borntraeger, Band 2,1931: Bestimmungstafeln (appendix), 1931; Band 1, H&lfte 1,1934.
2 MICROSCOPIC DETERMINATION OF ORE MINERALS

tion colors, paragenetic relations with other minerals, a list of localities 

where the mineral occurs, and a complete bibliography. The first 

volume is more general and contains a description of various microscopes, methods of polishing, determination of anisotropism and reflective power, photomicrography, and determination of mineral 

orientation. The appendix (Bestimmungstafeln), which is actually a 

third volume, is a table for the determination of minerals by means of 

their microscopic properties.

Not all the methods outlined in the texts cited have proved entirely 

satisfactory, and some discrimination is required in selecting from 

the methods described those winch are of greatest value. The writer 

has made an effort to select those methods which, in his opinion, are 

most useful and which at the same time do not require elaborate 

and expensive apparatus. The scheme of identification adopted is 

based on the following observations:

1. By means of polarized light, minerals crystallizing in the isometric system can be separated from those crystallizing in the other 

five systems.

2. By means of a needle, minerals can be graded as to hardness 

into two classes hard, or those which can be scratched with difficulty 

or not at all, and soft, or those which are easily scratched with a 

needle. The three grades of hardness described by Murdoch and by 

Davy and Farnham are accordingly modified by eliminating the 

"intermediate" class.

3. The systematic etching scheme described by Davy and Farnham, although far from perfect, is of great value in saving time. The 

etch tests are sufficient for the identification of some minerals, but for 

most minerals identifications based on this method must be checked 

by other methods.

4. Qualitative microchemical tests constitute the most reliable 

method available for identifying an ore mineral. These tests require 

considerable time. Unless the investigator has a clue as to what 

elements are present, he must test for the different elements in turn 

in a systematic way, analogous to the methods of ordinary qualitative 

analysis. The most effective and the quickest method of identifying 

a mineral is first to obtain all the data yielded by methods 1 to 3. 

This procedure will generally limit the designation to a choice of 

half a dozen or less minerals. The final choice will be governed by 

definite microchemical tests for one or more suspected elements.

The necessity for more accurate data than are available in the 

literature was brought out during the present investigations. Microchemical tests made on Murdoch's original suite of ore minerals and 

on the suite of specimens selected by the writer from the United States 

National Museum showed that over 20 percent of these minerals were
MICROSCOPIC TECHNIQUE 3

mislabeled. The necessity for careful selection of specimens is thus 

manifest.

The present investigation by no means eliminates all the errors 

resulting from incorrectly designated specimens. Microchemical tests 

are qualitative and not quantitative and are thus incapable of distinguishing between different minerals having the same elements. 

The writer cannot, at present, distinguish beegerite, Pb6Bi2S9 ; cosalite, 

Pb2Bi2S5 ; and lillianite, Pb3Bi2S6 . The only safe way of distinguishing 

between these minerals would be to obtain quantitative chemical 

analyses of selected specimens and try to obtain distinctive criteria 

on polished sections of these specimens by methods 1 to 3 outlined 

above. Usually but not always it suffices to obtain material from the 

type localities of the minerals. With such material it is not necessary 

to make analyses, as the original identification of the minerals from 

these localities is usually based on quantitative analyses. This plan 

is especially useful on minerals that are confined to one or two localities. 

For example, mineral from Rezbanya, Hungary, containing lead 

and bismuth and labeled "rezbanyite," is probably what it purports 

to be.

It should not be forgotten, however, that many published analyses, 

especially the earlier ones, were based on impure material. Before 

the reflecting microscope was applied to the study of ores many ore 

minerals were assumed to be pure because no impurities were visible 

in the hand specimen. Microscopic investigation lias shown the 

error of such an assumption. Even apparently pure crystals are by 

no means always pure. The correct formula for bornite escaped 

determination for over a hundred years because the crystals of apparently pure bornite analyzed were not pure but were later shown 

to be intermixtures of bornite and chalcopyrite. It thus becomes 

evident that no ore mineral should be analyzed until it has first been 

examined microscopically and shown to be pure. In the descriptions of minerals given in this bulletin, data obtained from analyzed 

material will be accompanied by reference to the analyses.

A suitable designation for the study of the ore minerals under the 

metallographic microscope has not yet been suggested. The term 

"mineralography" proposed by Murdoch is obviously not suitable, 

as the study embraces only a certain group of minerals. The term 

"mineragraphy" proposed by Whitehead 7 and used by Davy and 

Farnham and by Van der Veen is decidedly harsh and unpleasant. 

Notwithstanding this objection, this term has come into more widespread use than any other. The term "chalcography" used by 

Schneiderhohn is unsuitable, as it has already been applied to the 

art of engraving on copper.8 The most recent suggestion is "Erz-

' Whitehead, W. L., Notes on the technique of mineragraphy: Econ. Geology, vol. 12, p. 697,1917. 

8 Webster's New International Dictionary. This confusion in terminology was first noted by Mr. 

E. E, Fairbanks in an unpublished paper.
4 MICROSCOPIC DETERMINATION OF ORE MINERALS

mikroskopie" as proposed by Schneiderhohn and Kamdobr. This 

might be translated literally as "ore microscopy." The term is not 

adaptable to the English language, as the corresponding adjective 

cannot be formed. The writer prefers not to use any of the above 

terms and will retain the exact if cumbersome phrase "microscopic 

study of the ore minerals."

THE METALLOGRAPHIC MICROSCOPE

Any compound microscope can be converted into a metallographic 

microscope by means of a vertical illuminator, consisting of a short

FIGURE 1. Metallographic microscope with equipment for investigating polished sections, a, Spencer 

lamp, No. 374; (i, rubbing block; c, reagents for etch tests; d, vertical illuminator; e, polished section on 

modeling wax; ]t screw for racking stage up and down; g, polished section in brass mount; ft, lead filling; 

i, sleeve; i, nipple; k, flange: (, mounting cup.

tube in which is mounted a reflector. An opening in the side of the 

tube admits light to the reflector. The light after impinging on the 

reflector is thrown vertically downward through the objective and 

strikes the polished surface of the mineral to be examined. The 

light is then reflected vertically upward from the polished surface, 

passes through the objective and beliind or through the reflector, and 

after passing through the ocular reaches the eye.

There are two types of vertical illuminator, the difference depending on the kind of reflector used. The prism illuminator consists of 

a glass prism which occupies about one-half of the area of the tube. 

This arrangement enables part of the rays reflected from the surface 

to pass behind the reflector. (See fig. 2.) The glass-disk illuminator 

consists of a thin transparent glass disk which occupies practically
MICROSCOPIC TECHNIQUE

the entire area of the tube. In both types the reflector is rotatable 

in the tube and is turned until the clearest image appears.

The vertical illuminator screws on to the lower end of the microscope barrel, and the objective, in turn, screws on or is fastened by 

a spring clip to the lower end of the illuminator.

The glass-disk illuminator, also referred to as the plane-glass 

illuminator, gives a more even illumination than the prism illuminator, but only about half of the light striking the glass disk is reflected downward to the polished section; the rest of the light is 

absorbed by the disk or passes horizontally through it and is absorbed in the walls of the illuminator tube. The light impinging

M

FIGURE 2. a, Prism illuminator. (Courtesy of E. Leitz, Inc.) T, microscope barrel; K, prism; L, converging lens; 0, short-mounted objective; P, bundle of rays passing upward behind prism; M, polished 

surface; C, spring clip holding objective to illuminator barrel, b, Glass-disk illuminator. (Courtesy of 

Bausch & Lomb Optical Co.)

downward on the polished surface is reflected upward. On striking 

the plate-glass disk again, part of the upward-moving light is reflected 

back toward the lamp and leaves the microscope. A small proportion of the upward-moving light is absorbed by the disk itself, 

and the remainder gets through the disk and so reaches the eye. 

As a result of these losses only a small part, possibly less than onefourth, of the original intensity of the light is available for microscopic 

observation.

The prism illuminator reflects downward practically all the light 

that impinges on the prism. However, only part of the light reflected by the polished section reaches the eye, as the prism itself 

occupies half of the section of the tube, and all that part cf the light 

which, on its upward path, strikes the prism is reflected outside of 

the microscope and lost. Only that part of the upward-moving 

light that passes behind the illuminating prism reaches the eye. To 

get the maximum illumination the prism is turned so that the rays 

do not move exactly vertically downward but strike the surface
6 MICROSCOPIC DETEEMmATIOST OF ORE MINERALS

with a slight inclination from the vertical and in turn are reflected

with just enough inclination from the vertical to pass by the prism.

Van der Veen 9 compares the two types of illuminator as follows:

The specific intensity of the illumination by the prism illuminator is higher 

than that of the glass [disk] illuminator, because the bundle of rays, although 

divided into two parts, keeps its original intensity. In the plane-glass type 

every ray is reduced in intensity while the whole bundle remains intact. For 

this reason, the resolving power of the plane-glass illuminator is much higher 

than that of the prism illuminator. Therefore, the former is especially desirable 

for high magnification.

The writer prefers the prism type of illuminator for work with 

daylight and with ordinary artificial illumination (using an ordinary incandescent electric bulb and a daylight glass screen). This 

is satisfactory for observation with objectives of 32-, 24-, 16-, 8-, and 

4-millimeter focal length. With objectives of higher magnification 

tha'n the 4-millimeter lens and for microphotography the glass-disk 

illuminator is preferable. Strong artificial illumination is required 

for the higher magnifications. The writer finds the 6-volt 108-watt 

tungsten-ribbon Mazda lamp the most satisfactory. The arc lamp 

gives an intense light but one which varies in intensity.

Figure 1 shows the most common type of metallographic microscope. The stage can be moved vertically by means of a rack and 

pinion but is not rotatable. The microscope barrel with attached 

illuminator and objective remains fixed, and the polished surface is 

brought into focus by racking the stage up or down. The advantage 

of this arrangement is that the light source does not need to be 

shifted in position when the objective is changed. The distance of 

the polished surface above the glass slide that supports the specimen 

can likewise be changed without moving the microscope barrel or 

light source. The metallographic microscope of this type is entirely 

satisfactory for most work but has the disadvantage that it cannot be used in investigations requiring polarized light.

The writer prefers a petrographic microscope for use with reflected 

light. The illuminator can be attached and detached in a few seconds 

by means of a ring that fits in a clamp. In this manner the microscope 

can be changed from a petrographic to a metallographic microscope 

with a minimum of effort. Nearly all the work with reflected light 

is done with a short-mounted objective of 16-millimeter focal length. 

Consequently, the objective is screwed into the vertical illuminator 

and not detached from it when the illuminator is removed from the 

microscope.

The 16-millimeter objective is the best all-round lens for metallographic work for the following reasons:

1. It gives the most satisfactory field of view for the greatest 

number of mineral intergrowths that is, the size of mineral grains

8 Van der Veen, E. W., op. cit., p. 3.
MICROSCOPIC TECHNIQUE 7

in the greatest number of specimens is such that they are best observed 

with that lens.

2. It combines clearness of vision with depth of focus and freedom 

from boundary shadows. (See pi. 4.)

3. It is the best lens for use with polarized light. (See p. 69.)

Objectives of lower magnification than the 16-millimeter lens are 

more frequently used than those of higher magnification. The 24- 

millimeter and 32-millimeter lenses are the ones most frequently used. 

These two lenses do not have to be designed for use without the cover 

gla.ss and do not require special short mounts; the 24- and 32-millimeter petrographic objectives are entirely satisfactory for metallographic work.

If the mineral grains are so large that they require the use of 

an objective of still lower magnification, it is best to dispense with 

the use of the metallographic microscope altogether and view the 

polished section under the binocular microscope, using oblique light. 

By tilting the surface at the correct angle, a mirrorlike reflection is 

obtained which is similar to that produced by vertical illumination.

The 8-millimeter and 4-millimeter short-mounted objectives and 

more rarely the short-mounted 1.9-millimeter oil-immersion lens are 

occasionally used for higher magnifications, but the difficulty of obtaining a flat surface on a polished mineral specimen hinders the 

use of the oil-immersion lens. Fortunately the size of grain in mineral intergrowths is rarely so small that the oil-immersion lens is 

required.

The rotatable stage and tube analyzer of the petrographic microscope are necessary when polished sections are examined in polarized 

light.

For ordinary microscopic work, where the comparison microscope 

is not required, daylight furnishes satisfactory illumination. With 

some experience, the eye readily adjusts itself to changing strength of 

daylight. A north exposure or one shielded from direct sunlight is 

required. If trees, buildings, or other objects of any considerable 

size are in front of the microscope, the images of these objects are apt 

to be reflected on the surface of the mineral under the microscope. If 

an uninterrupted view cannot be obtained, artificial illumination is 

required.

The Spencer microscope lamp No. 370, now used by the writer at 

the University of Arizona, is highly satisfactory for both petrographic 

and metallographic microscopic work. This lamp has a diaphragm 

in front of the bulb by means of which a horizontal cylinder of light 

about 1 centimeter in diameter can be obtained, making a satisfactory 

illuminant for work in reflected light. For thin-section, work the 

lamp is tilted so that the light shines obliquely downward, and the 

diaphragm is opened to its full width of 6.5 centimeters.
8 MICROSCOPIC DETERMINATION OP ORE MINERALS

The lamp is provided with a 100-watt Mazda projection bulb. For 

ordinary observation with polarized light, a ground-glass "daylight" 

color screen is placed in front of the bulb. This gives color values 

approximating those of daylight. As the 110-volt current gives too 

bright a light for ordinary observation, a rheostat is placed in the 

circuit, which reduces its potential to 90 volts. This reduced voltage 

also prolongs the life of the bulb.

For polarized reflected light the full voltage is used and the groundglass "daylight" screen is removed. Where polarized reflected light 

is not required the lamp is equipped with a 60-volt frosted "daylight" 

Mazda bulb together with a "daylight" ground-glass screen. This 

bulb costs about 30 cents, as contrasted with $1.50 for the 100-watt 

projection bulb.

MOUNTING THE SPECIMEN

Mounting in modeling wax. The polished specimen is mounted by 

being pressed into a lump of modeling wax on a glass slide. A convenient size for the slide is 3% by IK inches by % inch. The polished 

surface is, of course, uppermost and must be parallel to the glass 

slide. A number of leveling devices are on the market, but none of 

them are as convenient as the home-made mounting cup made of 

2-inch pipe fittings shown in figure 1, which is similar to one illustrated 

by Davy and Farnham. 10

This consists of a 2-inch nipple 3 inches long filled with lead, a 

sleeve 2% inches long, and a flange. The upper surface of the nipple 

with its lead filling and the upper edge of the sleeve are machined so 

as to be at right angles with the length. The sleeve is rotated so that 

its upper edge is a convenient distance above the lead surface. The 

distance selected determines the distance between the polished surface 

and the upper surface of the slide. The section is pressed down by 

hand into the wax, the polished surface being approximately parallel 

to the glass slide; then the slide with wax and section is turned upside 

down, and the polished surface is lowered till it comes into contact 

with the lead surface of the cup. The slide is then pushed downward 

until it rests on the sleeve. The advantage of this leveling device is 

that the height of the polished surface above the slide can be adjusted 

at will.

Mounting in brass boxes. A much more effective method of mounting than the temporary emplacement in modeling wax is to cement 

the polished chip in a section of rectangular brass tubing by means of 

sealing wax. The polished sections thus mounted are easier to 

handle, and tiny fragments or fragile chips that could not be held 

steadily by hand or that would break to pieces during the polishing 

process are firmly gripped in the cement, so that the grinding can be 

continued to any required degree. A flatter surface is obtained when

10 Davy, W. M., and Farnham, 0. M., op. cit., p. 5.
MOUNTING THE SPECIMEN 9

the chip is mounted in the brass box than can be obtained on an unmounted chip, as the entire area of the box acts as a support for the 

specimen during grinding and polishing.

A rough flat surface is first ground on one side of the specimen 

before cementing it hi the brass tube. After the specimen is mounted, 

the grinding is continued and is followed by the polishing process. 

If desired, however, the specimen can be completely ground and 

polished before mounting in brass.

The rectangular tubing used measures 1% by 1% inches outside 

dimensions in cross section and is cut to %-inch lengths. The thickness 

of the brass is 1 millimeter. These tubes are furnished already cut by 

the American Brass Co., Waterbury, Conn., and cost about 3 cents 

each. Before the specimen is mounted, two conical indentations are 

punched in each of the longer sides of the tube to prevent the mounting 

medium from slipping in the brass. These indentations are driven in 

with a steel center punch, and to prevent inward bulging of the tubing 

when they are driven, it is placed over a steel bar which has a slot of 

triangular cross section cut on one face at right angles to the length 

of the bar.

Before the sealing wax is poured, the number and other designations of the specimen are stamped on the shorter side of the brass 

tubing by means of a hammer and % 2 -inch dies. The tube is supported while being stamped by a steel bar without the triangular 

slot.

When a specimen is to be mounted in sealing wax both the specimen and the tube must be warmed to about the temperature of the 

molten wax; otherwise the sealing wax will shrink away from the walls 

of the tube in cooling. When the tube is warmed before pouring "the 

sealing wax, the brass shrinks more than the wax and thus fits tightly 

to it.

The warming can be done on an electric hot plate but is most 

conveniently done in a warming oven. (See p. 10.) A steel plate 

9K by 10 inches by K inch with one machined surface supports the 

specimens and their brass mounts. A sheet of cardboard is placed 

on the steel plate, and the sections are placed face down on it. The 

brass tubes are placed over the sections in such a way as to center 

each section in its mount. The steel plate is then placed in the oven 

or on the hot plate until a temperature on its upper surface of about 

100° C. is attained. The steel plate is then withdrawn, and the molten 

sealing wax is poured into the brass tubes until it reaches a level just 

above the indentations. About 30 specimens can be mounted in one 

pouring of sealing wax. When cool, the cardboard is easily removed 

from the mounts.

Sealing wax tends to flow during hot weather, and it has been found 

advisable to give the wax a backing of plaster of paris. After the seal129151 40 2
10 MICROSCOPIC DETERMINATION OF ORE MINERALS

ing wax is cold the section is placed face down, and plaster of paris 

is poured until it almost fills the tube, about % inch of clearance being 

left in order that the mount will rest on its brass edges when righted.

A sealing wax made of 60 grams of flaked shellac, 45 grams of 

resin, and 23 cubic centimeters of turpentine has been found entirely 

satisfactory and is superior to sealing waxes on the market in that it is 

free from gritty filler. The ingredients are placed in a "granite-ware" 

saucepan about 5^ inches in diameter and 2% inches deep, and the 

pan is heated until vigorous bubbling sets in, the mixture being stirred 

from time to time with a tablespoon. When the ingredients are 

thoroughly mixed, the pan is removed from the heat source and the 

mixture stirred until it ceases to bubble. Bubbles on the surface of 

the mount that is, adjacent to the polished surface are objectionable, as they tend to collect grit during the subsequent grinding and 

polishing. The above proportions give enough sealing wax to mount 

about 10 polished sections. It is best to avoid preparing more wax 

than is to be used at one time. If wax is heated 2 or 3 times, so much 

of the volatile constituents is expelled that the wax becomes rubbery 

and must be discarded. Sections mounted as described above and 

kept away from the sun or other sources of heat will last several years 

without any appreciable deterioration of the sealing wax. The 

writer has sections that have been mounted 5 years, most of which 

are free from checking on the surface of the wax and others show it 

only slightly. Experience at Harvard has shown that eventually the 

sealing wax will disintegrate and the specimen will have to be remounted.

Portland cement has been suggested as a mounting medium, 11 but 

experience has shown that it is almost impossible to prevent particles 

of grit from breaking from the surface and scratching the softer 

minerals.

Warming oven. In the Geological Survey laboratory the warming is done in an oven equipped with electric heating coils. The 

oven measures 11 by 11 by 14% inches and was manufactured by 

the Central Scientific Co., Chicago, 111. By means of a thermostat 

a constant temperature is maintained. The temperature range is 

from 70° to 115° C.

A simple home-made oven will give good service. The design 

of this oven was suggested by Dr. Edwin P. Cox, formerly of the 

University of Oregon. It is a box constructed of metal and lined 

with asbestos to prevent the escape of heat. The box measures 16K 

by 12% by 12K inches, outside dimensions. Heating is accomplished 

by six 75-watt electric bulbs arranged in parallel. Each bulb can 

be turned on or off by screwing it in or out in its socket. Such an 

oven constructed at Harvard University gave a temperature after

" Short, M. N., Preparation of polished sections of ores: Ecori. Geology, vol. 21, p. 658,1926.
MOUNTING THE SPECIMEN 11

2 hours of 40° C. with one bulb burning and 120° C. with all six 

burning. The temperature maintained was almost constant. Intermediate temperatures were obtained by varying the number of bulbs 

in the circuit. This oven serves very well in impregnating a porous 

specimen with bakelite, as described on page 30.

Mounting in bakelite. The use of solid bakelite for mounting metallographic and ore specimens has increased greatly since the first edition 

of this bulletin. Solid bakelite has some decided advantages over 

sealing wax or Wood's metal. Once molded it withstands high 

temperatures and is chemically inert. It takes a good polish and is 

attractive in appearance. The disadvantages are the relatively high 

temperature and very high pressure necessary to mold it. This requires a rather expensive hydraulic press. Also, once the specimen 

is mounted it is removed from the bakelite with difficulty.

In several institutions where bakelite is used as a mounting medium, 

the press is home-made. Such a press has already been described 

by Krieger and Bird. 12 Although the cost of the materials is not over 

$30, construction involves a considerable expenditure of time, and it 

is doubtful if such a press would work as well at first as one made by 

an experienced manufacturer from a standard design.

The hydraulic press used by the United States Geological Survey 

(fig. 3) is made by Adolph I. Buehler, 228 North La Salle Street, 

Chicago, 111. (price, $120, 1936). It has given good service. The 

cylindrical mount is 1% inches in diameter and about % inch in height.

The molding tools proper are three in number. The specimen rests 

face down on a steel cylindrical disk \}{ inches in diameter by % inch 

in height. This is termed "base plate" by the manufacturer. A steel 

hollow cylinder l}( inches in interior diameter by 3}£ inches in height 

is placed over the base plate. The specimen is then covered with 

enough bakelite powder to make the combined volume of bakelite 

and specimen 28 cubic centimeters. The grade of bakelite used is 

BM 120. This is then covered with a solid cylinder 1% by 2% inches 

that serves as a piston to compress the bakelite. The upper surface 

of the piston is plane but its lower surface is convex downward. The 

mold assembly, consisting of base plate, hollow cylinder, and piston, 

is a part independent of the hydraulic press and can be removed. 

The assembly rests on an insulating plate of asbestos. This in turn 

is supported by a movable platen that is pushed upward by the hydraulic jack. The movable platen glides on two vertical rods. The 

upper crosspiece is stationary and acts as a buttress, against which 

the lower movable support pushes the mold assembly.

Surrounding the cylindrical mold is a heating unit consisting of a 

coil of wire in asbestos, operating on a 110-volt alternating or direct

" Krieger, Philip, and Bird, P. H., Mounting polished surfaces in bakelite: Econ. Geology, vol. 27, 

pp. 676-678,1932.
12 MICROSCOPIC DETERMINATION OF ORE MINERALS

current and capable of heating the cylinder and its contents to a 

temperature of about 170° C. The temperature actually used, however, is much lower. A thermometer fits into a cylindrical cavity in 

the piston. The press is then pumped up until a pressure on the 

bakelite of 1,000 pounds is attained. The heat is then turned on.

At about 60° C. the bakelite mel ts . This is indicated 

by a sudden lowering of the 

pressure-gage reading. The 

pressure is then increased to 

2,500 pounds and is held there 

until no further melting is indicated. It is then raised to 

3,500 pounds, and heating is 

continued until the temperature reaches 135° C.

The mold assembly is then 

removed from the press and 

quenched in water. The base 

plate, piston, and mounted 

specimen are then removed 

from the cylinder by putting 

the mold assembly in the hydraulic press for ejection under the short rod at the left, 

which projects downward 

from the upper cross piece. 

Below the short rod is a hole 

in the lower support, 1 1A 

inches in diameter. As the 

movable support is pushed 

upward, the base plate and 

bakelite mount are pushed 

out of the cylinder and 

through the hole in the 

support.

The press mounts, according to size, one or more specimens at a 

time, and the time required from start to finish for making a mount of 

the size above indicated is about 20 minutes.

A bakelite press specially designed for the Harvard process and 

described on page 34 mounts four specimens at one time.

Extreme care must be taken to clean out any fragments of bakelite 

adhering to the inside of the mold after the mount is removed. This 

can be done with a knife as the steel in the mold assembly is casehardened. Emery paper should not be used, as it might scratch the

FIGURE 3. Bakelite press. (From catalog of A. I. Buehler, 

Chicago.)
GRINDING AND POLISHING 13

steel. A thin film of beeswax applied from time to time aids in 

preventing the mount from sticking too tightly in the mold. The 

mold must be absolutely clean, and the polished section must not be 

impregnated with Canada balsam before mounting in bakelite, or the 

mount may stick so tightly that it cannot be removed by the hydraulic 

press.

GRINDING AND POLISHING

SAWING THE SPECIMEN

Diamond saw. The diamond saw has been used by lapidaries for 

many years to saw rock specimens and gem stones, but it has not yet 

come into general use in making thin and polished sections. Its use 

has been recommended by Davy and Farnham. 13 The saw consists 

of a metal disk ranging in diameter from 6 to 12 inches and in thickness from 0.02 to 0.03 inch, the edge of which is charged with diamond 

dust. The material used is copper, bronze, or soft steel. Saws purchased from dealers are believed to be cheaper in the long run. The 

diamond saw used by the Geological Survey is manufactured by the 

Felker Manufacturing Co., Torrance, Calif. The life of the saw is 

about 500 rock slides averaging 1 square inch in area, and its cost is 

about $4.60.

Vanderwilt 14 has described the construction of an inexpensive saw. 

A disk, of somewhat larger diameter than desired, is cut with shears 

from cold rolled copper 0.02 to 0.03 inch in thickness. The arbor hole 

is cut with an auger. First a small hole is made for the bit-feed screw; 

then the bit is turned until a depression is cut halfway through the 

disk from one side, after which the hole is completed by cutting 

through from the opposite side. The edge of the disk must be a 

little thicker than the body. This thickening is accomplished by 

tapping the edge lightly but firmly with a small hammer. The disk 

is trued by holding a file or piece of carborundum against its edge 

while it is turning on a shaft. For charging the edge with diamond 

dust a small case-hardened steel roller, which revolves on a shaft at 

the edge of a handle, is used. Detailed instructions for the construction of a similar diamond saw are given by Shaub. 15

Diamond dust used by Vanderwilt was purchased from the Arthur 

Crafts Co., 125 Summer Street, Boston, Mass., for $5 to $6 a carat. 

The size of grain found most satisfactory was No. 1, measuring from 

0.008 to 0.01 millimeter in diameter. The dust is mixed to a thin 

paste with olive oil. Crushed bort used by Shaub was obtained from 

the Diamond Drill Carbon Co. and J. K. Smit & Sons, Inc., both of 

New York, for $1.50 to $2.50 a carat.

" Davy, W. M., and Farnham, C. M., op. cit., p. 2.

» Vanderwilt. J. W., A simple diamond saw: Econ. Geology, vol., 25, p. 222, 1930.

» Shaub, B. M., An inexpensive rock-slicing machine: Econ. Geology, vol. 30, pp. 916-922, 1935.
14 MICROSCOPIC DETERMINATION OF ORE MINERALS

To charge the saw, the steel roller is coated with the diamond paste 

and pressed against the edge of the disk, which is turned several 

revolutions by hand. Then with the motor running, the charging is 

completed. The saw used by Vanderwilt is 6 inches in diameter and 

runs at 1,725 revolutions a minute. A stream of water is turned 

against the contact of the disk and the specimen.

The diamond saw described by Rosenbusch (Wulfing) 16 differs 

from that above described in that the dust is not charged uniformly 

around the periphery of the saw but is introduced in small nicks or 

notches cut at regular intervals around the periphery.

The notches are about 0.5 millimeter deep and are not parallel to the 

radii but make with them angles of 10° to 20°, uniformity of angle not 

being important. Diamond dust is mixed with thick oil and introduced into the notches with the linger. The notches are then closed 

by rotating the disk by hand in a 2-millimeter groove in an agate 

roller.

The diamond saw of Dake and Young 17 is similar to that described 

by Rosenbusch (Wulfing) but differs in some details. Instead of a 

stream of water playing on the edge of the saw to remove the cuttings, 

the saw runs in a mixture of equal parts of crankcase oil and kerosene. 

A pan of oil is so placed that the lower edge of the saw dips into it. 

The saw is made from a disk of Armco or automobile-fender steel, the 

periphery of which has % 2 -inch notches at intervals of about Ke inch. 

A suitable blade for cutting these notches can be ground from a 

hacksaw. The hacksaw is sharpened to a slightly dull chisel edge. 

A light hammer is used to drive the blade into the edge of the disk in 

making the notches. One carat of diamond bort powdered to 220 

mesh is mixed with a bulk of vaseline about the size of a pea. When 

this diamond paste has been carefully worked into the notches on the 

saw blade, the notches are closed by lightly tapping with a peen hammer. After this operation the saw is rotated in a groove in a casehardened steel roller to close the notches and grip the diamond paste 

better.

A little experience is needed to charge and operate a diamond saw 

properly, but the authors state that with proper precautions a 12- 

inch diamond saw will section at least 1,000 square niches of material 

of the hardness of agate.

The 6- to 12-inch saws have a thickness of 0.031 inch (No. 22, U. S. 

S. gage). Larger saws are somewhat thicker. The speed of the 6-inch 

saw is 600 revolutions a minute and that of the 12-inch saw about 300.

Dake 18 has commented on the diamond saw as follows:

Disks of sheet metal ready for charging with diamond dust can be purchased 

from any sheet-metal company at low cost. The saws are cut by a machine so

"8 Eosenbusch (Wulfing), Mikroskopische Physiographie, Band 1, Halite 1, p. 9, Stuttgart, 1924. 

» Dake, H. C., and Young,T. S., Complete and modern cabochon cutting technic: Mineralogist, vol. 3, 

p. 59, 1935. 

is Dake, H. C., written communication.
GRINDING AND POLISHING 15

that the periphery of the disk will be very true, and the arbor hole can be cut to 

any specified diameter. To cut a disk satisfactorily without the proper tools it a 

difficult matter.

The minor difficulties that attend the charging and using of a diamond saw are 

overcome by the operator by the time he has spoiled a few saws. When all the 

diamond grit has been torn from the periphery the disk can be repeatedly recharged until the saw becomes too small to be useful.

It is important to have the edge of the saw true prior to charging. A sharp 

edge of broken chalcedony makes an excellent tool for trimming off and truing 

the edge. This is done while the saw is rotating.

Sometimes a saw gets bent in such a manner that it tends to wobble with considerable side movement. To eliminate this difficulty a hardwood stick 2 by 1 

by 14 inches is pressed against the saw while it is in motion, starting near tha 

arbor and working toward the peripheiy. The pressure is lessened as the periphery is approached. This "massage" will reduce the wobble to a minimum.

A case-hardened steel roller is used not only to force the diamond grit into the 

edge of the disk but also to give the saw a "set." The roller slightly flattens the 

edge of the metal and thus enables the saw to cut its own clearance without binding 

and friction. The roller can be applied with rather heavy pressure. If the "set" 

is lacking on one side of the saw, the disk will tend to bend in the opposite direction, 

thus causing binding and cutting at an angle.

Some workers charge the diamond saw by merely smearing a little diamond 

paste on the edge. This is not nearly so effective as charging notches cut in the 

edge. It is also wasteful of diamond dust. Diamond saws can be purchased 

ready charged for $3 to $6, the price depending on the amount of diamond dust 

used and the size of the disk. A well-charged 12-inch disk should carry at least 

1 carat of diamond dust on the edge.

Corundum and silicon carbide saws. Saws similar to the diamond 

saw but constructed from artificial corundum (alundum) or silicon 

carbide (carborundum) bonded with bakelite have recently come into 

general use. Most of these saws are 6 inches in diameter and X« or %i 

inch in thickness. No metal is used in the construction of the disk, 

as the bonding is sufficiently tenacious to hold it rigid. Such saws are 

used in the same manner as diamond saws. They are cheaper than 

diamond saws and last much longer, as the entire disk is capable of 

cutting, whereas the effective cutting edge of the diamond saw is only 

%z inch or less in width. The disadvantages of the abrasive-bakelite 

saws are as follows:

1. They are somewhat brittle and sometimes fly to pieces when 

force is applied at an angle to the plane of the disk or when too much 

force is used on the periphery. At the University of Arizona the 

operator protects himself against such mishaps by wearing a fencing: 

mask and gloves.

2. They cut a wider groove than the diamond saw.

3. They are somewhat slower in cutting than the diamond saw.

The abrasive-bakelite saw used by the United States Geological 

Survey is 6 inches in diameter by %2 inch in thickness, is made of No. 

60 carborundum grit, and is sold by the National Grinding Wheel Co., 

North Tonawanda, N. Y., for about 65 cents. At 2,000 revolutions 

a minute it saws through a 1-inch granite drill core in 2 minutes or less.
16 MICROSCOPIC DETERMINATION OF ORE MINERALS

The abrasive-bakelite saw used by the writer at the University of 

Arizona is a Norton alundum, catalog No. X6-3760-07-T-2, sold by 

the Norton Co., Worcester, Mass., for about 45 cents. It is 6 inches 

in diameter by }ie inch in thickness. It has a finer grain than the saw 

described above and is a little slower but not so likely to shatter M 

fragile specimen. At 2,200 revolutions a minute it saws through a 

1-inch granite drill core in 2 to 1% minutes.

Both the diamond and abrasive-bakelite saws are superior to the 

lap in that they make possible successive cuts in the same direction.

Patton lathe Jeed. An excellent holder for feeding a rock to a diamond or abrasive-bakelite saw is described by Patton. 19 Some changes

"4 '

FIGURE 4. Diamond saw and lathe feed.

in his design have been incorporated in the holder constructed at the 

University of Arizona (fig. 4). The rock is gripped between two jaws 

made of 4-inch channel iron. The upper jaw is a simple channel 

section 1% inches long. The lower jaw is constructed by sawing a 

channel section at about 45° to its length and welding to it a similar 

section, the whole forming an elbow (pi. 1, a). The upper jaw is 

bolted to the upper end of the lower jaw in such a manner that the 

upper jaw swings freely while the lower jaw remains stationary 

(pi. 1, a). Attached to each jaw by screws is a wooden block. The 

rock is clamped tightly between these wooden blocks. The upper 

block (pi. 1, g) is simple in design, being cut to fit the channel iron. 

In the lower block (pi. 1, d, e, and/) are grooves to accommodate the 

sliding mechanism.

The jaws with the specimen can be moved in two directions at 

right angles to each other parallel to the plane of the diamond saw 

and parallel to its axis. As these motions are similar to those of a 

turning lathe, the entire apparatus may be designated a lathe feed.

» Patton, L. T., A holder for a diamond saw: Econ. Geology, vol. 29, p. 703,1935. . '.-.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 1

; H ^ ; .g. ; i

-f >P^'XS^ «J

H-/V-I 

i-- - ~-->, « /A/ / -^ Steel block with SteeWoctmtti ( fhreacfscf bore 

1^ smooth bore_

i^JSMfflffiMl \jjto\AAAAA'

'xs" board

-H

\Cutout portion 

/ for rivet

-\Ho/e

We/d

7 ^?

Elevation

Smooth_ 

ho/e

B/ock bolted 

to carriage -

Tongue fits slot 

of '/ower Jaw -

I- 7 *" |

"<W£- 

jr '/8">screw "T i Carriage -d--------^cr--- - - -«--Q.

y Plan

^

DETAIL OF PATTON LATHE FEED. 

o, 6, Assembly with exception of wooden jaw blocks; c, lower jaw, steel block, carriage, track, and feed screw; d, e, /, detail of lower wooden block; g, detail of upper wooden block; h, detail of carriage, steel block, and feed screw; i, detail of lower jaw
GRINDING AND POLISHING 17

The motion parallel to the saw is controlled by a track that screws 

onto a secure foundation. A board 1% by 5 by 12 inches makes a 

satisfactory foundation. The track and consequently the movement 

of the specimen must be exactly parallel to the saw to prevent binding. 

The track is 11% inches long and % inch high. Its bottom width is 

3% inches and the top has been cut so as to form a tongue whose cross 

section is shown in plate 1, a. The top of the tongue is 1% niches 

wide, the bottom 1% inches wide, and it is %2 inch high. A carriage 

6 inches long, 3 inches wide,- and 1 inch high slides on the track. A 

channel in the bottom of the carriage fits the tongue of the track 

(pi. 1, A). Because of the difficulty of getting a carriage that would 

slide freely and smoothly throughout the length of the track, the 

Texas Technological College laboratory made the channel about %2 

inch wider than the tongue of the track and placed between the two 

a ^-inch shim 6 inches long. The shim is held against the tongue by 

means of three %-inch screws, which are tight enough to keep the jaw 

assembly from wobbling but loose enough to allow free movement of 

the carriage. The motion of the carriage is controlled by a crank 

that is threaded as shown in plate 1, c. The jaw and carriage assembly 

is attached to the end of the crank by means of a % by %- by %-inch 

block, with a smooth %-inch bore. The crank turns freely in the 

bore, but horizontal motion of the crank is transmitted to the entire 

assembly. This motion is attained by another steel block % inch 

square in horizontal cross section and 1% inches in height countersunk into the track. This is bored with a %-inch female thread 

fitting that of the crank.

Motion parallel to the axis of the diamond saw is attained by means 

of a %-inch screw driven by a 6%-inch wheel. This wheel is manufactured by the South Bend Lathe Works, South Bend, Ind. The screw 

turns in a steel block 2% by 1% by 1% inches (pi. 1, A). This is bored 

with a %-inch female thread. At the left end of the screw is a cylindrical lug % inch in diameter and % inch in length which is attached 

firmly to the screw. The screw is smooth to the left of the lug. It 

turns freely in a smooth hole % inch in diameter cut in the end of the 

channel iron of the lower jaw. Hence the lug overlaps the hole in the 

channel iron, and as the screw turns counterclockwise, the lug presses 

against the channel iron and carries the entire jaw assembly with it, 

away from the saw. On the other hand, when the screw is turned 

clockwise the shoulder of the wheel presses against the outside of the 

channel iron and pushes the entire jaw assembly to the right, toward 

the saw (pi. 1, a). The block in which the %-inch screw turns is fixed 

firmly onto the carriage by two ^-inch screws (pi. 1, h). Hence the 

block has no motion parallel to the axis of the saw. On the bottom 

of the block is a tongue, rectangular in cross section, % inch wide and 

% inch high the thickness of the channel iro^i which fits snugly in
18 MICROSCOPIC DETERMINATION OF ORE MINERALS

a %- by 5-inch slot in the bottom of the channel (pi. 1, i). The block 

overlaps the slot % inch on each side of the slot, and the screw that 

clamps the block to the carriage is tightened sufficiently to prevent 

the jaw assembly from rocking, yet it permits the jaw assembly to 

move freely parallel to the axis of the saw. The manner in which 

this motion is controlled has already been described. The block that 

controls the %-inch screw must be attached to the carriage exactly at 

right angles to it; otherwise the screw will not be parallel to the axis 

of the saw.

The specimen is clamped tightly between the wooden blocks by 

means of a capstan-headed screw that turns in a female thread bored 

through the top of the strap (pi. 1, a). The strap is bent around the 

outside of the upper jaw, forming an inverted U, and swings freely on 

rivets inserted through the sides of the lower jaw. Upward return 

motion of the upper jaw is attained by means of a coiled or auto 

valve spring whose lower end is countersunk a short distance in the 

lower block (pi. 1, d) and whose upper end presses against the upper

block.

GRINDING AND POLISHING MACHINERY

Eardley-Hatton polishing table. To date most of the polishing 

wheels used in metallographic and mineragraphic laboratories have 

been of the Sauveur-Boylston type, described by Murdoch. 20 This 

wheel consists of two disks about 6 inches in diameter which are 

attached to the projecting ends of the armature shaft of an electric 

motor. The disks thus revolve in vertical planes. Cloths of different 

types are fastened to the disks by means of brass hoops. Sheeting 

or linen is generally used for coarser polishing powders such as finer 

grades of carborundum or alundum, chromic oxide, and rouge, and 

billiard cloth for magnesia or "black magnetic rouge." The disadvantages of a polishing wheel of this type are that the abrasive tends 

to fly off the wheel, and the effort required to hold the specimen 

against the vertical disk is much greater than against a horizontal 

wheel.

With the horizontal wheel gravity tends to retain the abrasive and 

aids the operator in maintaining a steady pressure of the specimen 

against the cloth.

Horizontal laps, most of which have been constructed locaUy, are 

used in several institutions.

One of the most inexpensive and yet most effective of these machines 

was designed by Prof. A. J. Eardley and constructed in the Department of Geology of the University of Michigan. This design was 

somewhat modified by J. H. Hatton, and a polishing table was constructed by him in the Department of Geology of the University of

!° Murdoch, Joseph, Microscopical determination of the opaque minerals, p. 19, New York, 1916.
GRINDING AND POLISHING 19

Arizona in September 1935. Hatton's paper 21 gives working drawings 

and directions for construction. The table completely equipped with 

two horizontally revolving laps costs about $110, the expense dependFIOUKE 5. Eardley-Hatton polishing table (top) and detail of mounting (bottom).

ing on the cost of labor. By increasing the length of the table 12 

inches a third lap can be installed at an additional expense of about $30. 

The table proper, constructed of wood, is made unusually strong in 

order to minimize vibration. The bearings supporting the shaft, 

which in turn supports the revolving lap, are the end plate and thrust 

bearing from a Delco-Remy generator used in the Plymouth automo-

» Hatton, J. H., An inexpensive table for polishing ores: Am. Mineralogist, vol. 21, pp. 800-809,1936.
20 MICROSCOPIC DETERMINATION OF ORE MINERALS

bile. The bearing hole into which the shaft fits is cut from a special 

hard steel, which is reamed with difficulty; hence the shaft must be 

turned so as to fit snugly in the bearing. The bearing has an inner 

diameter of 4 % 4 inch. The upper thrust bearing acts merely as a 

guide. The bushing is slightly smaller than is neccessary for the 

K-inch shaft and must be reamed to fit the shaft.

Individual motors are used to permit independent operation of 

each lap. These are K-horsepower Kenmore 110-volt alternatingcurrent motors of 1,750 revolutions a minute and are supplied by 

Sears, Roebuck & Co. The driver pulley is 5 inches in diameter. 

The driven pulley, which is 6% inches in diameter, turns the shaft 

1,350 revolutions a minute. Experience has shown this speed to be 

the most satisfactory for a 5-inch lap. A greater speed tends to 

pull the specimen from the hands and to hurl too much abrasive off 

the wheel, whereas excessively low speeds only prolong the time of 

polishing. Pulleys and the accompanying 42-inch V-shaped fan belts 

are likewise obtainable from Sears, Roebuck & Co. The motor is 

mounted on the back of the table in such a position that the slack of 

the belt is taken up.

The laps, which are brass castings machined on the upper surface, 

can be obtained from the Metallurgical Laboratory of the University 

of Michigan, Ann Arbor, for about $5 each (1936). The laps do not 

screw onto the shaft but have a tapered socket that fits onto the 

tapered upper end of the shaft. As all the laps of this type made at 

the University of Michigan have the same size of tapered socket, it 

is advisable to obtain one of the laps before having the shafts made 

so that the machinist can turn the shaft to fit. A close fit between 

the shaft of the motor and the socket of the lap prevents slipping 

during the polishing process. The lap can be removed almost instantaneously with a moderate upward jerk. The ease of changing laps 

makes it advisable to keep a lap for each type of abrasive, thus obviating the need for frequent changing of cloths.

The University of Michigan also makes laps of greater diameter, 

which have the same-sized socket as that used on the 5-inch lap and 

can therefore be used on this machine. However, a larger lap necessitates a slowing down of the motor to bring the peripheral speed 

down to that of the 5-inch lap. Larger laps are also unnecessarily 

wasteful of cloth, as the cloth must be replaced on the appearance of 

the smallest hole or tear.

The most satisfactory method of attaching the cloth to the lap is 

by the use of a % 6-inch fairly strong coiled spring made of No. 20 

wire. The spring is cut, and the ends are joined by wiring or welding 

or with a hook and eye. The cloth is easily put on and wrinkles can 

be readily removed by this method.
GKINDING AND POLISHING 21

The polishing machine is kept in the laboratory with the microscopes 

and opaque sections, thus allowing frequent polishings during identification of the mineral.

Suggestions for construction of grinding table. A grinding table may 

be constructed according to the same design as that of the polishing 

table. The grinding laps are usually 10 inches in diameter, hence 

wider holes must be provided on the table top. If a three-lap table 

is desired, the table should be lengthened by 15 inches. This would 

allow for a distance of 78 % inches (outside dimensions) between 

supporting posts and an over-all table-top length of 86 inches. The 

lap holes should be cut 15 inches in diameter with their centers 11% 

inches from the edge of the table and 25 inches apart.

The grinding laps are metal disks 1 inch thick. Brass laps like 

those used in the polishing machine are used as bases for the disks. 

The disks are attached to the brass laps by means of lag bolts % inch 

in diameter and Ks inch long. Four holes are drilled through the 

brass laps to accommodate four holes similarly placed in the disks. 

The holes in the brass laps are not threaded, but the holes in the disks 

have threads which fit similar threads at the ends of the bolts. The 

holes in the disks should not be over % inch deep, so that the life of 

the disks may be prolonged as much as possible.

Care must be taken to center the disk on the center of the shaft to 

avoid vibration and undue wear on the bearings.

The advantages of a grinding machine of this type are as follows:

1. Metal disks are less expensive than large laps that are provided 

with screw threads or other methods of attachment.

2. Worn or uneven disks can be machined easily.

If only two laps are provided, cast iron is preferable for coarse 

grinding and wrought iron for fine grinding. The cast-iron lap is 

made by casting a disk of the requisite diameter and thickness and 

then machining one surface and the periphery. The wrought-iron 

laps are cut from a 1-inch sheet of boiler plate, usually by means of 

an acetylene torch, and the periphery machined. If the surfaces 

are not sufficiently smooth, one of them must be machined.

The third lap, if provided, is made of copper and is used for the 

finest-grinding abrasives. Copper laps are used for this purpose at 

Columbia University and at the California Institute of Technology. 

Concentric grooves are cut on the surface of the copper to prevent 

grabbing of the specimens. Directions for cutting such grooves are 

given by Vanderwilt.22

The laps on the grinding table should not be run faster than 750 

revolutions a minute; otherwise the abrasive will be hurled off. The 

best set of pulleys to use with the'Kenmore motor described above is 

a 3-inch driver pulley and a 7-inch pulley.

" Vanderwilt, J. W., Improvements In the polishing of ores: Econ. Geology, vol. 23, p. 307,1938.
22 MICROSCOPIC DETERMINATION OF ORE MINERALS

ABRASIVES AND POLISHING POWDERS

For many years natural emery was the only abrasive used for grinding metallographic and ore specimens. It is still widely used, especially in optical grinding, and emery papers are extensively used to 

polish metallographic specimens. Since the invention of the electric 

furnace silicon carbide has largely replaced emery as an abrasive. 

This is sold under such trade names as "carborundum'' and "crystalon," each manufacturer having his own name. In addition to 

silicon carbide, artificial corundum is made in the electric furnace 

and is well known under the trade name "alundum.''

Examination under the microscope shows that most of the coarser 

abrasives are well sized, as they are classified by ordinary wire screens. 

But the mechanical methods of sizing the finer abrasives are not 

completely effective, and a small proportion of coarse particles is 

usually mixed with the predominant fine material. This is highly 

injurious, as coarse particles cut grooves across a surface that otherwise would have been well polished.

SIZING OF FINE ABRASIVES

The sizing of the finer abrasives is best accomplished by settling in 

water. The simplest method consists of mixing the abrasive with 

water in a cylindrical jar. The coarser particles settle to the bottom 

of the jar in a comparatively short time, while the finer particles 

remain suspended. The liquid with the suspended fine material is 

drawn off and used. This method is effective and quick but not sufficiently refined for use with the Harvard polishing process. Details 

of the method as used by the United States Geological Survey to 

classify chromic oxide and rouge are as follows: The material is mixed 

in about 20 times its weight of water and dumped into a tall cylindrical 

jar 7.2 centimeters in diameter and 56 centimeters in height. Enough 

water is added to nearly fill the jar. The contents are then well 

shaken and allowed to stand 10 minutes. The top 15 centimeters 

of the contents of the jar is withdrawn by means of a siphon, the end 

of which should start at the top and be lowered slowly so as not to 

disturb the lower sludge. The material drawn off is placed in a 2.5- 

liter bottle. The measuring jar is again nearly filled with water, and 

the contents are thoroughly stirred and allowed to stand 10 minutes. 

The top 15 centimeters is again siphoned off into the bottle containing 

the sludge already removed. This process can be repeated until the 

2.5-liter bottle is filled. The bottle is allowed to stand overnight, 

and the sludge settles to the bottom. The excess of clear water is 

siphoned off, and the sludge is ready for use. This sludge is termed 

"10-minute chrome," or rouge. It is obvious that the largest grains 

in the material remaining suspended 15 minutes would be smaller than 

those of the 10-minute suspension. Hence the size of the largest
GRINDING AND POLISHING 23

particles can be controlled. On the whole this method works very 

well, but occasionally an aggregation of fine grains surrounds a coarse 

grain and keeps it suspended longer than it would have been had its 

path been unimpeded.

Rodda method. A refinement of the above method has been described by Rodda.23 He calls attention to the necessity of thoroughly 

dispersing the particles before classifying them. This is accomplished 

by grinding in a ball or pebble mill for 2 hours. A little sodium silicate 

is added to the mixture as a dispersing agent. The mixture is dumped 

into a 2-liter settling cylinder, which is 5 inches in diameter. The 

mixture recommended is 1 cubic centimeter of 40-percent sodium 

silicate and about 0.5 liter of water for every 

100 grams of abrasive. Rodda's finest abrasive 

is a 4-hour suspension. The top inch is carefully 

siphoned off, care being taken not to allow the 

tip of the siphon to extend more than 1 inch below the surface. The remaining contents of the 

jar is allowed to stand another 4 hours, and then 

another inch is taken from the top. This is repeated until the level of the liquid approaches 

the caked sludge at the bottom of the cylinder. 

The caked material is then stirred while fresh 

water is added until the jar is nearly full. It ie 

then allowed to stand another 4 hours, and 1 

inch is drawn from the top. The process can be FIGURE e.-siphon tor 

repeated until the suspension is too thin for use. 

The abrasive thus obtained is that which 

falls in the liquid at the rate of 1 inch in 4 hours or slower.

The same process was repeated for 30 minutes, 3 minutes, and 20 

seconds, and it was found that the average grain size of each preparation was three times the si?e of the preceding one.

The form of siphon used by Rodda is shown in figure 6. The limb a 

is placed in the liquid to be siphoned. The mouth is placed at c, and 

suction is applied while the end of b is covered with the finger. When 

once the liquid starts flowing the finger and mouth are removed and 

the siphon functions normally.

Vanderwilt cZas^e/v- Commercial abrasives are not sufficiently 

well sized to use in the later stages of the Harvard polishing process. 

(See p. 37.) In order to prepare suitable abrasives of finer grain, 

Vanderwilt 24 devised a classifier to sort abrasives in accordance with 

their grain size. This consists of a series of siphons and bottles 

arranged in sequence as shown in figure 7. Jar 1 is placed high .to

23 Rodda, J. L., Preparation of graded abrasives for metallographic polishing: Am. Inst. Min. Met. Eng. 

Tech. Pub. 438, 1931. 

'* Vanderwilt, J. W., A laboratory method for grading abrasives: Econ. Geology, vol. 24, pp. 853-859,1029.

sives. (After Rodda.) 

text for explanation.

Sec
24 MICROSCOPIC DETERMINATION OF ORE MINERALS

furnish head for the flow of water. Dimensions of the jars are given 

in the table below. The size of jars 1 and 8 need not be exactly as 

specified, as they are not included in the closed-siphon system. In the 

apparatus set up by the present writer at the University of Arizona 

jar 1 is a tin cylinder 15 centimeters in diameter and 70 centimeters

FIGURE 7. Vanderwilt abrasive classifier, TJnivi;rt,;iy >! Arizona.

in height. A glass cylinder would be preferable, but one of this size 

is expensive. A vertical slot 7.5 centimeters wide is cut in the tin 

cylinder, and over this slot is cemented by means of "Arrowhead" 

cement a strip of celluloid, which permits sufficient visibility within 

the jar. In the writer's apparatus jar 8 is an ordinary water bucket. 

The tubes between the jars are 7 millimeters in interior diameter.

Data on Vanderwilt classifier

Jar

2... -_.------.---_.------.-... ._

3_____. ------.-.-..------.-.._..

5................ ..............

Diameter of 

jars 

(cm.)

12 

6 

7.9 

9.5 

12.7 

14.6 

16.5 

12

Inlet 

above 

bottom 

of jars 

(cm.)

24 

3 

3 

0.6 

0.6 

0.6

Upward 

velocity 

(cm. per 

hour)

15.1 

8.7 

6.1 

3.4 

2.6 

2.1

Size of grains 600 

alundum.Vanderwilt, 1929

Largest 

grain 

(mm.)

0.008 

.0047 

.0035 

.0025 

.002 

.0015

Most of 

material 

(mm.)

Size of grains 600 

alundum, Arizona, 

1938

Largest 

grain 

(mm.)

0.064 

.019 

.010 

.007 

.007 

.003 

.004

Most of 

material 

(mm.)

0.012 

.006 

.004 

.0025 

.001 

(') 

(')

i 0.001 or less.

Jars 2 to 7 are closed at the top by rubber stoppers, in each of 

which are bored three holes. Two of the holes admit the tubes leading 

to the jars on each side. Through the third hole is a short straight tube 

at the upper end of which is attached a short rubber tube closed by a
GRINDING AND POLISHING 25

stopcock. Opening this stopcock permits the air trapped at the top 

of the liquid to escape and the liquid below it to rise in the jar.

The abrasives most commonly used for classifying are 600 alundum 

and tin oxide. According to Vanderwilt it is best to let clear water 

flow through the classifier for several hours before the abrasives are 

added. In this way small leaks in. the siphon or air bubbles in the 

tubes can be eliminated.

The abrasive to be sized is then placed in jar 1 in the proportion of 

58 grams to the liter of water. The flow of the liquid is regulated at 

60 to 65 drops a minute by the pinchcock between jars 7 and 8.

The rate of settling of grains against an upward current of water is 

a function, of the size and density of the grains, but authorities differ 

as to the laws regulating this rate. In a general way, however, it 

can be stated that the faster the current the larger the grains carried 

upward. The table shows that the diameter of jars 2 to 7 increases 

progressively, hence the velocity of the flow decreases in proportion.

A motor rotates a long shaft to which is attached at the lower end a 

small propeller. This is placed near the bottom of jar 1, and the shaft 

is rotated at a speed just great enough to keep the mixture stirred up, 

or about 100 to 200 revolutions a minute. In the apparatus constructed by Vanderwilt the shaft is attached directly to the motor. 

To reduce the motor to this low speed, an adjustable rheostat is 

placed in the electric circuit. Vanderwilt describes a home-made 

water rheostat, but an ordinary coil rheostat would probably prove 

more satisfactory.

In the present writer's apparatus an 8-inch wooden pulley attached 

to the upper end of the shaft is used to reduce the speed (fig. 7). The 

advantage of this arrangement is that the motor can be run at its 

rated speed and is less liable to overheat. The motor shown is taken 

from a smaU electric fan. Resting in the bottom of jar 1 is a short 

iron bar, the ends of which are rounded so as to fit the jar. This bar 

can be withdrawn when the abrasive is removed and the jar cleaned. 

A cone on the lower end of the shaft fits into a conical depression 

bored in the center of this bar, thus making the shaft revolve in the 

center of the jar and preventing the propeller from striking the sides 

of the jar.

The bulk of the abrasive settles in jar 2. The finest material reaches 

jar 8. If it is desired to collect this fine material, the mixture is 

acidified with nitric acid when the jar is half full. Then enough concentrated acid is added to make the mixture one part of acid to 900 

parts of water. The fines will settle in a few hours, after which the 

upper part of the jar is siphoned off.

The classifier can be run continuously until one of the jars is filled 

with sized abrasive up to the bottom of the outlet tube. The flow is

129151 40 3
26 MICKOSCOPIC DETERMINATION OF ORE MINERALS

then shut off by the stopcock between jars 7 and 8, and all the rubber 

tubes between the jars are clamped to prevent abrasive from being 

carried from one jar to another. The filled jar is then removed from 

the system and emptied. It is then refilled with clear water, reconnected in the system, the clamps are loosened, and the flow is resumed. 

All joints must be kept airtight in order to avoid all air-water interfaces, which would invariably tend to collect larger grains by flotation 

and then at erratic times to dump these grains into jars farther 

along in the sequence.

The greatest difficulty encountered in the operation of the classifier 

is the collection of air bubbles at the tops of the tubes between the 

jars, especially between jars 1 and 2. The bubbles hinder the passage 

of the abrasive particles. The bubbles can be removed by opening the 

discharge stopcock (between jars 7 and 8) for a few seconds. The 

accelerated current carries over the bubbles to the next jar, where they 

collect above the liquid in the jar. The bending of glass into sharp 

turns usually results in serious constrictions which tend to trap air 

bubbles. Copper tubing of X-inch interior diameter is easily bent 

while cold into sharp curves with no noticeable decrease in diameter. 

In the writer's apparatus the curved parts of the tubes are constructed 

of copper tubing and the straight parts of glass, the two being joined 

by short sections of rubber tubing.

Although the classifier needs little attention during its operation, 

it should be inspected every few hours. Regulation of the flow calls 

for some skill. In the course of a few hours the flow tends to diminish, 

and the stopcock must be opened sufficiently to bring it back to normal. 

For instance, 8 hours after regulating the flow at 65 drops a minute it 

may have diminished to 40 drops a minute.

SIZES OF SOME WELL-KNOWN ABRASIVES

The following table summarizes the characteristics of several of the 

more important commercial abrasives. The size was measured 

microscopically by means of a micrometer ocular.

Characteristics of abrasives used in polishing mineral specimens

303 emery (American Optical Co.)-

Size (mm.)

Largest 

grain 

observed

0. 29! 

.20 

.Ofi^ 

.054 

.075

.072 

.045

Most of the 

material

±0. 156 

. 052- . 104 

.013- .036 

. 009- . 027 

.004- .01

. . 009- . 027 

. 009- . 018

Characteristics

Angular and well sized. 

Do. 

Do. 

Do. 

Contains a considerable proportion of 0.025 

mm., also much fine material, poorly 

sized. 

Angular and well sized. Largest fragments are not corundum but rock 

minerals. 

Do.
GRINDING AND POLISHING 27

These observations are more or less of a qualitative nature but 

serve to show the relative characters of the different abrasives. This 

is of some importance, for the trade names give no indication of real 

sizing of the material. These observations were first made by Sampson. 25 They have been checked and supplemented by the present 

writer and further checked by C. S. Ross.

POLISHING PROCESS AT THE GEOLOGICAL SURVEY

Procedure. The procedure outlined below is the one used in the 

Petrographic Laboratory of the Geological Survey for grinding and 

polishing unmounted specimens:

A. Removal of chip from hand specimen by chisel or with diamond saw.

B. Grinding of flat surface with 120 carborundum. Used only on hard ores; 

omitted on soft ores.

C. Grinding with F carborundum. This is the first grinding with soft ores 

and the second grinding with hard ores.

D. Grinding with 600 carborundum.

E. Polishing on muslin-covered lap with GOO alundum.

FI. Polishing on billiard cloth with magnesia oxide or black magnetic rouge. 

Omitted for some hard ores.

F2 . Polishing on muslin-covered lap with chromic oxide or rouge. Used in 

place of FI on some hard ores. Omitted for most ores.

A. The desired fragment of the specimen is first chipped off by 

means of a cold chisel. If a diamond or abrasive-bakelite saw is 

not available, a satisfactory surface can be prepared in a surprisingly 

short time by grinding on a horizontal lap with carborundum powder. 

Ordinarily the surface prepared should not exceed 1 square inch 

in area.

B. If the specimen consists of a large proportion of hard mineral 

such as pyrite, it is best to do the first rough grinding with 120 carborundum. This is continued until a surface of sufficient area is 

obtained. The specimen is then removed and carefully washed to 

remove adhering carborundum grit. The hands should likewise be 

cleaned carefully to prevent coarse grit from getting into the succeeding finer grinding material. If the specimen consists mostly of 

softer minerals and contains only a minor proportion of pyrite or 

other hard minerals, it is best to begin the grinding process with F 

carborundum, as the coarser grades tend to tear deep holes and 

gashes in the softer minerals. A horizontal steel lap 12 inches in 

diameter is used. Its speed is 1,150 revolutions a minute.

After grinding the first surface, corners and projections of the 

specimen should be ground off in order to avoid excessive grooving 

of the surface by fragments breaking loose during the succeeding 

stages. It is advisable to bevel back the edge of the surface at least 

60° from the face, especially on specimens that are to be mounted in

 Sampson, Edward, written communication.
28 MICROSCOPIC DETERMINATION OF ORE MINERALS

brass, as the mounting medium is more or less viscous and would have 

difficulty in flowing under a flatter beveled edge. The beveling is 

best done by holding the chip against the side of a rapidly rotating 

carborundum or emery wheel, which is kept wet to avoid heating. 

If such a wheel is not available the beveling can be done on the horizontal lap.

C. The section is then ground on a horizontal steel lap with F carborundum. The pits and scratches made by the coarser abrasive 

are removed, and a new series of pits and scratches, shallower than 

the first set, are made. Usually a minute suffices for this step. The 

speed of this lap is likewise 1,150 revolutions a minute.

D. The hands and specimens are carefully washed to remove any 

adhering F carborundum. The final grinding is done on a horizontal steel lap with 600 carborundum. The pits and scratches made by 

the F carborundum are ground out, and a still narrower and shallower series is made. Thirty seconds usually suffices for this stage 

of the grinding process. The section is then ready for polishing. 

The speed of this lap is likewise 1,150 revolutions a minute.

E. In the first stage of the polishing process 600 alundum is used 

on a lap covered with muslin or fine-textured linen. The linen is 

more expensive but requires changing less often because of its greater 

resistance to abrasion. The lap is 6 inches in diameter, and its speed 

is 1,150 revolutions a minute. The cloth is first wet, then stretched 

tightly over the surface of the lap by means of a hoop around the 

periphery or by means of strong elastic bands held in place by a 

groove around the periphery. The amount of time required for 

polishing with 600 F alundum varies with the specimen. It is best to 

polish for 1 minute at a time, observing the effect under the microscope at the end of each step. When no further improvement is 

noted, the section is ready for the next stage. Ordinarily the 600 F 

alundum accomplishes all it can do in 3 minutes. If pits remain 

at the end of that time, it is best to take them out on the chrome or 

rouge wheel. With some crumbly or very soft minerals the alundum 

keeps making new pits as fast as it removes the old ones. Experience 

is the best guide in determining when to discontinue polishing with 

600 F alundum.

FI. The final polishing is accomplished by means of either magnesia or "black magnetic rouge" on a horizontal lap covered with 

billiard cloth. The trade name of this cloth is Samson cloth, and it 

is supplied by J. Magann & Co., 78 Washington Street, Boston. 

Magnesia (MgO) is effective in polishing pyrite and minerals of intermediate hardness. It also gives a good polish to softer minerals 

but tends to form more scratches than black magnetic rouge. Magnesia is mixed with about 2 parts by volume of water and blown 

out of a wash bottle. Credit for discovering the value of magnesia
GRINDING AND POLISHING 29

as a polishing agent is due to Dr. Hans Schneiderhohn, of the University of Freiburg, Germany. He advises keeping the bottle tightly 

stoppered, as the magnesia tends to absorb carbon dioxide from the 

air, forming magnesium carbonate, which scratches the softer minerals. 

Magnesium oxide actually has a greater hardness than the carbonate 

but breaks down in polishing to an exceedingly fine state of subdivision. Magnesia will remove fine pits as well as scratches from 

pyrite. Five minutes or less usually suffices to give a perfect polish 

to pyrite. Additional time may be advisable for polishing some 

minerals such as marcasite, which are prismatic in habit and more 

or less porous. The writer has spent as much as 15 minutes to good 

advantage. Magnesia is ineffective in polishing specularite and some 

other minerals of equal or greater hardness; for these minerals chromic 

oxide or rouge is more satisfactory.

"Black magnetic rouge" is a polishing powder manufactured by 

Binney & Smith, 41 East Forty-second Street, New York. Its use 

was recommended to the Geological Survey by Prof. Alan M. Bateman. 

According to the manufacturers it is obtained by a patented process 

of precipitation and contains 99 percent of Fe203 . In general it has 

the same effect as magnesia, but it does not cut as fast. It is the 

best medium for polishing soft minerals, such as galena and chalcocite. 

It is more stable than magnesia and does not deteriorate when exposed 

to the air. It is mixed with a small proportion of water and applied 

to the cloth by means of a small paint brush. When used wet it 

will remove practically all the scratches, but in spite of care to prevent 

contamination by extraneous grit, a few scratches will persist, however 

long the polishing is continued. If the section is to be photographed, 

these last scratches can usually be removed by the following procedure, 

which was developed in the Geological Survey laboratory by F. S. 

Reed and J. L. Mergner.

Apply moist magnetic rouge to the rotating lap, and press the 

specimen very lightly against it, giving the specimen a slow rotation. 

Continue until the lap is entirely dry. Some skill and experience are 

required in manipulation. If polishing is continued too long after 

the lap is dry, the specimen tends to overheat. If polishing is stopped 

too soon the magnetic rouge tends to gum up and adhere to the 

specimen.

F2 . Polishing on a billiard cloth with magnesia or black magnetic 

rouge as above described is effective for by far the greater number of 

specimens. It fails, however, to give a good final polish to very 

hard minerals, such as specularite, chromite, franklinite, and the hard 

manganese oxides. For this purpose fine floated chromic oxide or 

rouge on a muslin- or linen-covered lap is recommended in place of 

magnesia as the final polishing agent. Neither fine floated chromic 

oxide nor fine floated rouge is available in the market, but both products
30 MICROSCOPIC DETERMINATION OF ORE MINERALS

are prepared in the Geological Survey laboratory by grinding in a 

small ball mill for 100 hours. The batch is removed from the ball 

mill and classified by the method described on page 22.

The effect produced by rouge is practically identical with that 

produced by chromic oxide, but the chromic oxide is somewhat faster 

in its polishing action. Both powders are very effective in polishing 

quartz, glass, and the silicate minerals.

General remarks. The average time for grinding and polishing 

an unmounted specimen by the process above described is 12 to 15 

minutes. If the specimen is mounted in brass, an additional 10 

minutes is required. The mounting should be done after the first 

flat surface is ground in other words, between steps B and C. If 

the first surface is ground with F carborundum, it should be mounted 

and then again ground with F carborundum until all edges of the 

brass tube rest evenly on the steel lap and the entire surface of the 

sealing wax has come into contact with the abrasive. The upper 

edges and corners of the brass mount should be beveled off after this 

stage of grinding is complete, in order to prevent the sharp edges of 

the brass from tearing the polishing cloth. This is done by holding 

the mount against the edge of a carborundum wheel. The wheel 

should be at least IK inches thick, as its cutting edge should overlap 

the specimen. The wheel should be kept wet in order to prevent 

overheating the sealing wax. The bevel should make an angle of 

about 45° with, the sides of the tube. Mounting in brass slows clown 

the polishing somewhat but gives a more nearly plane surface to the 

specimen and does not affect adversely the quality of the polish.

Impregnating the specimen. By far the greater proportion of 

specimens to be polished are strong enough to maintain themselves 

during the operations of grinding and polishing without noticeable 

disintegration. In ores that are porous and crumbly, however, the 

grains tend to work loose before they are completely polished, and 

when they are carried across the face of the specimen they groove and 

scratch it. This may be avoided by filling the pore spaces with some 

binding medium that is carried into the pore spaces by capillarity or 

under a vacuum. The binding medium must be dissolved in some 

fluid of low viscosity, in order that the resulting solution can readily 

enter the pores. The section is heated sufficiently to evaporate the 

solvent, leaving the binding medium in the pores. As impregnation 

is most effective at the outer surface of the specimen, it is advisable 

to give a flat surface to the specimen before it is impregnated. Some 

specimens are too crumbly to permit grinding even a rough flat surface. Such specimens should have a first impregnation before grinding and a second impregnation afterward.
GRINDING AND POLISHING 31

The technique of impregnation with bakelite varnish has been 

described by Koss, 26 who developed the method as used in making 

thin sections of friable rocks. In his paper in Economic Geology a 

method of impregnating specimens with bakelite under a vacuum is 

described.

The procedure described by Ross is highly successful with polished 

ores. The specimen is immersed in a dish containing the solution of 

bakelite in ether and alcohol or ether and acetone. The bakelite 

should be thinned to about the consistency of a thin watery sirup. 

The dish is covered to prevent too rapid evaporation of the solvent. 

The bakelite solution is drawn into the pores of the specimen by 

capillarity. After 4 to 8 hours the cover is taken off the dish, and 

the contents are exposed to the air over night. The solution is then 

about the consistency of unthinned bakelite or possibly still thicker. 

The specimen is then removed from the dish and placed with its flat 

side uppermost in a warming oven. Care should be taken to have a 

generous coating of the solution on the surface and to keep the surface 

horizontal so that the solution will not run off. A temperature of 

approximately 40° C. is maintained for about 24 hours. The temperature is then gradually raised to about 110° C. and maintained there 

an additional 24 hours. At the end of that time the bakelite is hard 

and resembles amber. It is futile to attempt to hasten the process 

of curing bakelite. The application of too much heat or a too rapid 

increase in the heat will cause its decomposition and the formation of 

bubbles, which tend to blow out material already in the pores.

The excess bakelite is ground oft' the surface on a horizontal lap, 

with F carborundum. The specimen is then examined under the 

microscope. If impregnation has not been sufficient, the entire 

process can be repeated, as cured bakelite is not affected by the solvent 

used. The second impregnation is usually not necessary, however, as 

bakelite holds the mineral grains firmly, and a specimen with 50 percent of its pore spaces filled with bakelite will take a satisfactory 

polish.

The method above described is, in the writer's opinion, the best 

method yet devised for impregnating a specimen. It has the disadvantage, however, that it requires at least 2% days for its 

completion.

Another method that is quick and sufficiently satisfactory for most 

purposes involves the use of "kollolith," an artificial product somewhat similar to Canada balsam; it is manufactured by Voigt & 

Hochgesang, Gottingen, Germany. It is prepared in two grades, 

hard and soft. The hard grade is recommended for impregnating 

the specimen. A piece of kollolith is cut or broken off from the

2' Ross, C. S., A method of preparing thin sections of friable rocks: Am. Jour. Sci., 5th ser., vol. 7, p. 

483, 1924; Methods of preparation of sedimentary materials for study: Econ. Geology, vol. 21, pp. 454-468, 

1926.
32 MICROSCOPIC DETERMINATION OF ORE MINERALS

tube in which it comes, placed in a porcelain dish, and heated until 

it melts. The specimen is ground with a rough flat surface and then 

immersed in the melt, which is boiled for 3 minutes. The specimen 

is then removed, care being taken to keep the flat surface uppermost 

so as to prevent the sirupy melted kollolith from running off. On 

cooling the kollolith again hardens.

If kollolith is not available, Canada balsam diluted with xylol forms 

a satisfactory substitute. The balsam is placed in an evaporating 

dish and slowly heated over a hot plate or Bunsen burner, and sufficient xylol is added to bring the mixture to a light sirupy consistency. 

If a Bunsen flame is used, it should be kept rather low, and a wire 

screen should be placed between the flame and the evaporating dish. 

If the mixture catches fire, the fire is easily extinguished by placing an 

asbestos board over the dish.

The specimen to be impregnated is immersed in the mixture, which 

is then heated to the boiling point. The volatile constituents are 

slowly boiled off. The mixture should be tested frequently by 

dipping the end of a match in it and slowly lifting the match. With 

insufficient cooking a drop adheres to the match, but as the proper 

consistency is approached, a long, slender thread is formed extending 

from the surface of the balsam to the match as the match is lifted. 

When the proper degree of cooking is attained, the thread is still 

flexible but can be broken by bending.

The melt is then cooled and becomes more viscous as it cools. 

When it has about the viscosity of a rather thick sirup, the specimens 

are withdrawn from the dish face up and cooled to room temperature. 

The balsam should be slightly sticky to the touch. If the balsam is 

overcooked, it is too brittle and tends to be pulled from the mineral 

during the polishing process.

HARVARD POLISHING PROCESS

Summary. The most effective method of polishing yet devised is 

that developed in the Laboratory of Mining Geology, Harvard University. After years of trial in the perfection of a method of polishing 

mechanically, certain fundamental principles were established and 

put to use. Vanderwilt 27 has described the method as it stood in 

1927. The method and the machine for executing it have been undergoing further improvement since that time. The method is based on 

two fundamental factors:

1. Polishing is a grinding process in which the grinding scratches 

are of submicroscopic width. To obtain a polish by fine grinding, 

the abrasives must be very carefully sized, and in the final grinding 

the diameter of the largest grains must not exceed 0.001 millimeter. 

Vanderwilt devised a method of sizing grains based on slow circulation

27 Vanderwilt, J. W., Improvements in the polishing of ores: Econ. Geology, vol. 23, p. 292,1928.
GRINDING AND POLISHING 33

of the sludge through a series ofji cylinders. (See p. 23.) This 

method is slow but gives a much more uniform product than the quick 

method described on page 22 of this paper.

2. Cloth laps are dispensed with, and the polishing is done on laps 

made of soft metal. Water is excluded in the final stages of the 

process, and the abrasive is carried in oil. The laps run at a low 

speed, the process from beginning to end requiring 3 to 12 hours. 

The grinding and polishing are all done by machinery; each machine 

holds six specimens, which are polished simultaneously.

This method gives a surface of almost no relief, even between 

minerals of great contrast in hardness, such as pyrite and galena. It 

is superior to the usual polishing methods for preparing surfaces for 

microscopic observations and photomicrography at high magnifications. It is particularly valuable in the investigation of gold ores in 

which the gold occurs as minute inclusions in hard minerals such as 

pyrite and arsenopyrite. By this means gold can be studied in ores 

of as low grade as 0.3 ounce to the ton. It is also markedly superior 

to older methods of polishing in the study of thin alteration zones 

between hard and soft minerals, such as zones of incipient chalcocite 

enrichment along boundaries between pyrite and chalcopyrite. The 

principal disadvantages of the method are that it requires expensive 

apparatus and more time and a higher degree of technical skill than 

the usual methods.

Principles of polishing. In the usual grinding and polishing procedure the grinding is done on a cast-iron plate, and the abrasion of the 

surface of the specimen is accomplished by grains of the abrasive 

rolling freely on the surface of the plate. The actual work of abrasion 

is to a large degree a bombardment of the surface of the specimen by 

these rolling particles. This causes pits and scars on the specimen 

and a shattering beneath the surface which may extend to a considerable depth. This fracturing and fissuring loosens particles on the surface, which are plucked out and removed. According to Vanderwilt 

it is impossible to polish by merely reducing the size of the pits produced by rolling grains even with the finest abrasives obtainable. 

Magnesia grains, for instance, with an average diameter of about 

0.0008 millimeter, produce pits in soft minerals when used on a hardmetal lap.

In the Harvard process all stages of grinding except the first are 

accomplished on soft-metal plates; these are made of soft ductile 

metals like copper, tin, and lead, which are readily charged with the 

abrasives. When the abrasive becomes embedded in the surface of 

the lap, it cannot be removed by washing, but is removed by planing 

off a layer from the lap with a lathe. The lap, with its embedded 

abrasive, grinds the specimen by cutting a series of grooves. By 

grinding on a series of soft-metal laps charged with successively finer-
34 MICROSCOPIC DETERMINATION OF ORE MINERALS

grained abrasives, these grooves finally become so narrow that a mirror 

surface results.

According to this view, which is by no means universally accepted, 

there is no essential difference between grinding and polishing, as 

polishing also produces a grooved surface, in which the scratches are 

of submicroscopic width. With a hard-metal lap a rolling abrasive 

grain is capable of cutting a groove equal to its full thickness; with a 

soft-metal lap, however, at least one-half the thickness of the abrasive 

grain is buried in the lap, and therefore it can cut the specimen less 

deeply than a grain of equal size rolling freely 011 a hard lap.

Polishing on a cloth-covered lap produces strong relief between 

hard and soft minerals. Because the fabric yields, it reaches the surface of the softer mineral, which continues to wear faster than an 

adjacent harder mineral. Hence, the longer the polishing is continued, 

the greater will be the relief produced. Moreover, polishing is a slow 

process at best, but this is counteracted in part by running the polishing wheel at a high speed. This accentuates the abrading action and 

produces new pits and other imperfections of the surface while others 

are being removed. As mentioned on page 42, polishing wheels are 

run at speeds as high as 1,800 revolutions a minute.

In the Harvard process the metal laps are unyielding, and the softer 

minerals cannot wear away faster than the harder ones. In other 

words, the polishing of the harder minerals regulates the time of 

polishing. The speed of the polishing laps does not exceed 150 

revolutions a minute, and the total length of time for grinding and 

polishing may amount to 12 hours. This length of time would make 

the process prohibitive in cost were it not for the fact that a machine 

has been devised that grinds or polishes six specimens simultaneously.

Graton-Vanderwilt polishing machine. The successful design and 

construction of the machine for grinding and polishing has been a 

triumph of persistent and painstaking endeavor coupled with scientific research and mechanical skill. The scientific features and the 

principles on which the design was based were worked out by L. C. 

Graton and J. W. Vanderwilt in the Laboratory of Mining Geology, 

Harvard University. Some details of the design were perfected by 

David Mann, formerly proprietor of the Mann Instrument Co. 

(now the Mico Corporation), of Cambridge, Mass., which constructed 

the machines.

Before the machine could be successfully designed, a suitable 

mounting medium for the specimens had to be found. Bakelite has 

proved to meet this need admirably. (See p. 11.) The specimen 

is placed face downward in the bottom of a mold, and the mold is 

then filled with powdered bakelite and placed in a hydraulic press 

that contains heating units. In the press used at Harvard four 

rectangular openings 1% by 1% inches in cross section are cut through
GRINDING AND POLISHING 35

a steel block 1% inches thick. Tightly clamped to this block is a solid 

steel block % inch thick. Four molds are thus formed from the two 

steel blocks, and four specimens can be mounted simultaneously. 

A steel die, whose cross section is the same as that of the mold, presses 

downward on each specimen and the bakelite powder covering it. 

The pressure is high, at least a ton to the square inch, and the temperature is held at about 130° C. for 14 minutes. At this temperature the 

bakelite first softens, then solidifies. The molds are then removed 

from the press, and the mounted 

specimens are taken from the 

molds. The dimensions of the 

mount are identical with those 

of the brass mount described on 

page 8 \% by 1% inches outside 

dimensions by % inch deep. At 

Harvard the specimens to be 

mounted receive a first flat surface by sawing through them 

with the diamond saw described 

on page 13 of this bulletin. At 

the Canada Department of 

Mines and Resources the first 

flat surface is obtained by grinding on a cast-iron plate (p. 38). 

Figures shows the most recent 

model of the Graton-Vanderwilt 

polishing machine as made by 

the Mico Corporation. This 

macliine differs from preceding 

models in many respects, but 

mainly in the use of ring-shaped 

laps and an improved method of 

holding specimens while polishing, 

kindly furnished by Mr. Mann:

A sturdy cast-iron box-shaped frame contains in its base a motor arranged 

with a shaft extension on each end. At one end a built-in gear-reduction unit 

with a vertical shaft carries a lap table rotating at a speed of about 85 revolutions a minute. Interchangeable ring laps 9% inches in outside diameter and 

3% inches in inside diameter may be placed on this table. Lead laps are supplied with a cast-iron backing, and laps of other materials, such as iron, tin, 

aluminum, and copper, are offered, each having certain special uses. The laps 

are provided with concentric grooves about 1 millimeter wide and about 0.75 

millimeter deep. Vanderwilt remarks that the grooving of laps is common 

practice in commercial grinding and polishing plants, but their importance has 

been overlooked in ore polishing. The grooves also serve to keep the lubricants 

and abrasives evenly distributed over the lap surface.

FIGURE 8. Graton-Vanderwilt polishing machine. 

(From catalog of Mico Corporation, Cambridge, 

Mass.)

The following description was
36 MICROSCOPIC DETERMINATION OF ORE MINERALS

Power to drive the specimen is taken from the opposite end of the motor by 

means of a V-belt drive to the top of the machine. A bevel-gear reduction unit 

transmits the power to a vertical shaft at the lower end of which is attached the 

geared specimen head. This head rotates at a speed of about 150 revolutions a 

minute and is bowl-shaped in order to catch dust particles and thus prevent 

their reaching the specimens below. Meshing with a fixed or sun gear are the 

six specimen spindle gears which rotate the specimen at about 75 revolutions a 

minute. The specimen spindles are. free to move vertically, and each carries a 

permanent loading weight of 2 pounds. Provision is made for locking each 

spindle in a top position at which it can be placed when rotating idly or when 

specimens are changed. Extra loading weights may be added at the top of each 

spindle. Projecting from the lower part of each permanent loading weight are 

two fork-shaped members which engage a gimbal device into which the specimens are clamped. This holds the specimens well below their centers of gravity 

and in intimate contact with the lap, with the result that the polished surfaces 

are very nearly optically flat.

The vertical shaft carrying the specimen head is drilled to permit oil from a 

cup on the top of the machine to be supplied to the specimen while being polished without the necessity of opening the celluloid dust guards with which the 

machine is supplied and which are not shown in the photograph.

The price of the machine complete with motor, six round or rectangular specimen holders, one cast-iron lap, one lead lap, and an 

assortment of abrasives and oil is $560 f. o. b. Cambridge, Mass. 

(1936). The price of extra lead laps is $8 each and cast-iron laps 

$6 each.

The use of a ring lap obviates the dishing of the center of the lap 

which occurred in the earlier design described by Vanderwilt, where 

the specimen passed across the center of the lap.

The Canada Department of Mines and Resources makes its own 

lead laps, which are 2% inches thick and allow 15 or more machinings 

before they are discarded. Electrolytic lead is preferable for their 

construction because of its superior softness.

Details of process as used at Harvard. Vanderwilt found that water 

cannot be used as a medium for carrying the abrasive when softmetal laps are used. After experimenting with various liquids, he 

found that the most satisfactory medium for this purpose is a mixture 

of three parts of kerosene to one part of mineral oil (nujol). The 

abrasive is mixed into a thick paste with the lubricant, and a ball of 

this paste 3 or 4 millimeters in diameter is spread evenly over the lap 

with a spatula. This amount suffices for about 3 or 4 hours of continuous grinding.

The lap is cleaned with a strong solution of sodium hydroxide 

(NaOH) and a stiff brush after a few hours of use.

The amount of oil to use on the lap requires some delicacy of adjustment. If not enough oil is used, both the lap and the surface of 

the specimen are damaged. Too much oil saves the lap but results 

in a badly scratched surface on the specimen. When the proper 

amount of oil is applied the lap appears practically dry but slightly
GRINDING AND POLISHING 37

greasy, and a piece of white paper touched to it shows a slight amount 

of oil. This balance between oil and abrasive is acquired only 

through experience. But a filter paper or .porous paper towel slightly 

moistened in oil applied from time to time, not oftener than once an 

hour, gives an approximation to the mixture desired."

The stages in the process as used on an average specimen at Harvard, according to Charles Fletcher, preparator for the Laboratory of 

Mining Geology, who has contributed much to the improvement of 

the present technique used there, are:

1. A first surface is prepared by sawing through the hand specimen 

with a diamond saw.

2. The specimen is mounted in bakelite by means of a hydraulic 

press.

3. If the surface is porous or crumbly it is impregnated with bakelite by a process similar to that described on page 31. The entire 

mount is not immersed in bakelite, but a solution of thin sirupy 

bakelite is applied to the upper surface of the specimen and the 

specimen is put in the oven. The curing starts with a temperature 

of about 40° C., which is gradually raised to about 90° and kept 

there for some hours.

4. The specimen is ground on a copper lap. If it contains a considerable percentage of hard minerals, No. 2 abrasive siphoned from 

600 alundum is used. If soft minerals predominate, No. 3 abrasive 

from 600 alundum is used. The average time of grinding is 20 

minutes.

5. First lead lap, Nos. 3 and 4 abrasive siphoned from 600 alundum. 

This eliminates most of the pits and scratches caused from step 4. 

The average time is 30 minutes.

6. Second lead lap, No. 5 abrasive siphoned from 600 alundum. 

The average time is 90 minutes.

7. Third lead lap, tin oxide is used as polishing abrasive. The 

commercial product siphoned from jars 2 and 3 is used. Average 

time is 15 minutes. Step 7 may be omitted if the pits and scratches 

are sufficiently removed in the preceding stages. The aggregate 

time for polishing the average specimen on the total cycle of laps is 

thus about 3 hours, for the most exacting results, such as required 

for high-power photography.

Four machines are connected to the same drive shaft. As each 

machine holds 6 specimens, 24 specimens can be ground or polished 

at one time. Thus, with all machines running at capacity, the average 

time charged against each specimen, including time for changing from 

lap to lap and the necessary interim control inspections under the 

microscope, is about 15 minutes. The machines require little attention after they start running, other than to keep the bearings oiled 

and to apply a little oil to the laps from time to time.
38 MICROSCOPIC DETERMINATION OF ORE MINERALS

Details of process used at Canada Department of Mines and Resources. Through the courtesy of Maurice Haycock, of the Mineragraphic Laboratory of the Canada Department of Mines and Resources 

at Ottawa, the writer was able to obtain some details of the process 

used there.

The two Graton-Vanderwilt machines in use, constructed by the 

Mico Corporation, differ in a few minor details from the machines used 

at Harvard. The head runs clockwise at 162 revolutions a minute, 

the specimens rotate on their own axes clockwise at 85 revolutions a 

minute, and the lap runs clockwise at 150 revolutions a minute.

The steps of the process are as follows:

1. Specimen is rough ground by hand on iron lap with 120 carborundum.

2. Rough grinding is completed on copper lap with 3 F carborundum.

3. Specimen is mounted in bakelite in hydraulic press. Details 

are practically the same as at Harvard.

4. Specimen is ground on iron lap in Graton-Vanderwilt machine. 

Abrasive used is product from jars 2, 3, and 4 of Vanderwilt classifier, 

using 600 alundum. Time of grinding, about 1 hour.

5. First lead lap with product of jar 3, using 600 alundum. Time, 

3 to 4 hours.

6. Second lead lap with product of jar 4, using 600 alundum. Time, 

3 to 4 hours.

7. Third lead lap with magnesia powder, commercial grade. Time, 

3 to 4 hours.

At the Canada Department of Mines and Resources most of the 

work is done on mill products, and here the advantage of the Vanderwilt process over ordinary hand polishing on a cloth lap is very marked. 

The mill products range in size from 40 mesh to submicroscopic and 

average about 200 mesh. The product is mixed with about seven 

times its volume of bakelite, which has been ground to pass 200 mesh. 

A cylindrical pellet is made of the mixture by pressing in a small steel 

mortar (fig. 9) under a pressure of about 1,000 pounds. The powder 

sticks together without a binder, and the pellet is placed in the bakelite mold, which has the same dimensions as that at Harvard. The 

mount is then made exactly like that of a polished chip in the hydraulic press. The polishing process is the same as that for a polished 

chip except that step 4 on the iron lap is omitted.

At the Canada Department of Mines and Resources the average 

output of one man, using two machines and polishing both ore chips 

and mill products, was seven.specimens a day over a period of several 

weeks.

General remarks. The hydraulic press with- heating units is manufactured by the Watson-Stillman Co. and costs about $500 (1929).
GRINDING AND POLISHING 39

A serious objection to the Harvard process is the high temperature 

and pressure required in mounting with bakelite. As already mentioned, a temperature of 130° C. is maintained for about 14 minutes. 

At this temperature certain intergrowths tend to form solid solutions.

Bateman and Lasky 28 have shown that if intergrowths of chalcocite 

and covellite are heated in an oven for 3 hours at 75° C. and atmospheric 

pressure, solid solutions between these minerals will form. The same 

solid solutions will form when the minerals are heated 1 hour at 100°- 

120° C. Solid solution, however, does not form below a superficial 

film a fraction of a millimeter thick.

According to Graton,29 experiments with chalcocite-covellite intermixtures have been made in which pieces from the same specimen were

FIGUEE 9. Mortar (a) and pestle (6).

mounted in bakelite and mounted in the cold. No difference between 

the two specimens could be 'detected. Probably there are other 

mineral pairs that will form solid solutions below the temperature of 

bakelite mounting.

Also, the high pressure used cracks certain fragile specimens and 

certain unusually brittle minerals. At Ottawa the Harvard process 

is not at present used for ores containing pyrrhotite.

All these problems have been solved at Harvard by substituting 

Wood's metal cores in bakelite shells when mounting minerals susceptible to shattering or alteration by the usual bakelite mounting 

process. The shells are prepared by mounting a rectangular plaster 

dummy in the bakelite mold as if it were a specimen. The dummy is 

then removed, leaving a bakelite shell of the usual l}{ by \% by % 

inches, outside dimensions, but containing a rectangular depression. 

In this depression the specimen is then mounted with Wood's metal in 

a manner analogous to that already described for mounting with sealing wax in brass boxes. Wood's metal is composed of bismuth 50 

percent; lead, 25 percent; tin, 12^ percent; and cadmium, 12% percent.

M Bateman, A. M., and Lasky, S. O., Covellite-chalcocite solid solution and exsolution: Econ. Geology, 

vol. 27, pp. 64, 73, 1932. 

2> Qraton, L. C., written communication.
40 MICROSCOPIC DETERMINATION OF ORE MINERALS

It melts at a temperature of 65.5° C. and flows around the specimen 

by gravity; no pressure is required as with powdered bakelite.

The first cost of the machinery is a handicap to the general adoption 

of the Harvard process. One man can tend four machines almost as 

well as one, and the labor cost would make the use of one machine 

alone relatively expensive. Two machines would give a better balance 

between first cost and operating expense. The cost of two machines, 

together with the bakelite press, would be about $2,000. A high 

degree of technical skill is required of the operator. A man of average 

intelligence can learn to use the process after an apprenticeship of a 

few weeks under a skilled technician, but it will be several months 

before he can work to the best advantage.

Notwithstanding the disadvantages above set forth, the Harvard 

process is the best that has been devised for polishing most ores.

POLISHING PROCESS AT COLUMBIA UNIVERSITY

Bird 30 gives the following description of the polishing process used 

at Columbia University.

A suitable chip is sawed or broken from the specimen and ground to shape on a 

cast-steel lap with 100 carborundum. The surface to be polished is further 

ground on a lap made of ordinary boiler plate with 3 F carborundum until all 

marks left by the first grinding are entirely removed. It is then ground on a 

grooved copper lap with American Optical Co.'s No. 303 emery abrasive. This 

grinding is continued until an appreciable thickness of the chip is removed to 

make sure of getting below any fractures or pits that may have been caused by 

the first two grindings. If the thickness of the chip permits, at least a millimeter 

is removed in this final grinding. All the above abrasives are suspended in water.

Every chip that is intended for a polished surface is impregnated before mounting, regardless of its physical appearance or condition, thus avoiding the trouble 

caused by plucking.

Bakelite resinoid (not varnish) No. BM-0014 is far superior to anything else 

tried as an impregnating medium. This sirupy material is diluted with about an 

equal amount of denatured alcohol. The chips, after having been ground on 

the copper lap, are placed in a small metal container and well covered with this 

thinned bakelite resinoid. They are then placed in an electric oven and heated at 

a temperature of 75° C. until the bakelite is about the consistency of thick sirup. 

The chips are then removed from the bakelite to a piece of cardboard and placed 

back in the oven at the same temperature until the bakelite can merely be dented 

with the fingernail. The temperature is then raised to 90° to 100° C. and maintained there until the bakelite is thoroughly cured. The time required for these 

three steps is about 12, 5, and 12 hours, respectively, or a total of nearly 30 hours. 

However, the amount of curing required can be judged better by observing the 

physical condition of the bakelite than by attempting to hold the treatment to a 

definite amount of time.

After the impregnation is complete the chip is again ground on the grooved 

copper lap with No. 303 emery in water, but only until the film of cured bakelite 

is removed. Further grinding will destroy the desirable results of impregnation, 

at least partly.

so Bird, P. H., written communication.
GRINDING AND POLISHING 41

The specimen is then mounted in bakelite. The mold used is constructed in 

the laboratory of Columbia University and has already been described,31 but 

some improvements in design have been made since the description was published. 

The mold is heated to only 120° C. instead of 140° C. as formerly. The bakelite 

now used is BM 45. The molding takes place under a pressure of 9 tons applied 

to the plunger, which has a diameter of 1% inches. The press is a Weaver hydraulic truck jack.

The bakelite mounts are 1% inches in diameter and about 1 inch thick and have 

absolutely parallel sides. In this respect they are superior to the brass mounts 

formerly used by the Geological Survey.

After the mounted specimen is removed from the mold, the surface to be 

polished is ground on the copper lap with No. 303% emery in ethylene glycol, but 

only sufficiently to remove the "burr" around the edge of the mount and possibly 

just a film from the specimen itself. The purpose of this grinding is to prevent 

the bakelite of the mount from protruding above the top of the specimen.

The first step in the polishing process itself is done on a 6;-inch lap covered with 

coarse canvas revolving about 1,200 revolutions a minute, \tith carborundum that 

will float 6 minutes. This carborundum is faster and just as satisfactory as 

chromic oxide. No attempt is made to hold the surface on this lap any specified 

length of time. The specimen is examined frequently, and polishing is continued 

until no further improvement is noted.

The second step of the polishing is done on the same kind of lap revolving at 

the same speed, but covered with a good grade of linen, which is much cheaper in 

the long run than muslin. The medium in this step is Norton "levigated alumina" 

that has been floated 15 to 20 minutes. The lap is thoroughly wet with water, a 

quantity of the alumina is squirted on from a bottle with a tube in the cork, and 

the polishing is begun immediately. No water is allowed to drip onto the lap 

during this step. The specimen is held on the lap until it seems to grip the linen, 

which is an indication that the lap is beginning to dry out. The specimen is then 

removed and examined. If necessary, the same procedure may be repeated 

several times, until the optimum results are obtained. The most important 

thing in this step is to stop at exactly the proper time. If polishing is carried too 

far the specimen will be unduly scratched; if not carried far enough, the highest 

degree of polish possible in this step will not be obtained.

The third and final step in the polishing process is done on another lap of the 

same size and speed as the other two but covered with a special grade of felt 

known as red bushing cloth, which is heavier and much tougher than billiard 

cloth and gives equally good results. This is supplied by the Continental Felt 

Co., 890 Broadway, New York, N. Y.; weight 22% ounces, width 52 inches, cost 

about $4 a yard.

The final polishing powder is black magnetic rouge. (See p. 29.) This is floated 

5 minutes before using, and the mixture of water and rouge in equal volumes is 

kept in a bottle. The best results are obtained by using liberal quantities of this 

material, which is rather inexpensive.

The final polish is applied as follows: The felt is dampened and tightly stretched 

over the lap. It is then brushed briskly with a stiff-bristled brush while the lap 

is running with a stream of water playing on the center. While the felt is still 

wet, and with a little water dropping from above onto the lap, a cake of ordinary 

toilet soap (Lifebuoy) is pressed firmly against the lap and held there until the 

surface is thoroughly filled with soap. The dripping water is turned off, a generous quantity of rouge is squirted onto the center of the lap, and the polishing is

31 Krieger, Philip, and Bird, P. H., Mounting Dolishflrt surfaces in bakelite: Econ. Geology, vol. 27, pp. 

675-678,1932.

129151 40 4
42 MICROSCOPIC DETERMINATION OF ORE MINERALS

begun. A good bit of pressure is used at first, and the specimen is held near the 

rim of the lap, but as the specimen is slowly moved toward the center of the lap 

the pressure is relieved until it is just touching. The specimen is then turned a 

little and the same motion followed through again. This is continued until the 

lap begins to dry. Here again the difficulty is to know just when to stop. When 

properly executed this procedure will produce a high degree of polish with few 

scratches.

POLISHING PROCESS AT THE UNIVERSITY OF MINNESOTA

In the process used at the University of Minnesota the first surface 

is ground on a horizontal wheel using a paste of 46 or 120 alundum. 

The flat surface thus obtained is beveled on a carborundum wheel. 

This is followed by hand grinding on a glass plate with alundum 5 F, 

followed on another plate with alundum 60 F. About 1 minute 

usually suffices for this step, but for some specimens more time can be 

spent to advantage. Polishing is accomplished on three vertical 

wheels driven at 1,800 revolutions a minute. The wheels are mounted 

with heavy linen or sheeting. The best grade of sheeting costs much 

less than linen and gives very satisfactory results. All abrasives are 

applied to these wheels with paint brushes dipped in the abrasive 

suspended in water. Alundum 60 F is used on the first wheel, 

chromic oxide (10 minutes) on the second, and either rouge (60 

minutes) or magnesia for the final polish. Schwartz,32 who describes 

this process, states that the average amount of time spent for specimens presenting no unusual difficulties is from 6 to 10 minutes.

METAIIOGRAPHIC POLISHING AT THE WATERTOWN ARSENAL

The technique of polishing iron, steel, arid other metals and alloys, 

as developed in metallography, differs from that of polishing brittle 

minerals, owing to the fact that the metals are sectile and tend to 

flow when pressed against the polishing lap. In metallographic polishing, the grinding and preliminary polishing stages are carried out 

dry on emery papers of successively finer grain size. The final polishing is done on cloth with wet abrasive and is similar to that already 

described for brittle minerals.

Metallographic polishing differs in detail as practiced in different 

laboratories. Inspection of the results of different procedures indicates that the polishing process used at the United States Arsenal, 

Watertown, Mass., is superior to other processes. The usual process, 

described on pages 27-37, has not been entirely satisfactory for polishing specimens containing native metals, so the writer submitted a 

suite of mineral specimens to the Watertown Arsenal to learn whether 

the process used there could be used on minerals. In addition to 

specimens containing native metals, others containing chalcocite, 

galena, and other soft minerals and some containing intergrowths of

32 Schwartz, Q. M., Preparation of polished sections of ores: Econ. Geology, vol. 22, p. 123,1927.
GRINDING AND POLISHING 43

hard and soft minerals were submitted. The results indicate that 

the Watertown process is superior to any other kind of hand polishing for native metals and soft minerals. For pyrite and other hard 

minerals the Watertown process gives excellent results but requires 

an hour or more for each specimen, whereas the usual wet polishing 

methods for minerals give a good polish on pyrite in much less time. 

Miss Norton,33 of the Watertown Arsenal, who perfected the 

Watertown process, describes it as follows:

Grinding and dry polishing. The specimen in the first step is leveled on a 

surface grinder, whose action is purely mechanical, thus avoiding the inequalities 

resulting when the specimen is held by hand.

In the next seven steps the specimen is ground by holding it against seven 

successive abrasive papers starting with relatively coarse and finishing with very 

fine. The papers used are aloxite 240, 320, and 400, obtainable from the Carbo- 

 rumdum Co., Niagara Falls, N. Y., at about 3 cents a standard sheet, 9 by 11 

inches, and emery 0, 00, 000, and worn 000 glazed with graphite. The emery 

papers, called "metallographic emery papers," are made by Behr-Manning Co., 

Troy, N. Y., and cost about 5 cents a standard sheet, 9 by 13% inches. The 

papers as used are 9 inches square and are attached to the center of a rotating 

horizontal plate by means of a J^-inch cap screw and a J^-inch washer. The 

periphery of the paper is not attached. The plate rotates at about 2,000 revolutions a minute.

A few seconds of grinding on each grade of paper is sufficient. Pressure on 

the sample is light, being just sufficient to hold the specimen against the wheel. 

The specimen is moved slowly from the center of the wheel toward the periphery.

The specimen is not rotated around its own axis; with the coarsest paper a 

series of paralleled grooves is cut tangential to the wheel. On the next finer 

paper the specimen is held in such a way as to cut a series of parallel grooves 

at right angles to the first series. On the third paper the grooves are cut parallel 

to those cut by the first paper. On the fourth paper the grooves are cut parallel 

to those of the second, and this sequence is continued to the finest paper. Each 

paper is discarded after using, as fragments of mineral embed themselves between 

the abrasive grains and hinder further proper cutting.

The last paper is prepared by rubbing a worn 000 emery paper with a stick 

of the purest graphite obtainable. Graphite supplied for electrodes is satisfactory. Graphite glazes over the 000 grits and minimizes their depth of cutting. 

They then produce a series of scratches that are finer than those made by the 

original 000 abrasive particles. The glazed 000 paper, therefore, constitutes an 

additional dry-grinding course and thus fulfills the most essential function of the 

polishing procedure here described the lessening of the amount of work on the 

wet laps by extending the dry-paper cutting to the finest possible stage.

Wet polishing. The last two steps are accomplished on cloth-covered horizontal stainless-steel laps, using two grades of carefully sized "levigated alumina," 

which is manufactured by the Norton Co., Worcester, Mass. Owing to their 

tendency to agglomerate, the particles are measured with difficulty. The commercial product is sized by stirring up 100 grams of abrasive in about half a liter 

of water to which has been added 1 cubic centimeter of 40 percent sodium 

silicate. The sodium silicate acts as a dispersing agent to deflocculate the 

abrasive.

The abrasive is then sized by the Rodda method (see p. 23), and two grades 

are obtained, comprising grains which remain suspended after 5 minutes and

83 Norton, M. R., written communication, published by permission of the War Department.
44 MICROSCOPIC DETERMINATION OF ORE MINERALS

after 4 hours of settling, respectively. The two grades are designated "coarser 

alumina" and "finer alumina."

In the ninth step the specimen is polished on the crepe side of a satin crepe 

fabric by means of coarser alumina. This satin crepe is manufactured by the 

Widder Silk Mills, Paterson, N. J., under the number 28W. The abrasive is 

used sparingly, the pressure is only that necessary to hold the specimen against 

the disk, and the moisture is restricted to such a degree that when the sample 

is taken off the lap, its surface may be seen to dry immediately. The wheel is 

rotated at about 200 revolutions a minute. With soft minerals the paper scratches 

are removed in 1 to 4 minutes.

In the last step the specimen is polished on satin or on the satin side of the 

above-described satin crepe fabric. The speed of the wheel is about 100 to 

200 revolutions a minute, the moisture is the same as in step 9, and the pressure 

is exceedingly light. In both step 9 and step 10 the specimen is rotated in a 

direction contrary to the direction of rotation of the wheel.

With soft minerals the marks of the coarser powder disappear in about 5 to 

10 minutes. Results, particularly with the soft minerals, are greatly improved 

if the wheel is kept stationary during the last minutes of polishing. The specimen 

is rotated by hand on the stationary wheel, never rubbed back and forth.

The specimen is examined frequently under the microscope to determine 

when each step is complete.

The foregoing outline is not a prescription for polishing a specimen. Its 

successful application depends on the experience, skill, and patience of the person 

who handles it. Speed, pressure, moisture, time, and other factors are variables 

which his good judgment alone must determine.

With minerals of medium and high hardness, steps 9 and 10 require about 

the time, in minutes, shown below.

rite... ........... .......-.... .

Pyrrhotite, pentlandite, magnetite,

Step 

9

GO

40

2

2

Stop 10 '

45

1.0

 M

10

10

Native bismuth ________ .. _ .

Native tellurium. ............ _ .....

Step 

9

2

2

2

2

2

2

Step 

10

10

r

r

r>

These periods are only approximations. It is impossible to state definitely 

that step 9 or step 10 will be completed in a certain number of minutes. They 

must be continued until the polish can no longer be improved. The time is 

therefore determined by the operator and by the polishing conditions.

Photomicrographs of specimens polished in the Watertown Arsenal 

are shown on plate 3.

PHOTOMICROGRAPHY OF POLISHED SECTIONS

Photomicrographic camera. Davy and Farnham 34 state that 

For the mining geologist, photographs of the ore minerals and the relations 

between them, as seen by vertical illumination, are almost indispensable in preparing reports. A well-chosen illustration will establish a point more conclusively in the minds of the readers than pages of written description. For the 

student of ore deposits, such photomicrographs are invaluable records to be 

preserved as reference material.

Davy, W. M., and Farnham, C. M., op. cit., p. 12.
PHOTOMICROGRAPHY OF POLISHED SECTIONS 45

Photomicrography is a very simple operation, far more so than 

is generally believed. All that is necessary is a camera box attached 

to the microscope, the objective and ocular of the microscope acting as a compound lens for the camera. The microscope itself 

is a projection instrument and will project the image of the object 

on which it is focused. If a sheet of white paper is held horizontally 

above the ocular of the ________________________ 

microscope, a circular image of this object is thrown 

on the paper. The diameter of the image depends 

upon the power of the ocular and the distance oi the 

paper above it. An ocular 

of low magnification, such 

as the No. 1 Leitz, projects 

a cone of rays of relatively 

small vertical angle. An 

ocular of higher magnification projects a cone of rays 

with a wider angle, and 

consequently at a given distance the image projected 

is larger. The diameter of 

the image varies directly 

with the distance from the 

ocular, because the image 

is the base of a right circular cone of rays. All that 

is necessary in photographing an image, then, is to 

place a photographic plate 

at a certain distance 

above the ocular of the 

microscope, the distance depending upon the size of the image desired, and to screen out all light other than that projected by the 

ocular of the microscope itself. The most essential requirement of 

the camera is that it be made rigid, to prevent vibration of the plate 

during exposure.

Figure 10 shows such an arrangement which gives excellent results. 

The camera is a 4- by 5-inch Folmer & Schwing plate camera from 

which the lens has been removed. This is supported on a tripod, 

the legs of which are screwed onto the camera box. The plate 

holder is 57 centimeters vertically above the base of the tripod and 

20 centimeters above the ocular of the microscope, with a 16-milliFIOUBE 10. Improvised microscope camera. The camera is 

held in a square frame supported by three legs. A Leitz 

lens for focusing the image rests on the plate holder.
46 MICROSCOPIC DETERMINATION OF ORE MINERALS

meter objective focused on a polished section mounted in brass. The 

tube length of the microscope with the vertical illuminator attached 

is 205 millimeters. The adjustable bellows provides for variations 

in focal distance. A black cloth provides a light-tight joint where 

the ocular enters the bellows. Differences in magnification are provided for by changing the objective or ocular. The No. 3 Leitz 

ocular just covers a 4- by 5-inch plate in the arrangement shown. 

With this ocular the different objectives give the following magnification: 32-millimeter Bausch & Lomb, 29; No. 2 Leitz (24-millimeter), 49; 16-millimeter Bausch & Lomb, 78; No. 4 Leitz (9-millimeter), 144; 4-millimeter Bausch & Lomb, 360.

This attachment does not provide a shutter for timing the exposure, 

which is accomplished by switching the electric light on and off. 

The exposure usually ranges from 1 second to 10 minutes, hence 

a shutter that will give exposures of fractions of a second is not 

required. * 

The image is first focused as well as possible at the top of the camera; i~ 

the ground glass is in the position that the photographic film will later 

occupy and gives an accurate view of the image as it will appear in the 

photograph. It also enables the operator to equalize the illumination 

by turning the vertical illuminator or shifting the lamp. The light 

should be as strong as possible, and no filter should be used while - 

arranging the object and equalizing the illumination. If the minerals Vdo not all show already, the effect produced by different color filters' is 

then tried out by testing them in succession, as described on pages 

51-55. The filter should be in place during both final focusing and 

exposure, as the achromatic objectives commonly used have slightly 

different focal lengths for the different color:,.

For an accurate focus the ground glass is inadequate. A trans- ^ 

parent plate of glass with several diagonal scratches made with a dia- +f, 

mond is much more satisfactory. The ground glass is removed from 

the camera, and the transparent glass put in its place. A transparent 

glass is conveniently obtained by soaking a photographic plate in hot 

water and scraping off the emulsion. A focusing magnifier magnifying 

three to seven times is first focused on the scratches on the transparent x 

glass. By means of the fine adjustment of the microscope, the image 

is then focused into the same plane as the scratches.

When viewed with the unaided eye no image is seen at first, and all 

that is visible is a strong spot of light in the ocular of the microscope. 

For this reason it is not generally known that it is possible to focus on 

a transparent glass. However, if a small hand lens is focused on the v 

glass and the eye placed in a certain position, to be ascertained by 

trial, a sharp image appears. The optical axis of the hand lens must ' 

be parallel to the ray of light at that point, hence it is best to focus at 

the center of the plate, where the plate and medial plane of the lens are
PHOTOMICROGKAPHY OF POLISHED SECTIONS 47

parallel. Some skill is necessary in detecting the image at first, but 

once successful the operator encounters no further trouble. With a 

little practice he almost instinctively locates the definite place where 

the image is seen. The image seen is only that part of the plate covered by the lens, but this image is sharp, and the amount of light 

required to give an observable image is only a small proportion of the 

light necessary to give a good image on the ground glass.

When the focus is obtained the transparent glass focusing plate is 

removed, and the plate holder with its enclosed photographic plate 

is inserted in its place. The slide of the plate holder is pulled out and 

the light turned on for the required exposure. The light is then 

turned off, the slide of the plate holder pushed back into place, and the 

plate holder removed and taken into a dark' room, where the negative 

is developed and fixed.

The Leitz "Makam" camera attachment, which, lias been used in 

the Geological Survey laboratory, is similar to the improvised camera 

above described but has some features of advantage. It consists of a 

camera box with an ocular. (The cost of this attachment is about 

$85.) The ocular of the camera fits into the barrel of the microscope 

and takes the place of the usual ocular of the microscope. This supports the camera and thus obviates the need of a tripod support. 

The camera has a horizontal eyepiece in a tube at right angles to the 

ocular. A prism placed in the optical axis of the camera just above 

the ocular throws the microscopic image through the eyepiece. The 

eyepiece can be moved in and out of its tube for the purpose of focusing. 

A lug attached to the eyepiece moves in a spiral slot, and by means 

of a circular vertical scale its position with the image in focus can be 

accurately located. The advantage of this arrangement is that it 

is not necessary to focus the image on the ground glass or transparent 

glass every time a picture is taken. The eyepiece is adjusted so that 

the image is in focus at the same time that it is in focus on the ground 

glass. Then when additional pictures are taken it is only necessary 

to focus the image in the eyepiece.

The eyepiece in the side tube can be adjusted for each user by focusing the cross hairs sharply with the revolving graduated collar. If 

the 'maximum intensity of illumination is required, the prism that 

throws the image through the side tube can be removed from the path 

of light by means of a wire release. If, however, moving objects, 

such as growing crystals, are being photographed, the prism may be 

left in the path and the exposure increased. The prism is a beamsplitting device that reflects only about 20 percent of the light into 

the side tube and allows the rest to go to the film. All the photographs 

of microchemical tests shown in part 4 of this bulletin were taken 

with this attachment. The size of the photographs is 3 % by 4% inches.
48 MICROSCOPIC DETERMINATION OF ORE MINERALS

As the camera attachment is an inverted pyramid, larger sizes would 

tend to make it more unstable and more readily affected by vibration. 

Satisfactory pictures with exposures as long as 12 minutes have 

been taken in the writer's laboratory with this attachment, though 

the laboratory is by no means free from vibration caused by heavy

FIGURE 11. Leitz "Makam" camera attachment.

machinery. With longer exposures some precaution should be taken 

to protect the camera and microscope from vibrations, by waiting 

until the machinery has shut down and nearby street traffic is not 

heavy.

As the distance between the ocular and the plate holder is fixed, 

each objective gives a fixed definite magnification as follows: 32- 

millimeter, 40; 24-millimeter, 68; 16-millimeter, 95; 9-millimeter,
PHOTOMICROGRAPHY OF POLISHED SECTIONS 49

170; 4-millimeter, 385. The tube length of the microscope with 

vertical illuminator attached is 215 millimeters.

Similar instruments are manufactured by both the Spencer Lens 

Co. and the Bausch & Lomb Optical Co. These have the added 

advantage that the camera body is supported on a heavy vertical rod

FIGURE 12. Vertical camera with observation eyepiece.

entirely independent of the microscope, with a decided increase in 

stability.

A large part of the photomicrography of both thin sections and 

polished sections at magnifications above 25 diameters at the United 

States Geological Survey is now done with the apparatus shown in 

figure 12. The arrangement for photographing polished sections 

consists of a standard petrographic microscope, a vertical illuminator 

with both prism and transparent plate reflectors, a direct-current clockfeed arc lamp, and a 4- by 5-inch plate camera with an observation 

eyepiece (made by Bausch & Lomb Optical Co.) for searching and 

focusing the specimen. The observation eyepiece operates in a manner quite similar to the side tube on the Leitz Makam camera described 

above.
50 MICROSCOPIC DETERMINATION OF ORE MINERALS
PHOTOMICROGRAPHY OF POLISHED SECTIONS 51

Many metallurgical laboratories and universities where much photomicrography of opaque specimens is carried on use the Le Chatelier 

model inverted microscope with camera attachment. This is manufactured by several firms, such as Leitz, Bausch & Lomb, and Zeiss. 

A Bausch & Lomb instrument used by the Geological Survey is shown 

in figure 13.

The polished surface is placed face down on the stage of the microscope; hence no mounting is necessary, and the object can be of any 

size that the stage can support without undue strain. The bellows is 

horizontal and has an extensible length of more than 3 feet. The 

camera box, bellows, microscope, and lamp are all supported on a 

firm horizontal standard, which in turn is suspended from springs. 

The standard vibrates freely from the springs, and the camera and 

other fixtures vibrate as a unit with it, but during these vibrations 

the camera ajid microscope maintain the same relative positions; 

consequently, there is no blurring of the picture. Vibrations in the 

building are transmitted in part to the springs of the instrument, 

but they have little or no effect on the picture. This instrument is 

better than the camera attachments already described for magnifications greater than 800 diameters. It is more convenient to operate, 

and, as the bellows has a wide range in length, magnifications of any 

desired magnitude can be taken. The instrument without objectives 

and ocular costs $880 (1935), however, and it is not very often that 

magnifications as high as bOO diameters will be required. Moreover, 

if the polished section has much relief, a magnification as high as this 

is not possible, as part of the picture will be out of focus. (See pi. 4, D.)

Films. Films have gradually but steadily improved in quality and 

variety, until at present they offer many advantages over plates, 

particularly for photomicrography. Films are more easily handled 

and stored than plates, are unbreakable, have much better nonhalation 

properties, and, in the form of film packs, are efficient timesavers both 

in the exposing of the film and in its subsequent processing. A panchromatic emulsion is not needed except in the exceedingly rare event 

of having to differentiate pink or red minerals from dark gray or black. 

A highly orthochromatic emulsion is preferable because it can be 

developed in red light sufficiently bright for convenient manipulation 

and is insensitive to the red rays for which the achromatic objectives 

are poorly corrected. Excellent emulsions in film packs are Eastman 

verichrome and Agfa plenachrome; on cut films, Eastman commercial 

ortho, Agfa plenachrome, and Defender pentagon. For the rare 

occasions when a panchromatic emulsion is necessary, Eastman 

panatomic film, obtainable either as cut film or film packs, is recommended.

Light filters. Photomicrographs of thin, sections usually show 

sufficient contrast between the component minerals if photographed
52 MICROSCOPIC DETERMINATION OF ORE MINERALS

in white light, and no color filter is required. In fact, in many 

thin sections the minerals have too much contrast in light or color 

to photograph well without screening out some of the component 

colors of white light. The resulting negatives may appear satisfactory, but the printing paper does not permit as much contrast 

a$ the negative. A light-colored mineral is dense in the photographic negative, whereas a dark-colored mineral is more transparent. 

If the contrast between the minerals in the negative is too great, 

the light-colored mineral will have to be printed so long in order to 

bring out the detail that the dark-colored mineral is overprinted and 

its detail obliterated. In this event some means should be taken to 

reduce the contrast between the minerals. This is accomplished by 

the use of red or yellow light instead of white. Inspection of many 

photomicrographs of thin sections in geologic papers indicates that 

this factor of overcontrast has not been sufficiently taken into 

consideration.

On the other hand, the greater part of the minerals in a polished 

section are of various shades of gray, and the contrast between the 

colors may be so slight that boundaries between the component 

minerals are almost invisible. Under such conditions means must be 

taken to increase the contrast between the component minerals. This 

is accomplished by screening out some of the colors of white light.

Color filters, or light filters, are commonly made in three forms 

(1) colored glass in which the color is incorporated in the glass melt;

(2) dyed gelatin sheets; and (3) dyed gelatin sheets cemented between 

two glass plates. Those of the first form are the most permanent but 

are relatively expensive and are obtainable in a smaller variety of 

colors than the other forms. The dyed gelatin sheets are inexpensive 

and are obtainable in more than a hundred color varieties but are 

easily damaged. Filters of the third type are obtainable in the same 

variety of color as the plain gelatin sheets and are much more durable. 

A great amount of useful quantitative data regarding the Wratten 

filters of this type is available.35

Liquid filters, which were once widely used, are now replaced by the 

more convenient types listed above.

Different color filters have selective absorption for different parts 

of the spectrum. When light passes through a green filter, for instance, 

the colors of the red end of the spectrum are absorbed, but the green, 

with some of the other colors near it in the spectrum, passes through. 

To get contrast between two minerals that are close together in the 

color spectrum, a filter that transmits light at the other end of the 

spectrum is chosen.

Let us take a specific example. Chalcopyrite is light yellow, and 

galena silvery white. It is desired to increase the contrast in color

36 Wratten light filters, 13th ed., revised, 91 pp., Rochester, N. Y., Eastman Kodak Co., 1934.
PHOTOMICROGRAPHY OF POLISHED SECTIONS 53

between them. If a yellow filter (the K2 Wratten, for instance) 

is used, the chalcopyrite reflects yellow light, and its color is not 

materially changed by light passed through the filter. Galena, being 

white, reflects a large part of the visible spectrum, including yellow; 

but only the yellow rays and those near it in wave length reach the 

eye, as many of the rays at the other end of the spectrum were 

absorbed by the filter. Hence both chalcopyrite and galena reflect 

yellow light, and the contrast between them is lessened. It is obvious 

that a yellow filter is not suitable for bringing out a contrast between 

two yellow minerals or between a yellow and a white mineral. On 

the other hand, when a blue filter is used for chalcopyrite and galena, 

the galena, which reflects most of the colors of the spectrum, will 

reflect the blue components. The chalcopyrite absorbs colors near 

the blue end of the spectrum, and, as only blue rays reach it, the 

mineral will appear dark. Hence in the photograph the chalcopyrite 

will be darker than the galena.

The Wratten filters, made by the Eastman Kodak Co., have been 

found highly satisfactory for this purpose. They are not completely 

monochromatic; indeed, this would be a disadvantage rather than an 

advantage. For instance, the blue filter transmits all the colors of 

the spectrum but not in the same proportion; it absorbs most of the 

red, transmitting only a small proportion of it. On the other hand, 

it transmits most of the blue but absorbs a small proportion.

The filters used by the writer are A, red; B, green; X2 , greenish 

yellow; K2 , yellow; G, deep yellow; and H, blue. Other colors have 

been recommended by different investigators, but the writer believes 

that the importance of having a particular shade of any color has 

been overestimated. A partial list of filters of this group that are 

suitable for different mineral combinations is as follows:

Chalcopyrite and galena, H is best; B very good.

Chalcopyrite and pyrite, H.

Chalcopyrite and chalcocite, B.

Chalcocite and bornite, any filter will do; no filter required.

Chalcocite, bornite, and chalcopyrite, B.

Mottled chalcocite, B needed.

Chalcocite and covellite, H (covellite prints dark, and means should be taken

to reduce the contrast). 

Specularite and magnetite, H.

Bornite and covellite, H is best; B gives very little contrast. 

Bornite and tennantite, B. 

Argentite and galena, B.

Sphalerite, K2 or Xgj with other filters this mineral appears too dark. 

Gangue minerals, K2 or Xa is best; with other filters transparent minerals print

too dark. 

The A and G filters are good for reducing contrast.

Apochromatic objectives are much to be preferred for photomicrography in ordinary light but are usually unsatisfactory for work
54 MICROSCOPIC DETERMINATION OF ORE MINERALS

in polarized light on account of the anomalous double refraction of the 

fluorite lenses used in them. Achromatic objectives, which are generally used, have slightly different focal lengths for the different 

colors and have their best spherical corrections in the yellow-green 

portion of the spectrum. Therefore the filter to be used should be 

in place for the final focusing. When no filter is needed for increasing 

contrast, the best image with achromatic objectives will be produced 

if the Wratten B and G filters are used. Usually the B filter alone 

gives satisfactory results.

The exposure factor of each filter with a particular kind of light is 

given by the manufacturer. This is the multiple of time required by 

the filter to give the same density of negative as the same picture 

taken with white light. For instance, the photomicrograph of pyrite 

and chalcopyrite shown in plate 4, B, was taken with the Bausch & 

Lomb inverted photomicrographic camera using a 16-millimeter objective and a tungsten incandescent lamp. The magnification is 112 

diameters. This required an exposure of 2 minutes with a B filter 

and only 20 seconds with no filter. The negatives obtained were of 

the same density. The exposure factor of filter B is therefore 6.

Exposure factors of Wratten filters 1

Filter......... .........

Wratten "M" plate... ...

/Tung.'____

\Arc2...... 

/Tung__... 

\Arc_ ......

fTung.....

\Arc. ......

fTung.....

\Arc. ......

A

2

5 

10

B

7

8 

8

8

5

q

8

15

C6

1.0

5 

20 

10

4

3

3 5

3

OH

30

it) 

55 

30

12

<)

9

7

E

2

3.5 

4 

fi

F

4

8 

10

?0

G

2

2.5 

2 

3

6

10

fi

12

H

14

12 

14

17

5

8

fi

9

X,

4

5 

3 

4

4

fi

5

7

X,

5

6 

4

5

5

7

fi

8

K"i

1.5

1.5 

1.5 

1 5

2

f, 5

f, 5

3

K",

1.5

2 

1.5 

?,

3

4

4

5

B+G

14

20 

8 

12

20

50

12

25

1 Photomicrography, 13th ed., p. 82, Eastn;an Kodak Co., 1935.

2 Tung., high intensity tungsten; arc, cored carbon arc.

The exposure factors vary with the speed of the emulsion and its 

spectral sensitivity, as well as with the color itself and the character of 

the light. It is best for the investigator to determine these items for 

himself experimentally. This is best done by dividing the negative 

into five strips and photographing each strip separately. The slide of 

the plate holder is pulled out until it covers only one-fifth of the 

width of the negative. With a colored pencil a mark across the 

opposite side of the slide is made where the slide first meets the slot 

through which it passes. The slide is then pushed until it covers 

two-fifths of the width of the negative, and another pencil mark is 

made on the opposite side of the slide. The same procedure is repeated 

for the other strips.

The exposures are made in geometrical rather than arithmetical 

ratio. For instance, the first strip is exposed for 10 seconds, the
PHOTOMICROGRAPHY OF POLISHED SECTIONS 55

second 20 seconds, the third 40 seconds, the fourth 80 seconds, and 

the fifth 160 seconds. If the last strip still looks underexposed when 

developed, the same procedure is followed with exposures of whole 

minutes for instance, 1, 2, 4, 8, and 16 minutes. The manner in 

which the strips are exposed is as follows:

The slide is pulled out until the entire plate is uncovered, and the 

plate is exposed 10 seconds. Then the slide is pushed one-fifth of the 

way across the plate, which is exposed another 10 seconds. One-fifth 

of the plate has been exposed 10 seconds, and four-fifths of it a total of 

20 seconds. Then the slide is pushed another fifth of the distance 

across the plate and the rest of the plate exposed 20 seconds. Threefifths of the plate will have been exposed a total of 40 seconds. Then 

the slide is pushed another fifth of the distance across the plate and 

the rest of the plate exposed another 40 seconds. The remaining twofifths of the plate will have been exposed a total of 80 seconds. The 

slide is then pushed until only one-fifth of the negative is uncovered 

and it is exposed an additional 80 seconds, making a total for this strip 

of 160 seconds. The plate is developed and examined, and the strip 

showing the desired density is chosen; or if the desired density would 

be between those of two adjacent strips, the mean between the two 

exposure factors is selected.

If an arc lamp is used instead of a tungsten incandescent lamp, the 

filter factors will be somewhat different from those given above and 

should be determined experimentally by the investigator. The arc 

lamp may be necessary in taking pictures with very high magnification.

Exposure formula. The time of exposure is affected by other 

factors, the most important of which are the numerical aperture of 

the lens used and the magnification. The aperture of a lens is defined 

as the angle of the widest cone of rays which the lens can utilize. The 

numerical aperture (usually marked on a lens by the letters N. A.) is 

defined as the product of the sine of one-half the angle of aperture and 

the refractive index of the medium (air, oil, etc.) in which the objective 

works. The longer the focus and the lower the magnification of a 

lens, the smaller will be its numerical aperture.

The working aperture of the objective is the aperture actually used 

in any particular set-up and usually varies from one-half to threequarters of the numerical aperture. It can be estimated by pulling 

out the ocular or sliding in the Bertrand lens and viewing the back 

lens of the objective. Only excellent objectives will permit use at full 

aperture. Although maximum resolution is obtained at full aperture, 

contrast is reduced, so that the aperture finally adopted is a compromise 

between sufficient resolution and necessary contrast. In the photomicrography of thin sections, working apertures of one-half to two-
56 MICROSCOPIC DETERMINATION OF ORE MINERALS

thirds the numerical aperture usually give best results. Large working apertures can be obtained only when the transparent plate reflector is used in the vertical illuminator. When the small excentric 

prism is used in the vertical illuminator the working aperture is always 

less than half the numerical aperture of the objective. The current 

models of both Leitz and Bausch & Lomb vertical illuminators have a 

short side tube containing an iris diaphragm and a condensing lens. 

The iris should be closed down until it can be seen in the field and 

should be focused as sharply as possible by moving the condensing 

lens on the vertical illuminator. Then the iris should be opened until 

its image is just outside the field of view. This materially reduces the 

glare resulting from reflections within the components of the objective 

and between the front lens and the specimen, with great improvement 

in the final image. An iris diaphragm immediately in front of the 

condenser on the lamp acts as an aperture stop and is used to control 

the working aperture.

The exposure of the negative varies inversely as the square of the 

working aperture. The following table gives the numerical apertures 

of the different lenses used by the writer and the relative exposure 

factors calculated in comparison with that for the 16-millimeter lens.

32-millimeter ________ -.-...

4-millimeter.. ___ ..- _ .....--.

N. A.

0.10

.20

.25

.45

.85

( ' V VN-A.;

100

25

16

5

1.4

Approximate

factor

6.2

1.7

1.0

 u L

"3.2 

08G=TTfi

11. O

The exposure of the negative varies directly with the square of the 

magnification. Hence the working aperture factor and the magnification factor tend to balance each other. To take a specific 

example: The negative of plate 4, B, was exposed 2 minutes with 

a B filter, a 16-millimeter lens, and a tungsten incandescent lamp 

with a magnification of 112. The calculated exposure required for 

a 32-millimeter lens to take the same picture with a magnification of

472

47 is 2 minutes X 6.2X7773;=2.2 minutes. The exposure actually i LZ

used, which was determined by exposing successive strips of the negative, was 3 minutes. (See pi. 4, A.}
PHOTOMICROGKAPHY OF POLISHED SECTIONS 57

The same procedure was applied to the other lenses, and the total 

results mav be tabulated as follows:

Lens Magnification

47 

70 

112 

200 

455

Exposure (minutes)

Calculated

2.2 

1.33 

2.0 

2.0 

2.85

Determined 

experimentally

3.0 

2.67 

2.0 

1.5 

2.0

A careful record of exposures should be kept by the investigator, 

and this will prove much more useful in determining future exposures 

than the factors above given.

Developing and printing. Panchromatic plates must be developed 

in total darkness or with the aid of the special Wratten green safe 

light. The writer prefers to work in total darkness, giving the 

development a specified time depending on the temperature. The 

plate is developed in a tray, then removed, rinsed in water, and 

placed in the fixing bath.

A very satisfactory developer is Eastman Universal, which is put 

up in tubes, each tube making sufficient solution for 12 films. Another 

satisfactory developer is Rodinal, which is prepared by the AgfaAnsco Corporation and sold in liquid form in 16-ounce bottles. Diluting 5 cubic centimeters of the solution by adding 95 cubic centimeters 

of water makes sufficient solution for a 4- by 5-inch tray. Rodinal developer oxidizes less rapidly than pyro developer and consequently 

may be used for several plates. If the developing is done within 3 

hours, six or more plates may be developed in the same solution. As 

more plates are developed the solution begins to lose its strength, and 

it is better to discard the solution than to add to the time of development purely by guesswork. The writer prefers to take and develop 

one picture at a time. This may seem to be less economical of time 

than taking several pictures at once and developing them together, 

but it is questionable if, in the long run, any time is gained by the 

latter procedure. By taking and developing the pictures one at a time 

errors in procedure or defects of apparatus or materials are revealed 

at once, and immediate steps can be taken to correct them.

The time of development depends on the temperature, as follows: 

80° F., 1.5 minutes; 75°, 2 minutes; 70°, 3 minutes; 65°, 4 minutes. 

It is best to use an acid fixing bath. Plates fixed in this bath do 

not require absolute darkness. Once they are placed in the fixing 

solution, a moderate amount of daylight can be admitted. A good 

formula is as follows: Sodium thiosulphate ("hypo"), 300 grams;

129151 40 5
58 MICROSCOPIC DETERMINATION OF ORE MINERALS

potassium metabisulphite, 25 grams; water, 1,000 cubic centimeters. 

This formula is satisfactory during the winter, when the wash water 

is cool. For summer use, or whenever the temperature of the wash 

water is above 65° F., the following formula is particularly recommended: Water (about 125° F.), 1.25 liters; hypo, 480 grams; sodium 

sulphite, desiccated, 30 grams; acetic acid (28 percent), 95 cubic 

centimeters; boric acid (crystals), 15 grams; potassium alum, 30 

grams; cold water to make 2 liters. Fixing is continued until the 

milkiness has disappeared from the emulsion when viewed from the 

opposite side of the plate. The negative is then washed in running 

water for half an hour and dried.

The choice of printing paper depends on the contrast between 

the minerals as seen in the negative. If this contrast is weak, a 

contrasty paper is required; a contrasty negative requires a paper 

giving less contrast. Glossy paper is recommended, as it shows more 

detail than the different matte papers. The prints are mounted on 

a ferrotype plate.

References. The subject of photomicrography is too extensive and 

involved to be described here in all its ramifications. A very excellent 

and concise introduction to the subject is given in the booklet "Photo- 

.micrography," published by the Eastman Kodak Co., 13th ed., 1935. 

Another is the "Handbook of photomicrography/' by H. Lloyd Hind 

and W. Brough Randies, published by George Rutledge & Sons, Ltd., 

London, 1927.
PART 2. PHYSICAL PROPERTIES AND MEASUREMENTS

COLOR

The color of a transparent mineral in reflected light depends largely 

on its index of refraction. Minerals of low index have low reflective 

power and absorb most of the incident light. The zeolites are nearly 

black, calcite and quartz are dark gray, sphalerite is light gray. In 

oblique incident light these minerals show internal reflection. This is 

favored by minute seams and cleavage cracks in the interior of the 

grain, which reflect the light. The internal reflection of a mineral 

usually gives the same color as the mineral itself in the hand specimen. 

Quartz is white, sphalerite brown, cuprite red, etc. The internal 

reflection is best seen by shutting off the light from the vertical 

illuminator and tilting the lamp so as to shine obliquely on the surface 

of the specimen.

Opaque minerals, if well polished, reflect nearly all the light impinging upon them and therefore, in vertically reflected light, appear 

lighter than the transparent minerals. Only a small proportion of 

the opaque minerals have distinct colors in vertically reflected light. 

By far the greater number are white or have various shades and tints 

of gray. The minerals with distinctive colors are recognizable at 

sight or after scratching with a needle. With practice, color memory 

may be developed so highly that the investigator can recognize at a 

glance some of the more common white and gray minerals, as well as 

those with more distinctive colors. Murdoch ' says with reason, 

"The beginner in this work may at the outset experience some difficulty in this matter of color determination, but it has been found that 

the ability to distinguish minute differences in color grows very rapidly with experience, and it is astonishing how much more can be seen 

after a week's work than at the start."

Color is perhaps the most valuable property of a mineral in aiding 

in its identification. By means of color, crystal habit, and relief, all 

the more common ore minerals, such as pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, galena, and hematite, can be recognized 

at sight. This is fortunate, for if it were necessary to go through the 

etching and microchemical tests for each mineral, the amount of time 

required to examine a suite of specimens would be prohibitive.

It is difficult to describe the color of a polished surface of a mineral 

in such a way that one not familiar with it can recognize it by its 

color. Talmage,2 in a very able discussion of the value of color, says:

Descriptions of color are subject to nearly as many interpretations as there are 

interpreters, no one of whom may get near to the idea that the describer intended

 Murdoch, Joseph, Microscopical determination of the opaque minerals, p. 28, New York, 1916. 

'Talmage, S. B., Diagnostic value of color in polished sections: Econ. Geology, vol. 20, p. 171, 1925.

59
60 MICROSCOPIC DETERMINATION OK ORE MINERALS

to convey. Likewise, descriptions of the same color by different investigators 

show variation. Take, for example, a very common mineral pyrrhotite. Its 

color is variously described as light bronze, cream, pale reddish yellow, pale 

creamy brown, bronze-yellow, and pale brownish cream, but no one can know 

just what it looks like from any or all of these descriptions. When once a man 

has worked with it, the term "pyrrhotite color" conveys to his mind a very 

definite idea.

With these statements the writer is in complete agreement. Pyrrhotite has a definite color, but it is elusive and defies accurate description. Once this color has been fixed in mind by examination of a

FIGURE 14. Comparison microscope.

sufficient number of polished surfaces of pyrrhotite, it can be recognized immediately. Cubanite (CuFe2S4 ) and perhaps one or two other 

minerals have the "pyrrhotite color."

The same thing is true of sphalerite, which has a peculiar gray 

color with a tinge of blue. With some practice this color can be 

recognized with unerring accuracy, but no description, however 

wordy, can convey to the mind an accurate mental picture of the 

"sphalerite color." Davy and Farnham are particularly fond of the 

description "cream color" and have applied it to pyrite, marcasite, 

pyrrhotite, cubanite, pentlandite, and several other minerals. In 

the writer's opinion, none of these minerals resemble cream in color. 

Pyrite, marcasite, pentlandite, and millerite closely resemble one 

, 'another in color and could be described as pale yellow.
PHYSICAL PROPERTIES AND MEASUREMENTS 61

It should not be forgotten that the ability to distinguish colors 

varies with different people; and this renders it difficult to apply 

any systematic classification on the basis of color, such as that set 

up in Murdoch's book.

The comparison microscope (fig. 14) is the best instrument for comparing colors of two minerals not in contact with each other. This 

consists of two fixed microscope tubes, each with its own objective 

and reflecting illuminator. Instead of two oculars there is only one, 

between the tubes, and the images from the objectives are projected 

into it by means of prisms. The ocular shows both surfaces under 

examination, each occupying half of the field. Carefully paired objectives and reflectors are necessary. Talmage recommends the glassdisk type of reflector rather than the prism, as it gives a more equal 

illumination. Uniformity of illumination is a necessity and is insured by obtaining polished sections of the same specimen of galena, 

placing one on each stage, and adjusting the light source and the reflectors until the appearance of the two halves of the single field, as 

viewed through the comparison eyepiece, is identical. 3

Work with the comparison microscope convinced Talmage that no 

two minerals, even among those classed as white and gray, have the 

same color, and hence that anyone, regardless of his color sensitiveness, could observe a color difference with such a microscope.

The comparison microscope also brings out the fact that the color 

of a mineral depends in part on its environment. According to 

Talmage: 4

Chalcopyrite, in a field of gray or white minerals, looks brass yellow; against 

pure native gold chalcopyrite looks a dirty olive green, and if adjacent to native 

copper, chalcopyrite looks nearly grass green. Tennantite and tetrahedrite are 

another pair almost indistinguishable. But by comparing the two with galena, 

the tennantite looks faintly olive green, while the tetrahedrite looks brown with 

the green suggestion.

Many attempts have been made to define actual color values for the 

ore minerals. These efforts have met with only partial success. 

Murdoch 5 says:

Except in the case of the distinctly colored minerals, the differences in color 

arc so slight than even spectroscopic measurements of the reflected wave length 

would probably be ineffective. The books on colors and color standards deal 

with far brighter colors than most of the tinted white minerals show. Ridgway's 

book on "Color standards and nomenclature," giving 1,100 named colors, is the 

best of this sort. He gives colored plates reproducing the shades and tints of the 

spectrum colors, with a white blank for comparison with the lightest tint of each 

color. Unfortunately, except for the distinctly colored minerals, which are 

relatively few in number, all the sulphide minerals have colors falling somewhere 

between pure white and his lightest tint, usually much nearer the white end, so 

his standards are of little use in this connection.

" Talmage, S. B., op. cit., p. 173.

1 Idem, p. 174.

 Murdoch, Joseph, op. cit., p. 28.
62 MICROSCOPIC DETERMINATION OF ORE MINERALS

Schneiderhohn 6 uses a drawing ocular or camera lucida, by means 

of which he reflects the image of the mineral on the white-paper 

margin of a color chart. This scheme seems to the writer to have 

less merit than direct comparison by means of a comparison microscope.

The writer suggests that the white and gray minerals be divided 

into four classes by comparison with native silver, the whitest, and 

sphalerite, the darkest, of the ore minerals, as follows:

Silver 

Galena

Argentite

/C 

Tetrahedrite<

^>V 

Sphalerite -^

Chalcocite, for instance, is darker than galena and lighter than 

argentite and thus belongs in class B. This scheme would require 

a comparison microscope, and for the present it is not used in the 

determinative tables.

For work with the comparison microscope daylight does not 

furnish a satisfactory illumination, as it varies too much during 

the day and is affected by cloudiness and other atmospheric changes. 

The artificial light Talmage recommends consists of a 75-watt 115- 

volt blue ground-glass lamp, screened by Wratten filters 78b and 78c. 

"This arrangement," he says, "gives a light value well within the 

normal daylight range and also gives good visual color values, minerals 

under this illumination looking just about as they do in daylight."

Minerals that exhibit colors other than shades of white or gray when 

viewed in vertically reflected light are listed in table 1, page 293.

Polished sections of many transparent minerals show distinctive 

colors when observed microscopically under oblique instead of vertically incident light. Seams, cracks, and cleavage planes below the 

surface of the section reflect back part of the light penetrating into 

the mineral. The color of a mineral under these conditions is usually 

nearly the same as that of the powder of the mineral. Internal reflections are also observed in vertically reflected polarized light 

(between crossed nicols) when the illumination is strong.

Minerals showing colored internal reflections are listed in table 2, 

page 293.

Nonmetallic minerals and some metallic minerals, such as sphalerite, huebnerite, and cassiterite, give powders ranging from white to 

light brown in color. Transparent or translucent minerals distinctly 

colored in the hand specimen that is, in ordinary oblique light 

usually yield powders of nearly the same color. These minerals are 

usually gray or bluish gray in reflected light. The powders are best 

obtained under the reflecting microscope by holding the needle at an

' Schneiderhohn, Hans, Mikroskopischen Bestimmung und Untersuchung von Erzen, p. 91, Berlin, 1922.
PHYSICAL PROPERTIES AND MEASUREMENTS 63

inclination to the polished surface and pushing it against the surface, 

at the same time giving the needle a rotary motion. The powder 

tends to collect as windrows on both sides of the groove.

Minerals that give characteristic colored powders are listed in 

table 3, page 294.

HARDNESS

There is lack of agreement among various authorities as to the 

definition of hardness. 7 Methods of measuring the relative hardness 

of materials mat be briefly outlined as follows:

1. Scratching with a needle or other sharp instrument.

2. Pressure exerted on the surface by means of a steel ball.

3. Impact on the surface by dropping a steel ball on it.

4. Grinding the surface on a steel lap, with emery or similar abrasive, the 

length of time required to grind away a certain weight of a mineral of standard 

size and shape affording a measure of the hardness.

5. Boring with a diamond point of standard size and shape, the number of 

revolutions of the drill required to bore a definite depth in the mineral affording 

a measure of the hardness.

Of the methods cited, those employing scratching are the most 

applicable to minerals in general and are the only ones suitable for 

use on polished surfaces under the microscope. The scratching 

method most used up to the present time is that described by Murdoch.8 A No. 10 Sharps needle is mounted in a handle 5 or 6 inches 

long, weighing a quarter of an ounce. The needle is brought into 

the field of view of the microscope and is held at an angle to the 

polished surface and dragged (not pushed) across the surface with its 

point inclined downward and to the rear. Murdoch recognized three 

grades of hardness, as follows:

1. Soft, or low hardness, including minerals that can be scratched with the 

weight of the handle alone or very easily scratched with light pressure.

2. Medium hardness, including minerals that can be scratched only faintly 

with slight pressure but easily with moderate or heavy pressure.

3. Hard, or high hardness, including minerals that can be scratched but 

slightly or not at all with heavy pressure.

The writer has used this method as well as he could for some years; 

but many serious errors have resulted from incorrect determinations 

based on it. A needle point will remain sharp for only a small 

number of hardness tests; it tends to curl like the toe of a ski, and 

thus its effectiveness for scratching is lessened.

The ore minerals exhibit all grades of hardness from the hardest to 

the softest, and any scheme of subdividing them is arbitrary; overlaps 

are inevitable. Moreover, some minerals vary considerably in hardness in different crystallographic directions.

7 For an excellent discussion of hardness in minerals see Talmage, S. B., Quantitative standards for hardness of the ore minerals: Econ. Geology, vol. 20, pp. 531-553,1925. 

s Murdoch, Joseph, op. cit, p. 29.
64 MICROSCOPIC DETERMINATION OF ORE MINERALS

There are more minerals which approximate the boundary between 

the medium and soft of Murdoch's three grades of hardness than 

between the medium and hard. A sharp needle will scratch chalcopyrite, tetrahedrite, and sphalerite, minerals listed by Murdoch and 

by Davy and Farnham as of medium hardness. The average worker 

is unable to decide whether these minerals scratch easily, thus falling 

in the "soft" group, or whether they scratch faintly, thus falling in 

the "medium" group.

For the reasons above set forth the writer had decided to dispense 

with the "medium" class of Murdoch and to place the minerals 

hitherto classed as of medium hardness in the "soft" class. In the 

scheme here adopted two grades of hardness are recognized:

1. Soft, including minerals which are scratched readily by a needle.

2. Hard, including minerals which are scratched with difficulty or not at all 

by a needle.

There are, no doubt, minerals as to which the investigator is puzzled 

to decide whether they scratch easily or with difficulty. This is inevitable, and experience is the only guide in aiding in a decision.

Observations of hardness as shown by resistance to wear are useful to a moderate extent. If two adjacent minerals are of different 

hardness the harder mineral will stand up in relief above the softer 

mineral. As polishing methods are improved this relief decreases. 

A scratch passing from a softer mineral to a hard is less distinct in 

the harder mineral than in the softer. In addition, an effect somewhat similar to the Becke ray used in petrography to detect differences of index of refraction between two adjacent transparent minerals is of value in determining differences of hardness between two 

adjacent opaque minerals. The harder mineral has a border that 

slopes downward toward the softer mineral. Light striking this 

border is reflected outside of the field of view. A dark band thus 

results. Adjoining this dark border on the side of the harder mineral 

and parallel to it is a line of light. When the tube of the microscope 

is raised, this bright line of light is seen to move from the hard to the 

soft mineral; when the tube is lowered the reverse takes place.9

An instrument for measuring the relative hardness of minerals in 

polished sections (fig. 15) has been described by Talmage. 10 It 

consists essentially of a diamond attached to the end of a graduated 

beam. The beam carries a sliding weight like that of a weighing scale. 

The edge of the diamond is wedge-shaped, like that of an ax. The 

beam rides horizontally on jeweled pivots and balances when the 

adjustable weight is at zero. The apparatus is so arranged that the 

scratching point is directly under the objective of the microscope,

' Van der Veen, E. W., Mineragraphy and ore deposition, p. 28, The Hague, 1925. 

10 Talmage, S. B., Quantitative standards for hardness of the ore minerals: Econ. Geology, vol. 20, pp. 

533-543, 1925.
PHYSICAL PROPERTIES AXD MEASUREMENTS 65

and the entire operation of scratching can be watched under the 

microscope. By means of the movable weight, Talmage determined 

the pressure necessary to produce a scratch of given character, called 

by him the standard scratch. The diamond remains stationary and 

the specimen is moved under it by means of a micrometer stage.

FIGURE 15. Talmage hardness machine. Detailed view of scratching mechanism. (After Talmage.

Courtesy of Economic Geology.)

Talmage divides the minerals into seven classes of hardness, as 

follows:

Classes of hardness

Designation

A. ..........

B

C. ..........

D. ._._.___._

Type mineral

Position 

of weight 

on beam

0.2

.7

1.0

Actual 

weight on 

scale 

(grams)

.520

Designation

E... ........

F

G... ........

Type mineral

Position 

of weight 

on beam

1.5

2.0

3.0

Actual 

weight on 

scale 

(grams)

0.755

1.035

According to Vanderwilt: n

This device gives highly reliable results that can be readily checked, provided 

the surfaces have an essentially scratchless polish. It is believed to yield far more 

accurate values of scratch hardness than any device hitherto employed. On 

badly scratched or pitted surfaces the scratch made by the diamond edge cannot 

be clearly seen and the test is not reliable.

Experiments made by the writer on Talmage's instrument indicate 

that the time required to make a reliable hardness test averages about 

15 minutes.

11 Vanderwilt, J. W., The nature of polished surfaces (doctor's thesis, Harvard University), p. 70. 1927.
66 MICROSCOPIC DETERMINATION OF ORE MINERALS

This instrument is not manufactured by any instrument maker at 

present, and its construction would involve some difficulty. It is not 

yet certain whether another instrument could be constructed that 

would duplicate Talmage's numerical results, but undoubtedly one 

similar to it could be made, and the figures corresponding to the type 

minerals in Talmage's table could be determined without difficulty.

Talmage's classes A, B, C, and D correspond to the "soft" group 

as used by the writer, and classes F and G to the "hard" group. 

Class E is about on the boundary between hard and soft, and minerals 

with that hardness are included in both classes.

The hardness determinations by Talmage are given in part 3 of this 

bulletin. A few of the determinations there given differ from those 

of Talmage, owing to the fact that for these minerals he obtained his 

results on mislabeled specimens in the Murdoch suite. The writer, 

using Talmage's machine, determined the hardness of a number of 

minerals not found in that suite, and these results are included in the 

tables. It is believed that the Talmage hardness determinations afford a mental picture of the relative hardness of different minerals, 

especially those in the "soft" class. Thus there would be no difficulty 

in getting a mental picture of the hardness of hessite (class A), which 

is about that of Talmage's type mineral, argentite, whereas millerite 

(class D) is about on the boundary between the "hard" and "soft" 

classes. The investigator, after some experience in scratching known 

minerals with an ordinary needle and looking up their Talmage 

hardness in the tables, will be able to subdivide minerals in the "soft" 

class roughly on the basis of hardness.

EXAMINATION OF OPAQUE MINERALS IN POLARIZED LIGHT

Summary. The most useful test that can be applied to determine 

an unknown opaque mineral is the test for anisotropism. The test 

is made in less than a minute, and the result is highly definite and 

instructive. In this test a nicol prism is placed between the light 

source and the vertical illuminator of the metallographic microscope; after reflection from the polished surface, the light passes 

through another nicol prism whose planes of vibration are nearly 

or exactly at right angles with the planes of vibration of the polarizing prism. Isotropic minerals cause no change in the polarization 

direction of the light impinging upon them. Anisotropic minerals, 

on the other hand, rotate the plane of polarization of the incident 

light, and when the light passes the analyzer of the microscope interference effects are set up. Quantitative optical data corresponding 

to those obtained on transparent minerals by means of the petrographic microscope cannot be obtained on opaque minerals. The 

amount of rotation produced can be measured but is only about 5° 

for the mineral causing the greatest rotation; all other minerals
PHYSICAL PROPERTIES AND MEASUREMENTS 67

show less than this amount. The results that are most valuable 

are qualitative. Minerals crystallizing in the isometric system and 

amorphous minerals are isotropic; minerals crystallizing in the 

other five crystal systems are anisotropic. Thus, a mineral can 

be quickly placed in one of two main classes by means of this test, 

and in the determinative tables here given this distinction is the 

first one made.

References. The pioneer work on the effects of polarized light 

on opaque minerals is that of Koenigsberger. 12 His apparatus makes 

use of only one nicol, the analyzer. Between the vertical illuminator 

and the analyzer is a Savart plate. He describes the anisotropism 

of a number of minerals.

A mathematical discussion of the optical principles involved in 

Koenigsberger's apparatus is given by Wright, 13 who describes certain improvements in the apparatus and advocates the use of the 

Wright biquartz wedge in place of the Savart plate.

A greatly increased interest in the possibilities of the method 

was caused by the work of Schneiderhohn. The apparatus that he 

devised is substantially that used by all subsequent investigators, 

including the present writer. A theoretical discussion of the phenomenon by Berek is given in Schneiderhohn's text. 14 A discussion of Schneiderhohn;s method, together with many original data, 

is given by Sampson. 15 Some of Schneiderhohn's theoretical conclusions are criticized by Van der Veen. 16 Excellent descriptions 

of the apparatus and directions for its use are given by Osborne 17 

and by Fairbanks. 18 The most recent and in many respects the 

most comprehensive description of the method is that of Sampson. 19

Apparatus. A petrographic microscope with an analyzing nicol 

in the barrel is the best instrument for investigating anisotropism. 

It is converted into a metallographic microscope by merely inserting 

a vertical illuminator between barrel and objective. The lower nicol 

of the petrographic microscope is not used in its original location 

but may be taken from its mounting and clamped in such a position that light passes horizontally through it and is polarized before 

reaching the vertical illuminator. It is preferable, however,"to leave

12 Koenigsberger, J., Ueber einen Apparat zur Erkennung und Messung optischer Anisotropie undurchsichtiger Substanzen und dessen Verwendung:Centralbl. Mineralogie, 1908, pp. 565-573, 597-605. (Translated in Winchell, N. H. and A. N., Elements of optical mineralogy, pp. 465-475, 1909.)

" Wright, F. E., Polarized light in the study of ores and metals: Am. Philos. Soc. Proc., vol. 58, pp. 401- 

447, 1919.

14 SchneiderhShn, Hans, Anleitung zur mikroskopischen Bestimmung und Untersuchung von Erzen und 

Aufbereituagsprodukten besonders im auffallenden Licht, pp. 65-71, Berlin, 1922.

" Sampson, Edward, Note on the determination of anisotropism in metallic minerals: Econ. Geology, vol. 

18, pp. 775-777,1923.

ID Van der Veen, J. W., Mineragraphy and ore deposition, pp. 32-36, The Hague, 1925.

17 Osborne, F. F., Technique in the investigation of iron ores: Econ. Geology, vol. 23, pp. 442-450,1928.

is Fairbanks, E. E., The laboratory investigation of ores, pp. 24-27, New York, 1928.

" Sampson, Edward, The determination of anisotropism in metallic minerals: Econ. Geology, vol. 21, 

pp. 412-423, 1929.
68 MICROSCOPIC DETERMINATION OF ORE MINERALS

the lower iiicol in its original place, in order to be able to change 

the instrument back to a petrographic microscope quickly. For a 

polarizer the writer uses a small iiicol (made by E. Leitz), which is 

attached directly to the vertical illuminator and can be rotated. 

(See fig. 15.) A cap analyzer can be used instead of an inside 

analyzer. This form is required when the Wright biquartz wedge 

(p. 73) is used. The cap analyzer cuts down the field of vision 

and is very tiring to the eye, and its use is not recommended except where unavoidable. In the writer's instrument the inside anTt,

FIGURE 16. Microscope arranged for use with polarized reflected light. Photographed by E. B. Dane, Jr. 

o, Bausch & Lomb petrographic microscope; 6, vertical illuminator; c, polished section in brass mount; 

d, polarizer attached to vertical illuminator; e, analyzer; /, blue ground glass; g, rotatable disk; A, small 

arc lamp.

alyzer is rotatable through an angle of 90°. This is more convenient 

than an analyzer fixed in position but is not absolutely necessary, 

as the same optical effect can be obtained by rotating the polarizing 

nicol. Sampson states that the plane of vibration of the polarizer 

should be parallel or afc right angles to the plane of the reflector of 

the vertical illuminator; if these planes are in an intermediate position serious disturbing effects are introduced. Ordinary light, such 

as is furnished by reflected daylight or the ordinary incandescent 

electric light, is inadequate for investigating opaque minerals in 

polarized light. A more intense illumination is needed, and a small 

arc lamp is very satisfactory for this purpose. The arc lamp used 

in the Harvard Laboratory of Economic Geology is made by E. 

Leitz and is known as the Lilliput arc lamp. (See fig. 16.) It is 

provided with a clockwork attachment for automatically feeding
PHYSICAL PKOPERT1ES AND MEASUREMENTS 69

the carbon to the electric arc; this attachment is convenient but not 

necessary. A rheostat giving about 13 ohms is necessary when 

using a lighting circuit of 110 volts. Direct current is superior to 

alternating current for this purpose.

When the analyzer is thrown out, this light is too strong for the 

eyes, and a ground glass is placed between light and microscope. 

In the apparatus shown in figure 16 this is placed on a rotatable 

disk by means of which it can quickly be placed in and out of the 

line of light.

The illumination unit furnished by Bausch & Lomb consists of a 

6-volt 108-watt Mazda tungsten-ribbon filament incandescent lamp 

with housing and a transformer for stepping the current down from 

110 volts. It is used with an alternating; current. The principal 

advantage of the incandescent lamp over the arc lamp is the steadiness of the illumination. On the other hand, the 6-volt 108-watt 

incandescent lamp has only a short life roughly 20 hours and is 

expensive, whereas carbons for the arc lamp are relatively cheap. 

The writer uses the tungsten-ribbon lamp above described for 

microphotography.

The illumination unit used in the laboratory of the Geological Survey for investigating anisotropism is the Busch microscope lamp, 

made in Rathenow, Germany, and sold by the Palo Co., New York. 

The price of this lamp is about $21. It is somewhat less expensive 

than the lamps already described and is highly satisfactory. It is 

furnished with a Mazda 110-volt 250-watt projection bulb using either 

alternating or direct current. This bulb is less expensive than the 

6-volt ribbon-filament bulb and has a longer life. It requires more 

manipulative skill, however, as the coiled filament gives a distinct 

image, and the lamp must be shifted in such a way as to eliminate this 

image. This unit is almost identical with that devised by Prof. A. M. 

Bateman, of Yale University, and described by Osborne in the paper 

already cited. The illumination unit used at the University of Arizona is the Spencer microscope lamp No. 370, which is described on 

page 7.

The 16-millimeter short-mounted objective and the 24-millimeter 

objective are by far the best for observations on opaque minerals in 

polarized light. An 8-millimeter objective also gives fair results 

but is inferior to those above mentioned. The 4-millimeter objective, 

which is the one most used for high magnification, is very unsatisfactory for this purpose, and with it only minerals of strong anisotropism 

exhibit the phenomenon. The writer can offer no explanation for 

this fact and lias not seen it mentioned elsewhere in the literature.

Details of the method. Opaque minerals belonging to the isometric 

system are isotropic when examined in the manner outlined above. 

If the polished section is turned 360° by rotating the stage of the
70 MICROSCOPIC DETERMINATION OF ORE MINERALS

microscope, isometric minerals show no change of color or change 

of intensity of light during the revolution. Minerals of the other five 

crystal systems, on the other hand, are anisotropic when examined 

in the same manner. Some of them show four extinctions during 

the revolution, thus behaving like anisotropic transparent minerals 

examined in polarized transmitted light on the petrographic microscope. By far the greater number of anisotropic opaque minerals 

will give only two extinctions to a revolution. The minerals noted 

thus far that give four extinctions are covellite, umangite, manganite, pyrolusite, hausmannite, stibnite, and molybdenite. This list is 

incomplete, and probably a few more minerals will be added to it 

in the course of time. This property of giving four extinctions is of 

great diagnostic value because so small a proportion of the total 

number of opaque anisotropic minerals exhibit it.

A few exceptions should be noted to the general rule that all isometric minerals are isotropic. Certain varieties of cobaltite exhibit 

weak anisotropism, as already noted by Schneiderhohn and Sampson. 

The same is true of certain varieties of hessite, argentite, cuprite, 

pyrite, bornite, and perhaps a few others. Most specimens of 

these minerals are isotropic, however. A possible cause of the 

anisotropism may be the presence in small proportion of accessory 

elements which are not found in most specimens of the mineral. A 

more probable theory, however, is that these minerals may have been 

isometric at the temperature of formation but have undergone inversion to a lower symmetry during cooling. It is known that chalcocite and other minerals exhibit this phenomenon, as do also some 

transparent isometric minerals, notably leucite. Amorphous minerals are also isotropic. Very fine grained anisotropic minerals are 

sometimes apparently isotropic. For instance, specularite is moderately anisotropic, but earthy red hematite, which is probably not 

-amorphous but minutely crystalline, appears isotropic.

Transparent minerals of high birefringence are anisotropic when 

examined in polished section, but those of low birefringence are apparently isotropic. The carbonate minerals can be identified at a 

glance and distinguished from quartz and other gangue minerals by 

this method. Light penetrates deeper into the transparent minerals, and only that which is reflected is visible. Small cracks and 

seams below the surface tend to disperse the light reflected and 

nullify the effects of polarization. In the determinative tables 

which follow it is noted that the ore minerals realgar and wurtzite 

are apparently isotropic.

The effect produced on polarized light by an anisotropic mineral 

is easily described. The nicols are first placed in the crossed position. This can be accomplished by observation on an isotropic 

mineral, such as galena. The analyzer is turned until the mineral
PHYSICAL PROPERTIES AND MEASUREMENTS 71

appears darkest. If an anisotropic mineral is now observed, it is 

found that the mineral is not in the position of extinction, but it will 

appear in that position if the analyzer is rotated a few degrees to 

the right or left. The anisotropic mineral has rotated the plane of 

polarization of the rays impinging upon it; rays striking the mineral with a "north-south" vibration are reflected with the direction 

of vibration of a small angle from that direction. The direction 

of vibration of the rays is no longer at right angles with the direction of vibration of the analyzer, and some of the light gets through. 

The angular rotation given to the analyzer to bring about extinction measures the angular rotation produced by the mineral.

It is found that in a rotation of the stage the mineral rotates the 

polarization plane first to the right, and after a 90° rotation of the 

stage the polarization plane is rotated to the left; in a complete 

rotation of the stage there are two rotations to the right and two to 

the left.

As already stated, usually only two extinctions are observed, and 

even with a mineral showing four extinctions, such as molybdenite, 

two of them 180° apart are sharp and clean-cut and the other two 

are less definite. This phenomenon, as Sampson points out, is due 

to absorption of light by the mineral; this absorption is stronger in 

certain directions than in others. It is analogous to pleochroism 

in a transparent mineral. Owing to this absorption, the positions 

of extinction during the revolution of the stage may be prolonged, 

and where the brightening of the mineral might otherwise have 

taken place, it is masked by absorption. However, if the Wright 

biquartz wedge plate is used, all minerals will give four extinctions, 

for with this plate actual rotation is the effective agent, and this 

is not modified by the intensity of the reflected light. From the 

reasoning set forth above it is concluded that there is no fundamental 

difference between anisotropic minerals which give two extinctions 

and those which give four extinctions to the revolution. The latter 

group is confined to minerals in which the color differences due to 

rotation of the plane of polarization prevail over the effect of color 

absorption. According to Goranson,20 every mineral regardless of its 

absorption gives four extinctions exactly 90° apart, provided the 

nicols are exactly crossed.

Absorption is selective on rays of different wave length. Some are 

absorbed more than others, the degree of absorption depending on the 

mineral. Hence the color of the mineral as seen in polarized light is 

usually very different from its color as seen in ordinary light.

In minerals of strong anisotropism differently oriented grains have 

entirely different colors. For instance, arsenopyrite shows colors

M Qoranson, E. A., The behavior of ore minerals in polarized light (doctor's thesis, Harvard University), 

p. 13, 1933.
72 MICROSCOPIC DETERMINATION OF ORE MINERALS

similar to those shown by thin sections of hypersthene, ranging from 

pink to blue. However, such evidence should be accepted with 

caution. The polarization colors vary with the adjustment of the 

nicols, the source of light (arc or incandescent), the intensity of 

illumination, and the perfection of polish, in addition to the effects 

produced by the mineral itself. Owing to these factors the writer 

has not been able to check the descriptions given in the tables of 

either Van der Veen or Farnham.

The amount of rotation produced by any particular anisotropic 

mineral ranges from zero to a maximum depending on the direction 

of the polished section with reference to the optical orientation of the 

mineral. Basal sections of hematite and molybdenite, for instance, 

are isotropic, whereas prismatic sections exhibit the greatest anisotropism for these minerals. Thus the maximum rotation of a mineral 

is a definite property of that mineral.

The measurement of such a constant, however, is attended with 

great difficulty, because the position is not clean-cut like that of a 

transparent mineral but is wavy and is thus harder to locate. To 

obtain this position a Wright biquartz wedge is necessary, and even 

with this a series of observations is required in order to obtain the 

exact angle by the method of averages. This must be extended to 

several grains, as the rotation of the polarization plane produced by 

the mineral depends on the orientation of the grain and tlius varies 

in the different grains cut by a polished section. On the other hand, 

it is a simple matter to pick out the grains giving the greatest rotation, by simply noting those that show the greatest contrast between 

light and dark positions as the stage is rotated.

In addition to the difficulty in obtaining the maximum angle of 

rotation of the plane of the polarization for any given mineral, there 

is the added objection that this angle, even if constant for any mineral, 

varies only a few degrees between the minerals with greatest polarization effect and those with least. Chalcopyrite, whose anisotropism 

is barely discernible, gives a rotation of a small fraction of a degree, 

whereas covellite, which has a greater polarization than any other 

mineral yet observed, rotates the angle only about 5°. The others 

are covered by this small range. Owing to these difficulties, the 

writer does not believe that it is practicable to use the maximum 

rotation as a constant to determine a mineral, as the maximum extinction perpendicular to "m" is used to determine a feldspar; but 

minerals can be subdivided readily into two groups, isotropic and 

anisotropic, and the latter can be subdivided by such general terms 

as weakly, moderately, and strongly anisotropic not exact terms, 

to be sure, but useful hints.
PHYSICAL PROPERTIES AZSTD MEASUREMENTS 73

Methods oj observation. 21 ADisotropism may be detected in three 

ways (1) with the analyzer in the crossed position; (2) with the 

analyzer slightly rotated from the crossed position; and (3) by the 

use of the Wright biquartz wedge and nicol.

The method with the analyzer crossed is the least sensitive of the 

three. As the stage is rotated, a strongly acting mineral will change 

from complete darkness to a very faint illumination.

If the analyzer is placed so that it is not exactly in the crossed 

position, the anisotropic effect can be seen better, particularly where 

several contiguous grains of the mineral he in the same field. With 

the analyzer thus turned, the mineral when in the four positions of 

no rotative effect will be in extinction, as if it were isotropic; but 

when in the position of greatest rotative effect it will be brighter or 

darker than it would have appeared had the analyzer not been turned, 

because the mineral by its rotative effect is adding to or subtracting 

from the effect of the turned analyzer. The advantage of this method 

of observation is similar to that obtained by using a gypsum plate with 

a petrographic microscope to secure a sensitive tint. The eye can 

better detect changes in the intensity of a moderate brightness than 

in that of almost complete darkness. In a polished section that 

shows several grains of a mineral the effect of anisotropism is readily 

seen, for one grain may be in the position of least illumination while 

its neighbor may be in the position of greatest brightness.

An effect similar to that, caused in a granular aggregate is produced in a single grain, by using a Wright biquartz wedge plate with 

the analyzer in the exactly crossed position. This compares two 

moderately bright fields. The plate contains two sections of quartz 

cut parallel to the base. One section is from a right-hand crystal, 

the other from a left. They are so mounted that the line separating 

them is a medial line in the microscope field. Half of the plate 

rotates light to the right, and half to the left. The combined quartz 

sections are ground to a wedge, so that the degree of rotation varies 

according to the thickness of the part of the plate used. The Wright 

wedge is used with a cap nicol. If light reflected up the microscope 

tube is plane-polarized, and the plane of polarization corresponds 

to the zero setting of the cap nicol, the two halves of the field will 

be equally illuminated. If, however, the reflected light has its plane 

of polarization turned from this zero position, as may be brought 

about by an anisotropic mineral, the intensity of illumination on 

opposite sides of the plate will vary according to whether the rotation caused by that side of the plate is the same as or opposite to 

that caused by the mineral. If the cap nicol is now rotated an 

amount equal to the rotation caused by the mineral, the illumination

11 This description of the three ways of detecting anisotropism is taken almost verbatim from Sampson's 

article (Econ. Geology, vol. 24, pp. 415-418,1929).

129151 40 6
74 MICROSCOPIC DETERMINATION OF ORE MINERALS

will again be equal. By this method the amount of rotation of the 

plane of polarization caused by the mineral may be measured quantitatively. However, the Wright biquartz wedge is useful principally 

for refined qualitative determinations, particularly when such determinations must be made on an isolated grain; with such a grain 

the effect of a slight change in the intensity of illumination is augmented by using the Wright plate, because the effect of the reflected 

light adds to the brightness on one side of the plate and detracts 

from that on the other side, according to whether the rotation caused 

by the mineral is the same as or opposite to that of the piece of rotating 

quartz in each half of the plate.

Adjustment of the instrument. The writer uses the second method 

described above exclusively. The Wright wedge is an effective 

method of determining anisotropism, but the use of the cap nicol 

is a serious handicap. The second method gives results equally 

good, permits the use of a tube analyzer, and dispenses with a change 

of ocular.

The question naturally arises as to how far from the crossed position the analyzer (or polarizer) should be turned. The writer finds 

a polished section of chalcopyrite from Sudbury, Ontario, of value 

in getting the best adjustment of nicols. This chalcopyrite shows 

multiple twinning similar to that of plagioclase feldspar, but the anisotropism is very feeble. The nicol (either polarizer or analyzer) 

is turned until there is a maximum color difference between adjacent 

lamellae. The darker lamellae will have a color about the same as 

that of bornite in ordinary light, and the lighter lamellae will be 

greenish yellow. This adjustment is found to be satisfactory for all 

anisotropic minerals. Figure 17 shows chalcopyrite that gives an 

effect identical with that of the Sudbury mineral.

Experience in teaching the subject demonstrates that making this 

adjustment is by far the most difficult task that the student will encounter. Sufficient practice alone will overcome this difficulty. The 

nicols are first set in approximately the crossed position. The polarizer and vertical illuminator are then pointed as directly at the light 

source as possible. The chalcopyrite surface is then observed through 

the microscope while one of the nicols is turned through a few degrees 

one way and then the other. If the appearance of the chalcopyrite lamellae is not satisfactory, the polarizer and vertical illuminator 

are swung a little to either side on the vertical axis of the illuminator, 

and the same process is repeated. This is continued until the chalcopyrite lamellae show the maximum contrast. Then the lamp 

is shifted to one side or the other until the illumkiation as seen in 

the microscopic image is evenly distributed. After some experience a 

student of average ability can make this adjustment in 5 minutes 

or less.
PHYSICAL PROPERTIES AND MEASUREMENTS 75

In the determinative table in part 3 of this bulletin polarization 

colors for the anisotropic minerals are noted. These colors were 

observed under illumination with a Spencer microscope lamp No. 370 

using a 100-watt Mazda projection bulb. The nicols were adjusted 

as described in the preceding paragraph. Too much reliance should 

not be placed on these color values, as with some minerals, such as 

pyrrhotite, the colors vary markedly when either nicol is rotated a 

fraction of a degree.

Sources of error. The surface of the mineral must be at right 

angles to the axis of the microscope that is, parallel to the stage 

otherwise there is a variation in the intensity of the light as the secFIGURE 17. Photomicrographs of ores taken in polarized reflected light. (After Sampson. Courtesy of 

Economic Geology.) A, Marcasite, showing polysynthetic twinning, in pyrite. Nicols nearly crossed. 

From material collected by A. W. Stickney and studied by him in the laboratories of the United States 

Geological Survey. This plate also illustrates that an imperfect polish does not interfere with clear-cut 

qualitative results. X60. B, Twinning in chaloopyrite from Bass prospects, Grand Canyon, Ariz. 

Nicols nearly crossed. Specimen unetched. X150.

tion is rotated. In a complete rotation of the stage the section will 

be tilted toward the light once and away from it once if the section 

and stage are not parallel. If the section is tilted toward the lamp, 

that part of the field of view toward the lamp is more strongly 

illuminated, and the other part is shaded. If the section is tilted 

away from the lamp (toward the observer), the reverse is true. This 

variation in intensity resembles the extinctions of anisotropic minerals, but it may be distinguished from anisotropism by the fact that 

there is only one "extinction" to the revolution and also by the fact 

that it can be observed with the analyzer out. If the polished sections are mounted on modeling wax they can be easily adjusted by
76 MICROSCOPIC DETERMINATION OF ORE MINERALS

means of the leveling cup (see p. 8) so that the surface is parallel 

to the stage. When the specimens are solidly mounted in brass 

boxes, the top and bottom of each box should be ground parallel.

If more than one grain of an anisotropic mineral is in the field of 

view, the above-mentioned possibility of error is obviated, because 

differently oriented grains of the mineral have sharp boundaries and 

show different colors or different shades of the same color under 

crossed nicols. Fortunately, most opaque minerals are fine-grained1 

that is, the size of individual grains is smaller than the field of 

view of the 16-millimeter objective, which is approximately 2 millimeters in diameter.

Poor polishing and irregularities of relief also lead to error in interpretation of polarization effects. This is particularly true of the 

hard minerals, such as chloanthite and other arsenides. Rotation 

of the stage gives rise to vague irregularities in illumination which 

the beginner is wont to interpret as anisotropism. Experience alone 

will enable him to discount these effects. Above all, grain boundaries 

should be looked for.

The causes of failure in the investigation of anisotropism are lack 

of adequate illumination, poor adjustment of the nicols, and pseudoextinctions due to lack of parallelism between polished surface and 

microscope stage or to irregularities of relief on the polished surface.

In classroom practice it has not been found necessary to equip 

each student with a polarizing apparatus for reflected light. It is 

sufficient to have one polarizing microscope centrally located in the 

classroom, and this will suffice for a class of eight or less.

ELECTROCONDUCTIVITY

Methods for determining the electrical conductivity of minerals 

have been described by Farnham,22 by Kerr and Cabeen,23 and by 

Harvey.24 The writer has not had any experience with these methods, but they promise to be of considerable assistance in the identification of the ore minerals. It appears from Harvey's work that the 

conductivity in some minerals varies enormously in different specimens, and this factor weakens the effectiveness of the method. According to Harvey, the principal cause of this variation lies in slight 

differences in chemical composition of the mineral.

MEASUREMENT OF REFLECTIVITY

The lack of numerical optical constants that can be determined 

microscopically for opaque minerals has long been felt by investiga22 Davy, W. M., and Farnham, C. M., op. cit., p. 8.

23 Kerr, P. P., and Cabeen, C. K., Electrical conductivity of the ore minerals: Econ. Geology, vol. 20, 

pp. 729-737,1925. 

M Harvey, R. D., Electrical conductivity and polished sections: Econ. Geology, vol. 23, p. 778,1928.
PHYSICAL PROPERTIES AND MEASUREMENTS 77

tors. The determination of such constants is much more difficult 

than with nonopaque minerals.

The measurement of reflectivity seemed to offer a possible means 

of supplying numerical constants that would afford a valuable aid in 

the determination of an unknown mineral. It was believed that 

reflectivity would prove to be a definite constant for an isotropic 

mineral and would be confined within narrow limits for an anisotropic 

mineral. With certain exceptions, which are described beyond, 

this belief has been verified.

Reflectivity is defined as the ratio of the intensity of light reflected 

from a given surface to the intensity of light vertically incident on 

that surface. Reflectivity has been exhaustively investigated by 

many scientists, notably Drude. 25

If the indices of refraction and absorption can be determined, the 

reflectivity can be calculated by a somewhat complex formula. 

These determinations have been made for several materials, both 

native metals and transparent minerals. Such determinations on 

opaque ore minerals have been comparatively few, however.

In recent years reflectivity has been measured experimentally by 

means of attachments to the metallographic microscope. Leaders 

in these investigations are Schneiderhohn and Ramdohr and their coworkers in Germany and Orcel and his coworkers in France. The 

most successful investigation in the United States has just been 

concluded by John H. Moses.

Microscopic methods for determination of reflectivity can be grouped 

into two classes:

1. Photometric methods. By means of a prism photometer the image of a 

mineral surface is brought into juxtaposition in the ocular with a field of light 

whose intensity can be varied. The intensity of the variable field is adjusted 

so that it is equal to that of the image of the mineral. The image of the mineral 

and that of the variable field appear to be two half circles separated by a vertical 

line through the axis of the ocular.

2. Photoelectric methods. By means of a photoelectric cell placed above the 

ocular of the microscope, the actual intensity of the light reflected from the 

mineral can be measured. This is compared with a measurement made on a 

standard mineral whose reflectivity is already known.

The two methods have about the same degree of accuracy, and 

determinations of reflectivity can be made with an error of not more 

than 1 or 2 percent. The determinations made by the two methods 

do not agree with each other, because the spectral sensitivity of the 

human eye differs from that of the photoelectric cell. The main 

objection to the methods is that two minerals of widely different 

chemical composition may have reflectivity values within 1 percent 

of each other. Hence, these methods cannot be relied upon to 

determine a mineral in the absence of other methods, but they may 

offer valuable confirmatory evidence.

2J Drude, P., The theory of optics, translated from the German hy Mann and Millikan, London, 1902.
78 MICROSCOPIC DETERMINATION OF ORE MINERALS 

THE SLIT MICROPHOTOMETER OF BEREK 26

In 1922 Schneiderhohii requested M. Jierek, who had designed 

many valuable attachments and improvements for the petrographic 

microscope, to construct a photometer ocular for the metallographic 

microscope. In 1923 a working model was constructed by means of 

which Schneiderhohn made some preliminary measurements. Improvements to the instrument rapidly followed, and in 1930 the inFIOURE 18. Microscope with Berek slit microphotometer attachment, s, Slit-shaped diaphragm; /, filter 

holder. (Courtesy of E. Leitz, Inc.)

strument was placed on the market by the firm of E. Leitz, Wetzlar, 

Germany. The microscope with the photometer attachment is shown 

in figure 18, and a sectional view of it in figure 19.

The incident light is split into two parts in a glass prism Wi with an 

oblique, semitransparent separation surface (fig. 19). The rays 

separated in this manner take different paths. Finally they are 

brought into juxtaposition by means of a photometer prism W2 , which 

is similar in construction to the Lummer-Broadhun cube. The part 

of light that passes the prism Wi traverses a rotatable polarizer P. 

Horizontal markings on its mounting indicate the positions in which 

its direction of vibration is vertical, 45°, and horizontal, respectively. 

After passing the polarizer the light goes through the collimator lens 

C, in front of which is an iris diaphragm J. The light then strikes 

the vertical illuminator and is thrown downward onto the specimen.

" The following description is based on that given in the text of Schneiderhobn and Ramdohr (Lehrbuch 

der Erzmikroskopie, Band 1, pp. 157-164,1934), the catalog of E. Leitz, Inc., and several written communications from Mr. H. W. Zieler, president of E. Leitz, Inc.
PHYSICAL PROPERTIES AND MEASUREMENTS 79

19. Sectional view of microscope and Berek slit microphotometer. (Courtesy of K Leitz, Inc.) 

See text for explanation.
80 MICROSCOPIC DETERMINATION OF ORE MINERALS

After reflection it travels upward into one half field of the photometer 

prism W2 . The image of the iris diaphragm J is thrown on the 

polished surface by means of the collimator lens C and the objective 

of the microscope. It serves to adjust the prism as well as to show 

the boundary of the illuminated field.

The other part of the light that enters the glass prism Wi goes 

upward through a tube parallel to the tube of the microscope; this 

tube contains a succession of diaphragms. The light then strikes the 

total reflecting prism R and goes through two polarizers, NI and N2 . 

From these it passes into one-half of the field of the photometer cube, 

where it is totally reflected and goes upward into the ocular.

The prism W\ does not divide the incident light into two portions 

of equal intensity. On the contrary, it is so designed that the part 

passing through the auxiliary tube exceeds in intensity that passing 

into the microscope itself. As a result, the light of the auxiliary system, even after being reflected three times and passing through two 

nicols, is still brighter than the light that is reflected from a mineral 

of high reflectivity such as native silver.

The two nicols NI and N2 afford the means by which the intensity 

of the light in the auxiliary system is controlled and measured. When 

their vibration directions are at'right angles, no light reaches the 

photometer prism. When their vibration directions are parallel 

the maximum amount of light reaches the photometer prism. In the 

parallel position the amount of this light is a little less than one-half 

of the light striking NI. (The ordinary rays of NI are deflected and 

absorbed in its mounting, and a small percentage of the extraordinary 

rays are absorbed and turned into heat by NI and N2 .) Likewise 

one-half of the rays reaching the polarizer P are absorbed by it. N2 

is rotatable, and its rotation is determined by means of a circular 

arc T attached to it. In contact with the movable arc is a fixed 

vernier which reads to 0.1°. The arc and polarizer are turned by 

means of a knob (not shown in fig. 19). The vernier is read with the 

aid of a lens L. A mirror illuminates the arc.

When the vernier reads 0°, the nicols are in the crossed position, 

and the half field of the auxiliary system as viewed in the ocular is 

black. When the vernier reads 90°, the nicols are in the parallel 

position, and the maximum brightness is seen in the ocular. Any 

intensity between these two extremes can be obtained by rotating N2 

the requisite distance from 0°. The intensity of light does not increase in direct proportion to the angle of rotation; on the contrary, 

it has been shown by mathematical analysis that the intensity 

increases in proportion to the square of the sine of the angle of rotation, 

As a result the intensity near the crossed position increases rapidly 

for a small increase in rotation, whereas near the 90° or parallel position 

the rate of increase is much less.
PHYSICAL PROPERTIES AND MEASUREMENTS 81

Another very essential part of the apparatus is a slit-shaped diaphragm which, is not attached to the microscope or auxiliary system 

but is supported on a vertical rod between the lamp and the auxiliary 

tube. (See fig. 18.) This diaphragm takes the place of the iris 

diaphragm usually found in front of the vertical illuminator. It 

serves two main purposes: (1) It permits a more accurate adjustment 

of the auxiliary system with respect to the microscope proper than is 

afforded by an iris diaphragm. (2) It eliminates rays of relatively 

high angle of incidence that strike the objective of the microscope, 

thus insuring that the rays which strike the mineral have a relatively 

low angle of incidence. This is important, as reflectivity is denned 

for zero angle of incidence. The width and height of the slit are 

controlled by accurate adjusting devices. The slit is oriented with 

its long dimension vertical. Color filters can be inserted at F (fig. 19) 

above the eyepiece. The filters recommended are red (Lifa No. 79), 

orange (Lifa No. 78), and green (Lifa No. 74). On account of the 

importance of the slit, the manufacturers have termed the entire 

apparatus the "slit microphotometer."

A neutral gray filter can be inserted into the auxiliary system. 

This serves to cut down the light that passes through this system. 

The reason for its use is as follows:

The amount of light reflected by some dark minerals, such as 

quartz, is so small that only a small rotation of N2 is necessary to 

match it. In this range a small rotation of the nicol N2 produces a 

large difference in the amount of light transmitted, and an accurate 

determination of the necessary rotation is difficult. When the 

intensity of light conveyed to NI is diminished by means of a gray 

filter, a larger rotation of N2 is required to produce a match with the 

light reflected by the mineral, and, as already explained, the rate of 

increase of light transmitted is slower than near the crossed position 

of the nicols. Therefore, a more accurate determination of the necessary rotation is possible.

In calibrating the instrument the measurement is first made on a 

standard mineral whose reflectivity is known. Then under the same 

conditions that is, the width and height of the slit, the adjustment 

of the auxiliary system, and the setting of the lamp and nicols the 

mineral whose reflectivity is to be determined is placed on the microscope, and N2 is rotated until a color match is obtained. The reflectivity of the mineral can be calculated by the formula already given, 

but the process is much simplified by coordinate paper supplied by 

the manufacturer. This is constructed in a manner analogous to 

that of logarithmic coordinate paper, so that a straight-line relationship exists between the angle of rotation and the reflectivity.
82 MICROSCOPIC DETERMINATION OF ORE MINERALS

For calibration of the instrument Berek recommended a basal 

plate of quartz 3 to 4 centimeters thick. More recently A. Cissarz 27 

has concluded that yellow sphalerite from Santander, Spain, is the 

most satisfactory standard for dry systems and galena for immersion 

systems. The figures for reflectivity as determined by him are given 

below.

Reflectivity as determined by A. Cissarz

Yellow sphalerite (in air): 

Cleavage surface (calculated).-...----...----.---..----.

Galena:

Green

Percent 

16.97

15.5

43. 3± . 2

28. 8± . 2

Orange

Percent 

16. 49

16.0

41. 6± .2

27. 1± . 2

Red

Percent 

16.15

15.6

40. 1 ± .2

25. 9± .2

An ordinary microscopic lamp is satisfactory for photometric 

measurements. Small variations in intensity during measurement are 

not particularly harmful, hence an ordinary alternating house circuit 

can be used. In this respect the photometric method is superior to 

that using the photoelectric cell, where constancy of voltage during 

measurement is absolutely essential. A good polish is recommended. 

Schneiderhohn and Ramdohr state that the results obtained are 

nearly independent of the degree of polish, but this statement is somewhat open to question.

The adjustment of the auxiliary system and slit diaphragm is a 

somewhat complicated and tedious process, but is fully described in 

the instructions supplied by the manufacturer.

Polarized light is always used. For isotropic minerals, the nicol P 

is turned to the 45° position and the measurement made at once. 

For anisotropic minerals, P is placed in the 0° position and the analyzer of the microscope (not shown in fig. 19) is inserted. The stage 

is revolved and the darkest of the extinction positions determined. 

With the mineral in this position, the analyzer is removed and the polarizer turned to the 45° position and reflectivity determined as already 

described. This gives a value termed by Berek "the mean value of 

the uniradial reflections." Apparently this value is independent of 

the orientation of the section relative to its crystal axes.

Colored light (usually green, orange, or red) is always used. This 

is especially important when measuring colored minerals. The eye 

can detect minute differences in light intensity when the color of two 

adjacent fields is the same. On the other hand, it is very difficult to 

measure differences in intensity when the colors are different. The 

use of a color filter minimizes such differences.

According to Schneiderhohn and Ramdohr, measurements made 

with the instrument are accurate to 1 or 2 percent if ordinary precau-

!7 Quoted by Schneiderhohn, Hans, and Ramdohr, Paul, op. cit., Band 1, p. 161.
PHYSICAL PROPERTIES AND MEASUREMENTS 83

tions are taken. By using a more elaborate procedure this figure can 

be cut to about 0.2 percent.

The method above described is complicated and requires a high 

degree of technical skill, as the personal element enters into determinations of relative color intensity. The instrument itself is expensive, 

costing approximately $334, according to most recent quotations of 

the Leitz Co. (1936). The method is being rapidly improved through 

the efforts of Schneiderhohn and Cissarz and offers much promise for 

the future.

Determinations of reflectivity as given by Schneiderhohn and 

Ramdohr are shown in table 4 (p. 294). These determinations were 

made some years ago, and the authors state that more recent determinations differ from those given by as much as 2 percent.

MEASUREMENT OF REFLECTIVITY WITH PHOTOELECTRIC CELLS

A second method of measuring reflectivity which is free from the 

personal equation or defects in the human eye is based on the photoelectric effect. Expressed in common language this is the transformation of light energy into electricity. Certain substances, such as 

metallic sodium, lose electrons when struck by light. Probably all 

substances show the effect to a greater or less degree, but those which 

show it markedly with visible light are relatively few.

There is an extremely simple relation between the intensity of 

illumination and the strength of the current produced. One is directly proportional to the other, and the graph expressing the relation 

is a straight line.

In practice, a polished section of a mineral whose reflectivity is 

known is used as a standard for calibration. It is placed on the stage 

of a reflecting microscope. The photoelectric cell is housed in an 

attachment that fits into the barrel of the microscope and takes the 

place of the ocular. The current produced by the standard mineral 

is measured. Immediately afterward the mineral whose reflectivity 

is unknown is placed on the microscope and measured in the same 

way. The lamp, the opening of the diaphragm in front of the illuminator, and other conditions, must be the same for both measurements, and the lamp must not be shifted between measurements, 

otherwise the comparisons cannot be made. If the current produced 

by the standard mineral is x amperes, and that produced by the other 

mineral is nx amperes, the reflectivity of the other mineral is just n 

times that of the standard.

The accepted explanation of the photoelectric effect is based on the 

quantum theory. A detailed explanation will not be given here but 

will be found in modern textbooks of physics. It is sufficient to state 

that the energy of the current produced is equal to the total energy 

of the light emission minus the work of separating the electrons from
84 MICROSCOPIC DETERMINATION OF ORE MINERALS

Ss .

the bombarded substance. This is known as Einstein's equation. 

Where the work of separation of electrons is large, the output will be 

correspondingly small.

Credit for the first application of the photoelectric cell to measurement of reflectivity is due to Dr. J. Orcel,28 of the Museum national 

d'histoire naturelle, Paris.

Two kinds of photoelectric cells have been used alkali cells and 

rectifying cells.

Alkali cells. The cell first described and used by Orcel is an alkali 

cell. This consists of a bulb of glass filled with argon under low

pressure. The interior of the bulb is coated 

with a film of potassium hydride (KH) deposited on a film of silver. This constitutes the cathode of the cell. When struck 

by light coming up the barrel of the microscope, it emits electrons that strike the 

anode T (fig. 20), a wire of tungsten bent 

in a loop.

Owing to the high internal resistance of 

the cell, the output is small and must be 

amplified. Two amplification circuits are 

used by Orcel and his coworkers. A simple 

type is shown in figure 21 and is sufficient 

for ordinary determinative measurements. 

The photoelectric cell is in series with a 

150- to 170-volt battery and a mirror 

galvanometer.

For more exact work, such as calibration 

of standard minerals, a thermionic tube is used in the amplifying 

circuit, as shown in figure 22.

The photoelectric cell C is connected in series with a 150- to 180- 

volt battery B2 and a resistance R. The anode of the cell is connected 

with grid G of the amplifying tube L, the filament F of which is heated 

by battery Bl} of 4 volts. The galvanometer D is connected with the 

screen grid G' and the plate P of the amplification lamp. To compensate the constant current flowing in the amplifying tube with the 

nonilluminated cell C, a compensating resistance p is connected as 

shown.

The sensitivity of the galvanometer is 10~9 to 10~10 amperes per 

millimeter of the scale at 1-meter distance. The amplification of the 

thermionic circuit is 100- to 150-fold.

Attached to the cell and below it is a total reflecting prism (P, fig. 

20) similar to that in the Leitz Makam photomicrographic camera.

2S Orcel, J., La mesure du pouvoir rfiflecteur de mineraux opaques a I'aide de la cellule photo-filectrique 

et, ses applications: Soc. franchise mine'ralogie Bull., torne 53, pp. 301-349, 1930; Acad. Sci. Paris Comptes 

rcndus, tome 185, p. 10.r>5; torne 187, p. 1141, 1928.

FIGURE 20. Alkali photoelectric cell. 

(After Orcel.)
PHYSICAL PROPERTIES A2STD MEASUREMENTS 85

(See p. 47.) This allows observation of the field of the microscope 

and accurate focusing of the specimen. The prism can be withdrawn 

from the axis of the microscope just before measurement with the 

cell. It possesses an adjustment by means of which it can be shifted

GalvQnome ter 

FIGUKE 21. Simple amplifying circuit for alkali cell.

in three different directions at right angles to each other. An iris 

diaphragm under the prism blocks off all light except that part 

reflected from the mineral whose reflectivity is to be measured.

r~AA#WWW °

FIGURE 22. Thermionic amplifying circuit for alkali cell. (After Orcel.) See text for explanation.

A very high polish is necessary for best results with this instrument. 

All focusing should be done with a movable stage in order to avoid 

shifting the barrel of the microscope between measurements.

The following crystalline substances are used as standards for calibration of the apparatus:

Reflectivity of standard minerals

Percent 

Sphalerite (yellow)_____________________________________ 17. 9

Tetrahedrite__.--_.._...___.__^.___._-__.__.._______._ 30. 0

Silicon___-.__-.__.-_..._.....__..__.._._._._____._.__ 42. 9

Galena.__-_-.-----.__.._._.___.__....._.._._.___.__._ 40. 0
86 MICROSCOPIC DETERMINATION OF ORE MINERALS

These figures represent averages of a series of very careful measurements.

The spectral sensitivity of the alkali cell shows a well-defined maximum at 465 millimicrons (blue); the sensitivity diminishes rapidly 

with variations from this wave length in either direction. Hence 

measurements with white light approximate rather closely those with 

monochromatic blue. Measurements with yellow and red light are 

inexact or impossible owing to lack of sensitivity to these colors.

All measurements are made with one nicol prism placed in front of 

the illuminator. Its plane of polarization is placed parallel to the 

plane of incidence of the prism.

A factor to be taken into consideration is extraneous light reflected 

from the upper surface of the microscope objective. This light is 

constant, but the galvanometer deviation that corresponds to it should 

be subtracted from the total deviation when a mineral is measured. 

The deviation produced by this extraneous light is measured by 

lowering the stage of the microscope or placing a black paper under 

the microscope. 29

Work with alkali cells is time-consuming on account of fatigue 

phenomena in the cell, which must be obviated by pauses for recovery. 

In addition, the lamp must be heated for an hour before constant 

results are obtained. Comparisons with the photometer ocular of 

Berek are possible only to a moderate degree, owing to the fact that 

the sensitivity of the human eye is greatest in the range of yellowish 

green (555 millimicrons); hence the sensitivity of the alkali cell does not 

correspond to that of the eye. In spite of these handicaps, consistent 

and reliable results have been obtained by Orcel and his coworkers, 

who have been carrying on active research work.

Rectifying cells. A second type of photoelectric cell differs from the 

alkali cell in that it does not require the introduction of extraneous 

electricity produced by batteries but has the property of converting 

light into electricity directly. It is known as a rectifying cell. The 

first investigations making use of a rectifying cell to determine 

reflectivity were made in 1932 by H. Ehrenberg under the direction of 

P. Ramdohr. A complete description of his results is given by 

Schneiderhohn and Ramdohr.30

In the rectifying cell the work of separating electrons is greatly 

reduced in comparison with that required in the alkali cell, and therefore the output of electricity produced by a given intensity of light is 

much greater.

The principle on which the rectifying cell is constructed is shown 

schematically in figure 23. It consists essentially of a very thin layer 

(U) of silver selenide or cuprous oxide between two electrodes. This is

28 Orcel, J., written communication.

»o Schneiderhohn, Hans, and Ramdohr, Paul, Lehrbuch der Erzmikroskopie, Band 1, pp. 168-178,1934.
PHYSICAL PROPERTIES AND MEASUREMENTS 87

known as the rectifying layer, as it is unipolar conducting. EI is the

electrode first struck by the light rays. It must allow them to pass

through in order to strike the rectifying layer. As none of the transparent substances are conductors, the rectifying layer is sprayed on a

metallic grid. Only the rays that strike the

openings in the grid affect the rectifying layer.

E2 is a second metal electrode . The light striking EI drives off electrons, which reach the

rectifying substance U and pass to the opposite electrode E2 . An electric current passes

from E2 to EI through a galvanometer.

The construction of an ocular cell is shown 

in figure 24. The figure is not drawn to scale, FIGURE ^.-construction of rectifyas the rectifying layer is only a few molecules ing cel1 - (Aftcr

J , . J m, and Kamdohr.)

thick. The ocular is a No. 4 penplane. I he explanation.

Sec text for

FIGURE 24. Cross section through rectifying cell. (After Schneiderhohn and Kamdohr.)

distance of the cell from the ocular lens is 9 centimeters.

The cell can be removed and a ground-glass disk installed in its 

place for the purpose of focusing the specimen. The change from the 

cell to the ground glass and vice versa is quickly accomplished by a 

horizontal sliding device attached to the top of the microscope barrel. 

After the focus is attained, the disk is removed, and the cell is placed in 

the axis of the microscope. The image will now fall on the lower 

electrode. Focusing is accomplished by moving the stage, in order 

not to disturb the adjustment of the microscope with the light.
88 MICROSCOPIC DETERMINATION OF ORE MINERALS

There is a direct proportional relationship between intensity of 

illumination and electric current up to 1,000 lux.31 Above that intensity the current approaches a limiting value. Hence there is a limit 

to the strength of light that can be used; fortunately this limit is well 

above that of ordinary microscopic illumination. The lamp used by 

Ehrenberg and Ramdohr is a 250-watt nitra attached to an ordinary 

110-volt alternating circuit.

The spectral sensitivity of the silver selenide rectifying cell reaches 

a maximum with yellow light of 575 millimicrons wave length, thus 

approaching that of the human eye more nearly than that of the 

alkali cell. The authors cited make the measurements with red, 

yellow, and green light. The light filters are placed directly in front 

of the vertical illuminator.

The galvanometer used is of the mirror type and is read by means of 

a telescope. Its sensitivity is 10~7 amperes.

It is not feasible to work without an ocular, owing to internal reflections set up in the barrel of the microscope.

There is a minimum size of grain below which determinations of 

reflectivity cannot be made. Only the image of the mineral to be 

measured must be projected on the ground glass; with an anisotropic 

mineral only the individual mineral that has a uniform optical orientation can be measured. With coarse-grained minerals the 16-millimeter objective can be used, but where the grain size is small the 

4-millimeter or oil-immersion objectives must be used, and with 

many specimens the field even with these high magnifications must be 

reduced by means of the iris diaphragm. Measurements with these 

high powers are not so accurate as with low powers. For exact 

measurements, the use of the new silvered prism, designed by Berek, 

is recommended by the authors.

The excellence of polish has an important influence on reflectivity. 

Scratches and pits deflect the light rays and diminish the percentage of 

the light that reaches the cell. In this respect the photoelectric cell is 

inferior to the photometer ocular, in which the measurement uses 

only that part of the specimen lying at the boundary between the 

two fields, and the errors above mentioned can be allowed for subjectively. It is necessary, therefore, to obtain as good a polish as possible 

before making measurements.

The calibration is accomplished by comparison with standard 

minerals in a manner similar to the calibration of the alkali cell.

Schneiderhohn and Ramdohr do not give tables of reflectivity 

determined by this instrument but promise to do so in the future. 

Experiments designed to eliminate some sources of error in the method 

are still under way. They state, however, that with ordinary pre31 A lux is the strength of illumination produced by one standard candle on 1 square meter of surface at a 

distance of 1 meter.
PHYSICAL PROPERTIES AND MEASUREMENTS 89

cautions and careful manipulation reflectivity can be measured with an 

accuracy of about 0.2 percent.

An interesting result of the published researches of Orcel was that 

almost at the same time that Ramdohr and Ehrenberg were working 

with the rectifying cell in Germany, J. H. Moses was conducting 

similar experiments at Harvard University. Moses was unaware of 

the work of Ramdohr and Ehrenberg, and consequently his results 

are entirely independent of theirs. The following description is 

abstracted from his thesis.32

Moses' apparatus is similar in many respects to that of Ramdohr 

and Ehrenberg but contains some important points of difference. 

The vertical illuminator is of the prism type with a small condenser. 

Attached to the barrel of the microscope and parallel to it is a metal 

disk through which a hole 1 millimeter in diameter has been bored. 

This hole is at the focus of the condenser and serves to control the area 

of the specimen illuminated. It is termed the "field diaphragm" by 

Moses.

The 4-millimeter objective is used for all measurements. The 

reasons for using it are that very small grains can be measured, and 

the perfection of polish is a vital factor. With the 4-millimeter 

objective it is possible to search for areas free from scratches and other 

imperfections. The principal objection to its use is that the angle of 

incidence for the most oblique ray is 58°. Moses discusses this factor 

in detail and concludes that it has little effect in measuring relative 

reflectivities.

The ocular is a No. 7.5. In the plane of the photocell this system 

gives a magnification of 120 diameters. The cell can be removed and 

a ground glass inserted in its place for the purpose of focusing the 

specimen. The arrangement is similar to that described for the Ramdohr-Ehrenberg apparatus (p. 86). A diaphragm is placed between 

the photocell and the ocular to control the size of the area of light 

striking the cell. The diameter of the opening is 16 millimeters. On 

the plane of the cell this diameter is a little smaller than the diameter 

of the image of the field diaphragm. The vertical illuminator is 

adjusted by turning it until the image of the field diaphragm covers 

the upper diaphragm. The cell used by Moses is a Westinghouse 

"Photox." It is of the cuprous oxide type.

The galvanometer is of the reflecting type and throws an image on 

a horizontal scale placed at a distance of 1 meter from it. It is made 

by the Leeds & Northrup Co., type 2500-g, and has a sensitivity of 

5 X 10~10 amperes per millimeter deflection. It has a critical damping 

resistance of 7,000 ohms, an external resistance of 470 ohms, and a 

period of 6.4 seconds. In order to read the galvanometer deflections,

11 Moses, J. H., The identification of opaque minerals by their reflecting power as measured photoelectrically (doctor's thesis, Harvard University), 1936.

129151 40 7
90 MICROSCOPIC DETERMINATION OF ORE MINERALS

JV 

/

a 50-centimeter ground-glass scale with 1-millimeter divisions marked 

on it was used. Both the scale and the galvanometer are supported ~ 

on a specially built vibration-free frame. In making photoelectric 

measurements it is extremely important that the illumination be constant during measurement. Also, as the determination of reflectivity 

for the mineral in question is compared with that of a standard, the 

strength of illumination must be the same for both measurements. 

A strong source of light is necessary, and it was found that an automobile headlight furnished the most satisfactory source. That 

selected was the "Tung-sol, 32-32 candle power, 6-8 volt, T 100" 

lamp manufactured by the Tung-Sol Lamp Works, Inc., Newark, N. J. 

It was found that the 110-volt alternating circuit available varied 

too much to be used with a transformer supplying 6 volts. Likewise, 

the direct-current system available at Harvard varied too much to be ( 

reduced by a rheostat to 6 volts. A 6-volt storage cell was used but 

was found to lose strength too fast. The difficulty was finally solved 

by connecting a 6-volt direct current (reduced from the 110-volt circuit) in series with a 6-volt storage battery. v

It was found immediately that the amount of light available from 

many different minerals was more than enough to deflect the gal- v 

vanometer beyond the limits of a meter scale. This difficulty was 

solved by placing a resistance of 4,470 ohms as a shunt across the galvanometer. A well-polished specimen of pyrite was chosen as a 

standard, and the resistance chosen was such that it gave a swing of 

a little more than 25 centimeters. The lamp is held in a light-tight 

box and is adjustable so that its distance relative to the field diaphragm 4 

can be varied. When the light beam from the galvanometer has come ^ 

to rest, the lamp is moved until the image reads exactly 25 centimeters. The pyrite is then removed, and the mineral to be determined is placed on the stage of the microscope and a reading obtained. "^ 

With a galvanometer of this type the deflection of the image obtained 

is exactly proportional to the strength of the current, and this, as -*- 

stated on page 83, is exactly proportional to the intensity of the light 

striking the cell. !

The cell surface is not uniformly responsive to light in different f 

places. Hence the sliding device is adjusted so that the cell is exactly 

centered over the center of the diaphragm. Moses differs from the 

European practice in that he uses unpolarized light. He gives his 

reasons for doing so as follows:

In spite of the undeniable evidence in the literature pointing out the importance 

of orientation on reflecting power and consequently the desirability of using plane- i 

polarized light when making measurements of reflecting power, the writer persisted in using nonpolarized light for all of his final measurements. His reasons 

were dependent on several factors. Firstly, one of the outstanding goals of.the 

entire problem was to develop a quick and easy method of opaque-mineral identification, and the complications introduced by the use of polarized light result in
PHYSICAL PROPERTIES AND MEASUREMENTS 91

a procedure which is anything but simple. Secondly, although realizing the 

similarity in the use of nonpolarized light in this case and in the case of determining 

the index of refraction in nonopaque minerals, it was believed (and this belief was 

eventually more or less verified) that the variations in the measurements with the 

orientation of the crystal would be very slight if strictly nonpolarized light were 

used. Unfortunately, it is difficult, if not impossible, to pass nonpolarized light 

through a microscope equipped with a vertical illuminator without producing a 

noticeable percentage of plane-polarized light. In spite of the presence of this 

unavoidable portion of plane-polarized light, only a few strongly anisotropic 

minerals showed a variation of as much as 1 percent on rotation of the microscope 

stage. Likewise, when the quality of the polished surface of a heterogeneously 

oriented group of grains of one mineral was constant over all, very little difference 

was found in the reflecting power measured on several different grains. A few 

minerals, including stibnite, when specially prepared specimens polished normal 

to the three crystal axes were used, did show a very noticeable difference in 

reflecting power by the three surfaces. The writer believes that such minerals 

are exceptional.

A third reason for using nonpolarized light for all the final measurements lay 

in the fact that, except when dealing with individual crystals, it is an extremely 

difficult and uncertain task to determine the position of the crystallographic axes 

when viewing a polished surface. And, in order to gain much useful information 

with plane-polarized light, knowledge of the position of the crystallographic axes 

is very essential.

The surface of the specimen must be exactly perpendicular to the 

axis of the microscope. In this respect unmounted polished sections 

leveled in modeling wax are superior to specimens mounted in bakelite 

or sealing wax. It has been found difficult to keep the top and bottom 

of such specimens absolutely parallel, especially if the specimen is 

ground by hand after mounting. According to Moses:

A difference of 1 to 2 percent was found in the galvanometer deflection when a 

block about 1 inch long was tilted by the thickness of a single sheet of typewriting 

paper, about 0.004 inch, under one end. Increasing the number of sheets increased 

the difference of the reading as compared with the normal position. Furthermore, 

as previously shown, when the polished surface is not strictly perpendicular to the 

tube of the microscope, there is a marked fluctuation in the galvanometer reading 

as the microscope stage is rotated. This phenomenon holds true even in the case 

of an isotropic mineral, such as galena, on a perfectly centered stage. On the 

other hand, no such fluctuation is observed when the surface is perpendicular to 

the microscope tube.

Moses' results as given in his thesis are expressed as galvanometer 

readings. They can be reduced to absolute reflectivity by multiplying 

each of his results by the fixed constant 2.29. This constant was 

obtained as follows: The reflectivity of galena as given by Orcel is 

44.3 percent and that given by Cissarz is 43.3. These figures, arrived 

at independently, agree very well. By taking the mean, 43.8, and 

dividing by Moses' galvanometer reading 19.1, the factor 2.29 is 

obtained.

The perfection of polish has an important bearing on the measured 

reflectivity. This is especially true of the native metals (see figures
92 MICROSCOPIC DETERMINATION OF ORE MINERALS

for silver and copper in table 4), but it also limits the accuracy of the 

results for the "metallic ores to ±1 percent. The precaution must be 

taken of obtaining the best polish that is permitted by the means at 

hand, and seeking under the microscope the best-polished area on the 

surface of the specimen. Owing to this limitation,' the figures for 

reflectivity given in table 4 (p. 294) cannot be regarded as absolute 

constants for the respective minerals.

SPECTROSCOPIC METHODS

The most delicate of all tests for the presence of an element is 

afforded by the spectroscope. By means of this instrument qualitative determinations can be made on quantities of material almost 

infinitely small. Several attempts have been made to combine the 

methods of spectroscopy with the study of polished sections. A very 

successful investigation by Haycock 33 is of unusual interest, in that 

he was able to estimate with considerable accuracy the relative 

atomic percentages of different elements in a mineral.

The quartz spectrograph is a modification of the spectroscope in 

which a prism of quartz takes the place of the usual glass prism, and 

which has a camera by means of which the image of the spectral lines 

may be photographed. The photographic plate obtained by photographing the spectrum of an unknown mineral is placed immediately 

above or below a plate made under the same conditions from a known 

element. If the spectral lines coincide, the known element is present 

in the mineral.

The quantitative estimation of elements in a substance is based on 

the following principles: (1) Other conditions remaining constant, the 

number and intensity of lines of an element recorded by the photographic plate decrease with decreasing concentration of that element 

in the source. (2) Other conditions remaining constant, the time 

required for the disappearance of all but the ultimate lines of an element depends upon the amount of that element in the source. The 

method employed was to compare the spectrum of the unknown 

mineral with the spectra produced by known amounts of the element 

to be determined. An estimate of the amount of the element was 

then made on the basis of intensity and number of lines. A unit volume of solution containing the unknown mineral was introduced into 

the arc and its spectrum photographed. The amount of solution used 

for every exposure was 0.002 cubic centimeter. This was estimated 

by filling a capillary tube to a given height.

A reference collection of photographs was made from solutions of 

given strength of all the metallic elements for whose presence tests 

were to be made. The standard strengths were normal, %N,

33 Haycock, M. H., The application of the quartz spectrograph to the study of opaque minerals: Econ. 

Geology, vol. 28, pp. 364-382, 1933.
. PHYSICAL PROPERTIES AND MEASUREMENTS 93

, KoN, XoN, XoN, ^0N, XooN, and ^ 0 oN. As five exposures could 

be made on one plate, two plates were needed for each element. 

Unfortunately sulphur does not furnish a satisfactory spectrum. In 

practice, therefore, it was not possible to determine the relative 

atomic ratio of sulphur to the metallic elements.

The material to be tested is bored from the polished surface of the 

specimen under microscopic observation. The boring is facilitated 

by a special drill devised by Haycock.34 This enabled amounts of 

material less than 0.25 millimeter in diameter to be removed and 

tested. With microchemical methods this amount would give a 

feeble test or none at all for most elements.

The material is taken into solution with nitric acid or aqua regia 

and then converted to the chloride by repeated evaporations with 

HC1. It is then taken into solution with 1: 8 HC1 and transferred 

to the tip of one of the graphite electrodes. This must be cold and 

dry when the solution is introduced. When the current is turned on, 

the spectrum is produced and photographed. Where tiny inclusions 

in a host are to be tested, enough areas are bored and the resulting 

material collected together to furnish an adequate amount for the 

test. For instance, tiny areas of argentite in galena can be tested, 

provided the galena is free from silver. Where the inclusions are small, 

it is impossible to avoid including small amounts of the host. Hence 

under these conditions it would not be possible to determine lead in 

a mineral included in galena, but it would be possible to test for all 

other elements except sulphur.

Let us assume that we are.testing a mineral containing both lead 

and silver. Let us assume, further, that the lead lines in the photograph corresponded to a solution KoN with respect to lead and that 

the silver lines corresponded to a solution %oN with respect to silver. 

The atomic ratio of lead to silver in the mineral would be 2:1. 

From the foregoing statements it is clear that all the metallic material must go into solution. If sulphur is present it will probably 

separate out. It is also clear that the conditions must be rigorously 

controlled. Haycock describes the factors to be considered as 

follows:

1. The length, diameter, and shape of the electrodes. Any difference in these 

factors will alter the temperature of the arc, both because of the effect on the 

potential drop and because of the rate of conduction of heat away from the 

points. Moreover, if sharp angles are present at the points, a high potential 

may occur and introduce spark lines into the spectrum.

2. Distance between the electrodes. A variation in distance between electrodes 

causes variation in the drop in potential.

3. Amperage of the current. Variation of the amperage causes variation in 

the drop in potential. It is to be noted that under the most favorable conditions, 

the arc will vary about 10 percent. Generally speaking, the higher the voltage

3( Haycock, M. H., A method for sampling minerals in polished sections: Econ. Geology, vol. 26, pp. 

'115-420, 1931.
94 MICROSCOPIC DETERMINATION OF ORE MINERALS

of the supply, the steadier the arc. Commensurate with safety of operation, the 

best results are thought to be obtained with voltages above 220, and many 

spectroscopists prefer voltages or 350 or even 700. The present work was done 

with a voltage of 110, higher voltages not being available.

4. Preparatory burning of the electrodes. Consistency on this part will produce 

electrodes of similar shape and porosity.

5. Volume of solution. This has been referred to previously.

6. Procedure of adding the solution to the electrode. It is necessary to take care 

that the solution is absorbed by the tip of the electrode and not allowed .to run 

down over the side. Therefore it must be added slowly.

7. Period of drying of impregnated electrode. Appreciable moisture remaining 

in the electrode will volatilize at the outset and affect the spectrum.

8. Position of the source with reference to the slit of the spectrograph. This is a 

common source of error, and care must be taken to keep the light accurately 

focused on and covering the slit.

9. Adjustments to the optical system of the spectrograph. Any changes in adjustment of slit and focus should be rigorously avoided.

10. Timing of exposures.

11. Type and make of plate. Variations in the sensitivity of photographic 

plates are different for different wave lengths, and a standard make of plate 

should be adopted and used consistently.

12. Development of the plate. Consistency in development is probably best 

obtained by following a time-temperature method, taking care to use developer 

of fixed concentration.

The quartz spectrograph used by Haycock is made by Adam 

Hilger, London. Of the three models made by Hilger large, intermediate, and small Haycock uses and recommends the intermediate 

model, because the' entire range of the spectrum can be shown on one 

exposure. It costs approximately $1,400. However, as mentioned 

in the description of microchemical tests p. (173), the extreme sensitivity 

of the spectrograph is often a handicap to the determination of the 

mineral, for elements are sometimes identified, such as copper in 

bismuthinite, which are not shown in the formula as ordinarily 

written. Aside from the expensive apparatus required, a further 

objection is that a long apprenticeship and great personal ability are 

necessary to produce good results.
PART 3. ETCH REACTIONS AND DETERMINATIVE TABLE

ETCH REACTIONS

The term "microchemical test" is used both by Murdoch and by 

Davy and Farnham to describe the etching effects produced by nitric 

acid and other reagents on the polished surface of the mineral to be 

investigated. It is believed that the term "etch test," used by Schneiderhohn and by Van der Veen, is preferable. The terms "microchemistry" and "microchemical test" have been used since 1877 to 

designate qualitative microscopic tests or reactions for elements. 

The tests are usually made on a glass slide. To avoid confusion, 

the term "microchemical test" will be confined in this bulletin to 

qualitative chemical tests made on a glass slide; tests made directly 

on the polished section will be referred to as "etch tests."

The etching effects or lack of effects of a set of chemical reagents 

of standard strength have been the principal means relied upon by 

all the above-named men to identify the unknown opaque minerals.

Murdoch "pigeonholes" the minerals according to their reactivity 

toward the following solutions: HN03 , 1:1; HN03 , concentrated; 

aqua regia; HC1, 1:1; HC1, concentrated; KCN, 20 percent; FeCl3 , 

20 percent; KOH, saturated solution. In his descriptions of the 

individual minerals, their reactions with the following solutions 

are also given: NH4OH, NaOH, iodine, (NH4 ) 2SX , K4Fe(CN) 6 , 

K3Fe(CN)fl , H2S04 , AgN03 , KC103 . Not all these reagents were 

used on any one mineral.

Davy and Farnham greatly simplified the scheme by using only 

four reagents to "pigeonhole" the minerals. These are HNO3 , 

1:1; HC1, 1:1; KCN, 20 percent; and FeCl3 , 20 percent. In thendetailed descriptions of the minerals the reactions produced by 

saturated solutions of KOH and HgCl2 are also given.

Both Schneiderhohn and Van der Veen have added a number 

of reagents to those already mentioned. Neither made any attempt at a systematic classification of minerals according to their 

reactivity toward chemical reagents, but both laid emphasis on 

reagents that bring out twinning and other internal structures in the 

mineral.

There are some strong objections against relying too much on 

etch behavior in identifying a mineral. The greatest trouble with 

the scheme is that some reactions are very feeble. Many of them, 

indeed, are so feeble that they will work sometimes and fail to work 

at other times, even on the same specimen. It is known that the

95
96 MICROSCOPIC DETERMINATION OF ORE MINERALS

susceptibility to etching is different in different orientations of the 

same mineral specimen, and orientation alone may be the chief 

factor in determining whether or not a reagent will etch a specimen. 

Slight differences in chemical composition of a mineral may cause 

some specimens to etch, whereas other specimens may be entirely 

negative to the same reagent. These variations were recognized 

specifically in the text of Davy and Farnham and in that of Van der 

Veen, but in the former the variations are not sufficiently considered, 

and the authors did not make the tests on a sufficient number of 

specimens of each mineral to reveal all the variations.

It is obvious that a mistake in interpreting any one reaction will 

cause the determinative scheme to fall down completely. When a 

reaction is weak some difficulty is experienced in deciding whether 

or not it is positive. The only solution of these difficulties is to 

place a mineral that is sometimes etched by a reagent and is sometimes negative toward it in both the "positive reaction" and "negative reaction" classes. For instance, tetrahedrite is sometimes etched 

by 1:1 HN03 and is sometimes inert toward it. It should be placed 

in both the "etched by HN03 " and "negative to HNO3 " groups. 

Sphalerite is usually negative to 1:1 HC1, but one or two specimens 

that gave faint reactions with HC1 have been observed. These 

facts should be noted in describing sphalerite, and it should be placed 

in both the "etched by HG1" and "negative to HC1" groups.

When two different minerals are covered by a drop of a reagent, 

an electric current is generated owing to the difference of potential 

between the minerals. As a result, the effect of the reagent is intensified on one of the minerals and diminished on the other. For 

instance, when bornite and galena in contact are covered with ferric 

chloride solution, the bornite reaction is stronger than it is with pure 

bornite alone; on the other hand, the galena adjacent to the bornite 

is entirely negative to ferric chloride solution, whereas pure galena 

alone gives a strong reaction with this solution. It is therefore 

advisable to find an area of the mineral larger than the drop to be 

applied and to prevent the drop from spreading over the contact 

with an adjacent mineral. It is not always possible to do this, and 

under such conditions a knowledge of the electrolytic effects produced by the different mineral associations is advisable, in order to 

discount them in identify ing the mineral. Fortunately in the great 

majority of mineral associations these electrolytic effects are too 

feeble to affect the etch reactions.

The manner of applying the reagents to the polished surface 

differs considerably. Schneiderhohn immerses the entire polished 

surface face down in a receptacle containing the reagent. In this 

manner the entire section is acted upon by the etching reagent, and
ETCH .REACTIONS 97

electrolytic effects cannot be avoided. In addition, the entire surface 

of the specimen must be repolished before other tests can be made.

Murdoch applies a drop of reagent directly to the polished surface 

by means of a small capillary pipette. This procedure is also followed by Davy and Farnham. The objections to the capillary tube 

are that it is fragile and easily broken, the ragged end tends to scratch 

the specimen, and the amount of liquid transferred is not easily 

controlled.

The procedure recommended by the writer is that of Prof. L. C. 

Graton, who uses a platinum wire with a small loop on one end of 

it. The other end of the wire is held by a 

glass rod or other suitable holder. The 

diameter of the wire used by the writer is 

0.32 millimeter, the distance from the end 

of the wire holder to the end of the loop is 

20 millimeters, and the inside diameter of 

the loop is 0.41 millimeter. The wire is 

bent with the loop at an angle to the rest 

of the. wire, forming a tiny golf club, as 

shown in figure 25. The loop is filled with 

reagent by means of a glass dropping rod, 

and with the eye to the microscope the 

loop is brought into the field of view and 

slowly lowered until the liquid comes into 

contact with the mineral. The liquid remains on the mineral when the loop is raised.

Van der Veen places the drop on the specimen and leaves it there 1 

minute, then washes the reagent off, and dries the specimen before it is 

placed under the microscope. He does not believe in microscopic observations on the specimen during etching, on the ground that fumes 

given off by the reagents are injurious to the microscope, especially the 

objective. The injurious effect of the reagent fumes has probably been 

overestimated by Van der Veen. The writer has used the same microscope objective in this work for 3 years and can observe no harmful 

effects.

If the specimen is observed by means of the microscope during 

etching, the position of the drop can be carefully controlled, and the 

specimen can be removed and washed before the etching has proceeded 

too far. For instance, the etch cleavage of chalcocite is brought out 

by KCN solution, but if etching is continued too long the etch structure is entirely destroyed and the surface turns black.

Either the 24-millimeter or the-16-millimeter objective is used for 

observation of the application of the drop and the etching. It is not 

advisable to use an objective of higher magnification when applying 

the etching liquids, as there is danger of the objective coming into conFIGUKE 25. Platinum wire for transferring drop of reagent to polished 

surface. Plane of loop is at an angle 

to rest of wire.
98 MICROSCOPIC DETERMINATION OF ORE MINERALS

tact with corrosive fluids. The fluid should be removed and the specimen washed before changing to objectives of higher magnification.

The time during which etching is continued varies with the mineral. Some minerals tarnish instantly with certain reagents, and 

the etching fluid can be removed immediately. The time limit set 

by the writer for the slower reactions is usually 1 minute.. If no 

effect can be observed at the end of 1 minute the test is considered 

negative. If the mineral is not completely etched at the end of 1 

minute, but some change is noticed on the surface, sufficient time is 

given to allow the effect to be completed. An exception is made of 

HN03 . With some minerals like jamesonite the etching with this 

reagent is sometimes slow in starting, but once started it proceeds 

rapidly. For this reason, if the HNO3 drop still has a negative effect 

at the end of 1 minute, it is preferably allowed to remain on the 

surface until it begins to dry up, or approximately 2 minutes.

The reagents recommended by Davy and Farnham are on the 

whole satisfactory and are therefore used by the writer. Adding to 

the number of reagents merely adds to the difficulties already mentioned without any compensating advantages. Any reagent used will 

strongly etch some minerals but will have no effect on others, and 

there is a group of minerals on which the effect will be between these 

extremes and with many of them weak and indefinite. The reagent 

recommended by Schneiderhohn, KMn04 in H2S04, is effective in 

bringing out structures in certain minerals, but it has the disadvantage just noted and in addition is messy and has to be made up 

fresh every few days. It is therefore not recommended for routine 

determinative etch tests.

Of the reagents used by Davy and Farnham, HN03 and HC1 

leave no residues on evaporation. KCN, FeCl3 , KOH, and HgCl2 , 

being dissolved salts, will each leave a residue on evaporation, and 

this sometimes forms an iridescent sediment which is closely similar to 

an etch tarnish. Owing to this difficulty it is not advisable to allow the reagent to evaporate to a point where it begins to drop its 

dissolved load. Consequently it is recommended that the saturated 

KOH solution be changed to a 40-percent solution. Mercuric chloride is more troublesome. Davy and Farnham recommend a saturated solution ^approximately 7 percent at ordinary temperatures). 

In a saturated solution mercurous chloride separates out, as well 

as mercuric chloride. Mercurous chloride tends to stick to the surface of a mineral and is hard to remove. Beginners especially are 

liable to confuse this precipitate with an etch reaction. For this reason 

the writer recommends a 5-percent solution of mercuric chloride 

(1 gram of HgCl2 dissolved in 19 cubic centimeters of water). Even 

with this concentration some mercurous chloride will slowly separate 

out, and it is advisable to filter the solution from time to time.
ETCH 11EACT10NS 99

The reagents used by the writer are as follows:

HN03 , 1:1 (1 part HNO3) specific gravity 1.42, to 1 part H2 0).

HC1, 1:1 (1 part HC1, specific gravity 1.19, to 1 part H2O).

KCN, 20 percent solution (by weight).

FeCla, 20 percent solution (by weight).

KOH, 40 percent solution (by weight).

HgCl2 , 5 percent solution (by weight).

Aqua regia (3 parts concentrated HC1 to 1 part concentrated HNOa).

H2 02 , commercial reagent.

Aqua regia is used only for minerals that are negative to the other 

solutions. The strength of the acids should be checked from time to 

time by means of a specific-gravity hydrometer.

The researches of Thiel have shown that most of the manganese 

oxide minerals break down hydrogen peroxide, H202 . A drop of 

the commercial reagent placed on the mineral effervesces, but the 

mineral itself is not etched. 1 According to Smitheringale,2 these 

reactions are less positive than is indicated in Thiel's paper. The 

present writer checked Thiel's results rather closely and found that 

if anything, the reactions are even more positive than indicated by 

Thiel. Evidently the quality of the reagent is a vital factor in the 

intensity of the test. The writer uses a 3 percent commercial solution. 

It is used only to confirm manganese oxides.

It is advisable to keep the HN03 and HC1 solutions in glass-stoppered dropping bottles similar to those used for refractive-index 

oils, as rubber stoppers are corroded by the acids. The other solutions, on the contrary, should not be kept in glass-stoppered bottles, 

as the liquid tends to evaporate, and the solid residue cements the 

stopper to the bottle. KOH will often form so tight a cement that 

the bottle will be broken in trying to remove the stopper. For the 

salt solutions other than the acids rubber stoppers are recommended. 

The stopper has a hole punched in it for the purpose of holding a 

glass dropping rod 4 millimeters in diameter. All the reagent 

bottles have a capacity of 15 cubic centimeters.

Minerals that effervesce when acted upon by 1: 1 HN03 are separated from those that are etched but do not effervesce. This is a 

useful distinction that is made both by Murdoch and by Davy and 

Farnham and is retained by the writer. For instance, niccolite 

and breithauptite, which otherwise cannot be distinguished except 

by chemical tests, are separated by the fact that niccolite effervesces 

with HN03 and breithauptite tarnishes without effervescence. The 

great difficulty with effervescence as a criterion is that the effervescence is sometimes very weak and liable to be overlooked. For instance, pyrite will give a slow effervescence, a few bubbles at a time,

1 Thiel, G. A., The manganese minerals; their identification and paragenesis: Econ. Geology, vol. 19, 

p. 107,1924.

3 Smitheriugale, W. V., Etching tests and X-ray examinations of some manganese minerals: Econ. 

Geology, vol. 24, p. 481, 1929.
100 MICROSCOPIC DETERMINATION OF ORE MINERALS

if the surface is smooth and well polished, but usually the surface 

is rough and the grains small, and under these circumstances, effervescence generally does not take place. Here again such a mineral 

may be placed in both the "tarnishes with effervescence" and 

"tarnishes without effervescence" classes.

The effect of calcite and other carbonates must be watched for. 

When a carbonate is present as a gangue mineral a vigorous effervescence produced by acids is often mistaken for a reaction on an 

adjacent ore mineral. This can be discounted by noting that the 

ore mineral is not tarnished. When an area on a polished section 

shows effervescence with both HN03 and HC1, the presence of 

calcite should be suspected.

A fountain-pen filler or medicine dropper is useful for removing a 

reagent drop from the specimen. The end of the filler is heated and 

drawn out to a point and broken off, giving a tube of small diameter. 

This is filled with water. When the etch reaction is complete, the 

polished section is removed from the microscope, and by means of the 

fountain-pen filler, a stream of water is squirted on that part of the 

surface of the section just etched, and the reagent drop is washed off. 

With care, only part of the surface need be wetted. The polished 

section is again brought under the microscope, and the area formerly 

covered by the etch drop is examined while still wet. The etch test 

is now complete, and the information is recorded.

The surface is then dried by means of a filter paper and rubbed on 

a chamois cloth which is mounted on a block of wood. Very fine 

floated rouge should be sprinkled on the surface of the rubbing 

block. (See p. 102.) The unknown mineral is then ready for the 

next etch test. The writer does not recommend examining the section after rubbing on the block, as recommended by Murdoch and 

by Davy and Farnham. In both books are to be found descriptions 

like the following: "Stains brown; rubs to a gray roughened surface." It has been found that the character of the surface after rubbing varies too much to be of use in identification tests. The rouge 

has an abrading action and if continued long enough would remove 

the etching from any surface.

In addition, some remnants of the reagents are likely to remain 

on the surface, even after washing. The reagent may have soaked 

into the cracks, pits, and pore spaces; or the residue after evaporation may adhere to the surface itself and remain there in spite of 

continued efforts to wash it off. Blotting and rubbing on a block 

only spread this residue over the surface, giving an iridescent film 

which, as experience has shown, is often mistaken for a positive 

etch reaction.

The .safest and most satisfactory method of making an etch test 

is to observe it microscopically from the time the drop of reagent
ETCH REACTIONS 101

is applied to the surface until the section is removed for the purpose 

of washing the reagent off. The effect can be observed right through 

the drop with all reagents except ferric chloride solution. This 

solution has a dense yellow color which may obscure the color of the 

etched surface. The effect of the ferric chloride can only be observed 

after washing it off, shaking the section to remove the excess water 

from the surface, and observing the section through the water remaining on its surface. It is never advisable, to blot the specimen or to 

dry it before making the final microscopic observation. The water 

itself usually contains enough dissolved matter to smear a film over 

the surface when dried. This smear can be removed only by rubbing 

it on the rouged chamois.

The character of the etching is well described in all the textbooks 

cited. Generally the effect is merely a discoloration of the surface. 

Where the etching action is strong the surface may be corroded, becoming rough or full of pits. With many minerals, scratches formed 

during polishing and invisible before etching are brought out by the 

etching process, owing to the fact that the scratches are widened by 

solution of the mineral.

With some minerals, notably chalcocite, etching brings out a series 

of closely spaced parallel cracks. This is known as "etch cleavage." 

Etch cleavage indicates the direction of greatest solution, and this 

does not necessarily indicate the direction of least molecular cohesion. 

For instance, the etch cleavage of orthorhombic chalcocite is parallel 

to the base, whereas the cohesion cleavage is parallel to the prism. 

In most specimens the etch cleavage probably indicates twinning 

planes. These are planes of structural weakness and are readily 

widened by the access of the reagents.3 In some specimens of 

cobaltite, for example these cracks indicate the boundaries between 

shells in zoned crystals.

The intensity of etching differs in different directions of a crystal. 

A random polished section cuts different grains of the same mineral 

in different crystallographic directions; consequently, when a polished surface is etched, adjacent grains will show different colors 

after etching. This effect is extremely useful in bringing out mineralgrain patterns. In addition to this differential etching, the spaces 

between adjacent grains are widened by solution, and spaces that were 

submicroscopic before etching may become visible after it.

With some minerals, certain reagents have no effect on the surface 

covered by the drop, but fumes from the liquid tarnish the area immediately contiguous to the periphery of the drop, thus forming an 

aureole around it. Usually this halo is soluble in water and disappears on washing the specimen. Under some conditions, however,

' Dr. Paul F. Kerr in a personal communication to the writer suggests that for many specimens the term 

"etch cleavage" is inaccurate and that where twinning planes are followed the term "etch parting" would 

be preferable.
102 MICHOSCOPIC DETERMINATION OF ORE MINERALS

it is permanent and thus becomes of additional diagnostic value. 

For instance, some silver minerals, such as argentite and stephanite, 

will form an insoluble aureole when etched- by 1:1 HC1, and this 

aureole is diagnostic of silver. These aureoles have been ignored by 

some authors and used by others. The present writer considers that 

a test resulting in an aureole is positive. Some nicety of judgment 

is required when an aureole is faint or when it does not entirely surround a drop. This is usually due to different orientation of adjacent 

grains. Such difficulties are unavoidable; the etching scheme is not 

perfect, and unless the effects are distinctive the mineral must be 

checked by other properties.

Some minerals show nothing but a halo of small drops of liquid 

adjacent to the main drop. These might be termed "sweat halos." 

They are ignored in the tables of etch reactions.

Probably the greatest source of error in making the etch tests 

arises from not having a clean surface to begin with. Some minerals 

like bornite tarnish very quickly, and with such minerals consistent 

results cannot be expected. It is remarkable how quickly a specimen 

can acquire a slight film of dirt or grease from contact with other 

objects. This film hinders a good contact between polished surface 

and liquid and may cause an otherwise positive reaction to fail. The 

obvious remedy is to make sure that any such film is removed before 

applying the reagent. The rouge block is usually effective in removing 

a film of dirt or grease, but it may fail to remove an oxidized tarnish 

unless rubbing is continued for some time. This rubbing, if continued, is certain to scratch the softer minerals. All laboratories where 

determinative work on the opaque ore minerals is carried on should 

be equipped with a burnishing wheel, using wet magnesia or black 

magnetic rouge on a billiard cloth. (See p. 29.) Such a wheel can 

readily be improvised from an electric fan by removing the propeller 

blades and mounting a wooden or steel disk on the shaft. By holding 

the specimen a few seconds on such a wheel the specimen is freed of 

any oxidized tarnish, grease film, or light etching. This wheel, however, is insufficient to remove a deep corrosion on the surface of the 

specimen, such as that made on galena by HN03 , which usually requires regrinding the surface on a steel lap.

Although the systematic etching scheme just described has many 

disadvantages and cannot be relied upon by a beginner, it is highly 

useful owing to the rapidity with which the tests can be made and 

the number of groups or "pigeonholes" that can be formed from 

a few reagents. A series of six etch tests can be completed, allowing for duplications of tests in doubtful reactions, in about 20 minutes. The ambiguities of etch behavior can for the most part be 

tabulated by a sufficient number of observations on specimens of the 

same mineral from different localities. The error resulting from
DETEBMINATIVE TABLE 103

mistaking a residue from the evaporation of a reagent for an etch 

tarnish can be avoided after a little experience.

It must be remembered that no set of tables of etch reactions will 

ever be perfect, however carefully they are made. Ninety-nine specimens of galena may be etched by HN03 , and the hundredth will not 

etch even after it is carefully burnished. Therefore, the etch tables 

cannot be relied upon absolutely for the final determination of a 

mineral, and a determination made by systematic etching must be 

checked by other properties, such as color, hardness, polarization or 

microchemical tests, or all of these.

In the table the terms "stain" and "tarnish" are used synonymously. 

The term "etch" indicates that the chemical action has removed 

material from the surface. When the reagent is removed the surface 

then appears corroded or pitted, or cracks are developed.

DETERMINATIVE TABLE

INTRODUCTION

In the following determinative table the minerals are divided into 

two principal classes soft and hard and each class is subdivided 

into isotropic and anisotropic. The soft minerals are given on pages 

105-110 and the hard minerals on pages 110-111.

In each of the main groups the minerals are further divided into 

"pigeonholes" by means of their etch reactions. The arrangement 

of these "pigeonholes" is the same as that of Davy and Farnham. 

The three principal subdivisions are based on the reactivity of the 

minerals toward nitric acid, as follows: (1) Effervesces with HN03 ; 

(2) reacts with HN03 without effervescence; (3) does not react with 

HN03 . Each of these subdivisions is further subdivided by reactivity toward HC1, KCN, and FeCl3 , in order.

An outline of the scheme is given on pages 105-111. In this outline 

the subdivisions based on reactivity toward nitric acid are indicated 

in the columns farthest to the left on each page. They are divided 

by means of horizontal lines extending entirely across the page. 

The subdivisions based on the remaining reagents are similarly 

arranged. The chemical symbol of the reagent alone, without further designation, indicates that the etch test is positive. If the 

reaction is negative, the abbreviation "neg." is given with/the chemical symbol. An inspection of the outline will show the systematic 

way in which the "pigeonholes" are arranged. In the detailed 

descriptions the reactions to KOH and HgCl2 are also given.

The following examples will illustrate the use of the tables:

1. The mineral is soft, anisotropic; color, galena white; etch reactions, HN03 effervesces, HC1 negative, KCN negative, FeCl3 stains 

light brown, KOH negative, HgCl2 negative. Reference to the tables 

shows that the mineral falls into the group on page 106. The de-
104 MICROSCOPIC DETERMINATION OF ORE MINERALS

scription fits any mineral in the group except calaverite, and if the 

observer's color perception is not good he might assume that it fits 

calaverite as well. To determine the mineral microchemical tests 

are necessary. It will also be seen that every mineral in that group 

contains arsenic, bismuth, or tellurium, hence they are the first elements to be tested for. Tests are made for other elements if necessary.

2. The mineral is soft, strongly anisotropic; gray in vertically reflected light; etch reactions, HNO3 negative, HC1 fumes tarnish 

faintly, KCN stains brown, FeCl3 negative, KOH stains iridescent, 

HgCl2 negative. The corresponding pigeonhole in the tables is the 

group on page 107, and the mineral is either stephanite or miargyrite. 

Scratching the mineral with a chisel-pointed needle (see p. 63) 

yields a ruby-red powder, which determines the mineral as miargyrite; 

therefore no microchemical tests are necessary. The HC1 reaction is 

evidently doubtful, and on the assumption that it is negative the 

mineral will be found in the group on page 107. Miargyrite is one 

of the minerals in that group, thus affording further confirmatory 

evidence.

The formulas given for minerals in the determinative table, with a 

few exceptions, are those of Wherry and Foshag.4 The descriptions 

of hollandite, jacobsite, and sitaparite are summarized from the 

excellent paper of Dunn.5

4 Wherry, E. T., The nomenclature and classification of sulfide minerals: AVashington Acad. Sci. Jour 

vol. 10, pp. 489-496,1920. Wherry, E. T., and Foshag, W. F., A new classification of the sulfo-salt minerals 

Idem, vol. 11, pp. 2-8, 1921.

8 Dunn, J. A., A study of some microscopical aspects of Indian manganese ores: Nat. Inst. Sci. India 

Trans., 1936, pp. 103-124.
OUTLINE OF DETERMINATIVE TABL

Soft minerals

 0

3

_L

-1 Reagents

x

3

HN03 

*> efl.

HC1

HC1 

neg.

KCN

KCN 

neg.

KCN

FeCl3

FeCls 

neg.

FeCls

FeCh 

neg.

FeCb

FeCls 

neg.

Isotropic

Mineral

Copper arsenides:

Chalcocite __ ....- ___ ....

Silver..........................

Alabandite _ ...------.-.----..

Chalcocite- ___ . __ ..-.-...

Silver.---.--..-.--..-.-..-.--..

Formula

CusAs .---_--.- __ .---....

Cu_. .-___.- .... .................

Cu2S. ............... _____ ....

A a

Acr

PbTe. ..... . ..... .........

PbS.... ....--.....- -...-......

MnS..... .........................

CueAs _ . __ ... ____ . ........

Cu....... .........................

Cu2S_ --...-..--.-.--........ ......

AgsSb - .-. .... ..

Ag... ...... -- -_.--- 

Page

112 

112 

112 

113 

113 

113 

113

114

114 

114 

115

115

117 

117 

117 

118 

118 

118 

119 

119

119

B

Ja

Pl

Se

Z

T

B

M

Tf

St
Soft minerals Continued

Reagents

HNO3 

eff.

HNO3

HC1 

neg.

HC1

KCN 

neg.

KCN

FeCU

FeCU 

neg.

FeCU

Isotropic

Mineral

Polydymite ------------- __

Violarite.. -. 

Antimony __ ...... _ . _ ..

Argentite. . . . ___ ....... _ ..

Dyscrasite...------------ _ ...

Hessite ____________ ..

Formula

AgsSb .

PbTe.. -..---.--.-. ------------

NijSi... 

ZnS - ------------------- ...

(Ni,Fe)sS4 

Sb... ..... . ................

AgjS....... ------------

AgjSb . .....................

AgjTe. ...................

Page

120 

120 

121 

121

123 

123 

123

125 

125 

125 

126

Ni

Ra

Te

Te

A

C

K

M

Sy

Ai

Bi

Ca

Em

G

Jo

M

Sy

Te

D

He

Po

St

U
DETERMINATIVE TABLE 107

^Sc5c5

S.Sb 5Ag 23-.--- 2Sb!S3.SbjO3.--'. SbaSs -... SAgjS.SbjSs-.. 

Stephanite.. Kermesite ...Stibnite..... Polybasite .

- 1*

;

^

in^-^

|

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3

Colusite _ .--..--..-....-

bil 

§

S 8 8 8°, °>°,

idIB

6PbS.Sb S.6SnS 232CuO.2PbO.V O 2

AgtS.FenSt.~~.

\ i

1

i

Cylindrite... Mottramite. ^Sternbergite.

CO

s r co'o

5?sei

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fil

S.PbS.Bi»S Ag23.

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a

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108 MICROSCOPIC DETERMINATION OF ORE MINERALS

3

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HC1

KCN

KCN

KCN

KCN

neg.

FeCh

FeCl3

neg.

FeCls

FeCh

FeCb

Fed.,

Argyrodite..-. ................. 

Klaprothite ____ -.. ._...._

Gold..........-..-.---..-.....

AgzS_._..__ __. . ... ..

3Ag2S.Sb2S3--.-.--- ---------------

MnS2 _-.... .......................

FesOi.. ......-.-..-......--.-...

Ag2S_. ............................

AgCl. ............................

AsS............ ......... ..........

4Ag2S.GeS2 . --..-_-.--.-.--_----. . 

3CusS.2BizSs- -------------------- .

3Cu2S.V2S5 --------------------

HgSe - ....--- -. ..

AU... .-.-.-.-.-.----_---....--...-

HgTe............. .......

147

147

148

149

149

150

151

152

152 

152

152
110 MICROSCOPIC DETERMINATION OF ORE MINERALS

O

O

Anisotropic

o

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b/l

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Andorite. ___...---. Bertbierite __ . ......

1010 

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a «

c = OO

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c a 

a £

&<'£

Livingstonite. _ ........ Miargyrite -...----.... __ 

Pyrrhotite. ________

id

|w

w a 

^^

Cu S.2PbS. z(Mn,Zn)O.2

Bournonite __ ........ Chalcophanite . _--.-.... Ohalconvrit.fi

\C. U3 CO CO CO CC

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HN03 

neg.

HC1

HC1 

neg.

HC1

KCN 

neg.

KCN 

neg.

KCN

neg.

FeCh 

neg.

FeCh

FeCl, 

neg.

FeCh

FeCh 

neg.

FeClj 

neg.

Gersder 

Pyrite

ffite.____ ...............

Psilome

Psilome

Skutter 

Kallilit 

Ullman 

Uranini 

WUlyai

Bravoil 

Cobalti 

Corynil 

Gersdoi 

Pyrite

J-. ._-..__.. _.._-..___

ite. ..................

te. .................. ...

nite.--. __ . __ --- ...

te. -.--.....--....-..-..

 ffite...... ....... ....

Willyamite.. _______ . ....

Frankli 

Magne

Reagents

HNOs 

neg.

HC1

neg.

KCN

neg.

FeCh 

neg.

Aq. 

reg.

Aq. 

reg. 

neg.

nite. ___ . __ ... ..... 

ite_. ...................

NiAsS.,1. ........................

FeSj............... ............

Ni(Sb,Bi)S. 

NiSbS. ... ...... ...... .-..-...

(Co,Ni)SbS. ......................

(Fe,Ni)Sj. _ .. ... 

Ni(As,Sb)S......-- .. . 

NiAsS........ ....... .............

(Co.Ni)SbS. - - - - ........

ZnO.FejOs-- --- - 

Fe3O4... 

161 

161

162

162

162 

163 

163 

163 

163

163 

164 

164 

164 

164 

164

165 

165

Isotropic

Mineral

Pyrite.-. --------------

Sperrylite.-. __ ... ...

Jacobsite... __ - __

Formula

FeSj

PtASj. ..... ...................

FejOs.. . . . ...

m(Ba,Mn)jMnO5+nFe4(MnOj)j__ 

MnO.FejOs-- ___ __ . __ ..

9MnjO3.4FejO3.MnOj.3CaO. ..

Page

165 

165

166 

166 

166 

166 

166 

166 

166 

167 

167

Loe

Py

Gl

Py

Ha

Fe

Go

Ho

Ilm

Ru

Sit

W
112 MICROSCOPIC DETERMINATION OF ORE MINERALS

SOFT MINERALS 

HNO3 eff., HC1, KCN, FeCl 3

ISOTROP1C

Copper arsenides. Textbooks in mineralogy usually describe three arsenides 

of copper domeykite (Cu3As), algodonite (CusAs), and whitneyite (Cu9As). 

Recent microscopic investigation has shown that algodonite is really an intergrowth of two distinct minerals which are closely similar in color and etch tests. 

In fact, the two constituents are rarely recognized under the microscope before 

etching, but they are brought out by reagents owing to the fact that one constituent 

etches more rapidly than the other. Similarly "whitneyite" comprises four 

distinct minerals, at least two of which are usually present in any specimen. It 

is not considered feasible for the average investigator to separate the individual 

constituents of algodonite or whitneyite; this requires considerable experience 

with these ores. It is possible, however, to distinguish between the different 

groups by following the descriptions given below. More complete details are 

given by Broderick. 6

KOH, HgClj

Domeykile (CusAs). Isometric; color white; hardness C . HNOs effervesces. 

turns dark gray, and brings out a triangular etch cleavage. HC1 brings out a 

triangular etch pattern; mineral stains slowly. KCN slowly brings out triangular 

etch pattern; surface does not stain. FeClj instantly stains black. KOH stains 

differentially iridescent and brings out triangular etch pattern. HgCl2 tarnishes 

black. Domeykite is an authentic mineral, readily distinguished from algodonite 

and whitneyite by its color. In oblique reflected light color has a suggestion of 

yellow. Fairly sectile but more brittle than other arsenides of copper.

Algodonite (CugAs). Isometric; color greenish gray; hardness C . 

effervesces and blackens; some varieties give parallel (possible cubic) etch cleavage. 

HCl, some specimens stain brown; with some specimens etch cleavage is developed 

but surface does not stain ; action sometimes requires more than 1 minute. KCN 

stains brown; with some varieties reagent effervesces and etch cleavage is developed; action sometimes requires more than 1 minute. FeCls instantly stains 

black. KOH slowly stains iridescent; some specimens negative. HgCl2 quickly 

stains brown. Algodonite is an intergrowth of two constituents, each a definite 

mineral, termed a and /3 algodonite. a algodonite is greenish gray, /3 algodonite 

has a suggestion of blue. With HCl a algodonite etches more rapidly than /3 algodonite. More sectile than domeykite. Algodonite intergrown with niccolite is 

termed "mohawkite."

Whitneyite (CugAs). Isometric; color pale coppery pink; hardness C . 

HNOs effervesces vigorously and stains iridescent; on some varieties brings out 

grain structure; with others etching brings out a distinct lamellar structure. 

HCl slowly stains gray; with some varieties action requires more than 1 minute. 

KCN slowly browns; with some varieties reagent effervesces slowly. FeCls 

instantly blackens. KOH slowly stains iridescent. HgCl2 instantly stains 

brown to black. Four distinct constituents have been recognized in whitneyite. 

These are termed a, /?, y, and 5 whitneyite. The first three are a distinctly lighter 

shade of pink than native copper, but the last is almost indistinguishable from 

copper in color. They vary in color from a, the lightest, to 5, the most distinctly 

coppery. Some of these constituents may be alloys of copper and arsenic rather 

than definite chemical compounds. Whitneyite is more brittle than native copper.

* Broderick, T. M., in Butler, B. S., and others, The copper deposits of Michigan: U.S.Geol. Survey 

Prof. Paper 144, p. 56, 1929.
DETERMINATIVE TABLE 113

In polishing, pits form on whitneyite, whereas they are almost completely absent 

on copper. On the other hand, polishing scratches are more numerous and 

deeper on copper than on whitneyite. The best test for whitneyite, however, is a 

microchemical test for arsenic.

Copper (Cu). Isometric; color coppery red; hardness B + . HN03 effervesces; 

some specimens tarnish brown; others seem to effervesce without tarnishing; 

surface roughens. HC1, some specimens slowly tarnish brown, others negative. 

KCN slowly browns. FeCl3 quickly darkens. KOH slowly tarnishes brown to 

iridescent. HgC^ quickly stains iridescent. Very sectile. Polishing gives 

multitude of parallel scratches which extend across specimen.

KOH neg.. HgCh

Arsenargentite (Ag3As). Isometric; color creamy white; hardness B. HN03 

effervesces and stains black. HC1 tarnishes iridescent. KCN tarnishes brown. 

FeCl3 instantly tarnishes iridescent. KOH negative. HgCl2 tarnishes brown. 

Almost sectile. Very rare. Identical with huntilite. From Silver Islet, Lake 

Superior.

Chalcocite (Ci^S). Orthorhombic; color bluish gray; hardness B. Very fine 

grained supergene chalcocite is apparently isotropic. Most chalcocite is weakly 

anisotropic. HN03 effervesces vigorously; mineral stains blue; with most 

specimens of orthorhombic chalcocite brings out parallel etch cleavage; very fine 

grained supergene chalcocite shows no etch cleavage. HCl negative; some varieties 

tarnish faintly. KCN quickly stains black; first brings out etch cleavage, but 

if solution is allowed to remain on surface, the etch cleavage is destroyed, and a 

rough, corroded surface remains. FeCl3 stains blue; with most specimens of 

orthorhombic chalcocite brings out parallel etch cleavage; very fine grained supergene chalcocite, however, does not show etch cleavage. KOH negative. HgClj, 

some specimens tarnish slightly; others negative.

Silver (Ag). Isometric; color silvery white; hardness B. HNOa effervesces 

few seconds; leaves white coating. HC1 fumes tarnish some specimens; on others 

deposit dark coating near periphery; some specimens negative. KCN tarnishes 

dark gray to brown, especially near periphery of drop; some specimens almost 

negative. FeCls instantly stains iridescent. KOH negative. HgCla stains 

brown to iridescent. Very sectile.

ANISOTROPIC 

KOH, HgClj

Rickardite (Cu3Te2). Crystal system unknown; color purplish red, similar to 

bornite; hardness B. Strong anisotropism; gives four extinctions per revolution. 

Interference color fiery orange, very similar to that of umangite. HN03 blackens 

with effervescence. HC1 etches gray to brown. KCN slowly bleaches gray. 

FeCls bleaches gray to brown. KOH tarnishes iridescent to black. HgCl2 slowly 

tarnishes purple. Very rare; reported only from Vulcan, Colo. Always intergrown with weissite and other tellurides; hence formula (Cu4Te3) given in textbooks is probably incorrect. Similarity to umangite indicates that Cu3Te2 is more 

probably correct.

Weissite (Cu2Te). Crystal system unknown; color light gray, about the same 

as chalcocite; hardness B. Weak anisotropism; interference colors pink, bluish 

gray, similar to chalcocite. HN03 effervesces and stains brown. HCl stains light 

brown. KCN stains light brown. FeCl 3 stains brown. KOH slowly stains 

brown. HgClz stains light brown. Specimen from Vulcan, Colo. Always associated with rickardite (Cu3Te2). Distinguished from rickardite by color. Formula
114 MICKOSCOPIC DETERMINATION OF ORE MINERALS

given in literature is Cu5Te3 . Similarity to chalcocite indicates that Cu2Te is more 

probably correct.

KOH neg., HgCI 2

Chalcocite (Cu2S). Orthorhombic; color bluish gray; hardness B. Most chalcocite is weakly anisotropic; polarization colors steel gray, bornite pink. HNOs 

effervesces vigorously; mineral stains blue; with most varieties brings out parallel 

etch structure; very fine grained supergene chalcocite, however, will not show etch 

cleavage; with fine-grained sooty chalcocite brings out grain boundaries only. 

HC1 negative; some varieties tarnish faintly. KCN quickly stains black; first 

brings out etch cleavage, but if drop is allowed to remain etch cleavage is destroyed, and a rough, corroded surface remains. FeCl3 stains blue; with most 

specimens brings out parallel etch cleavage. KOH negative. HgC^, some 

specimens tarnish slightly; others negative.

KOH neg., HgCl 2 neg.

Cuprite (Cu20). Isometric; color bluish gray; hardness D . Anisotropic; 

polarization colors purple, greenish blue, brown. HN03 effervesces and deposits 

coating of metallic copper. HC1 deposits white coating. KCN darkens and 

brings out parallel etch structure. FeCl3 tarnishes iridescent. KOH and HgCl 2 

negative. Ruby-red powder and internal reflection.

HN03 eff., HCI, KCN, FeCI3 neg.

ANISOTROPIC 

KOH, HgCh neg.

Stibnite (SbzSs). Orthorhombic; color gray; hardness B. Very strong anisotropism; polarization colors white, pinkish gray, pink, blue, brown, dark gray; 

many varieties show multiple twinning. HN03 stains differentially iridescent; 

some varieties effervesce slowly. HCI fumes tarnish; surface stains differentially 

brown after long etching; some varieties negative. KCN brings out scratchea 

and sometimes stains differentially light brown. FeCl3 negative. KOH instantly 

tarnishes brown and gives yellow coating; this is the most characteristic reaction 

for stibnite. HgCl2 negative. Prismatic. One perfect cleavage. Hardness and 

color vary somewhat with orientation, giving mottled appearance suggesting the 

presence of more than one mineral.

HNO3 eff., HCI, KCN neg., FeCl3

ISOTROPIC 

KOH neg., HgCb

Silver (Ag). Isometric; color silvery white; hardness B. HNO3 effervesces a 

few seconds and leaves white coating. HCI fumes tarnish some specimens; on 

others deposit dark coating near periphery; some specimens negative. KCN 

stains dark gray to brown, especially near periphery of drop; some specimens 

negative. FeCl 3 quickly stains iridescent. KOH negative. HgCl2 stains brown 

to iridescent. Very sectile.

KOH neg.. HgCh neg.

Altaite (PbTe). Isometric; color galena white; hardness B. HN03 effervesces 

and stains through iridescent to dark gray. HCI, some specimens tarnish iridescent, others negative. KCN negative. FeCl3 quickly tarnishes iridescent. KOH 

and HgCb negative. Cubic cleavage but less marked than galena; shows fewer 

triangular pits. Distinguished from galena by absence of H2S fumes.
DETERMINATIVE TABLE 115

Galena (PbS). Isometric; color galena white; hardness B. HN03 , a small percentage of galena specimens effervesce and turn black; most specimens blacken 

without effervescence. HC1 tarnishes iridescent. KCN negative. FeCl3 stains 

iridescent; with galena areas in contact with bornite, reaction is negative. KOH 

and HgClz negative. Perfect cubic cleavage; many specimens show abundant 

triangular pits. Galena specimens that effervesce with HN03 give off noticeable 

amounts of H2S.

ANISOTROPIC 

EOH neg., HgClj

Bismuth (Bi). Hexagonal; color light coppery pink; hardness A . Strong 

anisotropism; polarization colors brass yellow, bornite pink. HNO3 effervesces 

and darkens. HC1 slowly stains differentially black; sometimes brings out 

lamellar structure. KCN negative. FeCl3 instantly stains iridescent to black. 

KOH negative. HgCl2 stains brown in spots; doubtful. Perfect basal cleavage. 

Sectile but yields powder when scratched; polishing gives parallel scratches which 

extend across specimen.

Tetradymile (Bi2 (Te, 8)3). Hexagonal; color silvery white; hardness B. Anisotropic; polarization colors light to dark gray. HN03 effervesces and stains brown 

to black; action sometimes slow in starting. HC1 stains light brown to iridescent; 

on some areas action is very slow. KCN negative. FeCl3 stains iridescent. 

KOH negative. HgCl2, some varieties tarnish iridescent; others negative. Some 

sulphur usually replaces tellurium in the molecule. A sulphur-free variety is 

known as telluro-bismutite. Perfect basal cleavage.

KOH neg., HgClj neg.

Benjaminite ((Cu,Ag) 2S.2PbS.2Bi2S3). Orthorhombic; color galena white; hardness B. Anisotropic; polarization colors light to dark gray; shows multiple twinning. HN03 quickly effervesces and turns black. HCl fumes tarnish faintly; 

some areas negative. KCN negative. FeCl3 stains light brown; effect may be 

overlooked. KOH and HgCl2 negative. Good cleavage in one direction. Distinguished from aikinite only by microchemical test for silver. Reported only 

from Outlaw mine, Manhattan, Nev. Analyzed by E. V. Shannon (U. S. Nat. 

Mus. Proc., vol. 65, p. 1, 1924).

Melonite (NiTe2 ). Color pale yellow; hardness B. Strongly anisotropic; 

polarization colors white, light brown, dark brown. HNO3 instantly effervesces 

and stains black. HCl stains faint brown; not always observed. KCN negative. 

FeCl3 stains dark brown. KOH and HgCl2 negative. Very rare. A polished 

surface when tilted so as to get the reflection of daylight and viewed with the 

unaided eye has a distinct light coppery tinge, about that of bismuth.

Tellurium (Te). Hexagonal; color silvery white; hardness B. Strong anisotropism; polarization colors light to dark gray. HN03 stains black with effervescence. HCl fumes tarnish some varieties, others negative. KCN negative. 

FeCl3 slowly stains light brown; action weak and on some specimens practically 

negative. Perfect prismatic cleavage. Elongated cracks well developed. 

Scratches produced by polishing usually extend entirely across specimen.

HNO3 eff., HCl, KCN neg., FeCI3 neg.

ISOTROPIC 

KOH neg., HgCh neg.

Alabandite (MnS). Isometric; color gray, similar to sphalerite; hardness C. 

HN03 effervesces and gives off H2S; tarnishes brown; sometimes brings out grain 

structure. HCl effervesces and gives off H2S; tarnishes black. KCN, FeCl3 , 

KOH, and HgCl2 negative. Powder and internal reflection greenish yellow.
116 MICROSCOPIC DETERMINATION OF ORE MINERALS

ANISOTROPIC 

KOH. HgCIs neg.

Jamesonite (4PbS.FeS.3Sb2S3). Monoclinic; color galena white; hardness B + - 

Strong anisotropism; polarization colors light gray, dark brown, dark gray. 

HNO3 tarnishes iridescent with slow effervescence. HC1 fumes tarnish. KCN 

and FeCl3 negative. KOH tarnishes iridescent. HgCl2 negative. Prismatic. 

Perfect basal cleavage.

Plagionite (5PbS.4Sb2 S3). Monoclinic; color white; hardness C. Anisotropic; 

polarization colors light to dark gray. HNO3 effervesces and stains black. HC1 

fumes tarnish on some areas; other areas negative. KCN and FeCl3 negative. 

KOH stains gray to brown. HgCl3 negative. Prismatic. Very rare. Distinguished from zinkenite only by crystal form.

Semseyite (9PbS.4Sb2S3). Monoclinic; color gray; hardness C. Strong anisotropism; polarization colors light gray, bluish gray, brown, dark gray. HN03 

effervesces and stains black. HCl fumes tarnish a little; surface stains slightly 

in places. KCN and FeCl3 negative. KOH slowly stains iridescent; reaction 

sometimes requires more than 1 minute for effect to be perceptible. HgClj 

negative. Prismatic.

Zinkenite (PbS.Sb2S3). Orthorhombic; color gray; hardness C. Anisotropic; 

polarization colors white to dark gray. HNO3 effervesces and darkens. HCl 

fumes tarnish slightly. KCN and FeCl3 negative. KOH stains iridescent. 

HgCl2 negative. Prismatic. Has no distinct cleavage. Specimen from Wolfsberg, Harz, the type locality.

Troilite (FeS). Hexagonal; color somewhat similar to pyrrhotite but more 

yellowish; hardness D. Strong anisotropism; polarization colors white to reddish 

brown. HN03 , some specimens effervesce and give off H2 S; most specimens stain 

without effervescence. HCl effervesces and gives off H2 S. This action usually 

begins in less than 1 minute. On some areas, however, it may be as long as 3^ 

minutes before any action is observed. Once initiated, it proceeds vigorously. 

KCN and FeCl3 negative. KOH stains differentially iridescent; slow. HgCl2 

negative. Very rare in mineral deposits but sometimes found in meteorites.

KOH neg., HgCh

Bismuthinite (Bi2 S3). Orthorhombic; color galena white; hardness B + . 

Strong anisotropism; polarization colors white, brown, black. HN03 slowly 

effervesces and stains black. HCl fumes tarnish some specimens, others negative. 

KCN, FeCl3 , and KOH negative. HgCl2 stains light brown; some specimens 

negative. Prismatic. One perfect cleavage.

KOH neg., HgCh neg.

Bismidoplagionite (5PbS.4Bi2S3). Orthorhombic; color gray; hardness B + . 

Anisotropic; polarization colors white to dark gray. HNO3 quickly stains iridescent; effervesces slightly. HCl fumes tarnish; some areas stain iridescent; 

others negative. KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. Good 

cleavage in two directions. Reported only from Wickes, Mont. Analyzed by 

E. V. Shannon. 7 Cannizzarite (Pb3Bi5Su) is probably identical with bismutoplagionite.8

Boulangerite (5PbS.2Sb2 S3). Orthorhombic; color galena white; hardness B + . 

Strong anisotropism; polarization colors white, steel blue, brownish gray, dark 

gray; some specimens show shreddy or fibrous structure. HNO3 effervesces;

' U. S. Nat. Mus. Proc., vol. 58, p. 589,1920.

8 Wolfe, C. W., Cannizzarite and bismuthinite: Am. Mineralogist, vol. 23, p. 704,1038.
DETERMINATIVE TABLE 117

surface stains black; action sometimes slow in starting. HCl fumes tarnish slightly 

some specimens, others negative. KCN, FeCl3 , KOH, and HgCl2 negative. 

Prismatic. Distinguished from jamesonite by KOH test.

Geocronite (5PbS.Sb2S3). Orthorhombic; color gray, in contact with galena 

mineral has a greenish tinge; hardness B. Anisotropic; polarization colors 

pinkish brown, steel blue. HNO3 effervesces and stains black. HCl fumes 

tarnish but mineral does not stain. KCN, FeCl3 , KOH, and HgCl2 negative. 

Has at least one good cleavage. Gives a negative microchemical test for copper, 

which distinguishes it from meneghinite.

Meneghinite (4PbS.Sb2S3). Orthorhombic; color white; hardness C . Strong 

anisotropism; polarization colors light gray, pinkish brown, steel blue. HN03 

effervesces; surface stains black; usually reaction is slow in starting, but once 

started the etching rapidly advances as a wave from one side of the drop to the 

other. HCl fumes tarnish; some areas negative. KCN, FeCl3 , KOH, and HgCl2 

negative. Occurs as long, slender prisms vertically striated. Specimen examined 

came from Bottino, Tuscany, Italy, the type locality. Contains about 3 percent 

of copper, readily detected microchemically.

Tellurium (Te). Color silvery white; hardness B + . Strong anisotropism; 

polarization colors light to dark gray. HN03 stains black with effervescence. 

HCl fumes tarnish some areas, others negative. KCN negative. FeCl3 slowly 

stains light yellow; action slow and on some specimens practically negative. 

KOH negative. HgCl2 , some areas stain light brown; others negative. Perfect 

prismatic cleavage; elongated cracks well developed. Scratches produced by 

polishing usually extend entirely across specimen.

HNO3 eff., HCl neg., KCN, FeCl3

ISOTROPIC 

KOH, HgCh

Domeykite (Cu3 As). Isometric; color creamy gray; hardness C. HNOs effervesces and brings out lamellar structure. HCl brings out etch cleavage; some 

areas action very slow, reaction doubtful. KCN brings out etch cleavage. 

FeClj quickly stains iridescent. KOH stains iridescent and brings out lamellar 

structure. HgCl2 stains brown. Almost sectile but yields powder when scratched.

Algodonite (CueAs). Isometric; color greenish gray; hardness C . HNOj 

effervesces and blackens; some varieties give parallel (possibly cubic) etch cleavage. HCl, some specimens stain brown; other specimens develop etch cleavage 

but surface does not stain; action sometimes requires more than 1 minute. KCN 

stains brown; with some varieties reagent effervesces and etch cleavage is developed; action sometimes requires more than 1 minute. FeCl 3 instantly stains 

black. KOH slowly stains iridescent; some specimens negative. HgCl2 quickly 

stains brown. Algodonite is an intergrowth of two constituents, each a definite 

mineral, termed a and 0 algodonite. a algodonite is greenish gray. 0 algodonite 

has a suggestion of blue. With HCl, a algodonite etches more readily than 0 

algodonite. More sectile than domeykite. Algodonite intergrown with niccolite 

is termed "mohawkite."

Whilneyite (CuflAs). Isometric; color pale coppery pink; hardness C . 

HN03 effervesces vigorously and stains iridescent; on some varieties brings out 

grain structure; with others etching brings out a distinct lamellar structure. 

HCl slowly stains gray; with some varieties action requires more than 1 minute. 

KCN slowly browns; with some varieties reagent effervesces slowly. FeCl3
118 MICROSCOPIC DETERMINATION OF ORE MINERALS

\

instantly blackens. KOH slowly stains iridescent. HgCl2 instantly stains brown 

to black. Four distinct constituents have been recognized in whitneyite. These % 

have been termed a, 0, y, and « whitneyite. The first three are a distinctly lighter 

shade of pink than native copper, but the last is almost indistinguishable from ,* 

copper in color. They vary in color from «, the lightest, to s, the most distinctly 

coppery. Some of these constituents may be alloys of copper and arsenic rather -*' 

than definite chemical compounds. Whitneyite is more brittle than native copper. 

In polishing, pits form on whitneyite, whereas on copper they are almost completely absent. On the other hand, polishing scratches are more numerous and 

deeper on copper than on whitneyite. The best method of distinguishing whitneyite from copper is a microchemical test for arsenic. (For further details of the 

copper arsenides see p. 112.) (

-M*' 

Copper (Cu). Isometric; color coppery red; hardness B. HNOs effervesces;

some specimens tarnish brown, others seem to effervesce without tarnishing;

surface roughens. HC1, some specimens slowly stain brown, others negative.

KCN slowly browns. FeCls quickly darkens. KOH slowly stains brown to . \

iridescent. HgCl2 quickly stains iridescent to black. Very sectile; polishing

gives multitude of parallel scratches which extend across specimen. ^

KOH neg., HgCh ^

Chalcocite (Cu2S). Orthorhombic; color bluish gray; hardness B. Very fine 

. grained supergene chalcocite is apparently isotropic. Most chalcocite is weakly 

anisotropic. HNO3 effervesces vigorously; mineral stains blue; most specimens 

of orthorhombic chalcocite give parallel etch cleavage; very fine grained supergene < 

chalcocite, however, will not show etch cleavage. HC1 negative; some varieties \ 

tarnish faintly. KCN quickly stains black; first brings out etch cleavage, but if 

drop is allowed to remain, etch cleavage is destroyed and a rough, corroded surface ^,. 

remains. FeCls stains blue. KOH negative. HgCl2, some specimens tarnish 

slightly; others negative.

"Chalcocite" (Cu2S). Isometric; color bluish gray; hardness B. Orthorhombic 

chalcocite heated above 91° C. inverts to an isometric mineral with the same chemical composition. With specimens containing less than 8 percent of dissolved 

covellite the inversion is reversible. If the mineral contains 8 percent or more of 

dissolved covellite, the inversion of isometric to orthorhombic Cu2S does not take -' -v 

place when cooled below 91° C., and the isometric form persists at ordinary temperatures. A well-known locality for isometric Cu2S is Kennecott, Alaska. HNOg > 

effervesces, stains blue, and brings out octahedral etch cleavage. HC1 negative. 

KCN quickly stains black, giving a rough corroded surface. FeCls stains blue 

and brings out octahedral etch cleavage. KOH negative. HgCl 2 , some specimens tarnish slightly, others negative. This isometric mineral merits but has 

not received a species name.

Dyscrasite (Ag3Sb). Orthorhombic; color silvery white; hardness B to C. -<. 

Some varieties are isotropic, others are weakly anisotropic; some exhibit grating 

structure. HNOs effervesces vigorously; effervescence usually ceases in a few 

seconds; leaves white deposit; on some specimens brings out grating structure. 

HCl negative. KCN, some specimens slowly stain black; on some specimens 

brings out etch cleavage; usually requires at least 2 minutes before effect is perceptible; some specimens negative. FeCls quickly stains iridescent; on some 

specimens brings out etch cleavage. KOH negative. HgCl2 quickly stains 

brown. Chemical analyses, as well as physical and chemical properties, of 

dyscrasite vary widely. There is probably more than one mineral containing 

antimony and silver, analogous to the arsenides of copper. Ford 9 suggests that

 Ford, W. E., Textbook of mineralogy/p. 441,1932.
DETERMINATIVE TABLE 119

some specimens may be alloys of Ag3Sb with native silver or native antimony or 

both. Some specimens, especially those from Cobalt, Ontario, can be distinguished from native silver only by a microchemical test for Sb. See also description on page 120.

Silver (Ag). Isometric; color silvery white; hardness B. HNO3 effervesces a 

few seconds and leaves white coating. HC1 fumes tarnish some specimens; on 

others drop deposits dark coating near periphery; some specimens negative. KCN, 

mineral tarnishes light gray to brown, especially near periphery of drop; some 

specimens almost negative. FeCl 3 instantly tarnishes iridescent. KOH negative. 

HgCl2 stains brown to iridescent. Very sectile.

KOH ncg., HgCh neg.

Bornite (CujFeSi). Isometric; color pinkish brown; tarnished bornite is 

purple; hardness B. Usually isotropic, but some specimens show weak anomalous 

anisotropism. HN03 effervesces; surface stains yellowish brown; sometimes 

develops etch cleavage, which tends to form a bricklike pattern. HCl negative. 

KCN stains brown. FeCl3 stains orange. KOH and HgCl2 negative. Usually 

associated with chalcopyrite and other copper minerals.

ANISOTROPIC

KOH neg., HgCh

Chalcocite (Cu2S). Orthorhombic; color bluish gray; hardness B. Most 

chalcocite is weakly anisotropic; polarization colors steel gray, bornite pink. 

HNOj effervesces vigorously; mineral stains blue; with most varieties brings out 

parallel etch cleavage; very fine grained supergene chalcocite, however, will not 

show etch cleavage; with fine-grained sooty chalcocite brings out grain boundaries 

only. HCl negative; some varieties tarnish faintly. KCN quickly stains black; 

first brings out etch cleavage, but if drop is allowed to remain, etch cleavage is 

destroyed, and a rough, corroded surface remains. FeCl 3 stains blue; most 

specimens exhibit parallel etch cleavage. KOH negative. HgCh tarnishes 

slightly.

Dyscrasite (Ag3Sb). See description above.

KOH neg.. HgClj neg.

Bornite (CugFeS^. Isometric; color pinkish brown; hardness B. Most specimens are isotropic, but some exhibit weak anomalous anisotropism. HNOj 

effervesces; surface stains yellowish brown; sometimes develops etch cleavage, 

which tends to form a bricklike pattern. HCl negative. KCN stains brown. 

FeCla stains orange. KOH and HgCl2 negative. Usually associated with 

chalcopyrite and other copper minerals.

HNO3 eff., HCl neg., KCN, FeCl3 neg.

ISOTROPIC 

KOH neg., HgCh

Electrum (alloy of Au and Ag). Isometric; color light yellow; hardness B. 

HN03 fumes tarnish; effervesces slightly. HCl negative. KCN quickly stains 

dark. FeCl3 and KOH negative. HgClj stains iridescent to black. Very 

sectile.
120 MICROSCOPIC DETERMINATION OF ORE MINERALS

ANISOTROPIC 

KOH, HgCh neg.

Stibnite (Sb2S3). Orthorhombic; color gray; hardness B. Very strong anisotropism; polarization colors, white, pinkish gray; pink, blue, brown, dark gray; 

many varieties show multiple twinning. HN03 effervesces slowly and stains 

differentially iridescent; effervescence not always observed. HC1, some varieties 

stain differentially light brown after long etching; most varieties negative. KCN 

stains differentially light brown. FeCl3 negative. KOH instantly tarnishes 

brown and gives yellow coating; this is the most characteristic reaction for stibnite. HgCl2 negative. Prismatic; one perfect cleavage. Hardness and color 

vary somewhat with orientation, giving mottled appearance suggesting the presence of more than one mineral.

KOH neg., HgCh neg.

Klaprothite, also known as klaprotholite (3Cu2S.2Bi2S3). Probably isometric; 

color gray like tetrahedrite; in oblique light it has a pinkish-brown tinge; 

hardness C. Weak anisotropism, possibly anomalous. HN03 slowly stains 

brown; fumes tarnish; seems to be a slow effervescence; some areas negative. 

HC1 negative. KCN stains differentially iridescent. FeCls, KOH, and HgCl2 

negative. Specimen examined came from Butte, Mont. It was determined by 

F. B. Laney as klaprothite. It is intergrown with covellite in "mutual boundary" 

and subgraphic structures. °

HNO3 eff., HC1 neg., KCN neg., FeCI3

ISOTROPIC 

KOH, HgCls

Al(jodonite (CueAs). Isometric; color greenish gray; hardness C. HNOs 

effervesces and blackens;some varieties give parallel, possibly cubic etch cleavage. 

HC1, some specimens stain brown; other specimens develop etch cleavage, but 

surface does not stain; action sometimes requires more than 1 minute. KCN stains 

brown; with some varieties reagent effervesces and etch cleavage is developed; 

action sometimes requires more than 1 minute. FeCls instantly stains black. 

KOH slowly stains iridescent; some specimens negative. HgCl2 quickly stains 

brown. Algodonite is an intergrowth of two constituents, each a definite mineral, 

termed a and /3 algodonite. a algodonite is greenish gray, j3 algodonite has a suggestion of blue. With HC1 a algodonite etches more readily than /3 algodonite. 

More sectile than domeykite. Algodonite intergrown with niccolite is termed

"mohawkite."

KOH neg., HgCh

Dyscrasite (Ag3Sb). Orthorhombic; color silvery white; hardness B to C. 

Some varieties are isotropic, others are weakly anisotropic; some exhibit grating 

structure. HN03 effervesces vigorously; effervescence usually ceases in a few 

seconds; leaves white deposit; on some specimens brings out grating structure. 

HC1 negative. KCN, some specimens slowly stain black; on some specimens 

brings out etch cleavage; usually requires at least 2 minutes before effect is 

perceptible; some specimens negative. FeCl3 quickly stains iridescent; on some 

specimens brings out etch cleavage. KOH negative. HgCl2 quickly stains brown. 

Chemical analyses, as well as physical and chemical properties, of dyscrasite vary 

widely. There is probably more than one mineral containing antimony and 

silver, analogous to the arsenides of copper. Ford 10 suggests that some specimens 

may be alloys of Ag3Sb with native silver or native antimony or both. Some specimens, especially those from Cobalt, Ontario, can be distinguished from native 

silver only by a microchemical test for Sb. See also description on page 118.

10 Ford ,W. E., Textbook of mineralogy, p. 441, 1932.
DETERMINATIVE TABLE 121 

KOH neg., HgCl2 neg.

Arsenic (native) (As). Hexagonal; color gray; hardness C. Weakly auisotropic; polarization colors shades of gray; some of the fine-grained varieties may 

appear isotropic. HN03 slowly effervesces; surface stains iridescent. HC1 and 

KCN negative. FeCls quickly stains iridescent to black. KOH negative. 

HgCl2 slowly stains light brown; some areas almost negative. Good basal 

cleavage. Fairly sectile but gives powder when scratched.

Allaite (PbTe). Isometric; color galena white; hardness B. HN03 effervesces 

and stains through iridescent to dark gray. HCl, some specimens tarnish 

iridescent, others negative. KCN negative. FeCl3 quickly tarnishes iridescent. 

KOH and HgCl2 negative. Cubic cleavage but less marked than galena; shows 

fewer triangular pits. Distinguished from galena by the absence of H2 S fumes.

ANISOTROPIC

KOH neg., HgCIa

Arsenic (native) (As). See description above.

Dyscrasite (Ag3Sb). See description above.

Maucherile (Ni3As2). Tetragonal; color pinkish gray, about the same as cobaltifce; hardness D., Weak anisotropisni, contrasting markedly with niccolite, with 

which it is usually intergrown; polarization colors bornite pink of slightly different 

hues. HNOs instantly effervesces and stains black. HC1 and KCN negative. 

FeCl3 stains brown to iridescent and brings out a columnar or radial structure. 

KOH negative. HgCl2 slowly stains brown to iridescent. Resembles niccolite in 

color but is a lighter tinge of pink.

Niccolite (NiAs). Hexagonal; color coppery pink; hardness E. Strongly, 

anisotropic; polarization colors vivid yellow, steel blue, bornite pink. HNOs 

effervesces and tarnishes differentially. HC1 and KCN negative. FeCl3 slowly 

stains brown to iridescent. KOH negative. HgCl2 stains brown to iridescent. 

Niccolite in contact with breithauptite is distinctly buff. Breithauptite is more 

coppery in color and does not effervesce with HNOs.

Rammelsbergite (NiAs2 ). Orthorhombic; color white; hardness E. Strongly 

anisotropic; polarization colors pink, bornite pink, brown, light gray, greenish 

gray. Some specimens show multiple twinning. HN03 effervesces, stains dark, 

and brings out lamellar structure; leaves rough surface. HC1 and KCN negative. 

FeCl3 stains brown. KOH negative. HgCl2 slowly stains brown. Usually 

intergrown with niccolite or other nickel minerals.

Tellurium (Te). Color silvery white; hardness B + . Strong anisotropisni; 

polarization colors light to dark gray. HNOs tarnishes black with effervescence. 

HCl fumes tarnish some areas, other areas negative. KCN negative. FeCls 

stains faintly yellow; action weak and on some specimens practically negative. 

KOH negative. HgCl2 , some areas stain light brown, others negative. Perfect 

prismatic cleavage; elongated cracks well developed. Scratches produced by 

polishing usually extend entirely acioss specimen.

Tetradymite (Bi2 (Te,S)s). Hexagonal; color galena white; hardness B+. 

Anisotropic; polarization colors light to dark gray. HNOs effervesces; surface 

stains brown to black; action sometimes slow in starting. HCl slowly stains 

light brown to iridescent; some areas negative. KCN negative. FeCl3 stains 

iridescent. KOH negative. HgCl2 , some specimens stain iridescent, others 

negative. Perfect basal cleavage. The sulphur-free variety is known as tellurobismutite.

12915.1 40 0
122 MICROSCOPIC DETERMINATION OF ORE MINERALS

KOH neg., HgCh neg.

Aikinite (Cu2S.2PbS.Bi2 S3). Orthorhombic; color white; hardness C. Strongly 

anisotropic; polarization colors light gray, violet, brown, dark gray. HNOs 

effervesces; surface tarnishes iridescent to black. HCl and KCN negative. 

FeCIs stains light brown; action very slow and effect slight. KOH and HgCl2 

negative.

Benjaminite ((Cu,Ag) 2S.2PbS.2Bi2S3). Orthorhombic; color galena white; 

hardness B. Anisotropic; polarization colors light to dark gray; shows multiple 

twinning. HN03 quickly effervesces and turns black. HCl fumes tarnish faintly; 

some areas negative. KCN negative. FeCl3 stains light brown; effect may be 

overlooked. KOH and HgCl2 negative. Good cleavage in one direction. Distinguished from aikinite only by microchemical test for silver. Reported only 

from Outlaw mine, Manhattan, Nev. Analyzed by E. V. Shannon (U. S. Nat. 

Mus. Proc., vol. 65, p. 1, 1924).

Calaverite ((Au,Ag)Te2). Monoclinic; color light yellow; hardness C. Strongly 

anisotropic; polarization colors light gray, brown, dark gray; some specimens show 

multiple twinning. HN03 effervesces; surface stains black. HCl and KCN 

negative. FeCls, most specimens negative; some stain light brown. KOH and 

HgCU negative. Prismatic. Distinctions between calaverite and krennerite 

described by Short. 11

Galenobismutite (PbS.Bi2S3 ). Color galena white; hardness B. Anisotropic; 

polarization colors white to dark gray. HN03 effervesces and stains differentially 

iridescent; effervescence not always observed. HCl and KCN negative FeCls 

leaves slight yellow stain; some areas negative. KOH and HgCl2 negative. 

Prismatic cleavage. Specimen from Fahlun, Sweden. Gives microchemical tests 

for Pb, Bi, S, and Se.

Krennerite ((Au,Ag)Te2). Orthorhombic; color creamy white; hardness C. 

Strongly anisotropic; polarization colors light gray, yellow, brown; some grains 

show multiple twinning. HN03 effervesces and stains dark brown; some grains 

show etch cleavage. HCl and KCN negative. FeCl 3 stains light yellow. KOH 

and HgCl2 negative. Prismatic. Distinguished from sylvanite only by external 

crystal form. Distinctions between krennerite and calaverite described by 

Short."

Melonite (NiTe2 ). Color pale yellow; hardness B. Strongly anisotropic; 

polarization colors white, light brown, dark brow-n. HN03 instantly effervesces; 

surface stains black. HCl stains faint brown; not always observed. KCN negative. FeCl 3 stains dark brown. KOH and HgCl negative. Very rare. A polished 

surface when tilted so as to get the reflection of daylight and viewed with the 

unaided eye has a distinct light coppery tinge, about that of bismuth.

Sylvanite (AuAgTe4 ). Monoclinic; color creamy gray; hardness C. Anisotropic; polarization colors light gray, brownish gray, dark gray. HN03 slowly 

effervesces; surface stains iridescent; effervescence not always observed. HCl 

and KCN negative. FeClj sometimes leaves light-yellow stain; most areas negative. KOH and HgCl 2 negative. Lighter in color than calaverite. One perfect 

cleavage. Distinguished from krennerite only by crystal form. Distinctions 

between sylvanite, krennerite, and calaverite described by Short. 11

ii Short, M. N., Etch tests on calaverite, krennerite, and sylvanite: Am. Mineralogist, vol. 22, pp. 667-674, 

1937.
DETERMINATIVE TABLE 123 

HNO3 eff., HC1 neg. KCN neg., FeCl3 neg.

ISOTROPIC 

KOH neg., HgCh neg.

Polydymite (NisSO. Isometric; color brass yellow; hardness F. HN03 , slight 

effervescence; fumes tarnish; mineral slowly stains iridescent. HC1, drop turns 

yellow; mineral unaffected. KCN, FeCl3 , KOH, and HgCl2 negative. Color 

about same as that of pentlandite. The only genuine polydymite seen by the 

writer comes from Grunau, Saxony; the other specimens of so-called polydymite 

from Sudbury and elsewhere have proved to be violarite.

Sphalerite (ZnS). Isometric; color gray; hardness C . HN03 fumes tarnish; 

more rarely slow effervescence can be observed; on areas smaller than drop reaction is usually negative. HCl fumes tarnish some specimens, others negative; 

drop sometimes turns yellow, but surface is unaffected. KCN, FeCl3 , KOH, and 

HgCl2 negative. Aqua regia effervesces and stains dark brown; fumes tarnish. 

Some specimens show resin-colored internal reflection. Powder white to brown; 

if iron content is high, powder is metallic. Some specimens exhibit triangular pits 

from dodecahedral cleavage.

Violarite ((Ni,Fe) 3S4 ). Isometric; color violet; hardness C. HN03 slowly 

effervesces; surface stains slowly brown. HCl, KCN, FeCl3 , KOH, and HgCl 2 

negative. Some specimens exhibit triangular pits and cubic cleavage. Mineral is 

unstable and usually crumbly. Associated with and usually a supergene replacement product of pentlandite or pyrrhotite. 12

ANISOTROPJC

KOH, HgCb neg.

Jamesonite (4PbS.FpS.3Sb2S3). Monoclinic; color galena white; hardness B. 

Strong anisotropism; polarization colors light gray, dark brown, dark gray. 

HN03 tarnishes iridescent with slow effervescence. HCl fumes usually tarnish; 

sometimes negative. KCN and FeCl3 negative. KOH stains differentially 

iridescent. HgCl2 negative. Prismatic. Perfect basal cleavage.

Plagionite (5PbS.4Sb2 S3). Monoclinic; color gray; hardness C. Anisotropic; 

polarization colors white to dark gray. HN03 effervesces; surface stains iridescent. 

HCl negative; on some areas fumes tarnish slightly. KCN and FeCl 3 negative. 

KOH stains gray to brown. HgCl 2 negative. Distinguished from zinkenite only 

by its crystal form. Prismatic. Rare.

KOH neg., HgCh neg.

Aikinite (CuS.2PbS.Bi2 S3). Orthorhombic; color galena white; hardness C. 

Strong anisotropism; polarization colors light gray, violet, brown, dark gray. 

HN03 slowly effervesces; surface stains black. HCl negative. KCN negative. 

FeCl3 stains light brown; action slow and'effect slight. KOH and HgClo negative.

Bismuthinite (Bi2S3). Orthorhombic; color galena white; hardness B. Strong 

anisotropism; polarization colors white, brown, black. HN03 \effervesces; surface 

stains iridescent. HCl fumes tarnish some specimens; others negative. KCN, 

Feds, and KOH negative. HgCl2 , some specimens stain light brown; others 

negative. Prismatic. One perfect cleavage.

" See Short. M. N., and Shannon, E. V., Violarite and other rare nickel sulphides: Am. Mineralogist, 

vol. 15, pp. 2-7, 1930.
124 MICROSCOPIC DETERMINATION" OF ORE MINERALS

Boulangerite (5PbS.2Sb2S3). Orthorhombic; color galena white; hardness B. 

Strong anisotropism; polarization colors white, steel blue, brownish gray, dark 

gray. Some specimens show shreddy or fibrous structure. HN03 effervesces; 

surface stains black; action often slow in starting. HC1 fumes tarnish slightly 

some specimens; others negative. KCN, Feds, KOH, and HgCl2 negative. 

Prismatic. Distinguished from jamesonite by KOH test.

Calaveriie ((Au,Ag)Te2). Monoclinic; color light yellow; hardness C. Strong 

anisotropism. Polarization colors light gray, brown, dark gray; some specimens 

show multiple twinning. HN03 effervesces; surface stains black. HC1 and KCN 

negative. FeCl 3 , most specimens negative; some stain slowly light brown. KOH 

and HgCl2 negative. Prismatic.

Emplectite (CujS.P^Sa). Orthorhombic; color creamy; hardness C. Anisotropic; polarization colors light gray, purple, light brown. HN03 slowly effervesces; surface stains light brown; effervescence not always observed. HC1 negative; drop sometimes turns yellow but surface does not stain. KCN, FeC] 3, KOH, 

and HgCl2 negative. Aqua regia instantly effervesces and stains iridescent. 

Rare.

Guanajualite (Bi2 (Se,S)a). Orthorhombic; color gray; hardness B . Strong 

anisotropism; polarization colors white to dark gray. HNOa slowly effervesces 

and stains iridescent; fumes tarnish; effervescence not always observed. HC1, 

KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. Rare.

Guitermanile (3PbS.As2S3). Orthorhombic; color galena white; hardness C. 

Anisotropic; polarization colors reddish brown, steel blue; some areas show multiple twinning. HNOs effervesces slowly and stains iridescent; effervescence not 

always observed. HC1, KCN, FeCla, KOH, and HgCl2 negative. Specimen from 

Zuni mine, Silverton, Colo., the type locality. Rare.

Jordanite (4PbS.As2S3). Monoclinic; color galena white; hardness C. Anisotropic; polarization colors light gray, pinkish gray, violet, dark gray. HNOs, no 

action for 30 to 60 seconds, then quickly stains gray to brown with slow effervescence. HC1, KCN, FeCl3 , KOH, and HgCl2 negative. Very rare. Some 

specimens from Beuthen, Silesia, show circular cross sections.

Meneghinite (4PbS.Sb2 S3). Orthorhombic; color galena white; hardness C. 

Strong anisotropism; polarization colors light gray, pinkish brown, steel blue. 

HN03 effervesces; surface stains black; usually reaction is slow in starting, but 

once started it rapidly advances from one side of the drop to the other. HC1 

fumes tarnish; some areas negative. KCN, FeCla, KOH, and HgCl2 negative. 

Occurs as long, slender needles vertically striated. Contains about 3 percent of 

copper, readily detected microchemically.

Millerite (NiS). Hexagonal; color brass yellow, between color of pyrite and 

that of chalcopyrite; hardness E. Anisotropic; polarization colors light yellow to 

dark purple. HNOs effervesces slowly and stains brown; fumes tarnish; effervescence not always observed. HC1, KCN, FeCl 3 , and KOH negative. HgCl 2 , 

some specimens slowly stain brown, others negative. Usually but not invariably 

fibrous in habit.

Rezbanyile (Cu2S.3PbS.5Bi2Ss). Crystal system unknown; color galena white; 

hardness C. Strong anisotropism. HN03 effervesces strongly; surface turns black. 

HC1, KCN, FeCl3 , KOH, and HgCl2 negative. Specimen examined came from 

Rezbanya, Hungary, the type locality. Distinguished from other lead-bismuth 

compounds by the lack of prismatic habit. Rare.
DETERMINATIVE TABLE 125

Sylvanite (AuAgTe^. Monoclinic; color creamy gray; hardness C. Anisotropic; polarization colors light gray, brownish gray, dark gray. HNOs efl'ervesces and tarnishes iridescent; effervescence not always observed. HC1 and 

KCN negative. FeCls, some areas stain light yellow; most areas negative. KOH 

and HgClz negative. Lighter in color than calaverite. One perfect cleavage. 

Distinctions between sylvanite, calaverite, and krennerite described by Short. 13

'Tellurium (Te). Color silvery white; hardness B + . Strong anisotropism; 

polarization colors light to dark gray. HNOs stains black with effervescence. 

HC1 fumes tarnish some varieties; others negative. KCN negative. FeCls slowly 

stains light yellow; action slow and on some specimens practically negative. 

KOH negative. HgCl2, some specimens stain light brown; others negative. 

Perfect prismatic cleavage; elongated cracks well developed. Scratches produced 

by polishing usually extend entirely across specimen.

HNO3 , HCJ, KCN, FeCl3

ISOTROPIC 

KOH neg., HgCh

Antimony (Sb). Hexagonal; color silvery white; hardness B. Specimens from 

Borneo show some areas apparently isotropic; others weakly anisotropic. Specimens from Tulare County, Calif., and from Allemont, France, are weakly anisotropic. Some specimens exhibit multiple twinning. HNOs, on specimen from 

Borneo quickly stains black and brings out lamellar structure; specimen from 

Tulare County slowly stains brown; specimen from Allemont stains iridescent to 

black; action slower than with arsenic. HC1, on some specimens fumes tarnish 

and surface stains after long etching; most specimens negative. KCN, on specimen from Borneo some areas stain brown,1 most areas negative; specimens from 

Tulare County and Allemont negative. FeCls, specimen from Borneo stains 

brown; specimen from Tulare County stains black; specimen from Allemont 

slowly stains differentially iridescent after about 1 minute. KOH negative. 

HgCl2, specimen from Borneo stains brown; specimens from Tulare County and 

Allemont negative. Specimen from Tulare County is an arsenic-antimony alloy. 

Specimen from Allemont is a fine-grained graphic intergrowth between native 

antimony and native arsenic, the intergrowth being known as "allemontite." 13

Specimen from Borneo shows triangular pits.

Argentite (Ag2S). Isometric; color gray, hardness A. Most specimens are 

isotropic, but some show weak anomalous anisotropism. HNOs, some specimens 

slowly etch differentially; on others fumes tarnish but surface does not stain; 

some specimens negative. HC1 fumes tarnish; aureole will not wash off; this is 

the most distinctive reaction for argentite. KCN stains brown to black; some 

specimens develop structure. FeCls stains gray to black. KOH negative. HgClz 

quickly stains iridescent. Sectile. Yields filings instead of power when scratched. 

Polishing gives a multitude of fine scratches which extend entirely across specimen.

a dyscrasite (Ag3Sb). Orthorhombic; color silvery white; hardness B to C. 

Some specimens are isotropic; others appear weakly anisotropic; some exhibit 

grating structure. Specimen here described came from Broken Hill, Australia, 

and consists of an intergrowth of two minerals, both consisting of antimony and

is Short, M. N., Etch tests on calaverite, krennerite, and sylvanite: Am. Mineralogist, vol. 22, pp. G67- 

674,1937. 

'< Schneiderhohn, Hans, and Ramdohr, Paul, Lehrbuch der Erzmikroskopie, Band 2, p. 30, 1931.
126 MICROSCOPIC DETERMINATION OF ORE MINERALS.

silver. They are designated here a and /3 dyscrasite; a dyscrasite is somewhat 

more chemically reactive than /3 dyscrasite. HNOs, both varieties etch without 

effervescence and leave white coating. HC1 stains a dyscrasite light brown; 

/3 dyscrasite negative. KCN, both varieties stain iridescent to black, but a dyscrasite stains more quickly. FeCls, both varieties quickly stain iridescent. KOH, 

both varieties negative. HgCl2, both varieties stain brown. The two minerals 

of the intergrowth are closely similar in appearance, but a dyscrasite is somewhat 

more silvery in color than /3 dyscrasite. See also description on page 120.

Hessite (Ag2Te). Isometric; color gray; hardness A. Some specimens are 

isotropic; most, however, show anomalous anisotropism; polarization colors 

white, steel blue, borni'te pink; some areas show multiple twinning. HNOs 

stains iridescent to black. HCl slowly stains black; negative on some areas. 

KCN, some specimens slowly stain black; action usually requires more than 1 

minute and on some specimens negative. FeCl3 quickly stains iridescent. KOH 

negative. HgCl2 tarnishes light brown. Closely resembles argentite in appearance, but hessite will yield powder when scratched; argentite yields only metallic 

shavings.

ANISOTROPIC

KOH, HgCh

Polybasite (8Ag2S.Sb2S3). See description below.

Stephanite (5Ag2S.Sb2S3). Orthorhombic; color gray; hardness B. Strongly 

anisotropic; polarization colors light gray, steel gray, bornite pink. HNOs practically negative; after long etching fumes tarnish faintly and mineral seems to 

stain very light brown; reaction doubtful. HCl fumes tarnish; halo will not 

wash off; action slow and on some specimens practically negative. KCN stains 

dark gray to black and brings out scratches. FeCl3 practically negative; seems 

to stain light gray in places; solution leaves sediment, which rubs off easily. 

KOH quickly stains black. HgCl2 quickly stains brown to iridescent. Metallic 

powder. Best distinguished from polybasite by powder and lack of internal 

reflection.

._.- KOH neg., HgCh

Antimony (Sb). See description above.

Argentite (Ag2S). See description above.

a dyscrasite (Ag3Sb). See description above.

Hessite (Ag2Te). See description above.

Polybasite (8Ag2S.Sb2S3). Monoclinic; color gray; hardness C. Strongly 

anisotropic; interference colors brown, grayish, violet, yellowish green. HNOs 

fumes tarnish; some specimens negative. HCl fumes tarnish: halo does not 

wash off; action slow and on some specimens negative. KCN stains brown to 

black and brings out scratches. FeCls tarnishes iridescent; some specimens 

negative. KOH, some specimens slowly tarnish iridescent; most specimens negative. HgCl2 stains brown to black. Variability in etch behavior probably due 

to variable chemical composition. More or less copper always present. Powder 

dark ruby red; if particles are coarse, powder is black. Internal reflection red; 

hot always seen. Stephanite and pearceite always give black powder and show 

no internal reflection. All specimens examined by writer gave good microchemical tests for copper.

Stromeyerite ((Ag,Cu) 2S). Orthorhombic; color gray, similar to argentite; 

hardness B. Strong anisotropism; polarization colors light violet, purple, brown, 

black. HNOs stains through iridescent to black. HCl fumes tarnish; halo does 

not wash off; action sometimes almost negative; specimens from Silver King, 

Ariz., quickly stain black. KCN quickly stains black. FeCls quickly stains
DETERMINATIVE TABLE 127

iridescent. KOH negative. HgCl2 stains iridescent. Closely resembles argentite in color and etch tests; stromeyerite has stronger anisotropism and yields a 

powder when scratched.

Umangite ( 113862). Color purple, similar to slightly tarnished bornite; hardness B. Strongly anisotropic; gives four extinctions; interference color is a fiery 

orange very similar to that of klockmannite. HNOs fumes tarnish; surface tarnishes slightly bluish. HCl, surface tarnishes bluish. KCN, surface tarnishes 

differentially blue to dark gray, bringing out grain structure. FeCls, surface 

turns bluish. KOH negative. HgCl2 tarnishes to a covellite-like blue. Associated with clausthalite and klockmannite.

HNO3( HCI, KCN, FeC!3 neg.

KOH, HgCh

Pyrargyrite (3Ag2S.Sb2S3 ). Hexagonal; color bluish gray; hardness C. Moderately anisotropic, but some areas appear isotropic owing to transparency; polarization colors light gray, steel gray, brown; colors somewhat masked by internal 

reflection. HNOs, some specimens slowly stain gray to brown; on others fumes 

tarnish but surface does not stain; most specimens negative. HCl fumes tarnish; 

halo does not wash off; action slow in starting and on some specimens negative. 

KCN quickly stains brown to black and brings out scratches. FeCl3 negative; 

solution deposits sediment which does not wash off easily, but surface does not 

stain. KOH stains gray to iridescent. HgCl2 slowly stains brown. Ruby-red 

powder and internal reflection; lighter in color and more easily seen than in 

polybasite. In hand specimen color varies from dark red to black and mineral 

is distinguished from miargyrite with difficulty. Miargyrite has a darker-red 

powder and internal reflection than pyrargyrite. Proustite is a lighter red in 

hand specimen than pyrargyrite.

Colusite ((Cu,Fe,Mo,Sn) 4 (S,As,Te)3-4). Isometric; color bronzy gray; hardness C. HNOs tarnishes brown; etches to pitted surface. HCl tarnishes 

brown; fumes tarnish. KCN slight tarnish, which washes off; solution colors 

pink. FeCls, KOH, and HgCl2 negative. Associated with other copper ores 

at Butte, Mont., the only reported occurrence. 15

ANISOTKOPIC 

KOH, HgCh

Pyrargyrite (3Ag2S.Sb2S3). See description above.

Stephanite (5Ag2S.Sb2S3). Orthorhombic; color gray; hardness B. Strongly 

anisotropic; polarization colors light gray, steel gray, bornite pink. HNOs practically negative; after long etching fumes tarnish faintly and mineral stains very 

light brown; reaction doubtful. HCl fumes tarnish; halo will not wash off; 

action slow and on some specimens practically negative. KCN quickly stains 

black. FeCls practically negative; seems to stain light gray in places; solution 

leaves sediment which rubs off easily. KOH, some specimens stain iridescent to 

black; others negative. HgCl2 stains brown to iridescent. Metallic powder. 

Best distinguished from polybasite by powder and lack of internal reflection.

16 Landon, R. E., and Mogilnor, A. H., Colusite, a new mineral of the sphalerite grouty. Am. Mineralogist, 

vol. 18, pp. 528-533, 1933.
128 MICROSCOPIC DETERMINATION OF ORE MINERALS

KOH, HgCh neg.

Kermesite (2Sb2S3.Sb2 03). Monoclinic; color gray; hardness B. Strong anisotropism; polarization colors greenish yellow, purple. HNO3 tarnishes brown. 

HCl tarnishes light brown; some areas negative. KCN stains slightly light 

brown; doubtful. FeCl3 negative. KOH tarnishes and gives yellow coating; 

this is the most characteristic reaction for the mineral; reaction is identical with 

that for stibnite. HgCl2 negative. Prismatic. Red powder and red internal 

reflection (best seen with strong polarized light).

Stibnite (Sb2Ss). Orthorhombic; color gray; hardness B. Very strong anisotropism; polarization colors white, pinkish gray, pink, blue, brown, dark gray; many 

varieties show multiple twinning. HNOs effervesces slowly and stains differentially iridescent; effervescence not always observed. HCl, some varieties stain 

differentially light brown after long etching; most varieties negative. KCN 

stains differentially light brown. FeCl3 negative. KOH instantly tarnishes 

brown and gives yellow coating; this is the most characteristic reaction for stibnite. 

HgCl2 negative. Prismatic. One perfect cleavage. Hardness and color vary 

somewhat with orientation, giving mottled appearance that suggests the presence 

of more than one mineral.

KOH neg., HgCh

Polybasite (8Ag2S.Sb2S3). Monoclinic; color gray, hardness C . Strongly 

anisotropic; interference colors brown, grayish, violet, yellowish green. HNOs 

fumes tarnish; some specimens negative. HCl fumes tarnish; halo does not wash 

off; action slow and on some specimens negative. KCN stains brown to black and 

brings out scratches. FeCls tarnishes iridescent; some specimens negative. KOH, 

some specimens slowly tarnish iridescent; most specimens negative. HgCl2 stains 

brown to black. Variability in etch behavior probably due to variable chemical 

composition. More or less copper always present. Powder dark ruby red; if 

particles are coarse, powder is black. Internal reflection red; not always seen. 

Stephanite and pearceite always give black powder and show no internal reflection. 

All specimens examined by writer gave good microchemical tests for copper.

HNO3 , HCl, KCN neg., FeCl3

ISOTROPIC 

EOH neg., HgCh

Aguilarite (Ag2 (S,Se)). Isometric; color gray; hardness A. Some specimens 

are isotropic, but most are anisotropic. HN03 slowly stains light brown. HCl 

usually negative; fumes sometimes tarnish. KCN, some specimens stain differentially light brown, others negative. FeCl3 stains iridescent. KOH negative. 

HgCl2 stains iridescent. Sectile. Rare.

Lead (native) (Pb). Isometric; color gray, but tarnishes in a few hours, giving 

a yellowish, brownish, or iridescent coating; hardness A. HN03 quickly tarnishes 

black. HCl fumes tarnish and bring out scratches. KCN negative but seems to 

remove scratches. FeCl3 quickly stains iridescent to black. KOH negative but 

seems to remove tarnish. HgCl2 quickly stains black. Rare.

Petzite ((Ag,Au) 2Te). Isometric; color galena white; hardness A. HN03 

quickly stains iridescent. HCl stains iridescent; some specimens negative. KCN 

negative. FeCl3 quickly stains iridescent. KOH negative. HgCl2 slowly stains 

brown. Petzite is whiter than hessite and has a smoother surface. Hessite has 

a rough surface and resembles argentite closely. Petzite has a cubic cleavage 

and exhibits triangular pits, like galena, but the cleavage is less perfect than 

that of galena. Petzite is a much rarer mineral than hessite; most so-called
DETERMINATIVE TABLE 129

petzites examined] by the writer proved to be intergrowths of hessite with 

calaverite or other minerals.

KOH neg., HgCh neg.

Clausthalite (PbSe). Isometric; color galena white; hardness A . HN03 

tarnishes and leaves a coating that is brick red in oblique light; this distinguishes 

the mineral from galena. HCl slowly tarnishes light brown; if acid comes into 

contact with calcite, reaction may be negative. KCN negative. FeCls tarnishes 

iridescent; if reagent comes into contact with calcite, etching may require more 

than 1 minute. KOH and HgCl2 negative. Has cubic cleavage less perfect than 

galena. Usually shows individual triangular pits here and there. A cobaltiferous 

variety is known as "tilkerodite" and can be distinguished from ordinary clausthalite only by microchemical means.

Coloradoite (HgTe). Isometric; color pinkish gray; hardness C. HNOs slowly 

stains brown; some areas almost negative. HCl slowly stains dark gray to black. 

KCN negative. FeCla stains iridescent. KOH and HgCl2 negative. Rare.

Galena (PbS). Isometric; color galena white; hardness B. HNOs, small percentage of galena specimens effervesce and turn black; most specimens blacken 

without effervescence. HCl tarnishes brown to iridescent. KCN negative. 

FeCls tarnishes iridescent; this reaction is negative when galena is in contact with 

bornite. KOH and HgCl2 negative. Perfect cubic cleavage. Most specimens 

show parallel triangular pits.

ANISOTROPIC

KOH, HgCIs neg.

Cylindrite (6PbS.Sb2S3 .6SnS2). Tetragonal; color gray; hardness C . Strong 

anisotropism; polarization colors light to dark gray. HNOs fumes tarnish; surface 

stains iridescent. HCl fumes tarnish; stains slightly after etching 2 minutes; 

some areas negative. KCN negative. FeCls stains slightly light brown; doubtful. KOH stains light gray; in places iridescent. HgCl2 negative. Basal 

sections are more or less cylindrical and are made up of concentric shells. Prismatic sections are needlelike.

Mottramite (2Cu0.2PbO.V2 05 .H2 0). Orthorhombic; color gray, similar to 

sphalerite: hardness D. Weak anisotropism; polarization colors light to dark 

gray. HNOs quickly darkens differentially iridescent, brown -and black; brings 

out grain structure and leaves yellowish-green coating which rubs off; fumes 

tarnish differentially brown and do not wash off. HCl instantly blackens and 

leaves yellow coating which does not wash off. KCN negative. FeCls stains 

faint brown. KOH stains faint brown. HgCl2 negative. Yellow powder. Red 

and orange internal reflection. Radial fibrous structure.

Sterribergite (Ag2S.Fe4S5). Orthorhombic; color light brown; hardness B. 

Very strong anisotropism; shows without analyzing nicol; polarization colors light 

gray, pink, violet, brown, steel blue, black. HNOs fumes tarnish brown, do not 

wash off; surface slowly stains iridescent. HCl fumes tarnish slowly; tarnish does 

not wash off. KCN negative. FeCls stains iridescent. KOH stains differentially iridescent. HgCl2 negative; gives a deceptive "etching" owing to deposition of 

HgCl2 crystals. Habit prismatic. Some crystals curved like those of cylindrite. 

Usually associated with ruby silver minerals.

KOH neg., HgCb

Hessite (Ag2Te). Isometric; color gray, resembles argentite;'hardness A + . 

Most specimens show anomalous anisotropism; polarization colors white, steel
130 MICROSCOPIC DETERMINATION OF ORE MINERALS

blue, bornite pink; some areas show multiple twinning. HNOs stains black. 

HCl slowly stains black; some specimens negative. KCN, some specimens slowly 

stain black; action usually requires more than 1 minute and on some specimens 

negative. FeCls stains iridescent. KOH negative. HgCl2 stains brown to 

iridescent. Almost sectile but yields powder when scratched.

KOH neg., HgCh neg.

Alaskaite ((Pb,Ag2)S.Bi2S3). Orthorhombic; color gray; hardness C. Strong 

anisotropism; polarization colors white to dark gray. HNOs slowly stains dark. 

HCl fumes tarnish a little; doubtful. KCN negative. FeCls stains iridescent. 

KOH and HgCl2 negative. Argentiferous galenobismutite. Prismatic. Rare. 

Two analyses quoted by Dana give Ag 3 and 8 percent.

Cosalite (2PbS.Bi2Ss). Monoclinic, color galena white; hardness B. Strong 

anisotropism. HNOs stains black with slow (doubtful) effervescence. HCl 

slowly stains differentially gray to black. KCN negative. FeCls stains light 

brown. KOH and HgCl2 negative. Prismatic. Specimen from Boston Creek 

mine, Cobalt, Ontario, described and analyzed by T. L. Walker.

Schapbachite (Ag2S.PbS.Bi2Ss). Orthorhombic; color gray; hardness B 

Anisotropic; polarization colors light to dark gray. HNOs slowly stains brown. 

HCl stains differentially; some areas unaffected. KCN negative. FeCls stains 

iridescent. KOH and HgCl2 negative. From Schapbach, Baden, the only 

locality reported. Differs from alaskaite in its higher silver content (Ag 11.6 

percent).

HNO3 , HCl, KCN neg., FeCl3 neg.

ISOTEOPIC 

KOH neg., HgCh neg.

Hauerite (MnS2 ). Isometric; color gray; hardness E. HN03 fumes tarnish 

slowly; some areas practically negative. HCl solution turns brown; mineral 

stains light brown; fumes tarnish; some areas do not stain. KCN, FeCls, KOH, 

and HgClz negative. H202, slow effervescence; surface does not stain. Cubic 

cleavage; surface shows triangular pits. Red powder.

Sphalerite (ZnS). Isometric; color gray; hardness C. HNOs fumes tarnish; 

more rarely slow effervescence can be observed; on areas smaller than the drop 

reaction is Usually negative. HCl fumes sometimes tarnish; usually negative; 

drop sometimes turns yellow, but surface is unaffected. KCN, FeCl3 , KOH, and 

HgCl2 negative. Aqua regia effervesces and stains dark brown; fumes tarnish. 

Some specimens show resin-colored internal reflection. Dodecahedral cleavage; 

some specimens show triangular pits. Powder white to brown; if proportion of 

iron is high, powder is metallic.

ANISOTROPIC 

KOH, HgCh neg.

Andorite (Ag2S.2PbS.3Sb2Ss). Orthorhombic; color gray; hardness C. Anisotropic; polarization colors light to dark gray. HNOs fumes tarnish; some areas 

stain slowly brown, others negative. HCl fumes tarnish faintly; some specimens 

negative. KCN arid FeCls negative. KOH slowly stains brown. HgCl2 negative. Aqua regia effervesces vigorously and stains black; this is the most distinctive etch test. Almost invariably associated with stannite. Rare. Prismatic.

Cylindrite (6PbS.Sb2Ss.6SnS2). Tetragonal; color gray; hardness C . Strong 

anisotropism; polarization colors light to dark gray. HNOs fumes tarnish; sur-
DETERMINATIVE TABLE 131

face stains iridescent. HCl fumes tarnish; stains slightly after etching 2 minutes; 

some areas negative. KCN negative. FeCl3 stains slightly brown; doubtful. 

KOH stains light gray, in places iridescent. HgCl2 negative. Basal sections are 

more or less cylindrical and are made up of concentric shells. Prismatic sections 

are needlelike.

Franckeite (5PbS.Sb2 S3 .2SnS2). Color gray; hardness B. Strong anisotropism; 

polarization colors light to dark gray. HN03 stains differentially iridescent; 

fumes tarnish. HCl fumes tarnish slightly after long etching; doubtful. KCN 

and FeCl3 negative. KOH, some areas stain slightly; others negative. HgCl2 

negative. Strong cleavage in one direction.

Pyrrhotite (FeS]+). Hexagonal; color brownish or pinkish cream; hardness 

D . Strong anisotropism; polarization colors light gray, bluish gray, brown. 

HNOs fumes tarnish most specimens; surface usually stains slightly light brown; 

some specimens, especially those poorly polished, negative. HCl, on most specimens drop slowly turns yellow but surface does not stain; on a small proportion 

fumes tarnish and halo does not wash off; on specimens from Mary mine, Ducktown, Tenn., effervesces and brings out parallel etch cleavage; surface stains 

brown. Analysis of one of these specimens, insoluble 0.16, Fe 61.92, S 38.19, 

total 100.27 (George Steiger, Geol. Survey, analyst). KCN negative. FeCls 

negative. KOH, surface slowly turns brown, especially near periphery of drop. 

HgClj negative. Distinctly magnetic.

Ramdohrite (3PbS.Ag2S.3Sb2S3). Crystal system unknown; color galena white; 

hardness C. Anisotropic; polarization colors light to dark gray. HNOs slowly 

stains dark; some areas almost negative. HCl slowly stains brown; some areas 

almost negative. KCN and FeCls negative. KOH slowly stains faint brown; 

some areas negative. HgCl2 negative. Aqua regia effervesces vigorously and 

stains black. Prismatic. Rare.

Slernbergite (Ag^.Fe^Ss). Orthorhombic; color light brown; hardness B. Very 

strong anisotropism; shows without analyzing nicol; polarization colors light gray, 

pink, violet, brown, steel blue, black. HNOs fumes tarnish; surface slowly stains 

iridescent; some specimens practically negative. HCl fumes tarnish brown. KCN 

and FeCls negative. KOH stains differentially iridescent. HgCl2 negative; 

gives a deceptive "etching," owing to deposition of HgCl2 crystals. Habit 

prismatic. Some crystals curved like those of cylindrite. Usually associated 

with ruby silver minerals.

Troilite (FeS). Hexagonal; color somewhat similar to pyrrhotite but more 

yellowish; hardness D. Strong anisotropism; polarization colors white to reddish 

brown. HNOs, some specimens effervesce and give off H2S; other specimens 

stain without effervescence. HCl effervesces and gives off H2S; this reaction 

usually begins in less than 1 minute; on some areas, however, it may be as long as 

3K minutes before any action is observed; once initiated, it proceeds vigorously. 

KCN and FeCls negative. KOH stains differentially iridescent; slow. HgCl2 

negative. Very rare in mineral deposits but sometimes found in meteorites.

KOH neg., HgCh neg.

Bismutoplagionite (5PbS.4Bi2 S3). Orthorhombic; color gray; hardness B + . 

Anisotropic; polarization colors white to dark gray. HNOs quickly stains iridescent; effervesces slightly. HCl fumes tarnish; some areas stain iridescent; 

others negative. KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. Good 

cleavage in two directions. Reported only from Wickes, Mont. Analyzed by 

E. V. Shannon (U. S. Nat. Mus. Proc., vol. 58, p. 600, 1920.)
132 MICROSCOPIC DETERMINATION OF ORE MINERALS

Owyheeite (8PbS.2Ag2S.5Sb2S3). Orthorhombic; color gray; hardness B. 

Strong anisotropism. HNOs stains differentially iridescent. HC1 fumes-tarnish. 

KCN, FeCls, KOH, and HgCl2 negative. Prismatic structure. Shows triangular 

cleavage pits. Very rare. Specimen from Poorman mine, Silver City district, 

Idaho; analyzed by E. V. Shannon (Am. Mineralogist, vol. 6, pp. 82-83, 1921).

Teallite (SnS.PbS). Orthorhombic (?); color galena white; hardness C. Anisotropic; polarization colors light gray, brownish gray, dark gray, steel blue, violet. 

HNOs, some specimens slowly stain differentially brown; most specimens negative. 

HCl quickly tarnishes gray to brown; some areas give off H2S. KCN, FeCls, 

KOH, and HgCl2 negative. Aqua regia stains black and evolves H2 S. Sections 

are elongated parallel to the base and show perfect basal cleavage. Plates are 

flexible and curved like those of molybdenite.

Tenorite or melaconite (CuO). Monoclinic; color gray; hardness E . Moderately strong anisotropism; polarization colors white, dark purple, dark gray. 

HN03 fumes tarnish; surface slowly stains differentially brown; some specimens 

negative. HCl slowly stains differentially brown; some areas unaffected; drop 

colors green to brown. KCN, FeCls, KOH, and HgCl2 negative. Found in the 

oxidized zone of copper deposits. The massive variety, the more common, is 

melaconite; ind the prismatic variety tenorite. Melaconite commonly has more 

or less chemically combined manganese oxide and silica. If it contains more than 

about 5 percent of either of these two components the mineral is known as "copper 

pitch ore."

Hausmannite (MnO.Mn2Os). Tetragonal; color white with a faint tinge of 

blue; hardness D . Strongly anisotropic; most specimens show multiple twinning resembling that of plagioclase; polarization colors white, dark gray. HNOs 

fumes tarnish; some specimens negative. HCl tarnishes differentially brown in 

places; some areas negative; drop stains brown. KCN, FeCls, KOH, and HgClj 

negative. H2 02 effervesces slowly without staining surface. Good basal cleavage. Powder and internal reflection deep red. Takes a high polish.

HNOs, HCl neg., KCN, FeCi3

ISOTROPIC 

KOH, HgCIj

Pyrargyrite (3Ag2S.Sb2Ss). Hexagonal; color bluish gray; hardness C. Moderately strong anisotropism, but some areas appear isotropic owing to transparency; 

polarization colors light gray, steel gray, brown; colors somewhat masked by 

internal reflection. HNOa, some specimens slowly stain gray to brown; on others 

fumes tarnish but surface does not stain; most specimens negative. HCl fumes 

tarnish; halo does not wash off; action slow in starting and on some specimens 

negative. KCN quickly stains brown to black and brings out scratches. FeCls 

negative; solution deposits sediment which does not wash off easily, but surface 

does not stain. KOH stains gray to iridescent. HgCl2 slowly stains brown. 

Ruby-red powder and internal reflection, lighter in color and more easily seen than 

in polybasite. In hand specimen color varies from dark red to black and is 

distinguished from miargyrite with difficulty. Miargyrite has a darker-red 

powder and internal reflection than pyrargyrite. Proustite is a lighter red in 

hand specimen than pyrargyrite.

Canfieldite (4Ag2S.(Sn,Ge)S2 ). Isometric; color gray with pinkish tinge. Most 

areas isotropic, but some show anomalous anisotropism. HNOs slowly stains 

brown; reaction usually requires more than 1 minute before effect is perceptible. 

HCl negative. KCN stains brown or dark gray and develops etch cleavage.
DETERMINATIVE TABLE 133

FeCls deposits sediment which tends to cling to surface, but reaction should be 

termed"negative. KOH tarnishes iridescent; reaction usually requires more 

than 1 minute before effect is perceptible. HgCl2 tarnishes iridescent; some 

areas negative.

KOH neg.. HgCh

Antimony (Sb). Hexagonal; color silvery white; hardness B. Specimens from 

Borneo show some areas apparently isotropic, others weakly anisotropic. Specimens from Tulare County, Calif., and from Allemont, France, are weakly anisotropic. Some specimens exhibit multiple twinning. HNOa on specimen from 

Borneo quickly stains black and brings out lamellar structure; specimen from 

Tulare County slowly stains brown; specimen from Allemont stains iridescent 

to black; action slower than with arsenic. HC1, on some specimens fumes tarnish 

and surface stains after long etching; most specimens negative. KCN, on specimen from Borneo some areas stains brown; most areas negative; specimens from 

Tulare County and Allemont negative. FeGa, specimen from Borneo stains 

brown; specimen from Tulare County stains black; specimen from Allemont 

slowly stains differentially iridescent after about 1 minute. KOH negative. 

HgClz, specimen from Borneo stains brown; specimens from Tulare County and 

Allemont negative. Specimen from Tulare County is an arsenic-antimony alloy. 

Specimen from Allemont is a fine-grained intergrowth between native antimony 

and native arsenic, the intergrowth being known as "allemontite." 10 Specimen 

from Borneo shows triangular pits.

/3 dyscrasile (Ag3Sb). Orthorhombic; color silvery white; hardness B to C. 

Some specimens are isotropic, others appear weakly anisotropic; some exhibit 

grating structure. Specimen here described came from Broken Hill, Australia, 

and consists of an intergrowth of two minerals, both consisting of antimony and 

silver. They are designated here a and j3 dyscrasite; a dyscrasite is somewhat 

more chemically reactive than /3 dyscrasite. HNOs, both varieties etch without 

effervescence and leave white coating. HC1 stains a dyscrasite light brown; 

/3 dyscrasite negative. FeCls, both varieties quickly stain iridescent. KOH, 

both varieties negative. HgCl2, both varieties stain brown. The two minerals 

of the intergrowth are closely similar in appearance, but a dyscrasite is somewhat more silvery in color than /3 dyscrasite. See also description on page 125. 

Etch tests on dyscrasite from Andreasberg, Germany, are identical with those 

for /3 dyscrasite.

Petzite ((Ag,Au)2Te). Isotropic; color galena white; hardness A. HNO» 

quickly stains iridescent. HC1, some specimens stain iridescent, others negative. 

KCN, some specimens slowly stain light brown; most specimens negative. 

FeCls stains iridescent. KOH negative. HgCl2 slowly stains brown. Usually 

intergrown with hessite, altaite, or other tellurides. Petzite is whiter than hessite 

and has a smoother surface. Hessite has a rough surface and resembles argentite 

closely. Petzite has a cubic cleavage and exhibits triangular pits like galena, but 

the cleavage is less perfect than that of galena. Petzite is a much rarer mineral 

than hessite; most so-called petzites examined by the writer proved to be intergrowths of hessite with calaverite or other minerals.

ANISOTROPIC 

KOH, HgCla

Canfieldite (4Ag2S.SnS2 with some germanium). See description above. 

Proustite (3Ag2 S.As2S3). Hexagonal; color in reflected light bluish gray; hardness B . Anisotropic; polarization colors light to dark gray, largely masked

' Schneiderhohn, Hans, and Ramdohr, Paul, op. cit., Band 2, p. 30.
134 MICROSCOPIC DETERMINATION OF ORE MINERALS

by strong red internal reflection. HNOs fumes tarnish some specimens; most 

specimens negative. HC1 negative. KCN stains black and brings out scratches. 

FeCl3 , some specimens slowly stain gray to black; most specimens negative. 

KOH stains gray to black. HgCl2 stains brown. Color of powder ruby red. 

Color in hand specimen ruby red, in contrast to pyrargyrite, which is maroon 

to black.

Pyrargyrite (3Ag2S.Sb2S3). See description above.

Stephanite (SAgaS.SbjSs). Orthorhombic; color galena white; hardness B. 

Strongly anisotropic; polarization colors light gray, steel gray, bornite pink. 

HNOs practically negative; after long etching fumes tarnish faintly and mineral 

seems to stain very light brown; reaction doubtful. HC1 fumes tarnish; halo does 

not wash off; action slow and on some specimens practically negative. KCN 

quickly stains brown to black and brings out scratches. FeCla practically negative; seems to stain light gray in places; solution leaves sediment which rubs off 

easily. KOH, some specimens stain iridescent to black, others negative. HgCl2 

stains brown to iridescent. Metallic powder. Best distinguished from polybasite by powder and lack of internal reflection.

KOH neg.. HgCI 2

Aguilarile (Ag2 (S,Se)). Listed as isometric; color gray; hardness A. Strong 

anisotropism; polarization colors gray, violet, green, brown; brings out multiple 

twinning with curved or crinkly lamellae. HNOs stains differentially brown. 

HC1 negative. KCN, some specimens stain differentially light brown, others 

negative. FeCls quickly stains iridescent. KOH negative. HgCl2 stains differentially brown to iridescent. Shows different degrees of hardness in different 

crystallographic directions, giving surface a shreddy appearance. The strong 

anisotropism raises the question whether this mineral is isometric. Resembles 

argentite but yields powder when scratched. Reported only from Guanajuato, 

Mexico.

Antimony (Sb). Hexagonal; color silvery white; hardness B. Specimens from 

Borneo show some areas apparently isotropic, others weakly anisotropic. Specimens from Tulare County, Calif., and from Allemont, France, are weakly anisotropic. Some specimens exhibit multiple twinning. HN03 on specimen from 

Borneo quickly stains black and brings out lamellar structure; specimen from 

Tulare County slowly stains brown; specimen from Allemont stains iridescent to 

black; action slower than with arsenic. HC1, on some specimens fumes tarnish 

and surface stains after long etching; most specimens negative. KCN, on specimen from Borneo some areas stain brown; most areas negative; specimens from 

Tulare County and from Allemont negative. FeCl3 , specimen from Borneo 

stains brown; specimen from Tulare County stains black; specimen from Allemont 

slowly stains differentially iridescent after about 1 minute. KOH negative. HgCl2 , 

specimen from Borneo stains brown; specimens from Tulare County and from 

Allemont negative. Specimen from Tulare County is an arsenic-antimony alloy. 

Specimen from Allemont is a fine-grained intergrowth between native antimony 

and native arsenic, the intergrowth being known as "allemontite." 17 Specimen 

from Borneo shows triangular pits.

0 dyscrasite (Ag3Sb). Orthorhombic; color silvery white; hardness B to C. 

Some specimens appear isotropic, others weakly anisotropic; some exhibit grating 

structure. Specimen here described came from Broken Hill, Australia, and consists of an intergrowth of two minerals, both consisting of antimony and silver.

i 7 Schneiderhohn, Fans, and Ramdohr, Paul, op. cit., Band 2, p. 30.
DETERMINATIVE TABLE 135

They are designated here a and /? dyscrasite. HN03 , both varieties etch without effervescence and leave white coating. HC1 stains a dyscrasite light brown; 

j3 dyscrasite negative. KCN, both varieties stain iridescent to black, but a 

dyscrasite v stains more quickly. FeCl3 , both varieties quickly stain iridescent. 

KOH, both varieties negative. HgClz, both varieties stain brown. The two 

minerals of the intergrowth are closely similar in appearance, but <x dyscrasite is 

somewhat more silvery in color than 0 dyscrasite. See also description on page 125.

Naumannite ((Ag2,Pb) Se). Isometric; color gray; hardness B. Weak anomalous anisotropism. HN03 stains iridescent and brings out structure. HC1 

negative. KCN very slowly stains brown and brings out cleavage. FeCl3 instantly stains iridescent. KOH negative. HgCl2 stains brown to iridescent and 

brings out structure. Usually shows traces of cubic cleavage. Isomorphous with 

and usually indistinguishable from aguilarite.

Pearceite (8Ag2S.As2 S3) . Monoclinic; color white; hardness C. Strongly 

anisotropic; polarization colors bluish green, purple, dark brown. HNOa fumes 

tarnish. HC1 negative. KCN quickly stains dark gray. FeCl3 stains iridescent. 

KOH negative. HgCl2 stains iridescent. Powder black. Most specimens give 

good microchemical test for copper.

Polybasite (8Ag2S.Sb2Ss). Monoclinic; color gray; hardness C . Strongly 

anisotropic; interference colors brown, yellowish green, grayish violet. HN03 

fumes tarnish some specimens, others negative. HC1 fumes tarnish; halo does not 

wash off; action slow and on some specimens negative. KCN stains brown to 

black. FeCl3 , some specimens tarnish iridescent, others negative. KOH, some 

specimens slowly tarnish iridescent; most specimens negative. HgCl2 stains 

brown to black. Variability in etch behavior probably due to variable chemical 

composition. More or less copper is always present. Powder dark ruby red; 

if particles are coarse, powder appears black. Internal reflection red; not always 

seen. Stephanite and pearceite always give black powder and show no internal 

reflection. All specimens examined by writer gave good microchemical test for 

copper.

Eucairite (Cu2Se.Ag2 Se). Isometric; color galena white to buff; hardness B . 

Weakly anisotropic; polarization colors greenish yellow, bluish gray. HN03 

fumes tarnish strongly; surface slowly stains dark gray. HC1 negative. KCN 

instantly stains black. FeCl3 instantly tarnishes differentially iridescent. KOH 

negative. HgC^ stains pinkish violet. Associated with clausthalite and klockmannite.

HNO3 , HC1 neg., KCN, FeCl3 neg.

ISOTKOPIC 

KOH, HgCIs

Canfieldite (4Ag2S.(Sn,Ge)S2 ). Isometric; color gray with pinkish tinge. 

Most areas isotropic, but some show anomalous anisotropism. HN03 slowly 

stains brown; usually requires more than 1 minute to work. HC1 negative. 

KCN stains dark gray and brings out structure. FeCl3 deposits sediment which 

tends to cling to surface, but reaction should be termed negative. KOH tarnishes 

through iridescent to dark gray; reaction usually requires more than 1 minute. 

tarnishes iridescent; some areas negative.

Pyrargyrite (3Ag2S.Sb2S3 ). Hexagonal; color bluish gray; hardness C. Moderately strong anisotropism, but some specimens appear isotropic owing to transparency; polarization colors light gray, steel gray, brown; colors somewhat masked 

by internal reflection. HNOs; some specimens slowly stain gray to brown; on
136 MICROSCOPIC DETERMINATION" OF ORE MINERALS

others fumes tarnish, but surface does not stain; most specimens negative. HC1 

fumes tarnish; halo does not wash off; action slow in starting and on some specimens negative. KCN quickly stains brown to black and brings out scratches. 

FeCl3 negative; solution deposits sediment which does not wash off easily, but 

surface does not stain. KOH stains gray to iridescent. HgCl2 slowly stains 

brown. Ruby-red powder and internal reflection, lighter in color and more 

easily seen than in polybasite. In hand specimen color varies from dark red to 

black, and mineral is distinguished from miargyrite with difficulty. Miargyrite 

has a darker-red powder and internal reflection than pyrargyrite. Proustite is 

a lighter red in hand specimen than pyrargyrite.

KOH neg., HgCb

Argyrodite (4Ag2S.GeS2 ). Tetragonal, pseudo-isometric; color gray, similar to 

enargite; hardness C. Some specimens isotropic; others weakly anisotropic. 

HN03 fumes tarnish some areas faintly; usually negative. HC1 negative. KCN 

stains light brown to black and brings out scratches. FeCl3 and KOH negative. 

HgCl2 quickly stains iridescent.

Goldfieldile (6CuS.Sb2(S,Te)3). Isometric; color grayish white with a tinge of 

brown, almost identical with that of tetrahedrite; hardness B. HN03 fumes 

tarnish; drop turns brown. HC1 almost negative; some areas stain faint brown. 

KCN stains brown and etches rapidly, developing scratches and crystal structures. 

FeCl3 , KOH, and HgCl2 negative. No cleavage observed; mineral is brittle. 

Reported only from Goldfield, Nev. Above description by Tolman and Ambrose. 18

KOH neg., HgCIz neg.

Tennantile (5Cu2S.2(Cu,Fe)S.2As2 S3). Isometric; color gray; hardness D. 

HNO3 fumes tarnish. HC1 negative. KCN, some specimens stain slowly light 

brown, bringing out scratches; most specimens negative. FeCls, KOH, and 

HgCl2 negative. Color distinctly greenish where in contact with galena. Powder 

metallic in some specimens, reddish brown in others.

Tetrahedrite (5Cu2S.2(Cu,Fe)S.2Sb2S3 ). Isometric; color gray; hardness D. 

HNOs fumes tarnish; some specimens stain iridescent. HC1 negative. KCN, 

some specimens stain slowly light brown, bringing out scratches; most specimens 

negative. FeCl3 , KOH, and HgCl2 negative. Powder metallic in some specimens, 

reddish brown in others.

ANISOTHOFIC 

KOH, HgCla

Canfieldite (4Ag2S(Sn,Ge)S2). Isometric; color gray with pinkish tinge; hardness B. Most areas isotropic, but some show anomalous anisotropism. HNO? 

slowly stains brown; usually requires more than 1 minute to work; weak. HC1 

negative. KCN stains dark and brings out structure. FeCl3 deposits coating of 

ferric chloride which tends to cling to surface, but reaction should be termed negative. KOH tarnishes through iridescent to dark gray; reaction usually requires 

more than 1 minute. HgCl2 instantly tarnishes iridescent; some areas negative.

Klockmannite (CuSe). Hexagonal; color greenish gray; hardness C. Strong 

anisotropism; gives four extinctions per revolution; interference color fiery like 

covellite but orange rather than red. HN03 fumes tarnish. HC1 negative. 

KCN quickly stains black. FeCl3 negative. KOH slowly stains differentially 

brown; some areas negative. HgCl2 stains differentially blue. Mottled in

18 Tolman, C. F., and Ambrose, J. W., The rich ores of Goldfleld, Nev.: Econ. Geology, vol. 29, p. 274, 

1934.
DETERMINATIVE TABLE 137

appearance. Close resemblance to covellite indicates that the formula given 

above is correct. 19 Associated with clausthalite and other selenides.

Polybasite (8Ag2S.Sb2S3 ). Monoclinic; color gray; hardness C . Strongly 

anisotropic; interference colors yellowish green, brown, grayish violet. HN03 

fumes tarnish; some specimens negative. HC1 fumes tarnish; halo does not wash 

off; action slow and on some specimens negative. KCN stains brown to black 

and brings out scratches. FeCl3 usually tarnishes iridescent; some specimens 

negative. KOH, some specimens slowly tarnish iridescent; most specimens negative. HgCl2 stains brown to black. Variability in etch behavior probably due 

to variable chemical composition. More or less copper always present. Powder 

dark ruby red; if particles are coarse, powder appears black. Internal reflection 

red; not always seen. Stephanite and pearceite always give black powder and 

show no internal reflection. All specimens examined by writer gave good microchemical test for copper.

Proustite (3Ag2S.As2S3 ). Hexagonal; color bluish gray; hardness B. Anisotropic; 

polarization colors light to dark gray, largely masked by strong red internal 

reflection. HN03 fumes tarnish some specimens; others negative. HC1 negative. KCN stains black and brings out scratches. FeCl3 , some specimens slowly 

stain gray to black; most specimens negative. KOH stains gray to black. 

HgCl2 stains brown. Color of powder ruby red. Color in hand specimen ruby 

red, in contrast to pyrargyrite, which is maroon to black.

Pyrargyrite (3Ag2S.Sb2S3 ). Hexagonal; color bluish gray; hardness C. Moderately strong anisotropism, but some specimens appear isotropic owing to 

transparency; polarization colors light gray, steel gray, brown; colors somewhat 

masked by internal reflection. HNOs, some specimens slowly stain gray to brown ; 

on others fumes tarnish but surface does not stain; most specimens negative. 

HC1 fumes tarnish; halo does not wash off; action slow in starting and on some 

specimens negative. KCN quickly stains brown to black and brings out scratches. 

FeCls, negative; solution deposits sediment which does not wash off easily, but surface does not stain. KOH stains gray to iridescent. HgCl2 slowly stains brown. 

Ruby-red powder and internal reflection, lighter in color and more easily seen 

than in polybasite. In hand specimen color varies from dark red to black, and 

the mineral is distinguished from miargyrite with difficulty. Miargyrite lias a 

darker-red powder and internal reflection than pyrargyrite. Proustite is a lighter 

red in hand specimen than pyrargyrite.

KOH. HgCto ncg.

Kermesile (2Sb2S3 .Sb2O;i). Monoclinic; color gray; hardness U. Strong anisotropism; polarization colors greenish yellow, purple. HNOs tarnishes brown. 

HC1 tarnishes light brown; some areas negative. KCN stains slightly light 

brown; doubtful. FeCls negative. KOH tarnishes and gives yellow coating; 

this is the most characteristic^ reaction for the mineral; reaction is identical with 

that for stibnite. HgCl2 negative. Prismatic. Red powder and red internal 

reflection (best seen with strong polarized light).

Livingslonite (HgS.2Sb2S3). Orthorhombic; color lead gray; hardness C. 

Strong anisotropism; polarization colors white, steel blue, black. HN03 slowly 

stains brown to iridescent; some specimens negative. HCl negative. KCN 

stains brown and brings out scratches; some specimens negative. FeCl 3 negative. 

KOH instantly darkens, leaving yellow deposit; action highly characteristic,

" See also Ramdohr, Paul, Klochmannite, em neues naturaliches Kupferselenid: Centralbl. Mineralogie, 

1928, Abt. A, no. 7, pp. 225-232.

12915.1 40 10
138 MICROSCOPIC DETERMINATION OF ORE MINERALS

resembling that of stibnite. HgCl2 negative. Aqua regia effervesces and stains 

iridescent. Powder is red when finely divided. Bright-red internal reflection.

Miargyrite (Ag2S.Sb2S3). Monoclinic; color light gray, almost white; hardness 

B. Strong anisotropism; polarization colors white, bluish gray, light brown, 

dark brown. HN03 usually negative; sometimes stains slightly. HC1 fumes 

usually tarnish; halo does not wash off; action slow and sometimes negative. 

KCN slowly stains differentially dark gray; on some specimens action requires 

more than 1 minute. FeCl3 doubtful; solution deposits sediment which does not 

wash off easily, but surface does not stain. KOH quickly tarnishes differentially 

iridescent. HgCl2, some specimens stain slightly light brown; most specimens 

negative. Ruby-red powder and internal reflection. In contact with miargyrite, 

pyrargyrite is distinctly bluish. Miargyrite is more strongly anisotropic than 

pyrargyrite, with which it is almost invariably associated.

Slibnile (Sb2S3). Orthorhombic; color gray; hardness B. Very strong anisotropism; polarization colors white, pinkish gray, pink, blue, brown, dark gray; 

many varieties show multiple twinning. HNOs effervesces slowly and stains 

differentially iridescent; effervescence not always observed. HC1, some varieties 

stain differentially light brown after long etching; most varieties negative. KCN 

stains differentially light brown. FeCl3 negative. KOH instantly tarnishes 

brown and gives yellow coating; this is the most characteristic reaction for stibnite. HgCl2 negative. Prismatic. One perfect cleavage. Hardness and color 

vary somewhat Avith orientation, giving mottled appearance that suggests the 

presence of more than one mineral.

KOH neg., HgCh

Argyrodile (4Ag2S.GeS2 ). Tetragonal, pseudo-isometric; color gray, like 

enargite; hardness C. Some specimens isotropic; others weakly anisotropic. 

HN03 fumes sometimes tarnish faintly; usually negative. HC1 negative. KCN 

stains light brown to black and brings out scratches. FeCl3 and KOH negative. 

HgCl2 quickly tarnishes iridescent, the most characteristic reaction.

KOH neg., HgCh neg.

Covellite (CuS). Hexagonal; color indigo blue, purple when wet; hardness B-)-. 

Strong anisotropism; polarization colors white, blue, fiery orange to red. HNOs 

fumes tarnish some specimens, others negative. HC1 negative. KCN stains 

black. FeCl3 , KOH, and HgCl2 negative. Perfect basal cleavage. Usually of 

supergene origin, replacing chalcocite, bornite, and other copper sulphides, also 

frequently sphalerite or galena. In such associations covellite occurs as minute 

scales without crystal outlines. At Butte much covellite is hypogene and occurs 

as large hexagonal plates, in places several centimeters in diameter.

Chalcostibite (Cu2 S.Sb2S3 ). Orthorhombic; color galena white; hardness C. 

Anisotropic; polarization colors pinkish to greenish gray. HNOs fumes tarnish; 

mineral stains iridescent after long etching; some areas negative. HC1 negative. 

KCN slowly stains brown and brings out scratches; some specimens almost negative. FeCls, KOH, and HgCl2 negative. Lighter in color than tetrahedrite. 

Has well-marked cleavage.

Enargite (Cu2S.4CuS.As2S3). Orthorhombic; color, two varieties known, one-, 

gray, the other pinkish; the pinkish variety is known as "luzonite"; both varieties 

may be seen in the same specimen; hardness D . Strong anisotropism; polarization colors purple, gray, greenish yellow. HNO3 fumes tarnish faintly; sometimes negative. HC1 negative. KCN stains black and sometimes brings out a 

parallel structure. FeCl3 and KOH negative. HgCl2 sometimes stains slightly;
DETERMINATIVE TABLE 139

doubtful. In many places intergrown with tennantite. Prismatic. Perfect 

cleavage in two directions.

Famatinite (Cu2S.4CuS.Sb2S3 ). Orthorhombic; color pinkish gray, similar to 

pink enargite; hardness D. Strongly anisotropic; polarization colors greenish 

yellow, purplish red. HN03 stains differentially iridescent; fumes tarnish. HC1 

negative. KCN stains differentially dark; action slower than on enargite. 

FeCl3 , KOH, and HgCl2 negative. Prismatic cleavage. Isomorphous with 

enargite.

Klaprothite, also known as klaprotholite (3Cu2 S.2Bi2S3). Probably isometri,c; 

color gray, like tetrahedrite; in oblique light it has a pinkish-brown tinge; hardness C. Very weak anisotropism, possibly anomalous. HN03 slowly stains 

brown; fumes tarnish; some areas negative. HC1 negative. KGN stains differentially iridescent. FeCl3 , KOH, and HgCl2 negative. Spe.cimen from Butte, 

Mont., determined by F. B. Laney as klaprothite. It is intergrown with coarsegrained covellite in mutual-boundary and subgraphic structures.

HNO3 , KC1 neg., KCN neg., FeCl3

ISOTROPIC 

KOH neg.. HgCI 2

Antimony (Sb). Hexagonal; color silvery white; hardness B. Specimens from 

Borneo show some areas apparently isotropic, others weakly anisotropic. Specimens from Tulare County, Calif., and from Allemont, France, are weakly anisotropic. Some specimens exhibit multiple twinning. HN03 on specimen from 

Borneo quickly stains black and brings out lamellar structure; specimen from 

Tulare County slowly stains brown; specimen from Allemont stains iridescent 

to black; action slower than with arsenic. HC1, on some specimens fumes tarnish 

and surface stains after long etching; most specimens negative. KCN, on specimen from Borneo some areas stain brown; most areas negative. Specimens from 

Tulare County and from Allemont negative. FeCl 3 , specimen from Borneo 

stains brown; specimen from Tulare County stains black; specimen from Allemont slowly stains differentially iridescent after about 1 minute. KOH negative. HgCl 2 , specimen from Borneo stains brown; specimens from Tulare County 

and from Allemont negative. Specimen from Tulare County is an arsenicantimony alloy. Specimen from Allemont is a fine-grained intergrowth between 

native antimony and native arsenic; intergrowth is known as "allemontite." 20 

Specimen from Borneo shows triangular pits.

Dyscrasite (Ag:tSb). Orthorhombic; color silvery white; hardness B to C. 

Some specimens are isotropic, others weakly anisotropic. HN03 stains differentially iridescent; with some specimens brings out grating structure. HC1 

negative. KCN slowly stains brown; some areas negative. FeCl3 tarnishes 

iridescent. KOH negative. HgCl2 stains brown. Almost sectile but yields 

powder when scratched. The above description relates to specimen from Andreasberg, Germany. Etch tests are identical with those on /3 dyscrasite from 

Broken Hill, Australia. (See p. 125.) Compare also with description of specimen 

from Cobalt, Ontario (p. 118).

Freibergite (5(Cu, Ag) 2S.2(Cu, Fe)S.2Sb2S3 ). Isometric; color very light gray; 

hardness D. HN03 slowly stains brown. HC1 and KCN negative. FeCl 3 stains 

iridescent. KOH negative. HgCl 2 leaves spotty residue; almost negative. 

Specimens examined came from Randsburg, Calif., and Mayo district, Yukon. 

Both are relatively high in silver.

2 ° Schneiderhohn, Hans, and Ramdohr, Paul, Lehrbuch der Erzmikroskopie, Band 2, p. 30, 1931.
140 MICROSCOPIC -DETERMINATION OF ORE MINERALS

Petzite ((Ag,Au) 2Te). Isometric; color galena white; hardness A. HNO3 

quickly stains iridescent. HC1, some specimens stain iridescent, others negative. 

KCN negative. FeCl3 stains iridescent. KOH negative. HgCl2 slowly stains 

brown. Usually intergrown with hessite. Petzite is whiter than hessite and 

has a smoother surface. Hessite has a rough surface and resembles argentite 

closely. Petzite has a cubic cleavage and exhibits triangular pits like galena, but 

the cleavage is less perfect than that of galena. Petzite is a much rarer mineral 

than hessite; most so-called petzites examined by the writer proved to be intergrowths of hessite with calaverite or other minerals.

KOH neg., HgCh neg.

Clausthalile (PbSe). Isometric; color galena' white; hardness A . HN03 

tarnishes and leaves a coating which is brick red in oblique light; this distinguishes 

the mineral from galena. HCl slowly tarnishes brown; if acid conies into contact with calcite, reaction may be negative. KCN negative. FeCU tarnishes 

iridescent; if reagent comes into contact with calcite, etching may require more 

than 1 minute. KOH and HgCl2 negative. Has cubic cleavage less perfect 

than that of galena. Usually shows triangular pits here and there. A cobaltiferous variety is known as "tilkerodite" and can be distinguished from ordinary 

clausthalite only by microchemical means.

Coloradoite (HgTe). Isometric; color pinkish gray; hardness C. HN03 

slowly stains brown; some areas almost negative. HCl and KCN negative. 

FeCl3 stains differentially iridescent. KOH and HgCl2 negative.

Kallilite (Ni(Sb,Bi)S). Isometric; color gray; hardness D. HN03 tarnishes 

iridescent. HCl and KCN negative. FeCl3 stains faintly, almost negative. 

KOH and HgCl2 negative. From Schoenstein, Germany. Very rare. Two 

minerals occur in the specimen. One occupies greater proportion of total area, 

is soft and readily scratched. It gives etch tests as above and microchemical 

tests for Ni, Sb, and Bi. Sb greatly predominates over Bi. This is undoubtedly 

kallilite. The second mineral is unknown. It is harder than kallilite and 

cannot be scratched with a needle. It gives the same etch tests as kallilite, but 

the reactions are slower than those of kallilite. It gives microchemical tests for 

Ni, Co, and Sb. According to Thomson 21 kallilite is a mixture. The data given 

above indicate that kallilite is an authentic mineral.

Ullmanite (NiSbS). Isometric; color gray; hardness E. HN03 stains brown to 

iridescent. HCl and KCN negative. FeCls stains light brown. KOH and 

HgCJ 2 negative. Has well-defined cubic cleavage and shows triangular pits.

Willyamile (Co,Ni)SbS). Isometric; color gray; hardness E. HN03 tarnishes 

iridescent; seems to effervesce slightly. HCl and KCN negative. FeCl3 stains 

slightly; almost negative. KOH arid HgCl2 negative. From Broken Hill, 

Australia. Rare.

ANISOTROPIC 

KOH, HgCh neg.

Aramayoite (Ag2S.(Sb,Bi) 2S3). Orthorhombic (?); color gray; hardness C. 

Strong anisotropism; polarization colors light pink, steel blue. HNO3 stains 

iridescent; action slow in starting. HCl and KCN negative. FeCl3 stains faint 

yellow; reaction may be overlooked. KOH slowly stains iridescent. HgClj 

negative. Prismatic. Specimen from Chocaya, Bolivia, described by Spencer.22

« Thomson, Ell Is, A mineralographic study of the pyrite group: Toronto Univ. Studies, Oeol. ser., no. 

12, p. 36, 1921. 

" Spencer, L. J., Mineralog. Mag., vol. 21, pp. 152-162,1927.
DETERMINATIVE TABLE 141

Cylindrite (6PbS.Sb2S3.6SnS2 ). Tetragonal; color gray; hardness C . 

Strong anisotropism; polarization colors light to dark gray. HN03 fumes tarnish; 

surface slowly stains iridescent. HCl fumes tarnish; stains slightly after etching 

2 minutes; some areas negative. KCN negative. FeCl 3 seems to stain slightly; 

doubtful. KOH slowly stains brown to iridescent. HgCl2 negative. Prismatic. 

Basal sections are more or less cylindrical and are made up of concentric shells. 

Prismatic sections are needlelike.

KOH neg., HgCh

Aguilarite (Ag2 (S,Se)). Listed as isometric; color gray; hardness A. Strong 

anisotropism; polarization colors gray, violet, green, brown; brings out multiple 

twinning with curved or crinkly lamellae. HN03 quickly stains through iridescent 

to black. HCl negative. KCN, some specimens tarnish differentially light 

brown, others negative. FeCl 3 quickly stains iridescent. KOH negative. 

HgCl 2 quickly stains iridescent. Shows different degrees of hardness in different 

crystallographic directions, giving surface a shreddy appearance. The strong 

anisotropism raises the question whether this mineral is isometric. Resembles 

argentite but yields powder when scratched. Reported only from Guanajuato, 

Mexico.

Antimony (Sb). Hexagonal; color silvery white; hardness B. Specimens from 

Borneo show some areas apparently isotropic, others weakly anisotropic. Specimens from Tulare County, Calif., and from Allemont, France, are weakly anisotropic. Some specimens exhibit multiple twinning. HN03 on specimen from 

Borneo quickly stains black and brings out lamellar structure; specimen from 

Tulare County slowly stains brown; specimen from Allemont stains iridescent to 

black; action slower than with arsenic. HCl, on some specimens fumes tarnish 

and surface stains after long etching; most specimens negative. KCN, on specimen from Borneo some areas stain brown, most areas negative; specimens from 

Tulare County and Allemont negative. FeCl3 , specimen from Borneo stains 

brown; specimen from Tulare County stains black; specimen from Allemont 

slowly stains differentially iridescent after about 1 minute. KOH negative. 

HgCl2, specimen from Borneo stains brown; specimens from Tulare County and 

Allemont negative. Specimen from Tulare County is an arsenic-antimony 

alloy. Specimen from Allemont is a fine-grained intergrowth between native 

antimony and native arsenic; the intergrowth is known as "allemontite." 23

Specimen from Borneo shows triangular pits.

Breithauptite (NiSb). Hexagonal; color coppery pink; hardness E. Strong 

anisotropism; polarization colors light gray, bluish green, dark reddish brown. 

HNOs stains differentially iridescent. HCl and KCN negative. FeCl3 stains 

differentially iridescent. KOH negative. HgCl2 stains slightly, practically 

negative. Usually intergrown with niccolite and other arsenides of nickel and 

cobalt. In contact with niccolite, breithauptite is distinctly redder.

Dyscrnsite (Ag3Sb). Orthorhombic; color silvery white; hardness B to C. 

Some specimens are isotropic, others weakly anisotropic. HN03 stains differentially iridescent; with some specimens brings out grating structure. HCl 

negative. KCN slowly stains brown; some areas negative. FeCl3 tarnishes 

iridescent. KOH negative. HgCl2 stains brown. Almost sectile but yields 

powder when scratched. The above description relates to specimen from Andreasberg, Germany. Etch tests are identical with those on /? dyscrasite from Broken 

Hill, Australia. (See p. 125.) Compare also with description of specimen from 

Cobalt, Ontario (p. 118).

"Schneiderhohn, Hnns, and Ramdohr, Paul, Lehrbnch der Erzmikroskopip, Band 2, p. HO, lO.'tl.
142 MICROSCOPIC DETERMINATION OF ORE MINERALS

Plessite (Ag2Te). Isometric; color gray; hardness A-f. Most specimens of 

hessite exhibit anomalous anisotropism; polarization colors white, steel blue, 

bornite pink; some areas show multiple twinning. HNO3 stains black. HCl 

slowly stains black; negative on some areas. KCN, some specimens slowly stain 

black; action usually requires more than 1 minute and on some specimens negative. FeCl3 stains iridescent. KOH negative. HgCl2 stains brown to iridescent. 

Resembles argentite but yields powder when scratched.

KOH neg., HgCh

WMichenite (3Cu2 S.Bi2S3 ). Color gray; hardness C. HNO3 quickly tarnishes 

iridescent. HCl and KCN negative. FeCl3 stains light brown. KOH negative. 

HgCl2 stains slightly light brown. Specimen from Colquijirca, Peru; collected 

by Dr. Waldcmar Lindgren and analyzed at the Massachusetts Institute of 

Technology.

KOH neg., HgCh neg.

Aikinile (Cii 2S.2PbS.Bi 2 S3 ). Orthorhombic; color white; hardness C. Strong 

anisotropism; polarization colors light gray, violet, brown, dark gray. HNO:i , 

most areas effervesce and stain iridescent; some areas stain without effervescence. 

HCl and KCN negative. FeCl3 stains slightly very light brown; action weak and 

might be considered negative. KOH and HgCl2 negative.

Alaskaite ((Pb,Ag2 )S.Bi2S3). Orthorhombic; color gray; hardness C. Strong 

anisotropism. HN03 slowly stains dark. HCl fumes tarnish a little; doubtful. 

KCN negative. FeCl3 stains iridescent. KOH and HgCl2 negative. Argentiferous galenobismutite. Prismatic. Rare. Analysis of a specimen from Silverton, Colo., by J. G. Fairchild shows Pb 15.4; Ag 11.7, Cu 1.6, Bi 55.3, S 16.0.

Freieslebenite (2Ag2S.3PbS.2Sb2S3). Monoclinic; color galena white; hardness 

C. Strongly anistropic; polarization colors light gray, brownish gray, dark gray; 

some areas show multiple twinning. HNO3 practically negative; some areas seem 

to stain slightly after long etching. HCl and KCN negative. FeCl3 practically 

negative; seems to stain slightly. KOH and HgCl2 negative. Hand specimen 

shows striated crystals. Very rare. Specimen from Hiendelencia, Spain.

Galenobismutite (PbS.Bi2S3 ). Color galena white; hardness B. Anisotropic; 

polarization colors white to dark gray. HN03 effervesces and stains differentially ' 

iridescent; effervescence not always observed. HCl and KCN negative. FeCl 3 

leaves slight yellow stain; some areas not affected. KOH and HgCU negative. 

Prismatic cleavage. Specimen from Fahlun, Sweden. Gives microchemical tests 

for Pb, Bi, S, and Se.

Sylvanite (AuAgTe^. Monoclinic; color creamy white; hardness C. Aniso- V 

tropic; polarization colors light gray, brownish gray, dark gray. HNO3 effervesces 

and stains iridescent; effervescence not always observed. HCl and KCN negative. 

FeCl3 sometimes leaves light-yellow stain; usually negative. KOH and HgCl 2 ,. 

negative. One perfect cleavage. Distinctions between sylvanite, krennerite, 

and calaverite described by Short.24

HNO,, HCl neg., KCN neg., FeCl3 neg.

ISOTROFIC 

KOH neg., HgCh

Carrollite ((Co,Cu) 3 S4 ). Isometric; color creamy white, similar to linnaeite L 

but somewhat lighter; specimens from Rhodesia have a color similar to pyrrhotite 

but lighter; hardness E. HN03 fumes tarnish iridescent. HCl, KCN, FeCl3 ,

24 Short, M. N., Etch tests on calaverite, krennerite, and sylvanite: Am. Mineralogist, vol. 22, pp. 667- 

674,1937.
DETERMINATIVE TABLE 143

and KOH negative. HgCl2 stains brown to iridescent; action somewhat slow; 

specimen from Rhodesia negative. Microchemical tests on carefully selected 

areas indicate that carrollite is a distinct mineral and not a mixture of linnaeite 

and copper minerals, as has been averred. 25 From Carroll County, Md., and the 

N'Kana mine, Rhodesia.

Linnaeite (Co3 S4 ). Isometric; color creamy white; hardness E + . HN03 fumes 

tarnish. HCl, KCN, FeCl3 , and KOH negative. HgCl2 stains brown differentially; some areas almost negative.

Polydymite (Ni3 S4 ). Color light brass yellow; hardness F. Isotropic. HN03 

effervesces slightly and slowly tarnishes brown to iridescent; fumes tarnish. 

HCl, drop slowly turns yellow; surface unaffected. KCN, FeCl3 , KOH, and 

HgCl2 negative. Color about the same as that of pentlandite. The only genuine 

polydymite seen by the writer comes from Grunau, Saxony; the other specimens 

of so-called polydymite from Sudbury and elsewhere have proved to be violarite.

Siegenite ((Ni,Co) 3 S4 ). Isometric; color creamy white; hardness E + . HN03 

fumes tarnish strongly; surface slowly stains differentially brown. HCl, KCN, 

FeCl3 , and KOH negative. HgCl2 , some specimens stain slightly, others negative. 

Readily scratched by needle. Specimen from Carroll County, Md., shows good 

cubic cleavage. Specimens from Mine la Motte, Mo., and Heinrichsiegen, Germany, show no cleavage. Siegenite is more abundant than linnaeite. Indistinguishable from linnaeite except by microchemical tests for Ni and Co. The color 

is best seen by viewing the polished section with the unaided eye in oblique light, 

tilting it so as to get the reflection from a light source, preferably daylight from 

a north window.

KOH neg., HgCh neg.

Chalcopyrite (CuFeS2 ). Tetragonal; color brass yellow; hardness C. Most 

specimens are weakly but discernibly anisotropic, but a small proportion are 

apparently isotropic. HN03 fumes tarnish; some varieties negative. HCl, KCN, 

FeCl3 , KOH, and HgCl2 negative. Aqua icgia fumes tarnish differentially and 

bring out grain structure; area covered by drop usually stains slowly brown, but 

some areas almost unaffected. Easily scratched by needle, which distinguishes 

it from pyrite. Gives powder when scratched, in contrast to gold, which is sectile.

Freibergite (5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 ). Isometric; color gray; hardness D. 

HN03 stains iridescent; on some varieties fumes tarnish, but mineral covered by 

drop not affected. HCl, KCN, FeCI 3 , KOH, and HgCl2 negative. Silver-bearing 

tetrahedrite.

Pentlandite ((Fe,Ni)S). Isometric; color brass yellow; hardness D . HN03 

fumes tarnish; mineral slowly stains brown. HCl, KCN, FeCl3 , KOH, and HgCl2 

negative. Imperfect octahedral cleavage. Nonmagnetic. Almost invariably 

associated with pyrrhotite and chalcopyrite.

Sphalerite (ZnS). Isometric; color gray; hardness C+. HN03 fumes tarnish; 

more rarely slow effervescence can be observed; on areas smaller than drop reaction is usually negative. HCl fumes tarnish some specimens, others negative; 

drop sometimes turns yellow but surface is unaffected. KCN, FeCl3 , KOH, and 

HgCl2 negative. Aqua regia effervesces and stains dark brown; fumes tarnish. 

Some specimens show resin-colored internal reflection. Some show triangular 

pits from dodecahedral cleavage. Powder white to brown. If proportion of iron 

is high powder may be metallic.

» See Shannon, E. V., The identity of carrollite with linnaeite: Am. Jour. Sci., 5th ser., vol. 11, pp. 489- 

493, 3926.
144 MICROSCOPIC DETERMINATION OF ORE MINERALS

Stannite (Cu2S.FeS.81182). Tetragonal, pseudo-isometric; color light brownish 

gray; hardness D +. Most specimens show weak anisotropism; a small proportion 

are apparently isotropic. HN03 stains iridescent to black; fumes tarnish. HCl, 

KCN, FeCl3 , KOH, and HgCl2 negative. Usually associated with chalcopyrite.

Tennantite (5Cu2S.2(Cu,Fe)S.2As2S3). Isometric; color gray; tennantite in 

contact with galena shows distinct greenish tinge; hardness D. HNO3 fumes 

tarnish. HCl negative. KCN usually negative; stains light brown and brings 

out scratches on some specimens. FeCl3 , KOH, and HgCl2 negative. Some 

specimens yield reddish-brown powder, others black metallic powder.

Telrnhedrite (5Cu2S.2(Cu,Fe)S.281^283). Isometric; color gray; hardness D. 

HN03 fumes usually tarnish; some specimens tarnish iridescent, but usually 

requires more than 1 minute for test to work. HCl, KCN, FeCl3 , KOH, and 

HgCl 2 negative. Some specimens yield reddish-brown powder; others yield 

black metallic powder.

Ullmanite and kallilite. Reactions like willyamite. (For descriptions see p. 163.)

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HNOs tarnishes iridescent; seems to effervesce slightly. HCl and KCN negative. FeCl3 

seems to stain slightly; this reaction is easily overlooked. KOH and HgCl2 

negative.

Wurtzite (ZnS). Hexagonal; color dark gray; hardness D . Apparently 

isotropic. HN03 fumes tarnish. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. 

Aqua regia fumes tarnish; surface stains brown. Occurs most commonly as 

coatings on sphalerite and other sulphides. In the hand specimen, as in polished 

section, sphalerite and wurtzite are distinguished with difficulty. The only 

certain way is to examine crushed fragments microscopically between crossed 

nicols, using transmitted light. Sphalerite is isotropic, whereas wurtzite exhibits 

strong birefringence.

ANISOTROPIC

KOH, HgCI2 neg.

Andorite (Ag2S.2PbS.3Sb2S3). Orthorhombic; color gray; hardness C. 

Strongly anisotropic; polarization colors light to dark gray. HN03 fumes 

tarnish; some areas slowly stain brown, others negative. HCl fumes tarnish 

faintly; some specimens negative. KCN and FeCl3 negative. KOH slowly 

stains brown. HgCl2 negative. Aqua regia effervesces vigorously and stains 

black; this is the most distinctive etch test. Almost invariably associated 

with stannite. Rare. Prismatic.

Aramayoite (Ag2S.(Sb,Bi) 2S3 ). Orthorhombic (?); color gray; hardness C. 

Strong anisotropism; polarization colors light pink to steel blue. HNO3 stains 

iridescent; action slow in starting. HCl and KCN negative. FeCl3 stains faint 

yellow; reaction may be overlooked. KOH slowly stains iridescent. HgCl2 

negative. Prismatic. Specimen from Chocaya, Bolivia, described by Spencer.26

Berthierite (FeS.Sb2S3). Orthorhombic; color gray; hardness D . Strong 

anisotropism; some grains give four extinctions per revolution; polarization colors 

light gray, steel blue, dark brown. HN03 tarnishes differentially brown to 

iridescent; on some specimens reaction requires more than 1 minute before it is 

apparent. HCl negative; solution sometimes turns yellow. KCN and FeCl3 

negative. KOH tarnishes iridescent. HgCl2 negative. Aqua regia effervesces 

and stains light brown. Prismatic.

'« Spencer, L. J , Mineralog. Mag., vol. 21, pp. 156-162, 1927.
DETERMINATIVE TABLE 145

Cylindrite (6PbS.Sb2S3 .6SnS2). Tetragonal; color gray; hardness C . Strong 

anisotropism; polarization colors light to dark gray. HNOs fumes tarnish; 

surface slowly stains iridescent. HC1 fumes tarnish; stains slightly after etching 

2 minutes; some areas negative; KCN negative. FeCl3 seems to stain slightly; 

doubtful. KOH slowly stains brown to iridescent. HgCl2 negative. Prismatic. 

Basal sections are more or less cylindrical and are made up of concentric shells. 

Prismatic sections are needlelike.

Franckeite (5PbS.813283.281182). Color gray; hardness B. Strong anisotropism; 

polarization colors light to dark gray. HNO;t stains differentially iridescent; 

fumes tarnish. HCl fumes tarnish after long etching; doubtful. KCN and 

FeCls negative. KOH, some areas stain slightly, others negative. HgClj 

negative. Strong cleavage in one direction.

Kermesite (2Sb2S3 .Sb2O3). Monoclinic; color gray; hardness B. Strong anisotropism; polarization colors greenish yellow, purple. HNOs tarnishes light 

brown; some areas negative. HCl tarnishes light brown; some areas negative. 

KCN stains slightly light brown; doubtful. FeCls negative. KOH tarnishes 

and gives yellow coating; this is the most characteristic reaction for the mineral; 

reaction is identical with that for stibnite. HgCl 2 negative. Prismatic. Red 

powder and red internal reflection (best seen with strong polarized light).

Pyrrhotite (FeSj+). Hexagonal; color brownish or pinkish cream; hardness 

D . Strong anisotropism; polarization colors light gray, bluish gray, brown. 

HNOs slowly stains light brown; fumes tarnish; some specimens, especially those 

poorly polished, negative. HCl, on most specimens solution slowly turns yellow, 

but mineral is unaffected; on a small proportion fumes tarnish and halo does 

not wash off; on specimens from Mary mine, Ducktown, Term., effervesces, 

gives off H2S, and develops etch cleavage. KCN and FeCls negative. KOH, 

surface slowly turns brown, especially near periphery of drop. HgCl2 negative. 

Distinctly magnetic.

Vrbaite (Tl2S.3(As,Sb) 2S3). Orthorhombic; color gray; hardness C. Weak 

anisotropism. HN03 fumes tarnish. HCl, KCN, and FeCl3 negative. KOH 

instantly stains iridescent and brings out parallel etch cleavage. HgCl2 negative. 

Red powder and internal reflection. Only locality reported is Allchar, Macedonia.

KOH neg., HgCh

Millerite (NiS). Hexagonal; color brass yellow, between color of pyrite and 

that of chalcopyrite; hardness E. Strong anisotropism; polarization colors light 

yellow to dark purple. HNOs effervesces slowly and stains brown; fumes tarnish; effervescence not always observed. HCl, KCN, FeCl3 , and KOH negative. 

HgClj, some specimens slowly stain brown, others negative. Usually but not. 

invariably fibrous in habit.

KOH neg., HgCh neg.

Aikinite (CuS.2PbS.Bi2S3). Orthorhombic; color galena white; hardness C. 

Strong anisotropism; polarization colors light gray, violet, brown, dark gray. 

HN03 , most areas effervesce and stain iridescent to black; a few areas seem to 

stain without effervescence. HCl and KCN negative. FeCls stains slightly 

light brown; action weak and may be overlooked. KOH and HgCl2 negative.

Berthonite (2(Pb,Cii2)S.Sb2S3). Color gray, about like tetrahedrite; hardness 

C. HN03 fumes tarnish; mineral slowly stains brown to iridescent. Other 

reagents negative. Massive: no cleavage. From Tunisia. Described by 

Buttgenbach. 27

» Bnttgenbach, B., Soc. g<5ol. Belgique Annales, vol. 46, p. 212,1923.
146 MICROSCOPIC DETERMINATION OF ORE MINERALS

Bismutoplagionite (5PbS.4Bi2S3 ). Orthorhombic; color gray; hardness B + . 

Anisotropic; polarization colors white to dark gray. HNO3 quickly stains iridescent; effervesces slightly. HCl fumes tarnish; some areas stain iridescent; others 

negative. KCN, FeCl3 , KOH, and HgCl 2 negative. Prismatic. Good cleavage 

in two directions. Reported only from Wickes, Mont. Analyzed by E. V'. 

Shannon (U. S. Nat, Mus. Proc., vol. 58, p. 589, 1920).

Bournonite (Cu2S.2PbS.Sb2S3). Orthorhombic; color gray, about like tetrahedrite; hardness C . Anisotropic; polarization colors greenish gray, dark brown, 

purple; some specimens show multiple twinning. HN03 fumes tarnish slowly; 

some specimens tarnish slightly light brown; other specimens negative. HCl, 

KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia effervesces and stains iridescent. Distinguished from tetrahedrite by means of polarized light.

Clialcopyrite (CuFeS2). Tetragonal; color brass yellow; hardness C. Specimens from most localities are weakly anisotropic; those from a few localities, 

notably Insizwa, South Africa, show fairly strong anisotropism; polarization colors greenish yellow, bornite pink. Chalcopyrite associated with pyrrhotite usually shows multiple twinning in polarized light. HNO3 fumes tarnish; some 

varieties negative. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia 

fumes tarnish differentially and bring out grain structure; area covered by drop 

usually stains slowly brown, but some areas almost unaffected. Easily scratched 

by needle, which distinguishes it from pyrite; gives powder when scratched, in 

contrast to gold, which is sectile.

Chalcostibite (Cu2S.Sb2 S3 ). Orthorhombic; color white; hardness C. Anisotropic; polarization colors pinkish to greenish gray. HN03 fumes tarnish; surface 

stains iridescent after long etching; some areas negative. HCl negative. KCN 

slowly stains brown and biings out scratches; some areas almost negative. FeCl3 , 

KOH, and HgCl2 negative. Two good cleavages. Distinguished from tetrahedrite by means of polarized light.

Cubanite (Cu2S.Fe4S5). Orthorhombic; color brownish or pinkish cream; hardness C. Strong anisotropism; polarization colors yellow, grayish blue. HN03 , 

some specimens stain light brown; on some specimens fumes tarnish but surface 

does not stain; others negative. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. 

Invariably intergrown with chalcopyrite and usually associated with pyrrhotite. 

Color almost identical with that of pyrrhotite but a little yellower. Cubanite 

is weakly magnetic, less so than pyrrhotite. Cubanite shows no relief with 

chalcopyrite; pyrrhotite is harder than chalcopyrite and stands in relief above 

it. Also known as "chalmersite."

Emplectite (Cu2S.Bi2S3). Orthorhombic; color creamy gray; hardness C. Anisotropic; polarization colors light gray, purple, light brown. HN03 , most specimens effervesce and stain light brown; some specimens, however, stain without 

effervescence, and on some specimens the only effect is a brown halo made by the 

fumes. HCl negative; with some specimens the drop turns yellow, but the 

mineral does not stain. KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia 

instantly effervesces and stains iridescent. The color is the most characteristic 

property of this mineral.

Guanajuatite (Bi2 (Se,S) 3). Orthorhombic; color gray; hardness B + . Strong 

anisotropism; polarization colors white to dark gray. HN03 tarnishes differentially iridescent. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. 

Rare.
DETERMINATIVE TABLE 147

Guitermanite (3PbS.As2S3). Orthorhombic; color galena white; hardness C. 

Anisotropic; polarization colors reddish brown, steel blue; some areas show multiple twinning. HN03 slowly effervesces and stains iridescent; effervescence not 

always observed. HC1, KCN, FeCl3 , KOH, and HgCl2 negative. Specimen 

from Zuni mine, Silverton, Colo., the type locality. Rare.

Gratonite (Pb9As4 Si5). Hexagonal rhombohedral; color white; hardness B. Anisotropic; polarization colors blue, yellow; weaker anisotropisin than jordanite or 

guitermanite. HN03 effervesces abundantly after a short wait and stains iridescent to black. HC1, KCN, FeCl3 , and KOH negative. HgCl 2 somewhat doubtfully negative. Type material used in original study. From Cerro de Pasco, 

Peru.

Lorandite (TJ2S.As2S3). Monoclinic; color gray; hardness B. HN03 fumes 

give permanent tarnish; mineral stains slowly in places. HCl, KCN, FeCl 3 , 

KOH, and HgCl2 negative. Very rare; only locality reported Allchar, Macedonia. Red powder and internal reflection.

Nagyagite (Pb,Au,Te,S(?), sulphotelluride of Pb and Au). Orthorhombic; 

color bluish white; hardness B . Anisotropic; polarization colors light gray, 

brown, dark gray. HN03 slowly stains iridescent; some areas practically negative. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Habit platy. Plates are 

flexible and frequently curved like those of molybdenite. From Nagyag, Hungary. Formula doubtful.

Stannite (Cu2S.FeS.SnS2). Tetragonal; pseudo-isometric; color light brownish 

gray; hardness D + . Weak anisotropisin; polarization colors yellowish brown, 

light violet. HN03 stains iridescent to black; fumes tarnish. HCl, KCN, FeCl3 , 

KOH, and HgCl2 negative. Usually associated with chalcopyrite.

Sylvanite (AuAgTe.i). Monoclinic; color creamy gray; hardness C. Anisotropic; polarization colors light gray, brownish gray, dark gray. HN03 effervesces 

and stains iridescent; effervescence not always observed. HCl and KCN negative. FeCls, some areas stain light yellow, others negative. KOH and HgCl 2 

negative. Lighter in color than calaverite. One perfect cleavage. Distinctions 

between sylvanite, krennerite, and calaverite described by Short. 28

HN03 neg., HCl, KCN, FeCl3

ISOTROPIC 

KOH neg., HgClj

Polybasite (8Ag2S.Sb2S3). See description below (p. 151). 

Ar gentile (Ag2S). See description below.

ANISOTROPIC 

KOH, HgCh

Stephanite (5Ag2S.Sb2S3 ). Orthorhombic; color gray; hardness B. Strongly 

anisotropic; polarization colors light gray, steel gray, bornite pink. HN03 , after 

long etching fumes tarnish slightly and mineral seems to stain very light brown; 

reaction doubtful. HCl fumes tarnish; halo will not wash off; action slow and on 

some specimens practically negative. KCN stains dark gray to black and brings 

out scratches. FeCl3 practically negative; seems to stain light gray in places; 

solution leaves sediment which rubs off easily. KOH, some specimens stain 

iridescent to black; others negative. HgCl2 stains brown to iridescent. Metallic

38 Short, M. N., Etch tests on calaverite, krennerite, and sylvnnite: Am. Mineralogist, vol. 22, pp. 067- 

674, 1937.
148 MICROSCOPIC DETERMINATION OF ORE MINERALS

powder. Best distinguished from polybasite by powder and lack of internal 

reflection.

KOH, HgCh neg.

Miargyrite (Ag2S.Sb2S3). Monoclinic; color light gray, almost white; hardness 

B. Strong anisotropism; polarization colors white, bluish gray, light brown, dark 

brown. HN03 usually negative; sometimes stains slightly. HC1 fumes usually 

tarnish; halo does not wash off; action slow and sometimes negative. KCN 

slowly stains dark gray; on some specimens action requires more than 1 minute. 

FeCl3 doubtful; solution deposits sediment which does not wash off easily, but 

surface does not stain. KOH quickly tarnishes differentially iridescent. HgCl 2 , 

some specimens stain slightly light brown; most specimens negative. Powder 

and internal reflection ruby red. In contact with miargyrite, pyrargyrite is distinctly bluish. Miargyrite is more strongly anisotropic than pyrargyrite. It 

is almost invariably associated with pyrargyrite. Tested microchemically 

miargyrite shows only a trace of copper, in contrast with polybasite, which gives a 

strong test.

KOH neg., HgCh

Argentite (Ag2S). Isometric; color gray; some specimens aie isotropic, others 

weakly anisotropic. HNO3 , some specimens slowly etch differentially; on others 

fumes tarnish but surface does not stain; some specimens negative. HCl fumes 

tarnish brown to iridescent; halo does not wash off; this is the most characteristic 

etch test for argentite. KCN stains brown to black; some specimens develop 

structure. FeCl3 stains gray to black. KOH negative. HgCl 2 stains brown to 

iridescent. Very sectile. Yields filings instead of powder when scratched. 

Polishing gives a multitude of fine scratches which extend across specimen.

HN03 neg., HCl, KCN, FeCl3 neg.

ISOTROPIC 

KOH, HgCh

Pyrargyrite (3Ag2S.Sb2S3 ). Hexagonal; color bluish gray; hardness C. Weakly 

anisotropic, but some areas appear isotropic owing to transparency; polarization 

colors light to dark gray. HN03 , some specimens slowly stain gray to brown; on 

others fumes tarnish but surface does not stain; most specimens negative. HCl 

fumes tarnish; halo does not wash off; action slow in starting and on some specimens negative. KCN quickly stains brown to black and brings out scratches. 

FeCl3 negative; solution deposits sediment which does not wash off easily, but 

surface does not stain. KOH stains gray to iridescent. HgCl2 slowly stains 

brown. Ruby-red powder and internal reflection; lighter in color and more 

easily seen than in polybasite. In hand specimen color varies from dark red to 

black, and the mineral is distinguished from miargyrite with difficulty. Miargyrite has a darker-red powder and internal reflection than pyrargyrite. Proustite 

is a lighter red in hand specimen than pyrargyrite.

ANISOTROPIC

KOH, HgClj

Polybasite (8Ag2S.Sb2S3). See description below (p. 151).

Pyrargyrite (3Ag2 S.Sb2 S3 ). See description above.

Stephanite (5Ag2S.Sb2S3 ). Orthorhombic; color gray; hardness B. Strongly 

anisotropic; polarization colors light gray, steel gray, bornite pink. HN03 , after 

long etching fumes tarnish slightly and mineral seems to stain very light brown; 

reaction doubtful. HCl fumes tarnish; halo will not wash off; action slow and on 

some specimens practically negative. KCN stains dark gray to black and brings 

out scratches. FeCl3 practically negative; seems to stain light gray in places;
J)ETEBMI5TATIVE TABLE 149

solution leaves sediment which rubs off easily. KOH, some specimens stain 

iridescent to black, others negative. HgCl2 stains brown to iridescent. Metallic 

powder. Best distinguished from polybasite by powder and lack of internal 

reflection.

Miargyrite (Ag2S.Sb2S3). Monoclinic; color light gray, almost white; hardness 

B. Strong anisotropism; polarization colors white, bluish gray, light brown, dark 

brown. HN03 usually negative; sometimes stains slightly. HCl fumes usually 

tarnish; halo does not wash off; action slow and sometimes negative. KCN 

slowly stains dark gray; on some specimens action requires more than 1 minute. 

FeCl3 doubtful; solution deposits sediment which does not wash off easily, but 

surface does not stain. KOH quickly tarnishes differentially iridescent. HgCl2 , 

.some specimens stain slightly light brown; most specimens negative. Powder 

and internal reflection ruby red. In contact with miargyrite, pyrargyrite is distinctly bluish. Miargyrite is more strongly anisotropic than pyrargyrite. It is 

almost invariably associated with pyrargyrite. In microchemical tests shows 

only a trace of copper, in contrast with polybasite, which gives a strong test.

HN03 neg., HCl, KCN neg., FeCI3 neg.

ISOTBOPIC 

KOH neg., HgCh neg.

Hauerite (MnS2). Isometric; color gray; hardness E. HN03 fumes tarnish 

slowly; some areas negative. HCl solution turns brown; mineral stains light 

brown. KCN, FeC]3 , KOH, and HgCl2 negative. Triangular pits indicate cubic 

cleavage. Red powder.

Magnetite (FeO.Fe2 O3 ). Isometric; color gray; hardness F. HN03 negative. 

HCl, on smooth surfaces larger than drop fumes tarnish; solution turns yellow. 

KCN, FeCl 3 , KOH, and HgCl2 negative. Aqua regia solution turns yellow. 

Black powder when scratched. Powder is magnetic. (See description of chromite, p. 166.) Magnetite has the property of yielding somewhat to pressure. A 

needle drawn across a smooth surface will often leave a distinct mark. This is 

not a true scratch as the sides of the mark are unbroken. It will be found difficult to push the point of the needle diagonally against the surface and thus obtain 

a powder. Magnetite is properly a "hard" mineral but is placed in the "soft" 

group to obviate the difficulty caused by its anomalous behavior.

ANISOTIIOIMC

KOH, HfjCh neg.

AndorUv (Ag2S.2PbS.3Sb2S3). Orthorhombic; color gray; hardness C. Anisotropic; polarization colors light to dark gray. HN03 fumes tarnish; some areas 

slowly stain brown; others negative. HCl fumes tarnish faintly; some specimens 

negative. KCN and FeCl3 negative. KOH slowly stains brown. HgCl2 negative. Aqua regia effervesces vigorously and stains black; this is the most distinctive test. Almost invariably associated with stannite. Rare.

Slernbergite (Ag2S. 1^6485). Orthorhombic; color light brown; hardness B. 

Very strong anisotropism, shows without analyzing nicol; polarization colors 

light gray, pink, violet, brown, steel blue, black. HN03 fumes tarnish brown; 

tarnish does not wash off; surface slowly stains iridescent; some specimens practically negative. HCl fumes tarnish slowly; tarnish does not wash off. KCN 

negative. FeCl3 stains iridescent; some specimens negative. KOH stains differentially iridescent. HgCl2 negative; gives a deceptive "etching" owing to deposition of HgCJ2 crystals. Habit prismatic. Some crystals curved like those of 

cylindrite. Usually associated with ruby silver minerals.
150 MICROSCOPIC DETERMINATION OF ORE MINERALS

KOH neg., HgCh neg.

Delafossite (Cu20.Fe2O3). Color gray; hardness E. Strong anisotropism; 

shows radial structure. HNO3 negative. HC1 etches differentially black; drop 

turns yellow. KCN, FeCl3 , KOH, and HgCl2 negative. Black powder, showing 

tiny crystalline plates. Very rare.

Hausmannite (MnO.Mn2 03). Tetragonal; color white with a faint tinge of blue; 

hardness D . Strongly anisotropic; most specimens show multiple twinning 

resembling that of plagioclase; polarization colors white, dark gray. HNO3 fumes 

tarnish; some specimens negative. HC1 tarnishes differentially light brown; some 

areas negative; drop stains brown. KCN, FeCl3 , KOH, and HgCl2 negative. 

H 2 02 effervesces slowly without staining surface. Good basal cleavage. Powder 

and internal reflection deep red. Takes a high polish.

Manganite (Mn2 O3). Orthorhombic; color gray; hardness E + . Strongly anisotropic; gives four extinctions per revolution; polarization colors white, light gray, 

violet, brown, dark gray. HN03 negative. HC1 fumes tarnish; surface stains 

light brown. KCN, FeCl3 , KOH, and HgCl2 negative. H2O2 effervesces slowly 

without staining surface. Prismatic. Two perfect cleavages. Gives brown 

powder.

Melaconite or tenorite (Cu'O). Monoclinic; color gray; hardness E . Moderately strong anisotropism; polarization colors white, dark purple, dark gray. 

HN03 fumes tarnish; surface slowly stains differentially brown; some specimens 

negative. HCl slowly stains differentially brown; some areas unaffected; drop 

colors green to brown. KCN, FeCl3 , KOH, and HgCl2 negative. Found in the 

oxidized zone of copper deposits. The massive variety, the more common, is 

melaconite; and the prismatic variety tenorite. Melaconite commonly has more 

or less chemically combined manganese oxide and silica. If it has more than 

about 5 percent of either of these two components, the mineral is known as "copper 

pitch ore."

Pyrolusite (Mn02 ). Orthorhombic; color gray, almost white, similar to hematite; hardness D to E. Strongly anisotropic; gives four extinctions per revolution; 

polarization colors white to black. HN03 negative. HCl, drop colors brown; 

surface stains on some specimens, especially near periphery of drop; other specimens Jiegative. KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia same as 

HCl. H2 02 , drop effervesces vigorously, but surface does not stain. H2 02 + 

H2S04 , surface stains black. Gives black powder. Hand specimen sooty black. 

Prismatic; perfect prismatic cleavage. The apparently amorphous variety is 

seen to be a mat of tiny interlocking crystals. Some specimens soil fingers.

Teallite (SnS.PbS). Orthorhombic (?); color galena white; hardness C. Anisotropic; polarization colors light gray, brownish gray, dark gray, steel blue, violet. 

HN03 some specimens slowly stain differentially brown; most specimens negative. 

HCl quickly tarnishes gray to brown; some areas give off H2S. KCN, FeCl3 , 

KOH, and HgCl2 negative. Aqua regia stains black and evolves H2S. Sections 

are elongated parallel to the base and show perfect basal cleavage. Plates are 

flexible and curved like those of molybdenite.

HN03 neg., HCl neg., KCN, FeCl3

ISOTROPIC 

KOH neg., HgCb

Argentite (Ag2S). Isometric; color gray; hardness A. Some specimens are 

isotropic; others show anomalous anisotropism. HNOs and HCl fumes tarnish
DETERMINATIVE TABLE 151

most specimens, but some are negative. KCN stains differentially black and 

brings out structure. FeCls quickly stains black. KOH negative. HgCl 2 

quickly stains brown. Very sectile; does not yield powder.

AN1SOTUOPIC 

KOH, HgCh

Proustite (3Ag2S.As2S3). Hexagonal; color in reflected light bluish gray; 

hardness B . Anisotropic; polarization colors light to dark gray, largely masked 

by strong red internal reflection. HNOs fumes tarnish some specimens; most 

specimens negative. HCl negative. KCN stains black and brings out scratches, 

FeCls, some specimens slowly stain gray to black; most specimens negative. 

KOH stains gray to black. HgCl2 stains brown. Color of powder, ruby red. 

Color in hand specimen ruby red, in contrast to pyrargyrite, which is maroon to 

black.

KOH neg., HgCh

Argenlite (Ag2S). See description above.

Eucairite (Cu2Se.Ag2Se). Isometric; color galena white to buff gray; hardness 

B . Weakly anisotropic; polarization colors greenish yellow, bluish gray. 

HN03 fumes tarnish strongly; surface slowly turns dark gray; in presence of 

calcite reaction may be negative. HCl negative. KCN quickly stains black. 

FeCl3 stains iridescent; in presence of calcite reaction may be slow. KOH negative. HgCl2 stains pinkish violet. Associated with clausthalite, umangite, klockmannite, and other selenides.

Polybasite (8Ag2S.Sb2S3). Monoclinic; color gray; hardness C . Strongly 

anisotropic; interference colors brown, grayish violet, yellowish green. HNOs 

fumes tarnish; some specimens negative. HCl fumes tarnish; halo does not wash 

off; action slow and on some specimens negative. KCN stains brown to black. 

FeCl3 , some specimens stain iridescent, others negative. KOH, some specimens 

slowly tarnish iridescent; most specimens negative. HgCl2 stains brown to black. 

Variability in etch behavior probably due to variable chemical composition; more 

or less copper is always present. Powder dark ruby red; if particles are coarse, 

powder appears black. Internal reflection red; not always seen. Stephanite and 

pearceite always give black powder and show no internal reflection. All specimens examined by writer gave good microchetnical test for copper.

HNO3 neg., HCl neg., KCN, FeClg neg.

ISOTROI'IC 

KOH, HgCh

Pyrargyrite (3Ag2S.Sb2S3). Hexagonal; color bluish gray; hardness C. Most 

areas show moderately strong anisotropisni, but some areas appear isotropic, 

owing to transparency; polarization colors light gray, steel gray, brown; colors 

somewhat masked by internal reflection. HNOs, some specimens slowly stain 

gray to brown; on others fumes tarnish but surface does not stain; most specimens 

negative. HCl fumes tarnish; halo does not wash off; action slow in starting and 

on some specimens negative. KCN quickly stains brown to gray and brings out 

scratches. FeCl3 negative; solution deposits sediment which does not wash off 

easily, but surface does not stain. KOH stains gray to iridescent. HgCl2 slowly.'., 

stains brown. Ruby-red powder and internal reflection; lighter in color and 

more easily seen than in polybasite. Distinguished from miargyrite by a more' 

pronounced red color in hand specimen, miargyrite being practically black. Miar-; 

gyrite lias a darker-red powder and internal reflection than pyrargyrite. Prous-^ 

tite is ruby red in hand specimen and has a more pronounced internal reflection-; 

than pyrargyrite.
152 MICROSCOPIC DETERMINATION OF ORE MINERALS

KOH, HgCh neg.

Cerargyrite (AgCl). Isometric; color dark gray; hardness A. HN03 and HCl 

negative. KCN instantly stains dark gray to brown. FeCl3 negative. KOH 

stains brown. HgCl2 negative. Waxy yellow in oblique light, light-yellow 

internal reflection. Very sectile; the most characteristic quality of cerargyrite. 

Usually associated with native silver.

Realgar (AsS). Monoclinic; hardness B. Apparently isotropic; mineral is 

fairly transparent, and most of the incident light is absorbed. HNO3 and HCl 

negative. KCN fumes tarnish on some areas; others negative. FeCls negative. 

KOH instantly stains black. HgCl2 and aqua regia negative. Red internal 

reflection and powder.

KOH neg., HgCh

Argyrodite (4Ag2S.GeS2). Tetragonal, pseudo-isometric; color gray; hardness C. Some specimens isotropic; others weakly anisotropic. HN03 fumes 

tarnish some areas, others negative. HCl negative. KCN stains slowly brown 

to black and brings out scratches. FeCl3 doubtful or negative. KOH negative. 

HgCl2 tarnishes iridescent.

KOH neg., HgCh neg.

Gold (Au). Isometric; hardness B. HN03 and HCl negative. KCN, some 

specimens etch black, giving roughened surface; others negative. FeCl3 , KOH, 

and HgCl2 negative. Aqua regia, slight effervescence, which soon ceases; stains 

brown; fumes tarnish brown; colors may be masked somewhat by color of mineral 

itself.

Klaprothite, also known as klaprotholite (3Cu2S.2Bi2S3 ). Probably isometric; 

color gray, similar to tetrahedrite; in oblique light it has a pinkish-brown tinge; 

hardness C. Weak anisotropism, possibly anomalous. HN03 slowly stains 

brown; seems to be a slow effervescence; fumes-tarnish; some areas negative. 

HCl negative. KCN stains differentially iridescent. FeCls, KOH, and HgCl2 

negative. Specimen examined came from Butte, Mont., and was determined by 

F. B. Laney as klaprothite. It is intergrown with covellite in "mutual boundary" 

and subgraphic structures.

Sulvanite (3Cu2S.V2S5). Isometric; color galena white; hardness C. Some 

areas appear isotropic, others feebly anisotropic; polarization colors reddish 

brown and greenish gray. HN03 and HCl negative. KCN stains slightly 

and brings out scratches; some areas practically negative. FeCls, KOH, and 

HgCl 2 negative. Aqua regia stains brown. Cubic cleavage, not prominent. 

Very rare.

Tiemannite (HgSe). Isometric; color galena white; hardness B. HN03 and 

HCl negative. KCN, some areas tarnish iridescent, others negative. FeCl 3 

KOH, and HgCl 2 negative. Aqua regia effervesces and stains iridescent. Rare.

ANISOTROPIC

KOH. HgCb

Proustite (3Ag2S.As2S3 ). Hexagonal; color in reflected light, bluish gray; 

hardness B . Anisotropic; polarization colors light to dark gray, largely masked 

by strong red internal reflection. HNO3 fumes tarnish some specimens; most 

specimens negative. HCl negative. KCN stains black and brings out scratches. 

FeCl3 , some specimens slowly stain gray to black; most specimens negative. KOH 

stains gray to black. HgCl2 stains brown. Color of powder ruby red. Color 

in hand specimen ruby red, in contrast to pyrargyrite, which is maroon to black.
DETERMINATIVE TABLE 153

Pyrargyrile (3Ag2 S.Sb2S3 ). Hexagonal; color bluish gray; hardness C. Most 

areas show moderately strong anisotropism, but some areas appear isotropic 

owing to transparency; polarization colors light gray, steel gray, brown; colors 

somewhat masked by internal reflection. HN03 , some .specimens slowly stain 

gray to brown; on others fumes tarnish, but surface does not stain; most specimens negative. HCl fumes tarnish; halo does not wash off; action slow mTstarting and on some specimens negative. KCN quickly stains brown to gray and 

brings out scratches. FeCl3 negative; solution deposits sediment which does not 

wash off easily, but surface does not stain. KOH stains gray to iridescent. 

HgCl2 slowly stains brown. Ruby-red powder and internal reflection; lighter in 

color and more easily seen than in polybasite. Distinguished from miargyrite 

by a more pronounced red color in hand specimen, miargyrite being practically 

black. Miargyrite has a darker-red powder and internal reflection than pyrargyrite. Proustite is ruby red in hand specimen and has a more pronounced internal reflection than pyrargyrite.

Stephanite (5Ag2S.Sb2S3). Orthorhombic; color gray; hardness B. Strongly 

anisotropic; polarization colors light gray, steel gray, bornite pink. HNO3 , after 

long etching fumes tarnish slightly and mineral seems to stain very light brown; 

reaction doubtful. HCl fumes tarnish; halo will not.wash off; action may require more than 1 minute and on some specimens practically negative. KCN 

stains dark gray to black and brings out scratches. FeCl 3 practically negative; 

seems to stain light gray in places; solution leaves sediment which rubs off easily. 

KOH, some specimens stain iridescent to black, others negative. Hgd2 stains 

brown to iridescent. Metallic powder. Best distinguished from polybasite by 

powder and lack of internal reflection. :

KOH, HgCh neg.

Livingslonite (HgS.2Sb2S3 ). Orthorhombic; color gray; hardness C. Strongly 

anisotropic; polarization colors white, steel blue, black. HNO3 , some specimens 

slowly stain iridescent, others negative. HCl negative. KCN, some specimens 

stain brown, others negative. FeCl3 negative. KOH instantly darkens and leaves 

yellow deposit; reaction highly characteristic and resembles that of stibnite. 

HgCl2 negative. Aquaregia effervesces and stains iridescent. Powder red when 

finely divided. Red internal reflection. Prismatic. Rare.

Miargyrite (Ag2S.Sb2S3 ). Monoclinic; color light gray, almost white; hardness 

B. Strong, anisotropism; polarization colors white, bluish gray, light brown, 

dark brown. HN03 usually negative; sometimes stains slightly. HCl fumes 

usually tarnish; halo does not wash off; action slow and sometimes negative. 

KCN slowly stains dark gray; on some specimens action requires more than 1 

minute. FeCl3 doubtful; solution deposits sediment which does not wash off 

easily, but surface does not stain. KOH quickly tarnishes differentially iridescent. HgCl 2 , some specimens stain slightly light brown; most specimens negative. Powder and .internal reflection ruby red. In contact with miargyrite, 

pyrargyrite is distinctly bluish. Miargyrite is more strongly anisotropic than 

pyrargyrite. It is almost invariably associated with pyrargyrite. Tested microchemically, miargyrite shows only a trace of copper, in contrast with polybasite, 

which usually gives a strong test.

Orpiment (As2S3). Monoclinic; color gray in vertically incident light, yellow 

in oblique light; hardness B. Weakly anisotropic; polarization colors masked by 

internal reflection. HN03 and HCl negative. KCN etches and brings out 

structure. FeCl3 negative. KOH instantly stains dark brown. HgCl2 deposits 

sediment but does not etch surface. Yellow powder and internal reflection.

129151 39 11
154 MICROSCOPIC DETERMINATION OF ORE MINERALS

Samsonite (2Ag2S.MnS.Sb2S3 ). Monoclinic; color gray; hardness C. Weakly 

anisotropic; polarization colors greenish gray, purple. HNO3 negative. HC1 

negative. KCN slowly stains brown. FeCl3 negative. KOH quickly stains 

differentially iridescent. HgCl2 negative. Shows triangular pits. Red powder 

and internal reflection.

KOH neg., HgCh

Argyrodite (4Ag;S.GeS2 ). Isometric; color gray, similar to enargite; hardness C. 

Some specimens isotropic, others weakly anisotropic. HN03 fumes sometimes tarnish faintly; usually negative. HC1 negative. KCN stains light brown 

to black and brings out scratches. FeCl3 and KOH negative. HgCl 2 quickly 

stains iridescent, the most characteristic etch test.

Polybasite (8Ag2S.Sb2 S3). Monoclinic; color gray; hardness C. Strongly anisotropic; interference colors brown, grayish violet, yellowish green. HN03 fumes 

tarnish; some specimens negative. HC1 fumes tarnish; halo does not-wash off; 

action slow and on some specimens negative. KCN stains brown to black. 

FeCl3 , some specimens stain iridescent, others negative. KOH, some specimens 

slowly tarnish iridescent; most specimens negative. HgCl 2 stains brown to black. 

Variability in etch behavior probably due to variable chemical composition; 

more or less copper is always present. Powder dark ruby red; if particles are coarse, 

powder appears black. Internal reflection red; not always seen. Stephanite 

and pearceite always give black powder and show no internal reflection. All 

specimens examined by writer gave good microchemical test for copper.

KOH neg.. HgCh ncg.

Chalcoslibite (Cu2S.Sb2 S3 ). Orthorhombic; color galena white; hardness C. 

Anisotropic; polarization colors pinkish to greenish gray. HN03 tarnishes some 

specimens; on others fumes tarnish but surface does not stain; some specimens 

negative. .HCl negative. KCN slowly stains brown and brings out scratches; 

some specimens almost negative. FeCl3 , KOH, and HgCl2 negative. Lighter in 

color than tetrahedrite. Has well-marked cleavage.

KOH neg., HgCh neg.

Covellite (CuS). Hexagonal; color indigo blue; hardness B . Strong anisotropism; polarization colors white, fiery orange; four extinctions per revolution. 

HNO3 fumes tarnish some specimens, others negative. HCl negative. KCN 

stains black. FeCl3 , KOH, and HgCl2 negative. Perfect basal cleavage. Usually of supergene origin, replacing chalcocite, bornite, and other copper sulphides, 

frequently also sphalerite or galena. In such associations covellite occurs as 

minute scales without crystal outlines. At Butte much covellite is hypogene 

and occurs as large hexagonal plates, in places several centimeters in diameter.

Enargite (Cu2S.4CuS.As2S3). Orthorhombic; two varieties known, one colored 

gray, the other pinkish; the pinkish variety is known as "luzonite"; both varieties are sometimes to be seen in the same specimen; hardness D . Strongly 

anisotropic; polarization colors purple, gray, greenish yellow. HN03 fumes tarnish faintly; sometimes negative. HCl negative. KCN stains black and sometimes brings out a parallel structure. FeCl 3 and KOH negative. HgCl 2 sometimes stains slightly; doubtful. Commonly intergrown with tennantite. Prismatic. Perfect cleavage in two directions.

Sulvanite (3Cu2S.V2S5). Isometric; color galena white; hardness C. Some 

specimens appear isotropic, others feebly anisotropic; polarization colors reddish 

brown, greenish gray. HN03 and HCl negative. KCN stains slightly and 

brings out parallel scratches; some areas practically negative. FeCls, KOH, and 

HgCl2 negative. .Aqua regia stains brown. Cubic cleavage, not prominent. 

Very rare.
DETERMINATIVE TABLE 155 

HNO3 neg., HC1 neg., KCN neg., FeCl3

ISOTROPIC 

KOH neg., HgCh'neg.

Coloradoite (HgTe). Isometric; color gray with a distinct tinge of pink; hardness C . HN03 , some areas stain slowly light brown to iridescent; other areas 

negative. HC1 and KCN negative. FeCl 3 stains differentially iridescent. KOH 

and HgCl2 negative.

ANISOTROPIC

KOH, HgCh neg.

Keeleyite (PbS.Sb2S3). Orthorhombic; color gray; hardness C. Strong anisotropism. HN03 , HC1, and KCN negative. FeCl3 slowly tarnishes differentially. 

KOH stains black; brings out scratches. HgCl2 negative. Prismatic. Specimen 

from Oruro, Bolivia. Has same composition as zinkenite but different color and 

etch reactions.29

Freieslebenite (2Ag2S.3PbS.2Sb2S3). Monoclinic; color galena white; hardness 

C. Strongly anisotropic, polarization colors light gray, brownish gray, dark 

gray; some areas show multiple twinning. HNOs practically negative; some areas 

seem to stain slightly after long etching. HC1 and KCN negative. FeCJ 3 practically negative; seems to stain slightly. KOH and HgCl 2 negative. Hand 

specimen shows striated crystals. Very rare. Specimen is from Hiendelencia, 

Spain.

HNO3 neg., HCI neg., KCN neg., FeCl3 neg., aqua regia

ISOTROPIC 

KOH neg., HgCh neg.

Chalcopyrite (CuFeS2). Tetragonal, color brass yellow; hardness C. Most 

specimens are weakly but discernibly anisotropic; a small proportion are apparently isotropic. HN03 fumes tarnish; some specimens negative. HCI, KCN, 

FeCl3 , KOH, and HgCl2 negative. Aqua regia fumes tarnish differentially and 

bring out grain structure; area covered by drop usually stains slowly brown, but 

some areas unaffected. Easily scratched by needle, which distinguishes it from 

pyrite. Gives powder when scratched, in contrast to gold, which is sectile.

Germanite (Cu3 (Fe,Ge)S4). Isometric; color pinkish gray, about the same as 

pink enargite; hardness C. HN03 , HCI, KCN, FeCl3 , KOH, and HgCl2 negative. 

Aqua regia effervesces and slowly stains light brown.

Gold (Au). Isometric; hardness B. HN03 and HCI negative. KCN, some 

specimens etch black, giving roughened surface; others negative. FeCl 3 , KOH, 

and HgCl2 negative. Aqua regia slight effervescence, which soon ceases; stains 

brown; fumes tarnish brown; colors may be masked by color of mineral itself. 

Very sectile.

Metacinnabar (HgS). Isometric; color gray, black in oblique light; hardness 

B. Most specimens are isotropic, but some are weakly anisotropic and show 

multiple twinning. HN03 , HC1, KCN, FeCl3 , KOH, and HgCI 2 negative. Aqua 

regia. effervesces and turns black. Almost invariably intergrown with cinnabar. 

In polished sections metacinnabar has a smooth surface and gives a black powder, 

whereas cinnabar has a rougher surface and gives red powder and internal reflection.

28 See Shannon, E. V., and Short, M. N., A reexamination of the lead sulphosalt keeleyite from'Bolivia: 

Am. Mineralogist, vol. 12, pp. 405-408,1927: discussion, by E. T. Wherry, idem, vol. 13, pp. 29-30,1928,
156 MICROSCOPIC DETERMINATION OF ORE MINERALS

Sphalerite (ZnS). Isometric; color gray; hardness C + . HNO3 fumes usually 

tarnish; mineral stains light brown after long etching; more rarely slow effervescence can be observed; on areas smaller than drop reaction is usually negative. 

HC1 fumes tarnish some specimens, others negative; drop sometimes turns yellow 

but surface is unaffected. KCN, FeCl 3, KOH, and HgCl2 negative. Aqua regia 

effervesces and stains dark brown; fumes tarnish. Some specimens show resincolored internal reflection. Dodecahedral cleavage; some specimens show triangular pits. Powder white to brown. If proportion of iron is high, powder 

may be metallic.

Sulvanite (3Cu2S.V2S5). Isometric; color galena white; hardness C. Some 

specimens appear isotropic, others feebly anisotropic; polarization colors reddish 

brown, greenish gray. HN03 and HC1 negative. KCN stains slightly and 

brings out parallel scratches; some areas practically negative. FeQ3 , KOH, and 

HgCl2 negative. Aqua regia stains brown. Cubic cleavage, not prominent. 

Very rare.

Telrahedrite (5Cu2S.2(Cu,Fe)S.2Sb2S3). Isometric; color gray; hardness D. 

HNOs fumes usually tarnish; some specimens stain iridescent but usually requires 

more than 1 minute for test to work; some specimens negative. HC1, KCN, 

FeCl 3, KOH, and HgCl2 negative. Aqua regia fumes tarnish. Some specimens 

yield reddish-brown powder; others black metallic powder.

Tiemannite (HgSe). Isometric; color galena white; hardness B. HNOs, HC1, 

and KCN negative. FeCls slowly stains brown, some areas negative. KOH 

and HgCl2 negative. Aqua regia instantly effervesces and stains iridescent.

ANISOTROPIC

KOH, HgCh neg.

Andorite (Ag2S.2PbS.3Sb2S3). Orthorhombic; color gray, similar to tetrahedrite; hardness C. Anisotropic; polarization colors light to dark gray. HNOs 

fumes tarnish; some areas slowly stain brown; some specimens negative. HC1 

negative; fumes sometimes tarnish faintly. KCN and FeCl3 negative. KOH 

slowly stains brown. HgCl2 , some specimens slowly stain iridescent; others 

negative. Aqua regia effervesces vigorously and stains black; this is the most 

distinctive etch test. Almost invariably associated with stannite. Prismatic. 

Rare.

Berthierite (FeS.Sb2Ss). Orthorhombic; color gray; hardness D . Strongly 

anisotropic; some grains give four extinctions per revolution; polarization colors 

light gray, steel blue, dark brown. HN03 slowly stains iridescent; usually takes 

more than a minute before effect begins to show. HC1 negative; solution sometimes turns yellow. KCN and FeCl3 negative. KOH stains iridescent. HgCl2 

negative. Aqua regia effervesces and stains light brown. Prismatic.

Livingstonite (HgS.2Sb2S3 ). Orthorhombic; color gray; hardness C. Strongly 

anisotropic; polarization colors white, steel blue, black. HN03 , some specimens 

tarnish iridescent; others negative. HC1 negative. KCN, some specimens 

tarnish brown; others negative. FeCl3 negative. KOH instantly darkens, leaving yellow deposit; reaction highly characteristic and resembles that of stibnite. 

HgCl2 negative. Aqua regia effervesces and stains iridescent. Prismatic. 

Rare. Red powder.

Miargyrite (Ag2 S.Sb2 S3). Monoclini.c; color light gray, almost white; hardness 

B. Strong anisotropism; polarization colors white, bluish gray, light brown, dark 

brown. HNOs usually negative; sometimes stains slightly. HC1 fumes usually
DETERMINATIVE TABLE 157

tarnish; halo does not wash off; action slow and sometimes negative. KCN 

slowly stains dark gray; on some specimens action requires more than 1 minute. 

FeCl3 doubtful; solution-deposits sediment which does not wash off easily, but 

surface does not stain. KOH quickly tarnishes differentially iridescent. HgCl2 , 

some specimens stain slightly light brown; most specimens negative. Powder 

and internal reflection ruby red. In contact witho miargyrite, pyrargyrite is 

distinctly bluish. Miargyrite is more strongly anisotropic than pyrargyrite. It 

is almost invariably associated with pyrargyrite. Tested microchemically, 

miargyrite shows only a trace of copper, in contrast with polybasite, which usually 

gives a good test.

Pyrrhotite (FeSi+). Hexagonal; color brownish or pinkish cream; hardness D . 

Strong anisetropism; polarization colors light gray, bluish gray, brown. HNOa 

fumes tarnish most specimens; surface usually stains slightly light brown; some 

specimens, especially those pcorly polished, negative. HCl, on most specimens 

drop slowly turns yellow, but surface does not stain; on a small proportion fumes 

tarnish and halo does not wash off; on specimens from Mary mine, Ducktown, 

Tenn., effervesces, gives off H2S, and brings out etch cleavage. KCN negative. 

FeCl3 negative. KOH, surface slowly turns brown, especially near periphery of 

drop. HgCl2 negative. Distinctly magnetic.

KOH neg., HgCh neg.

Bournonite (Cu2S.2PbS.Sb2 S3 ).: Orthorhombic; color gray, about the same as 

tetrahedrite; hardness C . Anisotropic; polarization colors greenish gray, dark 

brown, purple; some specimens show multiple twinning. HN03 , usually fumes 

tarnish, some specimens negative. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. 

Aqua regia quickly effervesces and stains differentially iridescent.

Chalcophanite ((Mn,Zn)0.2Mn02.2H2 0). Orthorhombic; color gray. Strongly 

anisotropic; gives four extinctions per revolution. HN03 negative. HCl, drop 

colors brown; surface does not stain. KCN, FeCl 3 , KOH, and HgCl2 negative. 

H2 02 effervesces vigorously, but surface does not stain. Aqua regia, drop colors 

brown; surface stains differentially light brown. Prismatic.

Chalcopyrite (CuFeS2 ). Tetragonal; color brass yellow; hardness C. Specimens from most localities are weakly anisotropic; those from a few localities, 

notably Insizwa, South Africa, show fairly strong anisotropism; polarization 

colors greenish yellow, bornite pink. Chalcopyrite associated with pyrrhotite 

usually shows multiple twinning in polarized light. HN03 fumes tarnish; some 

varieties negative. HCl, KCN, FeCl3 , KOH, and HgCl 2 negative. Aqua regia 

fumes tarnish differentially and bring out grain structure; area covered by drop 

usually stains slowly brown, but some areas almost unaffected. Easily scratched 

by needle, which distinguishes it from pyrite; gives powder when scratched, in 

contrast to gold, which is sectile.

Cubanite (Cu2S.Fe4S5). Orthorhombic; color brownish or pinkish cream; 

hardness C. Strong anisotropism; polarization colors yellow, grayish blue. 

HN03 , some specimens stain light brown; on some specimens fumes tarnish, but 

surface does not stain; others negative. HCl, KCN, FeCl3 , KOH, and HgCl 2 

negative. Invariably intergrown with Chalcopyrite and usually associated with 

pyrrhotite. Color almost identical with that of pyrrhotite but a little yellower. 

Cubanite is weakly magnetic; less so than pyrrhotite. Cubanite shows no relief 

with Chalcopyrite; pyrrhotite is' harder than chalcopyrite and stands in relief 

above it. Also known as chalmersite.
158 MICROSCOPIC DETERMINATION OF ORE MINERALS

Cinnabar (HgS). Hexagonal; color bluish gray; hardness B. Anisotropic; 

polarization colors light to dark gray. HN03 , HC1, KCN, FeCl3 , KOH, and HgCl2 

negative. Aqua regia slowly effervesces and stains differentially iridescent. Red 

powder and internal reflection.

Freieslebenite (2Ag2 S.3PbS.2Sb2 S3). Monoclinic; color galena white; hardness C. Strongly anisotropic; polarization colors light gray, brownish gray, 

dark gray; some areas show multiple twinning. HNO3 practically negative; 

some areas seem to stain slightly after long etching. HC1 and KCN negative. 

FeCl3 practically negative; seems to stain slightly. KOH and HgCl2 negative. 

Aqua regia immediately effervesces and stains dark. Hand specimen shows 

striated crystals. Very rare. Specimen from Hiendelencia, Spain.

Metacinnabar (HgS). Isometric; color gray, black in oblique light; hardness 

B. , Most specimens are isotropic, but some are/weakly anisotropic and show 

multiple twinning. HNO3) HC1, KCN, FeCl3 , KOH, and HgCl 2 negative. Aqua 

regia effervesces and turns black. Almost invariably intergrown with cinnabar. 

In polished sections metacinna'bar has a smooth surface and gives a black 

powder, whereas cinnabar has a rougher surface and gives red powder and internal 

reflection.

Nagyagite (Pb,Au,Te,S(?)). Orthorhombic; color gray; hardness B. Anisotropic; polarization colors light gray, brown, dark gray. HNO3 slowly stains 

iridescent; some areas practically negative. HC1, KCN, FeCl3 , KOH, and HgCl2 

negative. Aqua regia stains differentially iridescent. Habit platy; plates are 

flexible like those of molybdenite. From Na-gyag, Hungary. Formula doubtful.

Sulvanile (3Cu2 S.V2S5). Isometric; color galena white; hardness C. Some 

specimens appear isotropic, others feebly anisotropic; polarization colors reddish 

brown, greenish gray. HN03 and HCl negative. KCN stains slightly and 

brings out scratches; some areas practically negative. FeCl 3 , KOH, and HgCl2 

negative. Aqua regia stains brown. Cubic cleavage, not prominent. Very 

rare.

HNO3 neg., HCl neg., KCN neg., FeCl 3 neg., aqua regia neg.

ISOTROPIC 

KOH, HgCh neg.

Realgar (AsS). Monoclinic; hardness B. Apparently isotropic; the mineral 

is fairly transparent, and most of the incident light is absorbed. HN03 and 

HCl negative. KCN fumes tarnish some areas; others negative. FeCl3 negative. KOH instantly stains black. HgCl2 and aqua regia negative. Red 

internal reflection and powder.

KOH neg., HgCh neg.

Magnetite (FeO.Fe2 03). Isometric; color gray; hardness F. HNO3 negative. 

HCl, on smooth surfaces larger than drop fumes tarnish; solution turns yellow. 

KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia solution turns yellow. 

Black powder when scratched. Powder is magnetic. (See description of chromite, p. 166). Magnetite has the property of yielding somewhat to pressure. A 

needle drawn across a smooth surface will often leave a distinct mark. This is 

not a true scratch, as the sides of the mark are unbroken. It will be found 

difficult to push the point of the needle diagonally against the surface and thus 

obtain a powder. Magnetite is properly a hard mineral but is placed in the 

soft group as well, in order to obviate the difficulty caused by its anomalous 

behavior.
DETERMINATIVE TABLE 159

ANISOTROPIC 

KOH neg., HgCh neg.

Chalcopyrite (CuFeS2 ). See description above (p. 43).

Graphite (C). Hexagonal; color gray with a brownish tinge; hardness A. 

Anisotropic; polarization colors light to dark gray; two extinctions per revolution, 

contrasting with molybdenite, which has four; basal sections are isotropic. 

Negative to all standard etch reagents. Perfect basal cleavage. Plates are 

flexible like those of molybdenite, and cross sections are likely to show thin 

curved bands.

Huebnerite (MnO.W03). Monoclinic; color gray, like sphalerite; hardness E. 

Strong anisotropism. Can readily be scratched by needle. Negative to all 

reagents. Gives yellow powder and dark-red internal reflection.

Molybdenite (MoS2 ). Hexagonal; color galena white; hardness B + . Strong 

anisotropism; polarization colors white, light violet, black; basal sections are 

isotropic; prismatic sections give nearly parallel extinction and four extinctions 

per revolution. Negative to all reagents. Perfect basal cleavage. Plates tend 

to be more or less curved.

Pyrolusite (MnC>2). Orthorhombic; color gray, almost white, similar to 

hematite; hardness D to E. Strongly anisotropic; gives four extinctions per 

revolution; polarization colors white to black. HN03 negative. HCl, drop 

colors brown; surface stains on some specimens, especially near periphery of 

drop; other specimens negative. KCN, FeCl 3 , KOH, and HgCl2 negative. Aqua 

regia same as HCl. H2O2 , drop effervesces vigorously, but surface does not 

stain. H 2O2 + H2SO.i, surface stains black. Gives black powder. Hand specimen sooty black. Prismatic; perfect prismatic cleavage. The apparently 

amorphous variety is seen to be a mat of tiny interlocking needles. Some specimens soil fingers.

KOH neg., HgCh neg.

Tungstenile (WS2). Hexagonal; color gray; hardness A. Strong anisotropism; 

gives four extinctions per revolution. Negative to all reagents. Easily scratched 

by thumb nail. Appears as a steel-blue smear along tiny fracture planes. Very 

fine grained, porous, and crumbly; polishes with difficulty. Resembles molybdenite but distinguished from it by its fineness of grain. Rare; reported only 

from Cottonwood, Utah. Analyzed and described by Wells.30

HARD MINERALS 

HNO3 eff., HCl neg., KCN neg., FeCl3

ISOTROPIC 

KOH neg., HgCh

Chloanthite-smaltite ((Ni,Co)As2). Isometric; color gray; hardness F. HN03 

effervesces vigorously and stains dark gray. HCl and KCN negative. FeClg 

stains differentially and brings out etch cleavage. KOH negative. HgCl2 stains 

differentially iridescent. Specimen from Schneeberg, Germany. Analysis by 

J. G. Fairchild, of the Geological Survey, shows Ni 15.2, Co 4.5, Fe 3.5, Cu 0.7, 

As 75.4, S 0.0, insoluble 0.0, total 99.3. This corresponds approximately to the 

formula (Ni,Co) 2As5, which can be expressed for purposes of comparison as 

(Ni,Co)As2 . 5 . See description of smaltite-chloanthite, below. The different 

varieties of the isomorphous series chloanthite-smaltite all give the same etch 

tests and can be distinguished only by microchemical methods. The end members 

are verv rare.

30 Wells, R. C., Washington Acad. Sci. Jour., vol. 7, pp. 596-599, 1917.
160 MICROSCOPIC DETERMINATION OF ORE MINERALS

Corynite (Ni(As,Sb)S). Isometric; color grayish white; hardness F. HNO3 

effervesces and brings out etch cleavage. HC1 and KCN negative. FeCl3 stains 

brown and brings out etch cleavage; on some specimens reaction is faint. KOH 

negative. HgCl2 , some specimens slowly stain brown to iridescent, others negative. Specimen from Olsa, Carinthia, Austria.

Plessite ((Ni,Fe)AsS). Isometric; color white; hardness F. Some specimens 

are isotropic; others show weak anomalous anisotropism. HNO3 effervesces and 

stains black. HCl and KCN negative. FeCl3 effervesces and stains differentially 

brown to iridescent. KOH negative. HgQ2 slowly stains light brown; some 

areas unaffected. Iron-bearing gersdorffite. Isotropic specimens are distinguished from gersdorffite only by a microchemical test for iron.

Smaltite-chloanthite ((Co,Ni)As2). Isometric; color gray; hardness F. HN03 

effervesces vigorously; surface stains differentially, bringing out concentric shell 

structure. HCl and KCN negative. FeCl3 stains differentially brown. KOH 

negative. HgQ2 stains brown to iridescent. Specimen from Schneeberg, 

Germany. Analysis by J. J. Fahey, of the Geological Survey, shows insoluble 

0.32, Ni 15.07, Co 15.83, Fe 3.69, Bi 0.86, As 63.42, total 99.19. This corresponds approximately to the formula (Co,Ni)Asi. 5 . Analyses of smaltitechloanthite show considerable variation, ranging from (Co,Ni)Asi.5 to (Co,Ni)Ass. 

The latter end member is termed skutterudite. Nickel-free varieties are rare. 

The average corresponds to about (Co,Ni)As2.

Skutterudite ((Co,Ni)As3 ). Isometric; color creamy; hardness F. HNOs 

effervesces and turns iridescent to black. HCl and KCN negative. FeCls 

stains brown. KOH and HgCl2 negative. Occurs as pyritohedrons with concentric shells. Specimen from type locality gave good microchemical tests for 

nickel and cobalt.

ANTSOTROPIC 

KOH neg.. HgCI 2

Niccolile (NiAs). Hexagonal; color coppery pink; hardness E. Strongly 

anisotropic; polarization colors vivid yellow, steel blue, bornite pink. HN03 

tarnishes differentially with effervescence. HCl and KCN negative. FeCla 

slowly stains differentially brown to iridescent. KOH negative. HgCl2 stains 

brown to iridescent. In contact with breithauptite, niccolite appears buff. 

Breithauptite is more coppery in color and does not effervesce with HN03 .

Plessite ((Ni,Fe)AsS). Isometric'; color white; hardness F. Some specimens 

are isotropic; others show weak anomalous anisotropism. HN03 effervesces and 

stains black. HCl and KCN negative. FeCl3 effervesces and stains differentially 

brown to iridescent. KOH negative. HgCl2 slowly stains light brown; some 

areas unaffected. Iron-bearing gersdorffite. Distinguished from gersdorffite 

by anisotropism and HgCl2 test.

Rammelsbergite (NiAs2 ). Orthorhombic; color white; hardness E. Strongly 

anisotropic; polarization colors pink, bornite pink, brown, light gray, greenish 

gray; some specimens show multiple twinning. HN03 effervesces, stains dark, 

and brings out lamellar structure; leaves rough surface. HCl and KCN negative. 

FeCl3 stains brown. KOH negative. HgCl2 slowly stains brown. Usually 

intergrown with niccolite or other nickel minerals.

Safflorile-loellingite ((Co,Fe)As2 ). Orthorhombic; color grayish white, identical 

with smaltite; hardness F. Strongly anisotropic; polarization colors yellow, 

purple, brown, steel blue. HN03 slowly effervesces, stains black, and brings out 

grain structure. HCl and KCN negative. FeCl3 stains differentially light brown
DETERMINATIVE TABLE 161

and brings out structure. KOH negative. HgCl 2 stains differentially light 

brown. Specimen from Cobalt, Ontario. Mineral has a spherulitic structure, 

shown on polished surface as aggregates of radiating crystals.

HNO3 eff., HC1 neg., KCN neg., FeCl3 neg.

ISOTROPIC 

KOH neg., HgClj neg.

Gersdorffde (NiAsS). Isometric; color gray, almost white; hardness F . 

HNO3 effervesces slowly and stains iridescent; HCl, KCN, FeCl 3 , KOH, and 

HgCl2 negative. Distinct cubic cleavage.

Pyrite (FeS2). Isometric; color pale brass yellow; hardness F. HN03 fumes 

tarnish; some specimens slowly effervesce; with small grains or poorly polished 

surfaces reaction may be entirely negative. HCl, KCN, FeCl3 , KOH, and HgClj 

negative. Aqua regia fumes tarnish; some specimens slowly effervesce. Not 

scratched by needle, which is best means of distinguishing from chalcopyrite.

ANISOTROPIC 

KOH neg., HgCh neg.

Arsenopyrite (FeAsS). Orthorhombic; color galena white; hardness F+. 

Anisotropic; polarization colors greenish yellow, brown, violet. HN03 slowly 

effervesces and stains differentially iridescent; on some specimens brings out 

parallel etch cleavage or zonal structure; effervescence not always observed. HCl, 

KCN, FeCl3 , KOH, and HgCl2 negative. Diamond-shaped cross sections frequently observed. Distinguished from loellingite by whiter color.

Glaucodot ((Co,Fe)AsS). Orthorhombic; color white; hardness F. Weak anisotropisrn; polarization colors pinkish gray to light gray. HNO» stains iridescent 

witli slow effervescence. HCl, KCN, FeCl;i , KOH, and HgClj negative.

Leucopyrite (Fe3As4). Orthorhombic; color white; hardness F. Strong 

anisotropism; polarization colors greenish yellow, purple, dark brown. HN03 

effervesces slowly and stains brown; solution deposits white octahedral crystals; 

effervescence not always observed. HCl, KCN, FeCl 3 , KOH, and HgCU negative. 

Distinguished with difficulty from arsenopyrite. Distinguished from loellingite 

only by chemical analysis.

Loellingite (FeAs2). Orthorhombic; color grayish white; hardness F . Strong 

anisotropism; polarization colors light yellow to deep blue. HN03 effervesces 

slowly and stains gray; on some specimens brings out fibrous or prismatic structure; solution deposits abundant white to black octahedra. HCl, KCN, FeCl 3 , 

KOH, HgClz negative. Does not show diamond-shaped cross-sections as does 

arsenopyrite.

Loellingite (nickeliferous) ((Fe,Ni)As2 ). Orthorhombic; color galena white; 

hardness E + , Strongly anisotropic. HN03 slowly effervesces; deposits colorless 

octahedral mineral; tarnishes iridescent. HCl, KCN, FeCl3 , KOH, and HgCl2 

negative. Etch tests practically the same as on ordinary loellingite. Microchemical tests show abundant nickel as well as iron.

Marcasite (FeS2). Orthorhombic; color pale brass yellow; hardness E + . 

Anisotropic; polarization colors vivid yellow, green, steel blue, brown. HN03 

stains brown to iridescent with slow effervescence; sometimes effervescence not 

observed. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. Marcasite 

is slightly paler in color than pyrite. Pyrite and marcasite can usually be distinguished by means of polarized light. This means is not infallible, however. 

(See under pyrite.)
162 MICROSCOPIC DETERMINATION OF ORB MINERALS

Pyrite (FeS2 ). Isometric; color pale brass yellow; hardness F. More than 99 

percent of pyrite specimens are isotropic; very rarely, however, polished sections 

show pyritohedrons with the color of pyrite rather than of marcasite, each pyritohedron being a mosaic of grains of weak anisotropism and diverse orientation. 

HNOs fumes tarnish; some specimens stain slightly; drop effervesces slowly. 

Other reagents negative. Specimens examined came from Cerro de Pasco, Peru. 

The color and the square rather than prismatic cross sections are sufficient 

criteria to identify mineral as pyrite.

HN03 , HC1, KCN neg., FeCl3

ISOTROPIC 

KOH neg., HgCh neg.

Psilomelane (Mn02.Mn2 O3 ). Amorphous; color gray; hardness D . Some 

areas appear isotropic; other areas are anisotropic, especially where they consist 

of radiating fibers; polarization colors white to black; four extinctions per revolution. HN03 stains light brown; fumes tarnish. HC1 stains brown to black; 

action more intense than that of HNO3 . KCN negative. FeCl3 , some specimens 

stain light brown; others negative. KOH and HgCl2 negative. H2 02 effervesces 

vigorously without etching. Massive, botryoidal, or reniform structure. Polished 

surfaces commonly show concentric banding.

ANISOTROPIC 

Psilomelane. See description above.

HNOs, HC1, KCN neg., FeCl3 neg.

ISOTROPIC

Psilomelane. See description above.

ANISOTROPIC

KOH neg.. HgCh neg.

Hausmannite (MnO.Mn2 03 ). Tetragonal; color white with a suggestion of 

blue; hardness D . Strongly anisotropic; most specimens show multiple twinning'resembling that of plagioclase; polarization colors white, dark gray. HN03 

fumes tarnish; some specimens negative. HC1 stains differentially brown; some 

areas negative; drop turns brown. KCN, FeCls, KOH, and HgCl2 negative. 

H2 O2 effervesces slowly without staining surface. Good basal cleavage. Powder 

and internal reflection brick red to deep red. Takes a high polish. Can be 

scratched with a sharp needle.

Psilomelane. See description above.

HNO3 , HC1 neg., KCN neg., FeCl3

ISOTROPIC 

KOH neg., HgCI 2

Skutterndite (ferriferous) ((Co,Ni,Fe)As3 ). Isometric; color gray; hardness 

D + . HNO3 slowly tarnishes iridescent; sometimes effervesces slightly. HCl 

and KCN negative. FeCl3 stains slightly and brings out scratches. KOH 

negative. HgCl2 leaves a brown deposit which rubs off easily. Specimen from 

Horace Porter mine, Gunnison County, Colo. Analysis by J. G. Fairchild, of 

the Geological Survey, shows Co 10.98, Ni 5.14, Fe 5.82, As 75.30, insoluble 1.44, 

total 98.68.
DETERMINATIVE TABLE 163 

KOH neg., HgCk neg.

Kallilite (Ni(Sb,Bi)S). Isometric; color gray; hardness D. HN03 tarnishes 

iridescent. HC1 and KCN negative. FeCl3 stains faintly, almost negative. 

KOH and HgCl2 negative. From Schoenstein, Germany. Very rare. According to Thomson,31 kallilite is a mixture. Two minerals occur in specimen. One, 

which occupies greater proportion of total area, is soft and readily scratched. It 

gives etch tests as above and microchemical tests for Ni, Sb, and Bi. Sb greatly 

predominates over Bi. This is undoubtedly kallilite. The second mineral is unknown. It is harder than kallilite and cannot be scratched with a needle. It 

gives the same etch tests as kallilite but the speed of the reactions is slower. 

It gives microchemical tests for Ni, Co, and Sb. The data above given 

indicate that kallilite is an authentic mineral.

Ullmanite (NiSbS). Isometric; color galena white; hardness E. HN03 stains 

brown to iridescent. HCl and KCN negative. FeCl3 stains light brown. KOH 

and HgCl2 negative. Distinctly softer than gersdorffite. Has well-defined 

cleavage and shows triangular pits.

Uraninite (uranate of U, Pb, etc.). Isometric; color dark gray; hardness G. 

HN03 stains differentially brown. HCl and KCN negative. FeCl3 slowly 

tarnishes to still darker brownish gray. KOH, HgCl2, and aqua regia negative. 

Massive. Powder greenish brown to black.

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HN03 tarnishes iridescent; seems to effervesce slightly. HCl and KCN negative. FeCl 3 

stains faintly, almost negative. KOH and HgCI 2 negative. From Broken Hill, 

Australia.

KOH neg., HgClj neg.

Breilhauptite (NiSb). Hexagonal; color coppery pink; hardness E. Strong 

anisotropism; polarization colors light gray, bluish green, dark reddish brown. 

HN03 stains differentially iridescent. HCl and KCN negative. FeCl 3 stains 

differentially iridescent. KOH negative. HgCl2 stains slightly, practically 

negative. Usually intergrown with niccolite or other arsenides of nickel and 

cobalt. In contact with niccolite, breithauptite is distinctly redder.

HNO3 , HCl neg., KCN neg., FeCI3 neg.

ISOTROPIC 

KOH neg.. HgCh neg.

Bravoite ((Fe,Ni)S2). Isometric; color pale violet; hardness E. HN03 fumes 

tarnish; mineral stains iridescent and brings out zonal structure. HCl, KCN, 

FeCl3 , KOH, and HgCl2 negative. Specimen from Mechernich, Germany. Occurs 

in zoned pyritohedrons, with alternate yellow and violet shells. The yellow shells 

are pyrite with only a trace of nickel, and the violet shells consist of bravoite. 

The analyses probably show too much iron in the bravoite, owing to the mechanical admixture of pyrite.32

31 Thomson, Ellis, A mineralographie study of the pyrite group: Toronto Univ. Studies, Oeol. ser., no. 

12, p. HO, 1921.

32 See Short, M. N., and Shannon, E. V., Violarite and .other rare nickel sulphides: Am. Mineralogist, 

vol. 15, pp. 12-14, 1930.
164 MICROSCOPIC DETERMINATION OF ORE MINERALS

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G. Some specimens isotropic; others weakly anisotropic. HNO3 , some specimens stain differentially iridescent after long etching and fumes tarnish; other specimens negative. 

HC1, KCN, FeCls, KOH, and HgCl2 negative. Usually crystallizes in cubes and 

pyritohedrons.

Corynite (Ni(As,Sb)S). Isometric; color gray; hardness E + . HN03 stains 

iridescent; fumes tarnish. HC1, KCN, FeCl3 , KOH, and HgCl2 negative. Specimen from Chatham, Conn. Analysis by J. G. Fairchild: Ni 31.6, Co 0.7, Fe 3.9, 

As 34.9, Sb 9.1, Bi 0.4, S 17.1, total 97.2. Etch tests differ from those of specimens from Olsa, Carinthia. (See p. 160.)

Gersdorffite (NiAsS). Isometric; color gray, almost white; hardness F . 

HN03 effervesces slowly and stains iridescent. HC1, KCN, FeCl3 , KOH, and 

HgCl2 negative. Distinct cubic cleavage.

Pyrite (FeS2 ). Isometric; color pale brass yellow; hardness F. HNO3 fumes 

tarnish; with some specimens drop slowly effervesces; with small grains or poorly 

polished surfaces reaction may be entirely negative. HC1, KCN, FeCl3 , KOH, 

and HgCl2 negative. Aqua regia fumes tarnish; with some specimens drop 

slowly effervesces. Not scratched by needle, which is best means of distinguishing 

from chalcopyrite.

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HNOs tarnishes iridescent; seems to effervesce slightly. HC1 and KCN negative. FeCla 

seems to stain faintly; this reaction is easily overlooked. KOH and HgCl 2 

negative. .Rare; only locality reported is Broken Hill, New South Wales.

ANISOTROl'IC 

KOH neg.. HgCIs neg.

Arsenopyrite (FeAsS). Orthorhombic; color galena white; hardness F+- 

Anisotropic; polarization colors greenish yellow, brown, violet. HNOs stains 

differentially iridescent; on some specimens brings out parallel etch cleavage or 

zonal structure. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Diamondshaped cross sections frequently observed. Distinguished from loellingite by 

whiter color.

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G. Some specimens isotropic; others faintly anisotropic; polarization colors gray, bornite pink. 

HN03 , some specimens stain differentially iridescent after long etching and fumes 

tarnish; other specimens negative. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. 

Usually crystallizes in cubes and pyritohedrons.

Glaucodot ((Co,Fe)AsS). Orthorhombic; color white; hardness F. Weak 

anisotropism; polarization colors pinkish gray to light gray. HNO3 stains iridescent with slow effervescence. HCl, KCN, FeCl3 , KOH, and HgCl2 negative.

Leucopyrite (Fe3As4). Orthorhombic; color white; hardness F. Strong anisotropism. HN03 effervesces slowly and stains brown; solution deposits white 

octahedral crystals; effervescence not always observed. HCl, KCN, FeCl3 , 

KOH, and HgCl2 negative. Distinguished with difficulty from arsenopyrite. 

Distinguished from loellingite only by chemical analysis.

Marcasite (FeS2 ). Orthorhombic; color pale brass yellow; hardness E+. 

Anisotropic; polarization colors vivid yellow, green, steel blue, brown. HNOs
DETERMINATIVE TABLE 165

stains brown to iridescent; with larger grains sometimes gives slow effervescence; 

with smaller grains effervescence usually not observed. HC1, KCN, FeCl 3 , KOH, 

and HgCls negative. Prismatic. Color paler than that of pyrite. Marcasite 

and pyrite can usually be distinguished by means of polarized light. This 

method is not infallible, however. (See under pyrite.)

Pyrite (FeS2 ). Isometric; color pale brass yellow; hardness F. More than 99 

percent of pyrite specimens are isotropic; very rarely, however, polished sections 

show pyritohedrons with the color of pyrite rather than of marcasite, each pyritohedron being a mosaic of weakly anisotropic grains. HN03 fumes tarnish. 

Other reagents negative. Specimen examined came from Cerro de Pasco, Peru. 

The color and the square rather than prismatic cross sections of crystals are 

sufficient criteria to identify the mineral as pyrite.

HNO3 neg., HC1, KCN neg., FeCl3 neg.

ISOTUOPIC 

KOH neg., HgCI 2 neg.

Franklinite (ZnO.Fe2Oa). Isometric; color gray, about the same as magnetite; 

hardness G. HNO3 negative. HC1 usually negative, but fumes sometimes tarnish; 

solution turns yellow. KCN, FeCl3 , KOH, and HgCl 2 negative. Aqua regia 

solution turns yellow. Dark-brown powder when scratched. Powder is somewhat magnetic. (See description of chromite, p. 166).

Magnetite (FeO.Fe2 03 ). Isometric; color gray; hardness F. HN03 negative. 

HC1, on smooth surfaces larger than drop fumes tarnish; solution turns yellow. 

KCN, FeCl3 , KOH, and HgCl 2 negative. Aqua regia, solution turns yellow. 

Black powder when scratched. Powder is magnetic. (See description of chromite, 

p. 166.)

ANISOTJIOPIC 

KOH neg., HgCh neg.

Hausmannite (MnO.Mn2 03 ). Tetragonal; color white with a suggestion of 

blue; hardness D . Strongly anisotropic; most specimens show multiple twinning resembling that of plagioclase; polarization colors white, dark gray. HN03 

fumes tarnish; some specimens negative. HC1 stains differentially light brown; 

some areas negative; drop stains brown. KCN, FeCl3 , KOH, and HgCl2 negative. 

H2 02 effervesces slowly without staining surface. Good basal cleavage. Powder 

and internal reflection brick red to deep red. Takes a high polish. Can be 

scratched with a sharp needle.

HNO3 neg., HCI neg., KCN neg., FeCl3 neg., aqua regia

ISOTKOPIC 

KOH neg.. HgCh neg.

Pyrite (FeS2 ). Isometric; color pale brass yellow; hardness F. HN03 fumes 

tarnish; some specimens slowly effervesce; with small grains or poorly polished 

surfaces reaction may be entirely negative. HCI, KCN, FeCl3 , KOH, and 

HgCl2 negative. Aqua regia fumes tarnish; some specimens slowly effervesce. 

Not scratched by needle, which is best means of distinguishing from chalcopyrite.

Sperrylite (PtAs2). Isometric; color white; hardness F. HN03 , HCI, KCN. 

FeCl 3 , KOH, arid HgCl2 negative. Aqua regia effervesces slightly and stains 

light brown in places. Occurs as small isometric crystals.
166 MICROSCOPIC DETERMINATION" OF ORE MINERALS 

HNO3 neg., HC1 neg., KCN neg., FeCI3 neg., aqua regia neg.

ISOTROPIC 

KOH neg., HgCh neg.

Braunite (4Mn0.3MnO2 .SiO2 ). Tetragonal; color gray; hardness G. Weakly 

anisotropic; effect may be overlooked. HNO3 , HC1, KCN, FeCl3 , KOH, HgCl2 , 

and aqua regia negative. H2 02 slowly effervesces without staining surface. 

Gives dark-brown powder when scratched. Frequently associated with hausmannite. Commonly in pseudo-octahedrons giving nearly equidimensional 

cross sections.

Chromite (FeO.Cr2 03). Isometric; color gray; hardness G. Negative to all 

reagents, including H2 02 . Chromite has a fairly light-brown powder; franklinite 

has a dark-brown and magnetite a black powder. Magnetite is much more 

strongly magnetic than chromite. Magnetite particles will stand on end when a 

magnetic needle approaches them. Chromite and franklinite will cling to a 

magnetized needle when the needle comes into contact with them.

Coballite (CoAsS). Isometric; color pinkish gray; hardness G. Some specimens isotropic; others show weak anisotropism. HN03 , on some specimens fumes 

tarnish and mineral stains differentially iridescent after long leaching; other 

specimens negative. HC1, KCN, FeCl 3 , KOH, HgCl 2 , and aqua regia negative. 

Usually crystallizes in cubes and pyritohedrons.

Franklinite (ZnO.Fe2 03). Isometric; color gray, about the same as magnetite; 

hardness G. HNO3 negative. HC1 usually negative, but fumes sometimes 

tarnish; solution turns yellow. KCN, FeCl 3 , KOH, and HgCl2 negative. Aqua 

regia, solution turns yellow. Dark-brown powder when scratched. Powder is 

somewhat magnetic. (See description of chromite.)

Hematite (Fe2 03). Hexagonal; color gray; hardness G. Most specimens are 

anisotropic; some specimens are so fine-grained as to appear isotropic. Negative 

to all reagents. Fine-grained and earthy varieties of hematite will give a red 

powder when scratched with a needle. Some specularite specimens, however, 

yield a metallic powder.

Hollandile (m(Ba,Mn) 2 Mn05 -|-nFe4(Mn05)). Tetragonal. Color white with 

an almost imperceptible yellow tint differing from the faint bluish tint of hematite. 

Hardness G but varies somewhat in different crystallographic directions. Some 

specimens are slightly softer and others harder than braunite. Anisotropic; 

polarization colors, gray, yellow, blue; basal sections are isotropic. Some specimens show lamellar twinning similar to that of hausmannite and hematite. 

HNO3 , HC1, KCN, FeCl3 , KOH, HgCl 2 , aqua regia, and H2 02 negative. SnCl 2 

positive. Always associated with other ores of manganese and in some specimens 

iutergroAvn with hematite. Hollandite has a strong cleavage parallel to (101). 

It resembles hematite closely, but hematite gives a negative and hollandite a 

positive etch test with SnCl 2 . Hematite has a red internal reflection, whereas 

hollandite has no internal reflection. Occurs in several localities in India.

Jacobsite (MnO.Fe2 03). Isometric. Color pale gray with an olive tint. 

Hardness F, a little less than that of braunite. Some varieties are isotropic and 

others feebly anisotropic. HN03 , HC1, KCN, FeCl 3 , KOH, HgCl2 , aqua regia, 

and H2 02 negative. From Jakobsberg, Sweden, Szolczva, Hungary, and several 

localities in India. Always associated with other manganese minerals. An 

intergrowth of jacobsite and hausmannite is known as "vredenburgite." In this 

intergrowth hausmannite occurs as lamellae arranged parallel to the octahedral
DETERMINATIVE TABLE 167

planes in jacobsite. In other specimens jacobsite is intergrown with hematite. 

Some specimens of jacobsite show deep-red internal reflection.

Magnetite (FeO.Fe203). Isometric; color gray; hardness F. HN03 negative. 

HC1 usually negative; on smooth surfaces larger than drop fumes tarnish. KCN, 

FeCl3 , KOH, and Hgdz negative. Aqua regia, solution turns yellow. Black 

powder when scratched. Powder is magnetic. (See description of chromite.)

Sitaparite (9Mn2 03.4Fe203.Mn02-3CaO). Probably tetragonal; color a distinctive yellow tint with a suggestion of olive; much lighter than jacobsite; 

hardness G; slightly harder than braunite. Weakly anisotropic; some grains 

apparently isotropic. HN03 , HC1, KCN, FeCl 3 , KOH, HgCl2, aqua regia, and 

H2 02 negative. SnClz slowly develops lamellar twinning. Occurs at Gowari 

Warhona, India, intimately intergrown with braunite; the braunite follows the 

twinning lamellae in sitaparite. Cleavage absent. Presence of sitaparite is 

indicated by bronzy luster apparent to naked eye. In some specimens sitaparite 

occurs as well-developed crystals with square and rectangular outline.

ANISOTUO'PIC 

KOH neg., HgClz nea.

Braunite (4Mn0.3Mii02 .Si02). Tetragonal; color gray; hardness G. Weakly 

anisotropic; effect may be overlooked. HN03 , HC1, KCN, FeCl 3 , KOH, HgCJ 2 , 

and aqua regia negative. H2 02 slowly effervesces without staining surface. 

Gives dark-brown powder when scratched. Frequently associated with liausmannite. Commonly in pseudo-octahedrons giving nearly equidimensional cross 

sections.

Cassiterite (Sn02). Tetragonal; color gray; hardness G. Anisotropic; polari/ation colors light to dark gray; boundaries between adjacent grains often straight 

edges. Negative to all reagents. Cannot be scratched by needle. Resincolored internal reflection; not always seen. Color of hand specimen white, 

yellow to black.

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G . Usually 

shows weak anomalous anisotropism; polarization colors gray, bornite pink. 

Some specimens negative to all reagents.

Ferberite (FeO.WOs). Monoclinic; color gray, like sphalerite; hardness E. 

Anisotropic; polarization colors yellow, brown, dark gray. Negative to all 

reagents. Powder dark, almost metaliic.

Goelhite (Fe2 03 .H20). Orthorhombic; color gray; hardness E. Moderately 

strong anisotropism. HNOs negative. HC1 doubtful; fumes tarnish slightly 

after long standing. KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia same 

as HC1. Gives yellow to orange-colored powder.

Hematite (specular) (Fe2 03 ). Hexagonal; color galena white; hardness G. 

Anisotropic; polarization colors light gray, dark brown, black; some grains show 

multiple twinning. Negative to all reagents. Habit platy.

Hollandite (m(Ba,Mn) 2Mn05+nFe4 (Mn05) 3). Tetragonal; color white with 

an almost imperceptible yellow tint differing from the faint bluish tint of hematite; 

hardness G but varies somewhat in different crystallographic directions. Some 

specimens are slightly softer and others harder than braunite. Anisotropic; 

polarization colors gray, yellow, blue; basal sections are isotropic. Some specimens show lamellar twinning similar to that of hausmannite and hematite. 

HN03, HC1, KCN, FeCl3 , KOH, HgCl2 , aqua regia, and H2 02 negative. SnClj
168 MICROSCOPIC DETERMINATION OF ORE MINERALS

positive. Always associated with other ores of manganese and in some specimens 

intergrown with hematite. Hollandite has a strong cleavage parallel to (101). 

It resembles hematite closely, but hematite gives a negative and hollandite a 

positive etch test 'with SnCl2 . Hematite has a red internal reflection, whereas 

hollandite has no internal reflection. Occurs in several localities in India.

Huebnerile (MnO.W03). Monoclinic; color gray, like sphalerite; hardness E. 

Strong anisotropism. Can readily be scratched by needle. Negative to all 

reagents. Gives yellow powder and dark-red internal reflection.

Ilmenite (FeO.Ti02 ). Hexagonal; color brownish gray; some specimens have 

suggestion of violet color; hardness G. Anisotropic; polarization colors light 

gray, dark brown. Negative to all reagents. Commonly intergrown with 

specularite and magnetite.

Jacobsite (MnO.Fe2 03). Isometric. Color pale gray with an olive tint. 

Hardness F, a little less than that of braunite. Some varieties are isotropic and 

others feebly anisotropic. HNO3 , HC1, KCN, FeCls, KOH, HgCl2, aqua regia, and 

H2 02 negative. From Jakobsberg, ^ Sweden, Szolczva, Hungary, and several 

localities in India. Always associated with other manganese minerals. An 

intergrowth of jacobsite and hausmannite is known as "vredenburgite." In this 

intergrowth hausmannite occurs as lamellae arranged parallel to the octahedral 

planes in jacobsite. In other specimens jacobsite is intergrown with hematite. 

Some specimens of jacobsite show deep-red internal reflection.

Polianite (MnO2 ). Tetragonal; color gray; hardness G. Strongly anisotropic; 

gives four extinctions. HNO3 , HC1, KCN, FeCl2 , KOH, HgCl, and aqua regia 

negative. H2 02 effervesces vigorously without staining surface. Prismatic. 

Very rare; found only at Flatten, Bohemia. Resembles pyrolusite closely but 

distinguished by hardness.

Rutile (Ti02). Tetragonal; color gray, similar to sphalerite; hardness G . 

Moderately strong anisotropism; four extinctions per revolution. Negative to all 

reagents. Orange to red internal reflection and powder. Almost invariably 

prismatic.

Sitaparile (9Mn2 03 .4Fe2 03.Mn02 .3CaO). Probably tetragonal: color a distinctive yellow tint with a suggestion of olive; much lighter than jacobsite; 

hardness G; slightly harder than braunite. Weakly anisotropic; some grains 

apparently isotropic. HN03 , HC1, KCN, FeCl3 , KOH, HgCl2 , aqua regia, and 

H2 02 negative. SnCl 2 slowly develops lamellar twinning. Occurs at Gowari 

Warhona, India, intimately intergrown with braunite; the braunite follows the 

twinning lamellae in sitaparite. Cleavage absent. Presence of sitaparite is 

indicated by bronzy luster apparent to naked eye. In some specimens sitaparite 

occurs as well-developed crystals with square and rectangular outline.

Wolframite ((Fe,Mn)O.W03 ). Monoclinic; color gray, like sphalerite; hardness 

E. Negative to all reagents. Shows cleavage in two directions. Gives darkbrown powder. Distinguished from huebnerite and ferberite only by chemical 

tests.

BINNENTHAL MINERALS

The following data cover minerals that have been reported only 

from Binnenthal, Switzerland. Data on lengenbachite and seligmannite are taken from Davy and Farnham's text. The present author 

was unable to obtain specimens of these minerals. The effect of
DETERMINATIVE TABLE 169

polarized light is inferred from the crystal system as given in Dana's 

System of mineralogy.

Dufrenoysile (2PbS.As2S3). Orthorhombic; color gray; hardness C. Weakly 

anisotropic; polarization colors shades of gray; shows multiple twinning. HN03 , 

HC1, KCN, and FeCl2 negative. KOH tarnishes iridescent and darkens. HgCl 2 

negative. Gives reddish-brown powder.

Baumhauerite (4PbS.3As2 S3 ). Monoclinic; color grayish white; hardness C. 

Anisotropic; polarization colors light gray, brown, dark gray; reveals multiple 

twinning. HNOs slowly tarnishes brown to black; brings out parallel etch 

cleavage; some areas unaffected. HC1, KCN, and FeCl3 negative. KOH tarnishes iridescent. HgCl 2 negative. Brittle. Gives red-brown powder. Perfect 

cleavage in one direction.

Lengenbachite (6PbS.(Ag,Cu) 2S.2As2S3 ). Triclinic; color gray; hardness B. 

Anisotropic. HNOs practically negative, but sometimes tarnishes slightly. 

HC1, KCN, FeCls, KOH, and HgCl 2 negative.

Rathite (3PbS.2As2 S3 ). Orthorhombic; color galena white; hardness C. Weak 

anisotropism. HN03 effervesces slowly and stains black; effervescence not 

always observed. HCl, KCN, and FeCl3 negative. KOH stains brown. HgCl2 

negative.

Seligmannite (Cu2S.2PbS.As2S3 ). Orthorhombic; color gray; hardness C. 

Anisotropic. HN03) HCl, KCN, FeCls, and HgCl2 negative. KOH slowly 

tarnishes iridescent.

DISCREDITED OR DOUBTFUL MINERALS 33

Arsenoferrile (FeAs2). Fine-grained loellingite.34 Specimen from Jachymov 

(Joachimsthal), Czechoslovakia. Contains 1.34 percent of copper according to 

an analysis by Dr. W. F. Foshag.

Chilenite (AgeBi). Specimen from Copiapo, Chile, the type locality, is an intergrowth of native silver and cuprite (Cu2 0).

Cliivialile (2PbS.3Bi2 S3 ). Specimens from Chiviato, Peru, the type locality, 

proved to be a mixture of bismuthinite and various copper minerals.

Comuccite. Formula not given by describer, but analysis is as follows: Pb 37.8(i, 

Fe 3.99, Sb 36.01, S 21.54, total 99.40. Specimen from San Giorgio, Sardinia, the 

type locality, is identical in physical properties and etch tests with jamesonite 

(4PbS.FeS.3Sb2S3).

Cuprobismutite (3Cu2S.4Bi2 S3). Specimen from Halls Valley, Colo., the type 

locality, is identical with emplectite (Cu2S.Bi2S3).

Dognacskaite (Cu, Bi, and S). Specimens from Dognacska, Hungary, the type 

locality, proved to be a mixture of bismuthinite and chalcocite and other copper 

sulphides.

Epiboulangerite (3PbS.Sb2S5). Identical in physical properties and etch tests 

with boulangerite. The formula Sb2Ss seems illogical, and the method of analysis 

employed was questionable.

Guejarite (Cu2S.2Sb2S3). Identical with chalcostibite (Cu2 S.Sb2 S3 ). (See 

Dana's System ^of mineralogy, Appendix I, p. 16.)

" The formula given after each mineral is the one which is usually given in the literature.

*' Buerger, M. J., The probable nonexistence of arsenoferrite: Am. Mineralogist, vol. 21, p. 70, 1936.

129151 40 12
170 MICROSCOPIC DETERMINATION OF OEE MINERALS

Hauchecornite ((Ni,Co)7(S,Sb,Bi) 8). Specimen from Hamm an der Sieg, 

Prussia, the type locality, is a mixture of two unknown minerals with the following 

properties:

1. Contains nickel, bismuth, and sulphur. Gave negative tests for cobalt and 

antimony. Color pink; hardness about E (Tahnage). Moderate anisotropism. 

HN03 slowly stains brown to iridescent. HC1, KCN, FeCl3 , KOH, and HgCl2 

negative.

2. Contains nickel, arsenic, antimony, and sulphur. Gave negative tests for 

cobalt and bismuth. Color gray with tinge of yellow; hardness about E. Strong 

anisotropism. HN03 slowly stains brown with slight effervescence. HC1, KCN, 

FeCl3 , KOH, and HgCl2 negative.

Hengleinile ((Fe,Ni,Co)S2). Specimen from Musen, Westphalia, the type 

locality, is a mixture of siegenite ((Ni.Co^S-i) and pyrite.

Kalgoorlite (HgAu2Ag6Tec). Specimen from Kalgoorlie, Western Australia, 

the type locality, proved to be coloradoite (HgTe).

Lehrbachite (PbSe,HgSe) and zorgite. Do not exist.35

Limonite (mixture of iron oxides). -Coarse-grained varieties are anisotropic; 

stalactitic limonite is usually amorphous and isotropic. Limonite is a mixture 

of oxides of iron, usually hematite and goethite; some varieties also contain jarosite. 

It is always of supergene origin and is most characteristically found in weathered 

outcrops. Gives yellow to orange-colored powder.

Matildiie (Ag2 S.Bi2Sa). Same as schapbachite.36

Onofrite (Hg(S,Se)). Specimen from Marysvale, Utah, is a mixture of tiemannite (HgSe) and sphalerite (ZnS).

Polyargyrite (12Ag2S.Sb2S3). Specimen from Wolfach, Baden, the type locality, 

is a mixture of argentite (Ag2 S) and tetrahedrite (5Cu2S.2(Cu,Fe)S.2Sb2S3).

Regnolite (5(CuS,FeS,ZnS).As2S3). Specimen from Cajamarca, Peru, the type 

locality, is tennantite (5Cu2S.2(Cu,Fe,Zn)S.2As2S3 ).

Tapalpite (3Ag2 (S,Te).Bi2 (S,Te)3)- Specimen from Tapalpa, Mexico, the type 

locality, is an intergrowth of tetradymite (Bi 2 (Te,S)3) and argentite (Ag2 S).

Turgile (2Fe2 03 .H2O). Is a fibrous variety of hematite (Fe2 0;t). 37

STAINING TESTS

In the determinative table no effort has been made to describe the 

exact chemical effect of subjecting the mineral surface to etching by 

liquids. Any test which left the surface of the specimen a different 

color or which left a corroded surface was termed a positive etch 

reaction. Likewise, the terms "stain" and "tarnish" are considered 

synonymous and come under the general heading of etching.

A more exact terminology has been developed by Gaudin.38 He has 

divided etching as follows: (1) Dissolution of the surface; (2) precipita35 Frebold, Georg, Ueber einige Selenerze und ihre Paragenesen in Harz: Centralbl. Mineralogie, 1927, 

Abt. A, pp. 16-32.

58 Ramdohr, Paul, Bleiglanz, Schapbachit, Matildit: Fortschr. Mineralogie, Band 20, pp. 56-57, 1936.

17 See Posnjak, E., and Merwin, H. E., The system ferric oxide-sulphur trioiide-water: Am. Chem. 

8oc. Jour., vol. 44, p. 1971, 1922.

39 Oaudin, A. M., Staining minerals for easier identification in quantitative mineragraphic problems: 

Econ. Geology, vol. 30, pp. 552-562,1935.
STAINING TESTS 171

tion on the surface of a coating differing in color from the mineral 

treated; (3) formation of transparent coatings that acquire interference colors. Combinations of any of these effects may occur and 

thus complicate the nature of the etching. Examples of dissolution 

of the surface are the corrosion of enargite by KCN and of chalcocite 

by HNO3 . A striking example of the deposition of a coating is the 

yellow coating deposited on stibnite by the KOH etch test. The formation of transparent coatings which display interference colors is exemplified by the iridescent tarnishing that is so commonly mentioned in 

the descriptions of minerals in the determinative table.

The object of Gaudin's investigation was to develop special staining 

reagents whereby it would be possible to determine very minute 

grains that were too small to isolate for microchemical tests. This is 

particularly important in investigating metallurgical products, especially mill tailings, where the ore minerals may not be seen in contact 

so that comparisons could be made. Where two minerals occur in 

contact, the eye is sensitive to slight color differences. Thus pyrrhotite 

and pentlandite are sufficiently different in ordinary ore specimens to 

involve no difficulty in distinguishing them. But where these minerals 

occur as tiny isolated grains in a bakelite mount of mill tailings the 

distinction is more difficult and very tiring to the eye. If, however, a 

reagent could be found that would stain one mineral a distinctive 

color and would stain the other mineral a different color or not stain 

it at all, numerical counts of these minerals in a polished section of mill 

products would be greatly facilitated. Gaudin decided that the third 

type of etching namely, the formation of iridescent transparent 

coatings offered the greatest possibilities of success. As these coatings are chiefly oxidation products, he sought for and experimented 

with strong oxidizing agents.

In practice it was found necessary to control all factors rigidly. The 

specimen is either unmounted or mounted in bakelite. It is immersed 

face down in & small beaker or dish, kept at the proper temperature on 

a water bath, and maintained there for a definite period of time. It 

is then; withdrawn with a noncorrosive forceps, washed under running 

water, then in a small stream of alcohol, and finally in a stream of 

diethyl ether, and then dried by a fan or by waving in air.

A very successful solution was made as follows: One part of chromic 

acid (Cr03 ) was first mixed with five parts of water; two parts of this 

solution was mixed with three parts of concentrated HC1; the mixture 

was diluted with water in the proportion of 1 to 2 and aged for half 

an hour or more before use.

Polished pentlandite exposed to this solution for 5 to 6 minutes at 

62° F. becomes brilliant blue in color; chalcopyrite is not changed in 

color, and pyrrhotite is darkened to a duller, darker bronze. If the 

period of etching is prolonged beyond 6 minutes, the pyrrhotite tends
172 MICROSCOPIC DETERMINATION OF ORE MINERALS

to take on brilliant colors. During 5 to 6 minutes of etching pyrite and 

arsenopyrite are unaffected and galena takes on a dull-yellow color.

The solution above described is also effective in distinguishing between enargite, tetrahedrite, and tennantite. A reaction time of 4 

to 5 minutes at 62° to 65° F. produces the following results: Enargite, 

unchanged; tetrahedrite, buff; tennantite, blue green. Solid solutions 

of tennantite in tetrahedrite exhibit colors ranging from blue green 

to dull buff through apple green, yellow green, and straw.

The staining method of Gaudin is more akin to the structure etching 

of Schneiderhohn than to the systematic etching scheme of Davy and 

Farnham, which is closely followed by the writer. As definite colors 

are sought for, the time of etching varies for different mineral associations and is not held to a definite time as specified in the determinative 

table. Experiments by Gaudin and his coworkers are being continued 

for the purpose of finding other staining reagents for other mineral 

associations. The method promises to become very valuable, especially 

in the study of metallurgical products and in the investigation of 

minerals in the polished section too small to isolate for microcbemical 

tests.
PART 4. MICROCHEMICAL METHODS

INTRODUCTION

Microchemical qualitative analysis consists in testing for one or 

more unknown elements in a solution by adding suitable reagents 

that will precipitate these elements, if present, out of the unknown 

solution. The reactions are carried out on a glass slide and observed 

under the microscope. The reagents usually chosen are those that 

will precipitate the elements as crystals of characteristic shape and 

color; the crystals are thus diagnostic of the elements sought. With 

only a few elements is the precipitate amorphous, and this is only 

because suitable crystalline precipitates are lacking. In a color reaction no precipitate is thrown down, but the unknown solution 

turns a characteristic color when the reagent is added. 

" Two great advantages possessed by microchemical analysis over 

ordinary qualitative analysis are the saving of time and the saving of 

material. Practically every microchemical analysis can be made in 

10 minutes or even less, whereas an ordinary qualitative analysis may 

consume hours in evaporations over a steam bath or in nitrations. 

Microchemical analyses also require much less material than ordinary 

chemical analyses. A microchemical analysis can usually be made 

on a quantity of material almost invisible to the unaided eye.

Microchemical methods are by no means new or modern, but they 

have been singularly neglected by the analytical chemist, owing to 

the prevailing belief that microscopic methods are intricate and require a long apprenticeship. They have received far greater recognition from European than from American chemists and petrologists.

The pioneer investigations in microchemistry were carried out by 

Boricky, 1 who worked out methods for determining the elements in 

rock-forming minerals. The methods used by Bofick£ were improved and expanded by Prof. H. Behrens,2 of the Polytechnic School 

of Delft, Holland, who developed microchemical methods for the 

identification of practically all the known solid elements. His book 

was later revised and translated into English by himself.3 Behrens 

deserves the credit of having done more to further the science of 

microchemistry than anyone else. Several editions of his textbook

' Boficky^ Emmanuel, Elementc einer neuen chemisch-mikroskopischen Mineral- und Oesteins-analyse: 

Nattmviss. Landesdurchf. Bohmen Archiv, Band 3, Prague, 1877. Translated into English by Winchell, 

N. H., Minnesota Geol. Survey Ann. Kept, (or 18!)0, vol. 19, pp. l-8u, 1892.

8 Behrens, II., Mikrochemische Methoden, Amsterdam, 1882.

3 Behrens, H., A manual of microchemieal analysis, London, 1894. (Now out of print.)

173
174 MICROSCOPIC DETERMINATION OF ORE MINERALS

have appeared, and the latest, in 1921, revised by Kley,4 is the foremost work on this subject in Europe.

The best text written in English is that of Chamot and Mason,5 of 

Cornell University, where Chamot has carried on instruction in 

microchemistry for more than 30 years. For many years Cornell 

was the only institution in the United States where microchemistry 

was taught. Kecently other institutions have taken it up, but it is 

regrettable that many of the leading universities still offer no courses 

in this subject.

Credit for the first application of microchemical methods to the 

identification of opaque minerals in polished sections is due to Dr. 

Waldemar Lindgren, who initiated instruction in this application .of 

the method at the Massachusetts Institute of Technology in 1920. 

The present writer first became acquainted with microchemical 

methods while studying under Professor Lindgren in 1922. The 

possibilities of the method were set forth in a timely editorial.6

In 1927 McKinstry 7 contributed a valuable article, references from 

which will be found in the description of the tests for arsenic (p. 208) 

and elsewhere in this bulletin.

Putnam, Roberts, and Selchow 8 are coauthors of a valuable series 

of articles on microchemical methods. These authors record the 

results of much new and original research work and give tests for some 

of the rarer elements. The present writer is indebted to them for the 

pyridine double bromide test for gold that appears on page 223.

PROCEDURE AND APPARATUS

The elements of a mineral are identified by means of microchemical 

tests made on a very small amount of material gouged out of the surface 

of the mineral. In order to be certain that the material tested is 

pure that is, does not contain extraneous minerals which may be 

intergrown with the mineral to be tested a polished surface is prepared and observed under a reflecting microscope, and the powder 

to be tested is gouged from this surface by means of a chisel-pointed 

needle. The powder thus obtained is collected by means of a sharpened match or splinter of soft wood, the point of which has been 

moistened and wiped nearly dry. The powder adheres to the damp 

wood. The match with its adhering powder is dipped into a drop of 

nitric acid or other suitable acid on a glass slide. The surface tension 

which holds the powder to the wood is broken, and the powder drops

4 Behrens, H., Mikrochemische Analyse, revised by P. D. C. Kley, Leipzig, Leopold Voss. 1921. (No 

translation into English has yet appeared.)

8 Chamot, E. M., and Mason, C. W.. Handbook of chemical microscopy, 2 vols., New York, 1931.

6 Lindgren, Waldemar, Microchemical reactions; Econ. Geology, vol. 19, p. 762,1924.

' McKinstry, H. E., Microchemical methods in the identification of opaque minerals: Econ. Geology, 

vol. 22, p. 830.1927.

s Putnam, P. C., Roberts, E. J., and Selchow, D. H., Contributions to determinate mineralogy: Am. 

Jour. Sci., 5th ser., vol. 15, pp. 89-102, 253-261, 423-430, 455-460,1928.
MICKOCHEMICAL METHODS 175

from the wood and falls to the bottom of the drop.9 The acid is 

evaporated off by holding the slide over a microflame. The residue 

is observed under the microscope to see if any of the original material 

has remained unattacked. If necessary, successive drops of the 

reagent can be applied and evaporated off until the material has been 

dissolved or broken up chemically. The residue is tested by means 

of microchemical tests.

In general, this method can be applied to any mineral, but it is 

most suitable to the opaque or nearly opaque ore minerals, because 

nearly all of these can be taken into solution by means of ordinary 

acids. The quantity of material tested depends on the delicacy of 

the test, but usually an amount which can be collected within an area 

on the polished surface from 0.17 to 0.35 millimeter in. diameter is 

sufficient. The textbooks used in this work are those of Chamot and 

Mason and of Behrens and Kley, already referred to. Neither was 

written primarily with the idea of adapting the methods to mineralogy, 

but the methods described in both are for the most part applicable 

to these tests. Chamot describes in great detail the effect that some 

interfering elements have on the tests. However, his descriptions do 

not exhaust the combinations of elements found in ore minerals. 

Behrens and Kley's book is written in German. It gives tests for 

practically every element found in ore minerals but is open to the 

objection that the effect of interfering elements is hardly mentioned. 

For instance, the thallium nitrate test for gold is given the preference 

over other tests. 10 It is a very beautiful and delicate test when 

applied to pure native gold but is valueless in the presence of tellurium ions.

The efforts of the present writer have been directed to the determination of the extent to which a test for an element in a mineral is 

interfered with by chemical compounds of other elements occurring 

in the same mineral. He has found, for instance, that the presence 

of copper ions does not interfere with the precipitation of silver as a 

bichromate, whereas lead is precipitated as bichromate simultaneously 

with silver bichromate and somewhat masks the recognition of the 

silver compound. Consequently, if lead is greatly in excess of silver 

it must be removed from the solution before proceeding with the 

silver test. In other tests a double compound of the interfering 

elements may be formed. For example, lead is usually precipitated 

as an iodide. This is honey yellow in color and crystallizes in welldefined plates. But if mere traces of bismuth ions are present in 

solution, the lead precipitate is entirely changed in character, and a 

rusty-brown amorphous precipitate results.

9 This method of transferring material from a polished section to a glass slide was originated by Dr. K.K. 

Landes while a student in the Laboratory of Mining Geology at Harvard University. 

' 'Behrens, H., and Kley, P. D. C., op. cit. (1921), p. 155.
176 MICROSCOPIC DETERMINATION" OF ORE MINERALS

A more detailed investigation of the mutual interference of elements 

has recently been completed by Fraser and Dreyer. 11 This is based 

largely on the systematic scheme published in the first edition of this 

bulletin. As mentioned elsewhere, Dr. Fraser is one of the originators 

of the scheme. Excerpts from their article are given in the outline 

of microchemical tests (p. 243) and elsewhere in this bulletin.

These interfering effects have made it necessary to experiment on 

each mineral until a suitable procedure could be developed to obviate 

the difficulties encountered. Usually a chemical separation is necessary. One or more of the elements in the drop is precipitated, leaving 

the other element or elements in solution. Methods of filtration on 

a microscopic scale were tried, especially the Behrens method described 

by Chamot and Mason. 12 These methods were found unsuitable 

owing to the small size of the drops used in the present investigation, 

the amount of liquid absorbed by the filter paper being too great. A 

suitable method of filtration was devised, involving the use of small 

capillary tubes. The precipitate in the drop is allowed to settle; 

water is carefully added to the drop in case the evaporation is rapid. 

Usually the precipitate clings to the glass slide after it has settled. 

The capillary tube is dipped into the drop, and the filtrate is drawn 

up into the tube by capillarity. The presence of a few particles of 

the precipitate in the liquid can be neglected. No difficulties from 

this source have been encountered by the writer. The tube is then 

moved to another place on the slide, and the liquid is removed by 

blowing on the other end of the tube. The lower end of the tube 

should be in contact with the slide before blowing the drop out of the 

tube; otherwise the drop tends to cling to the outside of the tube.

The precipitate is then washed with one or more drops of water 

or dilute acid to remove the remaining traces of the filtrate. Each drop 

is picked up by means of the capillary tube and either added to the 

filtrate or blown out onto a blotter and rejected.

Collecting the material. The polished section is placed in the field 

of the microscope and observed, using a short-mounted Bausch & 

Lomb 16-millimeter or Leitz No. 3 objective. Objectives of higher 

magnification focus too close to the surface of the specimen to permit 

the free manipulation of the needle.

An area of the mineral to be tested is then selected. It should be 

free from small inclusions of extraneous minerals. The diameter of 

the mineral should be a little larger than the minimum diameter required to give a satisfactory test, as it requires considerable skill to 

avoid pushing the needle over the mineral boundary.

The size of needle depends chiefly on the hardness of the mineral 

scratched. The needles used by the author range from Sharps No. 6

11 Fraser, H. J., and Dreyer, R. M., Mutual interference in the microchemical determination of ore minerals: Am. Mineralogist, vol. 22, pp. 949-976,1937. 

's Chamot, E. M., and Mason, C. W., op. cit., vol. 2, p. 11.
MICROCHEMICAL METHODS 177

to No. 12 in size. With the smaller sizes a smaller area can be scratched. 

The No. 12 needles are very easily broken, however, and can be used 

only on very soft minerals. For harder minerals, such as cobaltite, 

the force necessary to obtain a groove requires .a larger and stronger 

needle, and No. 6 has been found satisfactory.

The point is ground off by holding the needle diagonally against a 

rapidly rotating carborundum wheel. The surface obtained is a 

frustum of a cone. The contact between needle and wheel should be 

only momentary, a fraction of a second, as it is not necessary to grind 

off entirely the tapering point of the needle. The beveled surface 

should make an angle of about 60° with the axis of the needle. (See

fig. 26.)

The needle is then brought into the field of the microscope and 

the point is worked back and forth within the area until a groove 

or craterlike cavity is gouged. Beveled 

The depth of this cavity is

usually kept considerably less E"d vie» 

than the diameter of its inter- FIOUE. n-***^* * 1 * for scraping 

section with the surface of the

section a precaution necessary to avoid penetration into an adjoining underlying mineral.

Some difficulty is usually experienced in scratching the harder minerals. It is best to follow a small pit, seam, or abrasion on the surface. The needle wears rapidly and should be sharpened after every 

operation. In the softer minerals a roUing motion while pushing 

the needle is sometimes of help.

A sharpened match is highly effective in collecting the powder. A 

jagged point is to be avoided. The point should be smooth and regular in shape. A safety-razor blade has been found superior to a knife 

in sharpening the match. After wetting the end of the match, it 

should be wiped with a clean rag to remove the superfluous moisture 

before touching it to the specimen. If it is too wet, some liquid is 

likely to be transferred from the match to the specimen. A little 

practice will indicate the extent to which the moisture should be 

removed.

A platinum wire, in which a loop has been bent at its lower end, 

is the best instrument for applying the drops of the necessary reagents 

to the slide. The size of the drop should be kept to a minimum, and 

the deeper the drop with respect to its diameter on the slide the 

slower wiU be the rate of evaporation and the larger the growth of the 

crystals to be investigated. A wire of very small diameter bends too 

readily and is hard to keep in shape. A wire of comparatively large 

diameter does not readily permit the bending of a small loop. The 

diameter of the wire used by the writer is 0.35 millimeter, and the
178 MICEOSCOPIC DETERMINATION OF ORE MINERALS

inside diameter of the loop is 0.62 millimeter. The loop is formed by 

bending around a needle.

Glass slides. The chemical operations are carried out on a glass 

slide. The slide should be perfectly clean and free from scratches or 

abrasions. The slides used in mounting thin sections for petrographic 

work are satisfactory. Those used in this work are 25 by 45 millimeters in area and not over 1 millimeter thick. If heat is to be applied, 

the drop should be placed at one corner of the slide; otherwise the slide 

will almost certainly break.

Keeping a slide clean is much more difficult than would appear at 

first thought. The slightest trace of grease or other material on the 

slide causes the drop to spread and prevents certain manipulations 

necessary for the best performance of the tests. The usual sulphuric 

chromic cleansing mixture is occasionally used for cleaning slides. 

The writer uses a soap containing pumice. The soap is rubbed with a 

small wet sponge until the sponge is partly filled with lather. The 

slide is first rubbed with the soapy sponge, then held under a faucet 

supplying water as hot as can be comfortably endured. When the 

lather is completely removed by the hot water, the slide is dipped into 

a vessel containing distilled water in order to remove the remaining 

tap water. The slide is then removed, shaken vigorously to remove as 

much water as possible, and dried by rubbing with filter paper.

Drying the slide with a cloth tends to add lint or a film of grease 

to the slide. The clean slides are kept in a beaker and protected from 

dust by a watch-glass cover.

The tests given by Chamot and Mason 13 illustrate the difficulty in 

getting a perfectly clean slide. The extreme cleanliness required by 

the use of the ultramicroscope is not necessary, but the slide should be 

sufficiently clean to permit the performance of Chamot's method I. 

(See p. 181.) The following test is sufficient to determine this matter 

of cleanliness: Dip a clean platinum wire into a drop of water placed 

on the slide and draw the wire slowly out of the drop, keeping the 

end in contact with the surface of the slide. If a channel of liquid 

accompanies the wire, the slide is clean, for otherwise the liquid will 

not follow the wire.

Microburner. The ordinary Bunsen flame is not suitable for microchemical work, as it spreads the heat over too great an area. The 

somewhat elaborate auxiliary flame described by Chamot and Mason 14 

is likewise not essential. An improvised alcohol lamp made by the 

writer (fig. 27) is just as good and obviates the necessity of a gas 

connection. The temperature attained by the flame need not be 

much above the boiling point of the acids used. Chemical reactions

' 3 Chamot, E. M., and Mason, C. W., op. cit., vol. 1, p. 173. 

i« Chamot, E. M., and Mason, C. W., op. cit., vol. 1. p. 138.
MICROCHEMICAL METHODS 179

FT

fe'^:»?i^ £§$ *?

ii

 c

involving roasting or sublimation have not been utilized in the present 

investigation; all tests used are wet tests. The principal requirement 

of the lamp is that it furnish a flame of small cross section.

Reagents. A great deal more difficulty is experienced in getting a 

mineral in solution by evaporating a drop of acid containing the 

mineral on a glass slide than by placing the mineral in a test tube 

containing the reagent and boiling the reagent a minute or two. 

When heating the drop on a slide, a good part of the reagent is evaporated before it comes to boiling, and even after it reaches the boiling 

point it is entirely evaporated within a few seconds at the most. For 

this reason it is essential to have a drop of minimum diameter and 

maximum thickness rather than to have it spread, out over the slide.

Very few of the ore minerals thus far tested 

have .hitherto resisted attack by the reagents 

used. Some oxides, such as chromite, hematite, and cassiterite, cannot be brought into 

solution by nitric acid on a slide. Some sulphides, sulpharsenites, and sulphantimonites 

go into solution with great difficulty. For such 

minerals Chamot and Mason recommend the 

use of a small capsule or crucible covered with 

a glass slide, together with a suitable holder. 

In this way the reagent can be brought to boiling point and held there a minu te or so . ] 6 Pyr argyrite, which is almost insoluble when heated 

on a slide, was tested in a crucible and was easily 

broken up chemically. Above twice the quantity of material ordinarily used on a glass slide is necessary when 

using the capsule. The liquid is boiled to dryness. The capsule is 

allowed to cool, then leached with three or four drops of dilute acid. 

This is picked up by means of the capillary tube and transferred to a 

glass slide and there subjected to the usual treatment.

The reagent most frequently used is 1:1 nitric acid (1 volume of 

concentrated HN03 , specific gravity 1.42, plus 1 volume of H20). 

Aqua regia (3 volumes of concentrated HC1 plus 1 volume of concentrated HN03 ) is used where a strong oxidizing action is required. 

Where it is necessary to make use of a strong oxidizing action without 

introducing chlorine ions, fuming nitric acid is used. The stronger 

reagents have a tendency to spread over the slide, and it is greatly 

preferable to use 1:1 HN03 where possible. The reagent to be used 

is always determined by trial.

Capillary tubes. The process of separating a filtrate from a precipitate is described above (p. 176). The capillary tubes used by the 

writer are made from a tube of soft glass originally 3.6 millimeters in

" Chamot, E. M., and Mason, C. W., Handbook of Chemical Microscopy, vol. 2, p. 7, New York, 1931.

FIGURE 27. Improvised alcohol lamp, a, Tinfoil cover to 

keep cork from catching flro; 

b, brass tube r, cork; d, ink 

bottle; e, one or two strings.
180 MICROSCOPIC DETERMINATION OF ORE MINERALS

puter diameter, 2.2 millimeters in inner diameter, and about 16 centimeters long. The tube is held, one end in each hand, and heated in 

the middle over a hot flame of small cross section; when red hot, but 

before it begins to sag, it is quickly removed from the flame and elongated by pulling the hands away from each other. (See fig. 28.) 

Puring the heating the tube should be given a rotary motion in order 

to heat the cross section equally. The tube should not be elongated 

while still in the flame. An ordinary Bunsen flame is not suitable 

for this purpose, as it does not concentrate the heat in a sufficiently 

small cross section, and the improvised alcohol lamp already described 

does not give a hot enough flame. A blowpipe flame has been found 

most satisfactory. The writer uses a blowpipe operated by a power 

blast and fixed in a stand. An ordinary hand blowpipe serves just as

V

FIGURE 23. Capillary tube. About natural size.

well, but the services of an assistant who operates the blowpipe will 

be necessary, as the worker will need both of his hands to draw out 

the tubing.

Ordinarily the capillary tube thus obtained is longer than is convenient to work with and is therefore broken off at a point 8 to 10 

centimeters from the constriction. The inner diameter of the capillary part generally ranges from 0.06 to 0.12 millimeter. If the diameter is smaller than 0.06 millimeter it is difficult to blow the liquid out 

of the tube. If larger than 0.12 millimeter, the liquid is blown out 

too easily and tends to form a drop of large diameter in comparison 

to its volume. With a little practice satisfactory tubes can easily 

be made. The diameter of the capillary part is remarkably uniform 

over a comparatively great length, and it is still more remarkable 

that the walls of the tube rarely coalesce anywhere. Tubes 2 feet 

or more in length can be prepared in this manner with the exercise 

of a little skill. The longer the tube is drawn out the smaller the 

diameter tends to be. Usually the best practice is to draw it out about 

a foot and then break it off to a convenient length.

METHODS OF APPLYING TESTS

The criterion used in the recognition of nearly every element is 

the character of crystals formed by precipitation within the drop. 

It is therefore desirable to regulate the conditions of the tests so as 

to obtain crystals of maximum size and constant character. The 

reagent used in bringing about the precipitation should have a higher 

concentration than the unknown elements in the drop to be tested. 

If the concentration of ions that unite to form the precipitate is too 

great, the crystals will be small and poorly formed. If the concen-
MICHOCHEMICAL METHODS 181

tration is too low, the precipitate will not form at all. The 

medium at which the crystals will be of maximum size is determined 

by experiment. For instance, the crystals of silver bichromate ar<l 

of maximum size and most satisfactory in character when a smalj 

fragment of ammonium bichromate is placed in a 0.5 percent solution 

of silver nitrate. Similarly the crystals of copper mercuric thiocya-? 

nate, which are the most useful for the recognition of copper, are most 

satisfactory in size and shape when a solution of potassium mercurid 

thiocyanate is added to a 0.5 percent solution of copper nitrate. The 

writer believes that approximately this concentration is most satis* 

factory for most elements. Efforts have been made, therefore,- to 

start with a quantity of material that will yield approximately this 

concentration of the metallic element in the drop to be tested. If the 

concentration is too great the fact will be manifest at once by the 

abundance of the precipitate and its tendency to be amorphous. The 

test can therefore be repeated on new material, or the drop can b.e 

diluted and a portion picked up by the capillary tube, transferred to 

a new place on the slide, and tested. On the other hand, efforts have 

been made to ascertain the minimum amount of mineral necessary 

to give a definite positive test for the element in question. Chamot10 

says:

It is essential that reagents be always applied in the best possible manner 

and in concentrations and under conditions such as will lead to the separation 

of a solid crystalline phase in a very short period of time. It is therefore necessary that we first ascertain the best method of procedure for each particular 

reagent. Most failures to obtain satisfactory results when attempting microchemical reactions are due to a lack of appreciation of the importance of this fact. 

Manuals of microchemical analysis usually neglect to state definitely the best 

manner of adding a reagent to the drop to be tested, assuming that the investigator will ascertain for himself the conditions which will yield him products mogt 

easily identified. Under similar conditions as to concentration, acidity, and 

manner of reagent application, the crystalline phase will not only almost invariably separate with the same habit, but the crystals will usually develop to tljje 

same size and will lie upon the object slide in each experiment in the same positions 

with respect to faces.

Chamot and Mason describe the typical procedures used by them 

and designate them by roman numerals. Their nomenclature is 

followed by the present writer. Some of their procedures had no 

application in the writer's tests and are not here described. ;

Method I is the one most used in this investigation and is preferable 

to any other. 17 "A drop of the reagent is placed near a drop of the 

solution to be tested. By means of a platinum wire or drawn-out 

glass rod a tiny channel is made to flow from the reagent into the test 

drop, the slide being tipped to facilitate the flow, but under no condition should the two drops merge completely." If the slide is not

'» Chamot, E. M., Elementary chemical microscopy, p. 298, New York, 1921. 

17 Chamot, E. M., and Mason, C. W., op. cit., vol. 2, p. 30.
Slide

182 MICKOSCOPIC DETERMINATION OF ORE MINERALS

perfectly clean, it will be almost impossible to form a channel between 

the drops, as the liquid will not wet the glass. Except in a test for 

iron a sharp-pointed needle is better than a platinum wire or glass rod 

for joining the drops. A platinum wire of small diameter will bend 

too easily, and a tiny glass rod is too fragile. The presence of a little 

iron gained from the needle can be ignored in by far the greater proportion of tests. Where iron is objectionable a sharpened match 

serves very well to form the channel. The size of the drops used by 

the writer is smaller than that pictured by Chamot (see fig. 29), the 

maximum being 2 millimeters and the average 

about 1.5 millimeters. The distance between 

the edges of the drops is usually not more than 

1 millimeter.

In method II a tiny fragment of the solid 

reagent is added to a drop of the solution of the 

substance to be tested. This method is of 

frequent use. It furnishes a high concentration 

of ions of the reagent but is open to the objection 

that the quantity of reagent applied is not so 

easily controlled as when applied as a liquid of a

FIGURE ^y. Diagram showing '. *

chamot's method i. The definite concentration. Beginners almost 

reagent drop (r> has been made invariably use too large a fragment of the

to flow into the drop to be test- -, -, -, f i

ed (s) through a tiny channel reagent, and this causes a large proportion of the

(c>. The precipitate separates reagent to crystallize out in advance of the

precipitate desired and to mask it. The writer

crushes the solid reagent and spreads it over a dark surface and by 

means of a moist sharpened match picks up about the smallest fragment that can be seen by the unaided eye and transfers it to the center 

of the drop. If, after the microscopic observation, more reagent 

seems to be needed it can be supplied.

In method I-A a drop of the reagent is added to the test drop at 

its center. 18 An example of this method is given on page 178 of this 

paper. Where it is desirable to keep the drop from spreading, the 

test drop is slowly evaporated, and when it is nearly dry but still 

moist a drop of the reagent is added directly to it. An example of 

this procedure is the ammonium molybdate test for arsenic.

The operations of joining the drops or dropping the fragment in 

the test drop are best done under the microscope, using a Leitz No. 2 

or a Bausch & Lomb 32-millimeter objective. After the crystals of 

the desired precipitate begin to form, higher powers can be used. 

The Leitz No. 4 is the objective most used for final observation. It 

combines high magnification with clearness of vision and good peris Chamot, E. M., and Mason, C. W., op. cit., vol. 2, p. 31.
MICROCHEMICAL METHODS 183

spective. Objectives of higher magnification than this are only rarely 

needed.

In both of the textbooks cited more than one test is usually given 

for one element. The choice between tests is determined only by 

experiment. Some of the tests are of such great delicacy that they 

indicate the presence of an element when its proportion in a mineral 

is so small that it does not appear in the formula given in the usual 

texts. For instance, when chalcocite is taken into solution in nitric 

acid, the test drop will usually stain a solution of potassium mercuric 

thiocyanate pink, indicating the presence of iron in chalcocite. In 

such tests experience is the best guide in judging whether the element 

is present in sufficiently large amounts to be recorded in the formula. 

These tests can also be supplemented by tests of lower delicacy.

The microchemical method cannot be used to distinguish between 

two minerals having the same elements, such" as boulangerite and 

zinkenite, or petzite and sylvanite. Moreover, the susceptibility of 

this method is strictly limited, and reliable tests cannot be expected 

on quantities of material represented by areas on the section less than 

0.17 millimeter in diameter. It is useless to make tests on grains of 

a mineral so minute that they can be seen only with objectives of 

stronger magnification than the 16-millimeter lens. With such 

material the identity of a mineral must be based on other characteristics, such as behavior in polarized light, cleavage, color, hardness, and 

etching behavior.

SENSITIVITY OF MICROCHEMICAL TESTS

In. the outline preceding the description of each test is given the 

concentration of the most dilute solution that will yield a satisfactory reaction. It is recognized that to assign, a definite value for 

allowable dilution, is always more or less arbitrary. The figures 

given are based largely on the result of the writer's experience in 

teaching microchemistry to advanced students in economic geology. 

They are believed to represent the most dilute solutions that ordinarily will yield consistent results to the average student.

The molality of a solution with respect to an element is defined 

as the weight of the element in grams per liter of solution divided by 

the atomic weight of the element. For instance, a solution of lead 

acetate containing 0.379 gram of salt per 100 cubic centimeters is 

0.01 molal with respect to lead; likewise a solution of cobalt nitrate 

containing 0.291 gram of salt per 100 cubic centimeters is 0.01 molal 

with respect to cobalt. These two solutions have an equal number 

of atoms of the metallic element per unit volume.
184 MICROSCOPIC DETERMINATION OF ORE MINERALS

A comparison of the molalities of the limiting solutions, shown in 

the following table, gives a truer conception of the sensitivity of the 

tests than the concentrations of the solutions themselves:

Approximate sensitivities of microchemical tests

Element

Zinc............

Do 

Do 

Cobalt. ........

Do 

Do 

Do ..... .

Do. .....

Silver..........

Do...--...

Do. -.--.

Do .. 

DoDo . 

DoDo 

Do ..

Do .... .

Tin.. ..... ..

Do ...

Gold .

Do ... ...

Manganese ... ..

Test

Potassium mercuric thiocyanate.

..... do ..... ... ....... ........

Potassium mercuric thiocyanate.

Potassium mercuric thiocyanate.

Potassium mercuric thiocyanate.

Thiourea _______ . ..........

Ammonium molybdate.. ....... 

Potassium iodide __ .. __ -...

Cobalt mercuric thiocyanate. ...

Concentration of most dilute 

solution (grams of salt 

per 100 cubic centimeters 

of solution)

0.01 Zn(CjH3 02)2.3H20 

0.01 Cd (NO3 ) 2 .4H2O .....

0.01 Cu(NOs)2.3HjO.. _

0.02 CuCl2 .2H2 0....._ ... ...

0.02 Ni(N03) 2 .6H2 0 .........

..... do ......... ....

0.01 Co(N03)2.6H2O..... ....

0.02 Co(N03 )2.6H2O. .....

0.05 C6(N03) 2 .6H2O. . ....

0.02 Fe(N03)3.9H2O.._. ..

 ..do...... ..............

0.03 Fe(NOj) 8 .9HsO. .

0.03 AgNOs.. ... .... .

0.02AgNOs-- 

 .do.... ........ .

0.03 Se02. ___ --- . __ .

0.05 SeO2 . ... ... _. _ .

.... . do.. ............

0.01 (NH4)sAsO4.3H2O.. . 

0.02 (NH4)3AsO4.3H2O.. 

. .do. ..... ...........

0.02 SbClt. ...............

0.01 SbCls 

0.02 Bids 

0.01 BiCls 

0.01 SnCh- __ .. _ . __ ...

0.005 Pb(C2H3 02 ) 2 .3H2 0-... 

0.10 Pb(C2H3 02).3H20.. 

0.02 HAuCl4.H2O. ..........

0.10 HAuCl4.H2O... ... ..

0.005 HgCls-. .

0.02 MnCl 2 .4H2 0... ... ...

0.15 HtSOi.... ..............

Concentration of 

element

Grams per 

100 cubic 

centimeters

0. 0027 

.0033 

.0026 

.0095 

.0041 

.0041 

.0020 

.0041 

.0101 

.0028 

.0028 

.0041 

.0190 

.0127 

.0127 

.0356 

. . 0214 

.0356 

.01 

.01 

.0300 

.0061 

.0061 

.0107 

. 0053 

.0132 

.0066 

.0063 

.0027 

.0546 

.0016 

.0580 

.0037 

.0054 

.0545

Molalityof 

solution

0.00042 

. 00032 

. 00041 

. 00149 

. 00069 

. 00069 

. 00034 

.00069 

.00172 

.00050 

.00050 

.00074 

.00177 

.00118 

.00118 

. 00449 

.00270 

. 00449 

. 00078 

. 00078 

.00040 

. 00080 

. 00080 

. 00086 

. 00043 

. 00062 

.00031 

. 00053 

.00013 

. 00264 

. 00060 

. 00300 

.OC018 

.00101 

. 00154

REAGENTS

Below is a list of the reagents used.

Acids, ammonia, and hydrogen peroxide

1 : 1 HN03 1 volume of concentrated HN03 (specific gravity 1.42) added to 1

volume of H2 0.

1 : 7 HNO3 1 volume of 1 : 1 HN03 added to 3 volumes of H2 0. 

1 percent HN03 1 volume of concentrated HNO3 added to 99 volumes of H2 0. 

Aqua regia 1 volume of concentrated HN03 added to 3 volumes of concentrated

HC1 (specific gravity 1.09). 

Concentrated HC1.

1 : 5 HC1 1 volume of 1 : 1 HC1 added to 2 volumes of H2 0. 

5 percent HC1 5 volumes of 1 : 1 HCl added to 45 volumes of H2 0. 

Concentrated H2S04 . 

1 : 3 H2S04 . 

20 percent H2S04 . 

NH4OH concentrated solution. 

20 percent NH^OH 1 volume of concentrated NH4OH added to 4 volumes of

H2 0.
MICROCHEMICAL METHODS 185

Pyridine-hydrobrornic acid 1 volume of pyridine added to 9 volumes of 40 percent 

HBr.

Salt solutions

Dimethyl glyoxime 2 percent solution in alcohol.

Potassium mercuric thiocyanate 3 percent solution in water. (See directions

for preparing salt below.)

Stannous chloride 2 percent solution in 1 : 5 HC1. 

Ammonium molybdate 1.5 percent solution in 1 : 7 HN03 . 

Sodium sulphide saturated solution.

Cinchonine reagent 1 gram cinchoriine in 8 grams of 1 : 1 HC1. 

Diphenyl carbazide 1 percent solution in alcohol. 

Benzidine reagent 0.05 gram solid in 10 cubic centimeters of glacial acetic acid

and 90 cubic centimeters of water. 

Ammonium bichromate reagent 0.1 gram K2Cr2 07 in 33 cubic centimeters of

water.

Potassium thiocyanate 10 percent solution in water.

Rhodanine reagent 0.03 percent dimethyl-amino-benzol-rhoclanine in acetone. 

Thiourea 10 percent solution in water.

Solids

Calcium acetate Ca(C2H3 02) 2 . Metallic zinc Zn.

Soda-niter flux 19 parts Na2C03 and 1

part KNO3 . 

Hydroquinone. 

Thiourea SC(NH2). 

Fluorite CaF2 . 

Sodium chloride NaCl. 

Ammonium chloride NH4 C1.

Cesium chloride CsCl. 

Potassium iodide KI. 

Potassium nitrite KN02 . 

Potassium ferrocyanide lv4Fe(CN)o. 

Cobalt nitrate Co(N03 ) 2 .6H2 0. 

Potassium thiocyanate KCNS. 

Ammonium bichromate (NH4 ) 2Cr2 07 . 

Rubidium chloride RbCl. 

Copper acetate Cu(C2H3 02) 2 . 

Sodium bismuthate NaBi03 .

Potassium mercuric thiocyanate is not purchasable in the market 

but can be made up in the laboratory as follows:

Dissolve 2 grams of metallic mercury in 20 cubic centimeters of 1 : 1 HN03 . Mercury goes readily into solution without the application of heat. Dilute to 

about 100 cubic centimeters. Prepare a saturated solution of ferric alum by 

adding an excess of the solid reagent to about 5 cubic centimeters of water. 

Filter off the undissolved solid and add 1 : 1 HN03 a drop at a time to the ferric 

alum solution until the reddish color is removed. Pour about 1 cubic centimeter 

of the ferric alum solution, which serves as the indicator in the subsequent titration, into the mercuric nitrate solution.

Prepare a 10 percent solution of potassium thiocyanate by dissolving exactly 

10 grams of KCNS in water and adding enough water to make exactly 100 cubic 

centimeters of solution.

The mercury is now completely precipitated as thiocyanate by titrating KCNS 

into the beaker containing the mercuric nitrate. The reaction proceeds as follows:

Hg+2KCNS = Hg(CNS) 2 +2K 

200.6 + 194.2 =316.6 + 78.2

(The nitrate ions are ignored, as the same quantity occurs on both sides of the 

equation.) 19

'» This procedure is used for determining mercury in gravimetric analysis. See Hillebrand, W. F., and 

Lundell, O. E. F., Applied inorganic analysis, p. 172, New York, 1929.

129151 40 13
186 MICROSCOPIC DETERMINATION OF ORE MINERALS

The weight of KCNS required according to the above reaction is calculated as 

follows:

200.6 : 194.2 = 2 :x

£=1.94 grams or 19.4 cubic centimeters of KCNS solution

If the KCNS reagent is fresh, a milky-white precipitate of mercuric thiocyanate 

is formed when the first drop of KCNS is added to the mercuric nitrate. If the 

KCNS reagent is not fresh, as much as 5 cubic centimeters of KCNS may be 

required before the first precipitate of mercuric thiocyanate appears. The solution should be stirred vigorously as the titration proceeds. When the titration is 

complete, an additional drop of KCNS will turn the solution pink owing to the 

formation of the soluble red ferric thiocyanate from the ferric ion in the indicator.

In a typical preparation made by the present writer, 19.5 cubic centimeters of 

KCNS was required to complete the precipitation of the mercury as thiocyanate. 

The precipitate is then filtered, washed, and dried in a weighed 50 cubic centimeter 

beaker over a steam bath. When nearly dry the beaker is then removed from the 

bath, placed in a desiccator, and left for several hours. The beaker and contents 

are then weighed and the weight of the precipitate obtained. In the preparation 

above described 2.82 grams of Hg(CNS) 2 precipitate was obtained, as compared 

with a theoretical amount of 3.16 grams.

If more KCNS solution is added to the mercuric thiocyanate precipitate, this 

goes back into solution, with the formation of the double thiocyanate of potassium 

and mercury, according to the following reaction:

Hg(CNS) 2 + 2KCNS = K2Hg(CNS) 4 

316.6+194.2 = 510.8

The theoretical weight of KCNS necessary to complete this reaction with the 

2.82 grams of Hg(CNS)2 is as follows:

316.6 : 194.2 = 2.82 : x

x= 1.73 grams KCNS

The solution (17.3 cubic centimeters) is now added to the Hg(CNS)2 precipitate 

and the beaker warmed over a steam bath. The solution should not be heated 

to the boiling point, but only until the solution appears to be complete. With 

stale KCNS reagent, some elemental sulphur may be dropped and the completion 

of the above reaction obscured. An excess of KCNS is to be avoided. A small 

deficiency does no harm. It is better to use the calculated amount of KCNS 

necessary to complete the formation of the double thiocyanate and then if a 

small quantity of Hg(CNS) 2 remains undissolved, it can be filtered off.

In the above-mentioned preparation, 2.82 + 1.73 = 4.55 grams of K2Hg(CNS)4 

was obtained. While the solution was still warm it was poured into a 200-cubiccentimeter beaker and enough water was added to make a total of 150 cubic 

centimeters of solution, or a 3 percent solution of K2Hg(CNS)4. The solution 

was then carefully filtered.

The solution should be tested by merging a drop of it into a drop of 1 percent 

cobalt nitrate solution, weakly acidified with HNOs. A blue precipitate should 

form immediately and should consist of isolated or interlocking needles. (See 

pi. 7, A.) If precipitation is delayed and branching forms appear at the edges of 

the drop (see pi. 14, .4), an excess of KCNS in the reagent is indicated. In this 

event it is probably quicker and more satisfactory to start from the beginning 

again than to attempt to purify the reagent by recrystallizing it from water. 

The entire procedure, with the exception of the final evaporation, does not require 

more than 30 minutes. The reagent should also be tested by merging it with a 

drop of 0.01 percent cobalt nitrate solution. When the drop approaches dry ness
M1CROCHBMICAL METHODS 187

tiny blue cobalt mercuric thiocyanate prisms will appear, provided the reagent is 

of satisfactory purity. Copper, iron, and zinc do not require the same degree of 

purity of reagent as cobalt.

The preceding list of reagents comprises 13 acids and other liquids 

leaving no residue on evaporation, 12 salt solutions, and 18 solids. 

The liquid reagents are best kept in 15-cubiccentimeter glass-stoppered dropping bottles. 

(See fig. 30.) Care must be taken, however, to 

remove the glass dropping rods from the bottles 

from time to time, as the rods tend to become 

"frozen" to the necks of the bottles owing to evaporation of the salts.20 The solids are kept in 

cylindrical phials with cork stoppers.

.Ground-glass 

surface

TESTS MADE DIRECTLY 

SLIDE

ON A GLASS

FIGURE 30. Glass-stoppered 

dropping bottle. About 

natural size.

The order in which the elements are arranged 

in the following pages is the same as the order 

in which they appear in the systematic scheme on page 152.

ZINC

Potassium mercuric thiocyanate test. 

Reagent: 3% potassium mercuric thiocyanate solution in H2 0.

Product: Zinc mercuric thiocyanate (Zn(CNS)2.Hg(CNS)2), white feathery crosses.

Limit: 0.01 % zinc nitrate solution. 21

This is the only test used by the writer for the determination of zinc. 

Another test, the sodium carbonate reaction described by Chamot, 

is characteristic when obtained, but the writer has never had much 

success with it, as the conditions under which the zinc carbonate 

tetrahedrons form are not always obtainable. The potassium mercuric 

thiocyanate test, on the other hand, is reliable, delicate, and easily 

controlled. The mineral is taken into solution with 1 : 1 HNO3 or 

aqua regia. The residue is leached with a drop of 1 : 7 or 1 percent 

HN03 . The drop is transferred and merged with a drop of potassium 

mercuric thiocyanate solution, using method I of Chamot and Mason. 

(See p. 181.) The acidity of the nitric acid depends on the presence or 

absence of other metallic elements. When zinc alone is present or 

greatly predominates over other metals, as in testing for sphalerite, 

the test seems to work a little better with the stronger acid. In the 

systematic scheme described on pages 266-292, the weaker acid is 

called for.

20 Glass-stoppered dropping bottles similar to that shown in the sketch may be obtained from Arthur H 

Thomas Co., Philadelphia,.Pa., for approximately 60 cents each. 

31 This outline was devised by Putnam, Roberts, and Selchow. See articles already cited.
188 MICROSCOPIC DETERMINATION OF ORE MINERALS

Potassium mercuric thiocyanate (K2Hg(CNS) 4 ; see p. 187) is the 

most useful reagent yet discovered for microchemical determination 

of the ore minerals. It yields delicate and satisfactory tests with 

solutions containing zinc, copper, cobalt, nickel, iron, silver, and gold, 

providing that only one metal is present in the solutions. With 

lead the reaction is less satisfactory. The reactions with the different 

metals are tabulated on page 259.

The acetates of. these metals work equally well when acidified with 

HN03 .

All reactions involving the use of potassium mercuric thiocyanate 

succeed best in the presence of HN03 , although they work fairly well 

with neutral solutions. Potassium mercuric thiocyanate does not 

precipitate zinc, copper, etc., from alkaline solutions.

Tests were made to determine the delicacy of the zinc reaction by 

successively diluting a solution of zinc acetate in 1 percent HN03 . 

The solution was free from cadmium.

1%: The solution gave a strong reaction but was too concentrated. Amorphous grains and distorted forms predominate.22 

0.5%: Highly satisfactory. Feathery crosses predominate. 

0.1%: Highly satisfactory. The simpler forms predominate. 

0.05%: Satisfactory. Simple crosses, some with arrowhead points common. 

0.01 %: Test distinct but requires time. A few small simple crosses are observed. 

This is probably the limit of allowable dilution.

The most common forms are illustrated in plate 5.

The presence of other metals in addition to zinc in the unknown 

solution modifies the results above given.

Iron does not materially alter the shape of the zinc mercuric thiocyanate crystals but stains them a light lavender to purplish red, the 

color depending on the concentration of iron. This color is best observed in reflected light. Most specimens of sphalerite when tested 

in this way show the presence of more or less iron.

Copper has a far-reaching effect on the zinc reaction, the two elements tending to form isomorphous crystals. If one element is in 

excess of the other, part of the predominating element will unite with 

all of the subordinate element to form mixed crystals, and the remainder of the predominating element will form pure crystals. Acetates of zinc and copper acidified with HN03 were mixed and tested 

with K2Hg(CNS) 4 , with the following results:

0.5% zinc acetate 23 and 0.005% copper acetate. Pure zinc crystals alone form. 

No sign of copper.

81 In this paper solutions are usually expressed, for convenience, as percentages of the dissolved salt. This 

is slightly inaccurate, because 1 cubic centimeter of the solution weighs more than 1 gram. The concentrations as given are actually grams of salt per 100 cubic centimeters of solution.

» This solution is actually 1 cubic centimeter of 1 percent zinc acetate added to 1 cubic centimeter of 0.01 

percent copper acetate. The percentages as given above are percentages of each salt expressed in terms of 

total solution.
MICROCHEMICAL METHODS 189

0.5% zinc acetate and 0.01% copper acetate. Almost no evidence of copper.

Some of the zinc crosses show a faint violet tinge in reflected light. 

0.5% zinc acetate and 0.02% copper acetate. Same as normal zinc test, but

crystals are tinged violet in reflected light; almost opaque in transmitted light.

In addition to crosses, simpler crystals also seen. (See fig. 31, a.) 

0.5% zinc acetate and 0.05% copper acetate. Same as preceding, but color is a

light violet. Simpler crosses predominate over feathered crosses. (See fig.

31, 6.) 

0.5 % zinc acetate and 0.1 % copper acetate. Isolated purple prisms like those shown

in figure 31, c. Crystals fairly transparent. Crosses like preceding subordinate. 

0.5% zinc acetate and 0.25%

copper acetate. Isolated

prisms like preceding most

characteristic. Color violet to purple; some opaque.

No crosses observed. 

0.5% zinc acetate and 0.5%

copper acetate. Clumps

of prisms which have the

same form as the copper

compound but are dark

purple. In addition some

normal yellowish - green

copper prisms are present.

(See fig. 31, d.) 

0.25% zinc acetate and 0.5%

copper acetate. Typical

copper crystals greatly predominate, but in addition

there are clumps of sporelike prisms, opaque in

transmitted light and dark

purple in reflected light. 

0.1% zinc acetate and 0.5%

copper acetate. Same as

above, but clumps of mixed

crystals are smaller and

less numerous. (See fig. r e

 , , FIGURE 31 . Zinc and copper mercuric thiocyanate crystals. 31, e.)

0.05% zinc acetate and 0.5% copper acetate. Similar to preceding, but mixed

crystals still less numerous. 

0.02% zinc acetate and 0.5% copper acetate. No indication of zinc.

The conclusion is that a very small percentage of copper in a fairly 

concentrated zinc solution is readily detected by this method; the 

presence of a small percentage of zinc in a fairly concentrated copper 

solution is also easily detected, although the sensitivity of the test is 

less than when the zinc predominates.

Cobalt and zinc also form mixed crystals. Mixtures of cobalt and 

zinc nitrates yielded the following results:

0.5% zinc nitrate and 0.5% cobalt nitrate. Almost no indication of the presence 

of zinc. Some deep-blue crosses and V-shaped crystals have the forms of the 

normal zinc compound. Normal deep-blue cobalt prisms predominate.

o a
190 MICROSCOPIC DETERMINATION OF ORE MINERALS

0.5% zinc nitrate and 0.1% cobalt nitrate. Zinc crosses predominate. Most of

them are normal, but some are tinged with blue. 

0.5% zinc nitrate and 0.05% cobalt nitrate. Zinc crosses, most of them normal

but a few tinged light blue. 

0.5% zinc nitrate and 0.02% cobalt nitrate. Normal zinc crosses, a few of which

are tinged pale blue in reflected light. About the limit of possible dilution for

cobalt.

The conclusion is that a small percentage of cobalt can be detected 

in a concentrated zinc solution but zinc cannot be detected in a strong 

cobalt solution. The cobalt should be removed by means of potassium 

nitrite (p. 197) before testing for zinc with potassium mercuric thiocyanate.

CADMIUM

Potassium mercuric thiocyanate test. 

Reagent: 3% K2Hg(CNS) 4 in H2 0.

Product: Cadmium mercuric thiocyanate in colorless prisms.

Limit: 0.01% cadmium nitrate solution.

The test for cadmium is carried out precisely like the corresponding 

test for zinc. The mineral is taken into solution with 1:1 HN03 , the 

residue is leached with a drop of 1:7 or 1 percent HN03 , transferred to 

another place on the slide, and merged with a drop of potassium 

mercuric thiocyanate solution according to method I.

According to Chamot and Mason: 24

The cadmium salt Cd(CNS)2.Hg(CNS) 2 separates more slowly (than zinc) in 

brilliant colorless orthorhombic prisms usually several times as long as broad, one 

end usually pyramidal, the other beveled or truncated (probably sphenoidal or 

bisphenoidal); an almost constant peculiarity is the presence of cavities near each 

end of the prisms. As in the case of zinc, the habit of the cadmium double 

thiocyanate is peculiar to the element and thus affords a most valuable test. 

The prisms are single and are rarely grouped or in aggregates unless formed in 

concentrated solutions.

COPPER

Potassium mercuric thiocyanate test. 

Reagent: 3% K2Hg(CNS) 4 in H2 0.

Product: Copper mercuric thiocyanate in greenish-yellow mosslike aggregates

and prisms. 

Limit: 0.01% copper nitrate solution.

The test for copper is carried out precisely like the corresponding 

test for zinc. Method I (p. 178) is followed. A drop of potassium 

mercuric thiocyanate solution is merged into the drop containing 

copper ions in 1:7 or 1 percent HNO3 . Clumps of mosslike or fernlike 

dendrites are precipitated in solutions of relatively high concentration. 

The color of the aggregate is yellow with a tinge of green. Where the 

dilution is greater, aggregates of acicular prisms form. These give 

parallel extinction in polarized light. They are generally light

» Chamot, E. M., and Mason, C. W., op. cit., vol. 2, p. 125.
MICROCHEMICAL METHODS 191

greenish yellow, but some of the thicker crystals may be distinctly 

olive green. (See pi. 6.)

In addition to the above-mentioned crystals, certain opaque forms, 

usually rounded or hexagonal in outline, occasionally appear. Crystals 

of the usual thiocyanate compound sprout from a large proportion of 

these black forms.

Extraneous elements, for the most part at least, do not seem to 

interfere with the copper test. Copper, however, may interfere with 

tests for other elements. It is frequently desirable to separate the 

copper from some other elements that are soluble in nitric acid. ' This 

can be done by taking the copper into solution by means of a drop of 

ammonia, transferring the drop to a different position on the slide, 

evaporating off the ammonia, and taking the copper into solution 

again by means of 1:7 nitric acid. The copper is then proved by 

means of potassium mercuric thiocyanate, as already described. If 

the concentration of copper is sufficiently great in the ammoniacal 

solution the familiar blue color is readily recognized, and the final test 

is not absolutely necessary. It should be remembered that nickel, 

cobalt, zinc, silver, arsenic, and some other elements are also soluble in 

ammonia. None of these seem to interfere with the copper test.

The potassium mercuric thiocyanate test for copper is so satisfactory 

and so nearly "foolproof" that none other is needed. The only fault 

that can be found with it is that some minerals which are not supposed 

to contain copper yield good tests for it. For instance, a good copper 

test can usually be obtained from bismuthinite. The student should 

be on guard for results of this kind and ignore faint tests.

Potassium iodide test. 

Reagent: Solid potassium iodide.

Product: Copper iodide, an amorphous precipitate.

Limit: 0.02% copper chloride.

If a fragment of potassium iodide is placed in a 1:5 HC1 solution 

containing copper, a finely divided light-yellow amorphous precipitate 

of cuprous iodide (Cul) is formed. The larger grains are disk-shaped 

or plates with outlines suggesting hexagons. This reaction is sensitive 

and useful, in that it often appears in tests for other elements, especially for antimony, bismuth, and tin. Copper should be eliminated 

before testing for these elements. When the copper iodide precipitate 

appears, it is best to wash the original residue with successive drops of 1 

percent HN03 and test each one for copper withK2Hg(CNS) 4 solution 

until a negative reaction is obtained. Then proceed with the test for 

Sb, Bi, or Sn.

Cesium chloride. When a fragment of cesium chloride is placed 

in a 1:5 HC1 solution containing copper ions, orange or golden-colored 

needles form at the edge of the drop. The precipitate does not form in 

solutions more dilute than 0.10 percent copper chloride. This result,
192 MICROSCOPIC DETERMINATION OF ORE MINERALS

like that with potassium iodide, sometimes appears in tests for other 

elements and does not interfere to any extent with the tests for Sb, Bi, 

orSn.

When successive fragments of KI and CsCl are placed in a 1:5 

HC1 solution containing copper, irregular, more or less rounded grains 

form. These are opaque in transmitted light and white in obliquely 

reflected light. In addition long colorless prisms with ragged sides 

sometimes form. It is best to remove all copper before making the 

KI and CsCl test for antimony or bismuth.

NICKEL

Dimethyl glyoxime test. 

Reagent: 2% dimethyl glyoxime solution in alcohol. 

Product: Nickel dimethyl glyoxime (Ni(C4.H 7^02)2). 

Limit: 0.02% nickel nitrate.

The familiar dimethyl glyoxime test used in qualitative analysis is 

also the best for microchemical tests. This reagent is not soluble in 

water but is readily soluble in alcohol and other organic solvents. A 

2 percent solution of the reagent in alcohol is the most satisfactory 

strength. The solution continually precipitates white crystalline 

dimethyl glyoxime, which sinks and tends to form a cake. For this 

reason it is best to avoid shaking or stirring the solution. The dimethyl glyoxime crystals are easily recognized under the microscope 

and do not obscure the nickel test.

The precipitation takes place in neutral, weakly alkaline, and weak 

acetic acid solutions. It is most satisfactory in an ammoniacal solution. No precipitation occurs in HN03 or HC1 solutions. Cobalt 

retards the formation of the precipitate, and therefore if much cobalt 

is present it should be removed by starting with an acetic acid solution, precipitating the cobalt with potassium nitrite, and testing the 

nitrate with dimethyl glyoxime.

The familiar pink precipitate is usually amorphous at first, but a 

felty mass of tiny pink needles soon forms. When observed with a 

high power these needles are seen to be strongly pleochroic between 

colorless and pink. With the analyzer inserted, the interference color 

is usually light blue.

In testing an unknown mineral for nickel the material is dissolved 

in 1 : 1 HN03 or aqua regia. The residue is leached with 1 : 7 HN03 , 

and the filtrate is transferred to a new place on the slide and evaporated over the microflame. A drop of 20 percent NH4OH is added to 

the residue from the evaporation in order to precipitate any iron 

present. If iron were left in solution it would likewise be thrown 

down by dimethyl glyoxime as a reddish precipitate. Hence it is advisable to eliminate the iron by precipitation with ammonia before making 

the nickel test. The ferric hydroxide formed includes a considerable 

percentage of nickel, but most of the nickel remains in solution. The
MTCROCHEMICAL METHODS 193

gelatinous ferric hydroxide does not interfere with the formation or 

observation of the nickel precipitate.

Alcohol has a low surface tension and tends to spread out over a 

wide area on the slide. If a drop of the reagent (dimethyl glyoxime 

in alcohol) is carefully added to a drop of 20 percent ammonia containing the nickel by just touching the platinum loop containing the 

reagent to the top of the ammonia drop on the slide, this spreading 

tendency can be avoided. Some practice is necessary to accomplish 

this. The glass slide must be absolutely clean. The test is so nearly 

foolproof, however, that the pink nickel precipitate will appear even 

if the alcohol spreads out over a considerable area. Dimethyl 

glyoxime gave a good test on a 0.02 percent solution but a poor test 

on a 0.01 percent solution of nickel nitrate.

Potassium mercuric thiocyanate test. 

Reagent: 3%K2Hg(CNS) 4 inH2 0.

Product: Nickel mercuric thiocyanate (NiHg(CNS)4).

Limit: 0.02%Ni(N03) 2 solution.

Potassium mercuric thiocyanate added to a 1:7 HN03 solution 

containing nickel ions precipitates round spherulites that are light 

brown in transmitted light and white in reflected light. Sometimes 

an amorphous white powder is first precipitated at the edge of the 

drop, but it tends to redissolve with the formation of the spherulites. 

The nickel mercuric thiocyanate is more soluble than the corresponding cobalt precipitate and does not form till the drop is partly evaporated. The spherulites are tinged pink if iron is present in solution. 

(See pi. 7, D.} This test is exceedingly useful in that the nickel 

spherulites often appear when testing for cobalt and iron.

COBALT

Potassium mercuric thiocyanate test. 

Reagent: 3% K2Hg(CNS) 4 in H2 0.

Product: Cobalt mercuric thiocyanate (CoHg(CNS)4), in indigo-blue prisms.

Limit: 0.01% cobalt nitrate solution.

Tests for cobalt are carried out exactly like those for copper and 

zinc. The precipitate separates as dark-blue prisms, usually in 

clusters. (See pi. 7, A) With dilute solutions (0.05 per cent or less 

of Co(N03 ) 2 ), blue spherulites also form. The formation of spherulites has not been sufficiently emphasized by Chamot and is not mentioned by Behrens and Kley. If much nickel is present the cobalt 

mercuric thiocyanate will not form as prisms but will appear at the 

edge of the drop as blue spherulites. (See pi. 7, CO. If the ratio 

of cobalt to nickel is relatively high the spherulites will show radiating fibers, each giving parallel extinction. With still higher ratios 

incipient prisms form at the surface of the spherulite, and finally a 

mass of radiating quills something like a prickly pear results. The
194 MICROSCOPIC DETERMINATION OF ORE MINERALS

presence of other elements also causes the formation of the spherulites. Potassium cobalt nitrite when taken into solution with nitric 

acid and tested with potassium mercuric thiocyanate will cause the 

formation of the spherulites.

Tests with mixed solutions of nickel and cobalt nitrates indicate 

that some cobalt is taken into the nickel spherulites without materially 

changing their color, and the rest of the cobalt forms the usual blue 

spherulites. Hence in minerals in which the nickel is largely in excess 

of cobalt blue spherulites may not appear. This became evident when 

testing some specimens of siegenite (Ni,Co) 3S4 , and chloanthitesmaltite (Ni,Co)As2 , which gave good cobalt tests with CsCl but none 

with K2Hg(CNS)4 . The cesium chloride test (p. 191) should be made 

as a confirmatory test when the mineral contains much nickel or iron.

Although the potassium mercuric thiocyanate test for cobalt is 

as delicate as the corresponding test for copper, the cobalt compound 

tends to supersaturate and frequently does not begin to be precipitated until evaporation of the drop is well advanced. This is particularly true when spherulites predominate over prisms. Once 

formed, however, the precipitate is not readily taken into solution 

by diluting the drop with water or nitric acid.

The spherulites may appear anywhere within the drop but are 

most commonly found at the circumference. At first thought it 

would seem that the absence of the more typical prisms and the 

formation of blue spherulites in their place is a serious drawback 

to this test. However, one soon becomes familiar with their appearance and accepts them as proof of the presence of cobalt. Experiments performed on mixtures of cobalt and nickel chlorides indicate 

that if the ratio of nickel to cobalt is greater than 1:5, only spherulites will form. If the ratio is less than that, some well-formed 

crystals will be produced in addition to the spherulites.

A comparatively strong nitric acid solution (1:7 HN03 ) gives 

the best results, although the test will work in a neutral, weak nitric 

or acetic acid solution. It does not work well in hydrochloric or 

sulphuric acid. With weak nitric acid the precipitate tends to assume branching and mosslike forms rather than prisms.

Experiments were made to determine the sensitiveness of the 

cobalt test by successively diluting a solution of cobalt nitrate.

1.0%: Excellent test. 

0.5%: Even better.

0.1%: Good test but requires a minute or more. The crystals appear at the 

edge of the drop. Most of the precipitate appears as small formless 

rounded grains. The blue color is sufficiently diagnostic. 

0.05%: Similar to preceding.

0.01%: Blue spherulites first appear at the edge of the drop. After a while 

tiny needles also appear, both singly and in clusters. This is probably close to the limit of possible dilution.
MICROCHEMICAL METHODS 195

The formation of the triple thiocyanate of cobalt, zinc, and mercury is described on page 189. Copper and cobalt do not combine 

to form mixed crystals under the same conditions, but each forms its 

own precipitate. Copper comes down first in characteristic yellowish-green crystals, and cobalt later in blue prisms and spherulites.

Iron does not interfere with the formation of typical cobalt prisms 

unless in relatively concentrated solution. Ferric ions stain the thiocyanate solution reddish but do not form an insoluble compound. 

Mixtures of ferric and cobalt nitrates in 1 : 7 HN03 in various proportions were tested as follows:

0.5% Co(N03) 2 and 0.05% Fe (N03) 3 Normal cobalt prisms.

0.5% Co(N03) 2 and 0.10% Fe(N03)3. Normal cobalt prisms predominate; also

some formless grains much darker but distinctly blue in strong transverse

illumination.

0.5% Co(N03) 2 and 0.25% Fe(N03 ) 3 . Cobalt prisms; some normal, but most of 

 them very dark blue. The blue color appears with strong illumination. 

0.5% Co(N03) 2 and 0.5% Fe(NOs)3. Same as preceding, but solution turns

very pink, and prisms are envelope-shaped. 

0.25% Co(N03) 2 and 0.5% Fe(N03)3. Same as preceding. Most of the prisms

are opaque, but a few normal cobalt prisms are also seen. 

0.1% Co(NO3) 2 and 0.5% Fe(N03) 3 . Cobalt prisms black but show blue at thin

edges.

0.05% Co(N03) 2 and 0.5% Fe(N03 ) 3 Solution strong red. Prisms light purplish to black. No sign of blue. 

0.02% Co(N03 ) 2 and 0.5% Fe(N03) 3 Solution red to purple. Small seedlike

grains purplish to black.

0.05% Co(N03) 2 and 2.5% Fe(N03) 3 Very good test. Prisms purple to black. 

0.05% Co(N03) 2 and 5.0% Fe(N03) 3 . Black prisms and spherulites. Some of

the thinnest prisms are purplish.

Summarizing the above experiments shows that a moderate concentration of iron does not interfere with the thiocyanate test for 

cobalt, but a high concentration modifies the shape and color of the 

precipitate. As iron occurs in most cobalt minerals, especially the 

arsenides, the student should familiarize himself with the appearance 

of the mixed crystals. A potassium mercuric thiocyanate test yielding a red solution and purple to black prisms calls for a confirmatory 

test for cobalt by the cesium chloride method.

Cesium chloride test. 

Reagent: Solid cesium chloride (CsCl).

Product: Cesium cobalt double chloride (3CsCl.CoCl2) in large blue soluble

prisms. 

Limit: 0.02% cobalt nitrate solution.

The double chloride of cesium and cobalt forms in a concentrated 

solution of cobalt ions in 1 : 5 HC1 when solid CsCl is introduced into 

the test drop. The mineral is broken down with three or four applications of 1 : 1 HN03 . The residue is leached with a drop of 1:5 HC1 

and the drop is transferred to another place on the slide. A small 

fragment of CsCl is introduced into the drop. The crystals formed
196 MICROSCOPIC DETERMINATION OF ORE MINERALS

are long needles; prisms, some with irregular and others with arrowshaped terminations; knife-shaped crystals; irregular grains. These 

are of considerable size, usually much larger than the crystals formed 

in the thiocyanate test. Their color is usually blue with a distinct 

suggestion of green. They belong to the tetragonal system. 25

This precipitate is very soluble and forms only in concentrated 

solutions, but fortunately the test can be made in very dilute solutions 

if the drop is allowed to evaporate nearly to dryness.

Iron, nickel, and arsenic, the elements most commonly associated 

with cobalt, do not interfere with the test. Iron and nickel each forms 

its own double chloride in the presence of cobalt. The iron compound 

appears before the cobalt. The iron crystals appear first at the 

periphery of the drop and come down successively closer to the center 

until the drop is dry. Common forms of the cesium-iron double 

chloride are bipyramids resembling octahedrons, hexagons, and thin 

diamond-shaped plates. The bipyramids are golden brown and 

resemble the corresponding tellurium compound except for the color. 

(Cesium chlorotellurite is honey yellow.) The hexagons and diamond-shaped plates are orange and resemble those formed in the 

double iodide test for antimony, but the cesium-iron double chloride 

comes down in the absence of KI, and hence antimony and iron are not 

mistaken for each other in this test.

The cobalt compound usually does not appear until the iron compound is completely precipitated. Apparently the two are not isomorphous, and the presence of a large excess of iron has no effect on 

the color of the cobalt compound.

When the test is made in a drop containing nickel ions to the exclusion of other metallic ions, the cesium-nickel double chloride does not 

begin to come down until the drop is nearly dry. A fine granular 

white to light-yellow amorphous precipitate then slowly appears at 

the edge of the drop. If cobalt is associated with nickel, the cesiumnickel double chloride is entirely changed in character and is distinctly 

crystalline. Yellow elongated slivers, pointed at both ends, and boatshaped crystals predominate. They are considerably smaller than 

those of the cobalt compound. Apparently a minor proportion of 

cobalt enters into the nickel compound, but after the nickel compound 

has all precipitated, just before the drop dries up, the usual blue 

cobalt compound appears. The color is unaffected by the presence 

of nickel.

When iron and nickel ions are present in the drop to the exclusion 

of cobalt, the nickel test does not appear. The crystals formed 

resemble in every respect those of iron alone, and it is evident that 

nickel enters into the compound isomorphously. Minerals tested

" Chamot, E. M., and Mason, C. W., Handbook of chemical microscopy, vol. 2, p. 141,1931.
MICBOCHEMICAL METHODS 197

with the above-stated result were pentlandite, (Ni,Fe)S, and violarite, 

(Ni,Fe)3S4 .

The cesium chloride test for cobalt fails completely in the presence 

of even a small proportion of copper. The crystals formed are grass 

green instead of blue. A specimen of siegenite, (Ni,Co)3S4 , from Mine 

La Motte, Mo., which gave no copper test by the thiocyanate method, 

yielded green crystals with cesium chloride. The absence of a 

thiocyanate test for copper indicates that the copper content of this 

siegenite is probably less than 1 percent.

Specimens of siegenite from other localities yielded good cobalt and 

nickel tests with cesium chloride. Some of them failed to give cobalt 

tests with the thiocyanate reagent.

In conclusion, the cesium chloride test for cobalt is particularly 

valuable when testing minerals high in iron or nickel or both. Although the test has long been known, Fraser and Dreyer 20 are entitled 

to credit for recognition of this important advantage.

Potassium nitrite test. 

Reagent: Solid potassium nitrite (KN02).

Product: Potassium cobalt nitrite (3KN02 .Co(N02 ) 2 .l]£H2 0) in small yellow

isometric cubes. 

Limit: 0.5% Co(N03) 2 .

The potassium nitrite test for cobalt is described in detail both by 

Chamot and Mason and by Behrens and Kley. The solution containing the cobalt, generally a nitric acid solution, is evaporated to dryness. 

It is then redissolved in a drop of 1 percent acetic acid, a fragment of 

potassium nitrite is added, and the drop is gently heated but not 

sufficiently to evaporate it. Potassium cobalt nitrite separates out 

in tiny yellow cubes, so minute as to simulate an amorphous or 

finely granular deposit. With high magnification the grains seem 

almost spherical and greatly resemble those of ammonium arsenimolybdate, described on page 209.

Behrens and Kley *7 state that this reaction is especially valuable 

for separating cobalt from other metals, because in the absence of 

elements of the barium group only cobalt is precipitated.

The test would be very useful indeed if it were as reliable as these 

comments would indicate. The present writer cannot agree with their 

conclusions. The results of many days of experimentation with 

cobalt solutions of various strengths indicate that the test is unreliable 

when the concentration of cobalt is less than 0.5 percent of cobalt 

nitrate. There is evidently a strong tendency for the double nitrite 

to supersaturate, and it may not come down at all. The test requires 

a great deal of manipulative skill. The fragment of KN02 must not 

be too large, and the solution must not be heated too much, or it will

2 « Fraser, H. 3., and Dreyer, R. M., Mutual interference in the microchemical determination of ore minerals: Am. Mineralogist, vol. 22, pp. 949-976, 1937. 

» Behrens, H., and Kley, P. D. C., op. cit., p. 77.
198 MICROSCOPIC DETERMINATION- OF ORE MINERALS

evaporate before the precipitate has a chance to come down. For 

these reasons this test is not recommended as a test for cobalt, but it 

is occasionally useful in removing an excess of cobalt before testing 

for other elements.

IRON

Potassium mercuric thiocyanate test. 

Reagent: 3% K2Hg(CNS) 4 in H2 0.

Product: No precipitate forms, but solution turns pink.

Limit: 0.02% Fe(N03) 3 .

Any thiocyanate solution will give a delicate test for iron. The 

iron must be in the ferric state in this as in the other tests described 

in this paper. As a matter of fact, the iron is certain to be left in 

the ferric state if the mineral to be determined is taken into solution 

or broken down by means of nitric acid or aqua regia.

The residue is then leached with 1: 7 HN03 , and the drop is transferred to another place on the slide and joined to a drop of the reagent by method I (p. 182). Ferric nitrate solutions of 0.2 percent 

or greater concentration immediately turn pink near the junction of 

the two solutions. If the concentration is less than 0.2 percent the 

color may not appeal until the test drop is nearly dry. The pink to 

purple color then appears at the periphery of the drop.

This test is very valuable in that it often appears when testing for 

other elements. Many minerals containing 1 percent or more of iron 

will often give a distinct thiocyanate test for iron when, as a matter 

of fact, iron is not expressed in the formulas usually given for these 

minerals. This is not an objection to the test but merely records the 

fact that more or less iron is found in many ore minerals that are not 

usually classed as iron-bearing.

Potassium ferrocyanide test. 

Reagent: Solid potassium ferrocyanide (K4Fe(CN) 4). 

Product: Amorphous blue precipitate (prussian blue). 

Limit: 0.02% ferric nitrate or chloride solution.

A fragment of potassium ferrocyanide when added to a solution 

containing ferric ions will give the familiar prussian-blue precipitate. 

It is amorphous but sufficiently characteristic in color to be diagnostic. 

When transverse illumination is used the color is likely to be greenish. 

The blue color is best observed by tilting the mirror and observing 

the precipitate in oblique illumination.

The greatest objection to this test is that many elements are precipitated by potassium ferrocyanide, and if any of them are present 

with the iron the blue color of the iron precipitate may be obscured. 

In the presence of a predominance of nickel or cobalt, for instance, 

some other test for iron should be used.

The ferrocyanide test works in neutral or acid solutions but is 

much more sensitive in the presence of acid. The writer could not
MICRO CHEMICAL METHODS 199

get a blue precipitate with neutral ferric nitrate solutions in a concentration of less than 0.1 percent, but by adding HN03 the solution 

could be diluted to 0.02 percent and still give a satisfactory test.

The test works equally well in 1:7 HN03 and 1:5 HC1 solutions. 

The acid should not be more concentrated than this, as the reagent 

itself contains iron in the molecule, and a strong acid would tend to 

liberate it and thus contaminate the test.

Ammonium hydroxide test. 

Reagent: NH4OH solution.

Product: Gelatinous ferric hydroxide.

Limit: About 0.03% ferric nitrate or chloride.

When ammonia is added to a neutral ferric salt or added in excess 

to an acid ferric solution, the familiar yellow to orange-colored gelatinous ferric hydroxide precipitate immediately forms. In order to 

avoid making the drop too large it is best to evaporate off nearly all 

the excess of acid and then add the ammonia directly to the still 

moist residue.

The precipitate tends to cling to the glass. It can be washed by 

three or four successive drops of water, taken again into solution 

with 1:7 HN03 , and the drop tested for iron with a fragment of 

potassium ferrocyanide. In this manner iron can be separated from 

copper, nickel, and many other elements soluble in ammonia. Ordinarily, however, the ferric hydroxide precipitate is so distinctive in 

appearance that no confirmatory test is needed.

Summary. None of the tests for iron described above precipitate 

the iron in definite crystals. However, all the tests are sufficiently 

distinctive to identify the element. The choice of a test depends 

chiefly on the other elements present. In acid solutions containing 

unknown elements the first test is usually made with potassium mercuric thiocyanate, and hence this constitutes the most valuable test 

for iron. The potassium ferrocyanide test is the most distinctive 

of the four and can be made in the absence of other elements precipitated by the reagent. The ammonium hydroxide test is of value in. 

separating iron from copper or nickel. The ferric hydroxide formed, 

if insufficient in quantity to be distinctive in appearance, can be redissolved in HN03 or HC1 and tested for iron with K4Fe(CN)6 .

SILVER

A large number of silver minerals occur in nature, but many of 

them are rare. The most abundant sources of silver, however, are 

certain copper and lead minerals, such as tetrahedrite, tennantite, 

and galena. Careful microscopic examination of these ore minerals 

indicates that in most localities the silver is contained in the molecule itself and not as definite silver minerals included in the basemetal minerals.
200 MICROSCOPIC DETERMINATION OF OEE MINERALS

All the silver minerals tested by the writer are broken down by 

1 : 1 HN03 , but some of them entail considerable difficulty. Pyrargyrite, for instance, usually requires about 10 successive drops of 

nitric acid, each evaporated to dryness over a microflame, before 

enough material is available in soluble form for testing. It is exceedingly important that no chlorine ions be introduced, as silver 

chloride is immediately precipitated and once formed cannot be 

broken down by subsequent treatment with nitric acid. Hence in 

transferring material from the polished sulphide to the glass slide 

the match should be moistened with distilled water not by touching it to the tongue, as the saliva contains enough sodium chloride 

to precipitate part of the silver when the match is introduced into 

the test drop. The nitric acid should be tested for its freedom 

from chlorine ions by pouring a little of it into silver nitrate solution. If any cloudiness appears, the acid should be rejected. These 

precautions are not necessary if the silver chloride test is used.

Ammonium bichromate test.28 

Reagent: Solid ammonium bichromate

Product: Silver bichromate in ruby-red triclinic crystals; sometimes also silver

chromate in ruby-red orthorhombic crystals. 

Limit: 0.03% AgN03 .

The mineral is dissolved in 1 : 1 nitric acid. The excess acid is 

carefully evaporated off, avoiding overheating the residue. The 

residue is leached with 1 percent nitric acid. The drop is transferred to a different place on the slide and tested with a fragment 

of ammonium bichromate by method II. The silver -separates as 

bichromate and chromate. Both compounds are dark ruby red. 

The bichromate is triclinic, and the plates give inclined extinction 

in polarized light. The chromate is orthorhombic and shows parallel 

extinction. The bichromate shows little or no pleochroism, whereas 

the chromate shows strong pleochroism; thin chromate plates are 

blood red when the elongation is parallel to the plane of polarization 

and yellow when perpendicular to it. Usually the bichromate predominates over the chromate if the silver nitrate solution is pure.

In the presence of sulphuric, selenious, or tellurious acids and other 

extraneous ions the silver tends to precipitate as chromate. The 

bichromate shows an astonishing variety of forms, a few of which are 

shown in plate 8. The color of the crystals and their solubility in 

ammonia are distinctive criteria. With a weaker concentration of 

silver it is necessary to use a very small fragment of ammonium bichromate just enough to give part of the drop a faint tinge of yellow.

When the drop has nearly evaporated the yellowish ammonium 

bichromate residue usually obscures the reddish silver compounds, 

and it is necessary to add a drop of 1 percent HN03 in order to take

38 Potassium bichromate gives an identical test and is equally satisfactory.
M1CROCHEMICAL METHODS 201

the excess ammonium bichromate into solution again. Both silver 

compounds are insoluble in weak nitric acid after they have once 

formed.

Copper does not interfere with the bichromate test for silver unless, 

it is present greatly in excess of the silver. The writer was unable 

to get a bichromate test for silver in tennantite which assayed 1 percent of Ag. Some freibergite specimens, however, gave good bichromate tests for silver. No analyses of these specimens were available. 

A solution containing 2.5 percent of Cu(N03 ) 2 and 0.05 percent of 

AgN03 gave a good silver test, but a solution containing 2.5 percent 

of Cu(N03 ) 2 and 0.025 percent of AgN03 gave a negative silver 

test.

Lead is precipitated as chromate, usually as an amorphous yellow 

powder. It does not prevent the formation of the red silver bichromate and chromate crystals, but an abundance of the yellow powder 

may interfere with the observations of the silver crystals. A solution 

containing 0.5 percent of Pb(N03 ) 2 and 0.05 percent of AgN03 gave 

a good bichromate test for silver. None of the specimens of argentiferous galena tested by the writer yielded a bichromate test for 

silver.

The ammonium bichromate test for silver is highly distinctive, and 

when the concentration of silver is comparatively high the test is 

almost spectacular. On the other hand, certain solutions seem to reduce the sensitivity of the test greatly. Also it requires considerable 

skill and experience to get the conditions just right. One source of 

error, getting chloride ions into the test drop, has already been mentioned. Another error commonly made by beginners is mistaking 

ammonium bichromate itself for the silver compound. The ammonium bichromate is orange-colored and very soluble, whereas the silver compound is red and once formed is highly stable in 1 percent 

HNO3 . To make certain that the observer sees the silver compound, 

he should allow the drop to evaporate almost to dryness, then add a 

drop of 1 percent HN03 to the residue. The ammonium bichromate 

goes back into solution, and any silver bichromate and chromate 

crystals present are not affected.

Again it should be emphasized that when the concentration of 

silver is low (less than 0.1 percent of AgN03 ) the crystals of silver 

bichromate will be very small, needlelike, but usually irregular, and 

will form at the edge of the crop. (See pi. 8, D.) The color of 

these crystals is the main criterion. If the crystals are very small, 

the color may best be observed in obliquely reflected light.

The student should make a 0.03 percent AgN03 solution in 1 percent HN03 and prove his skill by making successive tests until consistent positive results are obtained.

129151 40 14
202 MICROSCOPIC DETERMINATION OF ORE MINERALS

Chloride test. 

Reagents: 1:5 HC1 and concentrated NH4OH.

Product: Silver chloride in minute colorless isometric crystals.

Limit: 0.02% AgN03 .

The mineral is broken down chemically by successive drops of 

1:1 HN03 . A drop of 1:5 HC1 is applied to the dried residue. Any 

silver present is immediately precipitated as chloride. Silver chloride is so insoluble in water and in acids that it is completely amorphous. It can be made to crystallize by dissolving in concentrated 

ammonia and allowing the solution to evaporate. As evaporation 

proceeds tiny highly refracting colorless isometric silver chloride 

crystals begin forming at the periphery of the drop, and precipitation proceeds toward the center until the drop dries up. If the 

concentration in silver is comparatively high, the crystals will assume 

complex forms, such as combinations of cube and hexoctahedron, 

cube and octahedron, and, more rarely, the tetrahedron. Viewed with 

a 4-millimeter objective many of these crystals are very beautiful. 

Often triangular and pseudohexagonal plates can be observed. When 

the concentration of silver is comparatively low simpler forms predominate. The cube is the most characteristic form, but the octahedron is frequently formed.

Some elements have a detrimenta! effect on the test, notably copper 

and tellurium. Copper causes the precipitate to come down as very 

small black skeletal crystals. When these appear copper should be 

tested for. If found, the silver test should be repeated, and the 

silver chloride should be leached with successive drops of 1:7 HN03 

until the drop fails to give a copper test with potassium mercuric 

thiocyanate. Then the silver chloride should be leached with a drop 

of water to remove the nitric acid. It is then dissolved in ammonia.

If tellurium is present, the silver chloride should be leached by 

successive drops of 1: 5 HC1, and each drop removed and tested by 

means of a fragment of CsCl. When the drop does not give an 

immediate precipitate with cesium chloride, the silver chloride is sufficiently free from tellurium to proceed with the silver test. It is 

then washed with a drop of water and taken into solution with 

NH4OH.

Mercurous mercury and lead are the other common elements of 

group I (qualitative analysis), which are precipitated by chlorine 

ions. Neither dissolves in ammonia. Mercurous chloride turns 

black; lead chloride is converted to the insoluble hydrate, usually 

retaining the needle forms of the lead chloride pseudomorphously. 

Formerly the writer preferred to leach the silver chloride with ammonia and transfer the ammonia drop to another place on the slide 

for observation. It has been found better to leave the ammonia drop 

on the original residue; the crystals seem to be larger and better
MICROCHEMICAL METHODS 203

formed in this manner. To avoid confusion between the white particles of the original residue and the silver chloride crystals the 

ammonia drop is made large enough to spread out a little beyond the 

edge of the residue. Then the microscope is trained on the edge of 

the ammonia drop until the first crystals of silver chloride form. 

A specimen of tennantite from Bonanza, Colo., was analyzed and 

found to contain 1.00 percent of Ag. It was tested in the manner 

above described and yielded well-formed cubes of AgCl. However, 

the writer was not able to obtain a satisfactory chloride test for silver 

by making up a solution containing 0.5 percent each of Cu(N03 ) 2 

and AgN03 in H20. The silver yielded an abundant amorphous 

white precipitate when HC1 was added to the solution. This precipitate was washed with five successive drops of 1:7 HN03 and one 

of water. When dissolved in NH4OH, the forms reprecipitated were 

small, black, and skeletal. The writer cannot explain this discrepancy.

Mixtures of Pb(N03 ) 2 and AgN03 were tested by this method. 

The lead chloride occludes some of the silver chloride formed and 

so reduces the sensitivity of the test somewhat, although not seriously. A solution containing 0.5 percent of Pb(N03 ) 2 and 0.05 percent of AgNO3 gave well-formed cubes. One containing 0.5 percent 

of Pb(N03 )2 and 0.025 percent of AgN03 gave a negative test. Several specimens of galena associated with silver minerals were tested 

in the same way, but all of them gave negative silver tests.

It frequently happens that in making this test minute black formless grains come down when the ammonia drop evaporates. Commonly these are probably silver chloride grains, but in the absence 

of definite isometric forms it is safest to consider the test indecisive.

The silver chloride test does not give the satisfaction that the 

bichromate test affords. The crystals are small and colorless. However, a little experience with this method imparts a surprising amount 

of confidence in it, owing to the unique chemical behavior of silver 

chloride. The test is more sensitive than the bichromate test, but 

its chief value lies in the fact that silver can be separated from elements, notably lead, that interfere with the bichromate test. Familiarity with the chloride test is therefore recommended, although 

for most purposes the bichromate test is best.

A curdy white precipitate suddenly appearing when a drop of 

HC1 is introduced in the course of a microchemical test gives a decided indication of the presence of silver. This precipitate should be 

cleaned by suitable acids and further tested by means of ammonia.

Potassium mercuric thiocyanate test. 

Reagents: 3% K,Hg(CNS) 4 in H2 0.

Product: Curdy white amorphous precipitate (KCNS.AgCNS).

Limit: 0.02% AgN03.
204 MICROSCOPIC DETERMINATION OF ORE MINERALS

The mineral is broken down with 1:1 HNO3 . The residue is 

leached with a drop of 1:7 HNO3 , the drop transferred to another 

place on the slide and joined to a drop of potassium mercuric thiocyanate (method I). An amorphous cloudy white precipitate forms. 

This reaction is useful in that the precipitate often appears incidentally ID tests for other elements. So far as the writer is aware no 

other element gives a similar precipitate with potassium mercuric 

thiocyanate. However, when it appears, the presence of silver should 

be confirmed by the bichromate test.

The potassium mercuric thiocyanate test for silver works particularly well in the presence of copper. The silver precipita'te comes 

down before the copper precipitate and may be seen as a horseshoeshaped wave of white grains spreading out in front of the copper 

precipitate. The silver precipitate is best seen in obliquely reflected

light.

SELENIUM

Stannous chloride test. 

Reagent: 2% SnCl2 solution in 1: 5 HC1. 

Product: Native selenium, a brick-red powder. 

Limit: 0.05% Se02 in H2 0.

The red color of native selenium is very characteristic and when 

obtained no additional test is necessary to confirm the presence of 

the element. Most selenides are readily broken down by nitric acid. 

Usually metallic selenium is thrown down at once and can be observed best in oblique light. When once precipitated it cannot be 

taken into solution with nitric acid or aqua regia. With some compounds, such as claustbalite, some metallic selenium is precipitated, 

but most of it goes into solution as selenious acid (H2Se03 ). 

r<*,The usual procedure is as follows: Break down the mineral with 1:1 

HNO3 and leach the residue with a drop of 1:5 HC1. A red tinge in 

the residue indicates selenium. To confirm it, transfer the filtrate 

and merge with it a drop of 2 percent SnCl2 solution (method I). Selenium will be precipitated as a brick-red powder. This test is highly 

satisfactory, but the precipitate is amorphous, and it is best to use a 

larger amount of mineral than when testing for tellurium. An area 

on the mineral 0.35 millimeter in diameter is usually sufficient. Gold, 

mercury, and tellurium, if present in solution, will likewise be thrown 

down but rarely obscure the red color of the precipitate.

Potassium iodide test. 

Reagent: Solid potassium iodide (KI).

Product: Selenium iodide (SeI4), a reddish-brown powder.

Limit: 0.03% Se02 in H2 0.

The mineral is broken down by repeated applications of 1:1 HNO3 . 

Usually some of the selenium will be precipitated as the element, 

and the remainder will form the acid H2SeO3 , which remains as a
MICRO CHEMICAL METHODS 205

white powder when the solution is evaporated. The residue from 

evaporation is leached with a drop of 1:5 HCI, and the drop is transferred. A fragment of KI is placed in the center of the drop. A 

reddish-brown amorphous precipitate of SeI4 is thrown down immediately and spreads toward the periphery of the drop as a widening 

ring. (See description of the corresponding test for tellurium, p. 207.) 

The color is very similar to that of the corresponding Te precipitate 

but is a lighter shade of brown.

Although the iodide reaction for selenium is more sensitive than the 

stannous chloride test, the color of the iodide precipitate is less 

characteristic than that of the native element. The iodide test is 

of value in that it is sometimes encountered in testing for other 

elements, notably antimony, bismuth, and tin.

Cesium chloride gives no precipitate with selenious acid, and this 

fact constitutes the best means of distinguishing between selenium 

and tellurium. When the iodide precipitate is obtained, it is best 

to take more material into solution with 1:5 HCI. Usually enough 

selenious or tellurious acid remains in the residue from the preceding 

test to yield another satisfactory test for the respective element. If 

not, fresh mineral powder is dissolved in 1:1 HN03 , and the residue 

is leached with a drop of 1:5 HCI. A fragment of cesium chloride 

is added. A precipitate of honey-yellow isometric crystals proves 

tellurium. If no precipitate forms, the solution contains selenium. 

This is confirmed by merging the drop with a drop of stannous chloride 

solution; a brick-red precipitate proves selenium.

Thiourea test. 

Reagent: 10% thiourea, SC(NHj), in H2 0.

Product: Deep-red, finely divided amorphous precipitate of unknown composition.

Limit: 0.05% SeO2 .

This test was discovered and described by Evans.29 

The precipitating reagent is the oiganic compound thiourea. The 

reagent drop is merged into a drop of 1: 5 HCI, containing the unknown 

elements (method I). If selenium is present, a fine amorphous brilliant-red precipitate forms in the test drop near its junction with the 

reagent drop. The precipitate spreads toward the opposite edge in a 

narrow horseshoe-shaped band. By transmitted light the precipitate 

ranges in color from dark blue to purplish red, but hi oblique reflected 

light the precipitate is ruby red.

The sensitivity of the test is of the same order of magnitude as that 

of the SnCl2 test described above. The volumes of precipitate formed 

by the two reagents are approximately equal, but the thiourea precipitate consists of smaller particles than the stannous chloride precipitate, and therefore, with dilute solutions containing selenious acid, 

the stannous chloride precipitate is observed more easily. For both

M Evans. M. H., A new microchemical test for selenium: Am. Mineralogist, vol. 22, pp. 1128-1130, 1937.
206 MICROSCOPIC DETERMINATION OF ORE MINERALS

tests the 16-millimeter objective is the best for observation in oblique 

reflected light.

Of the common elements tested, only copper, gold, mercury, and 

bismuth interfere with the thiourea test for selenium.

Copper produces a finely divided, feathery precipitate, white by 

reflected and brownish by transmitted light. Mercury produces a 

colorless crystalline precipitate. Bismuth forms a clear yellow solution with thiourea. This is recommended by Watson 30 as a spot test 

for bismuth. Gold forms an immediate blue-black precipitate which 

dissolves and does not interfere further.

These reactions are seldom strong enough to mask the brilliantly 

colored selenium precipitate, but interference by these elements can 

be largely obviated by the following procedure:

The mineral is decomposed with a drop of 1:1 HN03 . Part of 

the selenium is precipitated as the element. This forms either a red 

agglutinated mass or a red powder. The rest of the selenium is 

converted to the acid H2Se03 . The elements associated with selenium are converted to mtrates; an exception is gold, which is deposited 

as the element if present. The original residue is leached with a drop 

of 1 percent HNO3 followed by a drop of water. Most of the nitrates 

of copper and mercury are removed in this manner, and the concentration of the remaining part is too low to interfere with the subsequent 

selenium test. The original residue is then tested for selenium by 

leaching with a drop of 1:5 HC1, transferring the drop, and testing 

with thiourea as described above. Bismuth is largely carried over in 

the 1:5 HC1 drop with the selenium. The yellow color of the 

bismuth masks the selenium precipitate to a small extent. Selenium 

will mask bismuth, but if the test drop is filtered, or the precipitate 

allowed to settle, both elements can readily be recognized.

The reactions of thiourea with 25 metallic elements are presented 

on page 264.

The thiourea test is particularly valuable for detecting selenium 

in the presence of tellurium; the tellurium gives no reaction with 

thiourea, whereas stannous chloride precipitates tellurium as a black 

powder, which obscures to a large degree the red selenium precipitate. 

On the other hand, stannous chloride gives no precipitate with either 

copper or bismuth and is more satisfactory than thiourea for tests of 

most minerals containing selenium.

TELLURIUM

Cesium chloride test. 

Reagent: Solid cesium chloride (CsCl). 

Product: Cesium chlorotellurite (CszTeCU). 

Limit: 0.01% Te in 1:5 HC1.

'»Watson, J. A., Colour reactions in the microcheinical determination of minerals: Mineralog. Mng., 

vol. 24, No. 148, pp. 21-24, March 1935.
MICKOCHEMICAL METHODS 207

The mineral is broken down by 1:1 HN03 . The residue is leached 

with a drop of 1:5 HC1. The drop is then transferred and tested 

with a fragment of cesium chloride, by method II. Well-formed 

lemon-yellow isometric crystals are precipitated. If the concentration of tellurium is low the crystals begin to appear at the edge of 

the drop and the precipitation proceeds toward its center.

In concentrated solutions octahedrons predominate in the precipitate. With lower concentrations pseudohexagonal plates and triangles are more common. These show no double refraction and are 

probably isometric. (See pi. 9, A, B.) The composition of the 

crystals may be confirmed, as stated by Behrens and Kley, by adding 

a small fragment of KI to the center of the drop. If the amount 

added is small the crystals are stained brown, but if greater quantities are added the crystals are completely destroyed, and a precipitate of small amorphous brown grains is formed.

The principal difficulty with the tellurium test is that of getting 

the material into solution. A minimum of heat is required, otherwise anhydrous tellurous acid (H2Te03 ) forms instead of the normal 

compound H2Te03 .nH20. The anhydrous variety is only sparingly 

soluble in even the strongest acids.

This test is exceedingly satisfactory in every respect. It is simple 

and reliable, and the product .is highly characteristic.

The test does not work well in neutral or weakly acidic solutions. 

The strength of acid given above was determined by experiment, 

as follows: 0.5 gram of metallic tellurium was placed in a beaker 

containing 20 cubic centimeters of 1:1 HN03 , and the solution 

evaporated to dryness on a steam bath. The white powder, 

H2Te03 .nH2O, was taken into solution in 17 cubic centimeters of 

1:1 HC1. When dissolved, 33 cubic centimeters of H2O was added 

to the solution, the result being 1 percent of Te in 1:5 HC1.

Potassium iodide test. Potassium iodide added to a solution containing H2Te03 precipitates a finely divided amorphous chocolatebrown powder consisting in part of the tetra-iodide of tellurium 

(TeI4) and in part of the native element. When the KI is added 

the precipitate comes down immediately in the place where the 

fragment is added; then the precipitate spreads out toward the 

periphery of the drop in a widening ring. The precipitate in the 

center of the ring is redissolved, forming a liquid with a peculiar 

greenish tinge in transmitted light. This liquid is surrounded by 

the ring of brownish precipitate. The reaction is not easy to describe 

but is highly characteristic and when once observed will never be 

forgotten.

Selenious acid gives exactly the same reaction with KI. The precipitate is of a lighter shade of brown than the corresponding tellurium precipitate, and when the quantity precipitated is small
208 MICROSCOPIC DETERMINATION OF ORE MINERALS

the prevailing tint may be pinkish or orange-colored. The inexperienced observer, however, should make confirmatory tests on a 

fresh drop of test solution. (The solvent is, of course, 1:5 HC1.) 

First a fragment of CsCl is added to the drop. If a honey-yellow 

precipitate of isometric crystals forms, tellurium is present. If no 

precipitate forms, the drop is merged with a drop of stannous chloride 

solution (see p. 204); a brick-red precipitate proves selenium.

All the amorphous precipitates described here are best observed in 

obliquely reflected light, with a 16-millimeter objective.

The iodide test for tellurium is of particular value in that it is 

sometimes encountered in testing for other elements. It is the main 

reliance for identifying both selenium and tellurium in the systematic scheme of analysis described on pages 255-257.

ARSENIC

The reactions for arsenic recommended in the textbooks on microchemistry can be summarized as follows:

1. The production of arsine. This test is highly recommended by 

Chamot and Mason and is an adaptation of the familiar Marsh test 

for arsenic used in qualitative chemical analysis. It is undoubtedly 

exceedingly sensitive but requires a somewhat complicated apparatus 31 and is therefore not used in this work, as the ammonium molybdate test described below is highly satisfactory and can be made on a 

glass slide.

2. The formation of silver arsenate. The red crystals formed by 

adding silver nitrate to an arsenate solution are characteristic, but 

the test solution must be nearly neutral, and, moreover, many other 

elements interfere with this test.

3. The formation of ammonium-magnesium arsenate. This compound is the one commonly used in quantitative analysis. It ie 

recommended by Behrens and Kley as a microchemical test. A grain 

of magnesium acetate is added to an ammoniacal arsenate solution. 

The crystals formed are colorless but are characteristic in shape.32 

It is the writer's experience that this test requires conditions restricted in range. Although it is sensitive and was obtained on a 

0.01 percent ammonium arsenate solution, the crystals so formed 

were blunt square prisms rather than the more distinctive envelopeshaped crystals depicted by Behrens and Kley, which were obtained 

only in relatively concentrated solutions. The test requires alkaline 

conditions and is therefore less convenient than one that will work 

in acid solutions.

4. The formation of ammonium arseni-molybdate. This test is 

the same as the corresponding phosphorus test used in qualitative

31 Chamot, E. M., and Mason, C. W., op. cit., vol. 2, p. 199.

32 Behrens, H., and Kley, P. D. C., op. cit., p. 130, fig. 93.
MICBOCHEMICAL METHODS ' 209

analysis. According to Behrens and Kley it is far inferior to the 

ammonium-magnesium arsenate test just described, but the writer's 

experience is decidedly to the contrary. The molybdate test is 

entirely satisfactory and is recommended for the following reasons: 

It is at least as sensitive as the ammonium-magnesium arsenate test 

and gives a good reaction with 0.01 percent ammonium or sodium 

arsenate; it is effective in strongly acid solutions; the crystals obtained 

are yellow and characteristic in shape; only two elements (phosphorus and silicon) interfere with the test; the test works over a great 

range of conditions and is almost "foolproof." 

Ammonium molybdate test. 

Reagent: 1.5% ammonium molybdate in 1:7 HNOs.

Product: Ammonium arseni-molybdate in tiny yellow isometric crystals.

Limit: 0.01% (NH4)3As04 .3H2 0.

The mineral is decomposed on a glass slide by successive drops of 

1:1 HN03 or aqua regia. Arsenic compounds in general are more 

easily broken down by aqua regia than by nitric acid, but if antimony is also to be tested for it is better to use nitric acid. Both 

acids have a strongly oxidizing action and convert arsenic to the 

pentavalent state.

The residue is leached with a drop of 1:7 HN03 . Antimony remains behind; arsenic goes into solution as arsenic acid, H3As04 . 

The drop is transferred to a new place on the slide and slowly 

evaporated over the microflame. When nearly dry a drop of the 

1.5 per cent ammonium molybdate solution is added to the residue. 

This is evaporated very slowly by holding the slide at least 4 inches 

above the microflame and drawing it in and out of the heat until 

evaporation is complete. When dry, the excess ammonium molybdate is dissolved by adding a drop of 1:7 HN03 to the residue. Tiny 

yellow octahedrons of ammonium arseni-molybdate remain if arsenic 

is present in the unknown solution. (See pi. 9, C, D.}

These crystals are isometric. Octahedrons predominate but usually 

other isometric forms such as the trisoctahedron are combined with 

the octahedron. The crystals are usually very small and are often so 

nearly spherical that the individual forms cannot be distinguished. 

Twins, especially on the spinel law, are common, and tiny crosses 

seem to suggest other systems than the isometric. The crystals are, 

however, isotropic in polarized light. The color is distinctive a 

light lemon yellow and is best observed in oblique light, using an 

8-millimeter objective and turning aside the reflecting mirror so as 

to cut off the transverse illumination. The crystals are readily 

soluble in NH4OH but are not easily recrystallized.

Ammonium arseni-molybdate is precipitated slowly and only partly 

in cold solutions, in contrast to the corresponding phosphorus compound, which is precipitated rapidly and completely under the same
210 MICROSCOPIC DETERMINATION OF ORE MINERALS

conditions. Few if 'any ore minerals having a metallic luster contain 

phosphorus.

Bismuth, according to Behrens and Kley, will form a precipitate 

identical in appearance with the corresponding arsenic compound. 

The writer cannot confirm this statement. Tests carried out on different concentrations of bismuth nitrate, using the same procedure 

as the test for arsenic, failed to give any precipitate.

Silicic acid is completely removed by evaporating the mineral to 

dryness with HN03 ; insoluble Si02 separates out of the solution.

The ammonium molybdate test for arsenic does not interfere with 

the usual tests for other elements. For instance, after the arsenic 

test already outlined is made, the unknown solution can be tested for 

cobalt or copper by placing a drop of potassium mercuric thiocyanate 

directly on top of the 1 : 7 HN03 solution containing the excess of 

ammonium molybdate. In fact, the copper or cobalt mercuric 

thiocyanate forms better crystals when arsenic has been removed 

from solution than it does when arsenic is present.

Antimony and arsenic aie common associates but are easily separated by means of nitric acid. In testing for arsenic, it is customary 

to test for antimony as well. If bismuth is present it will be indicated 

in the test for antimony.

To throw down ammonium arseni-molybdate there must be a considerable excess of ammonium molybdate. However, if too much 

ammonium molybdate is present some will separate out in advance of 

the arseni-molybdate and obscure it. For this reason it is preferable 

to use an ammonium molybdate solution of definite strength rather 

than to introduce the solid reagent. The test works in acid or neutral 

solutions, but the ammonium arseni-molybdate crystals are most characteristic when formed in a nitric acid solution. The writer has 

found by experiments with solutions of various strengths that a 1.5 

percent solution of ammonium molybdate in 1:7 HN03 forms the 

most satisfactory reagent.

Potassium iodide test. 

Reagent: Solid KI.

Product: Arsenic tri-iodide (Asia), an amorphous yellow powder.

Limit: 0.02% (NH4) 3 As04 .3H2 0 in 1:5 HC1.

The mineral is broken down by means of 1:1 HN03 or aqua regia. 

The residue is leached with a drop of 1:5 HC1, and the drop transferred to another place on the slide. A fragment of KI is placed in 

the drop. A yellow to orange-colored precipitate forms if arsenic is 

present. Usually this is amorphous, but if the concentration of 

arsenic is fairly high, yellow hexagons will form. These are almost 

indistinguishable from lead iodide.

The color of the amorphous arsenic precipitate resembles that of 

the copper iodide precipitate but is a darker shade of yellow. The
MICKO CHEMICAL METHODS 211

color is best observed in obliquely reflected light, with a 16-millimeter 

objective.

With solutions of greater concentration than 0.10 percent ammonium arsenate the precipitation is immediate; with lesser concentration, the precipitate comes down at the edge of the drop a few seconds 

later.

Under the same conditions antimony colors the drop yellow but 

gives no precipitate.

The .'odide test is not as distinctive as the molybdate test for arsenic 

but is very sensitive and is frequently encountered in making the 

cesium double iodide test for antimony or bismuth.

Cesium double iodide test. 

Reagents: Solid KI and CsCl.

Product: Arsenic cesium double iodide (AsIa.SCsI) in orange-colored hexagons

or formless grains. 

Limit: 0.02% (NH4) 3AsO4.3H2 0 in 1: 5 HCl.

The mineral is broken down by 1:1 HN03 or aqua regia. The 

residue is leached with a drop of 1:5 HCl, and the drop transferred. 

A fragment of KI is added, then one of CsCl. The KI gives an 

amorphous precipitate, as described in the preceding test. When the 

CsCl fragment is added the arsenic iodide precipitate is replaced by 

a deep orange-colored precipitate which is probably the cesium arsenic 

double iodide. This is usually amorphous, but if the concentration 

in arsenic is relatively high, orange-colored hexagons will sometimes 

form in addition to the amorphous precipitate. These hexagons are 

identical in shape and color with those of the corresponding antimony 

compound. (See p. 214.) This would apparently be an exceedingly 

good test for arsenic were it not for the danger of confusing it with 

the antimony test.

In contrast with antimony, cesium chloride alone does not give an 

immediate precipitate with arsenic, hence in testing for both arsenic 

and antimony it would be better to add the cesium chloride before 

the potassium iodide. The fact that potassium iodide alone precipitates arsenic out of an HCl solution is also of great value in distinguishing between arsenic and antimony. When the yellow arsenic 

iodide precipitate appears the original residue should be leached with 

successive drops of 1:7 HN03 until all the arsenic is removed before 

testing for antimony. When the arsenic is removed from the original 

residue this can be tested for antimony by leaching with a drop of 1:5 

HCl. This is transferred and a fragment of KI added. If a yellow 

amorphous precipitate forms, arsenic is still present and the purification is not complete. If no precipitate forms and the solution stains 

yellow, the presence of antimony is indicated. It may be confirmed 

by adding a fragment of CsCl. Orange-colored hexagons will then 

prove antimony and not arsenic. If no yellow precipitate appears
212 MICROSCOPIC DETERMINATION OF ORE MINERALS

when KI is added and the solution does not turn yellow, antimony 

cannot be tested for by the cesium double iodide method, because 

orange-colored hexagons would mean either arsenic or antimony. 

The process should be repeated, using the cesium double chloride test 

for antimony (p. 214).

These facts are summarized as follows:

KI alone gives an amorphous yellow precipitate with arsenic.

KI alone gives no precipitate with antimony. It colors a moderately concentrated antimony solution light yellow but is without effect on a dilute 

solution.

CsCl alone does not give an immediate precipitate with arsenic (p. 211).

CsCl alone gives an immediate precipitate with antimony.

The cesium double iodide reaction with arsenic is not mentioned in 

the literature. For reasons already given it is regarded as a nuisance 

by the present writer. However, a complete understanding of it is of 

vital importance when dealing with arsenic and antimony minerals.

ANTIMONY AND BISMUTH

The microchernical tests for antimony and bismuth are so similar 

that some experience is necessary to distinguish between them. If 

either element is in the solution to the exclusion of the other, no difficulty is experienced in its identification. If both elements are present, however, they tend to form isomorphous crystals. Hence it is 

believed best to describe the two elements together.

Bismuth compounds are readily soluble in HN03 , with the formation of the nitrate Bi(N03 ) 3 . This will remain in solution in even 

weak concentrations of HN03 , but in the absence of acids it is hydrolyzed by H20, precipitating the oxy-nitrate BiON03 . These bismuth 

compounds are readily soluble in 1:5 HC1, with the formation of 

BiCl3 . The chloride is likewise hydrolyzed by pure water and 

precipitates BiOCl. In testing for bismuth the ore mineral containing it is first broken down by 1:1 HN03 . The bismuth nitrate 

resulting is taken into solution by 1:5 HC1, as the microchernical tests 

to be described work better in a hydrochloric than in a nitrate solution.

Antimony differs from bismuth in that its compounds are not readily soluble in nitric acid. Antimony compounds, however, are almost 

without exception easily broken down by 1:1 HN03 , the 'antimony 

being oxidized to Sb2 05 . This is a white powder comparatively 

insoluble in 1:7 HN03 but unfortunately sufficiently soluble to give a 

microchernical test for antimony. Therefore nitric acid cannot be 

relied on to separate antimony and bismuth in the residue obtained 

by evaporating a compound containing the two elements down to 

dryness with 1:1 HNO3 . Sb2O5 is readily soluble in 1:5 HC1. Usually not all the antimony goes into solution in the first drop; successive drops can be applied to the same residue and an antimony test
M1CROCHEMICAL METHODS 213

obtained from each. Hence if too much antimony is present in the 

first drop to give a satisfactory test, as shown by a dense amorphous 

precipitate, the second or third drop of 1:5 HC1 applied to the original 

residue will usually exhibit well-formed crystals when tested in the 

same way.

The microchemical tests recommended for antimony and bismuth 

are (a) the formation of the double chloride with cesium (SbCl3 .3CsCl 

or BiCl3 .3CsCl) and (6) the formation of the double iodide with 

cesium (SbI3 .CsCl or BiI3 .CsCl). Both tests have their advantages 

and disadvantages. If both antimony and bismuth are present in the 

same compound it is recommended that both tests be made so that 

the good points of each test can be combined.

Double chloride test. 

Reagent: Solid cesium chloride (CsCl).

Product: Antimony (or bismuth) cesium chloride (SbCl3 .3CsCl) in colorless

highly refracting plates. 

Limit: 0.02% SbCl3 solution; 0.02% BiCl3 solution.

The mineral is broken down by placing it in a drop of 1 : 1 HN03 

and evaporating to dryness. The residue is leached in a drop of 1 : 5 

HC1, the drop is transferred to another place on the slide, and a 

fragment of CsCl introduced into the solution (method II).

Antimony forms star-shaped crystals, also hexagonal plates, some 

with radiating spokes. The forms exhibited are identical in shape 

with those of the double iodide. (See p. 215.) Those of the double 

iodide are strongly colored, however, whereas the double chloride 

crystals are transparent and colorless. Most of the double chloride 

precipitate is amorphous. The best-formed crystals are found at 

the edge of the mass, the center of the mass usually comprising formless, more or less equidimensional grains.

Bismuth forms transparent rhombs that resemble cleavage flakes 

of calcite. (See pi. 11, <7.) In addition to the rhombs, petal-shaped 

and knifeblade-shaped crystals sometimes form, more rarely hexagons 

resembling the corresponding antimony compound. In solutions 

containing bismuth, however, hexagons have never been observed 

alone, except with very dilute bismuth chloride solutions (0.02 percent or less BiCl3 ). Almost invariably rhombs are also present.

Bismuth-cesium chloride invariably forms very thin transparent 

plates. They are so thin that they show little or no birefringence in 

polarized light. In fact, at times they are almost invisible in ordinary light but are readily detected owing to their refringence. By 

raising and lowering the objective the Becke line becomes prominent. 

The plates are highly reflecting, however, and sometimes exhibit 

iridescent colors owing to interference between the waves reflected 

from top and bottom surfaces. This is marked in polarized light, 

and the effect has been mistaken for birefringence. It disappears
214 MICROSCOPIC DETERMINATION OF ORE MINERALS

when oblique light is shielded from the slide by means of the hand 

or other object. The bismuth compound has a much stronger crystallizing tendency than the corresponding antimony compound, and 

amorphous grains are rare.

To summarize the above observations, colorless hexagons, stars, 

and formless grains indicate antimony; very thin, colorless rhombs, 

petals, and blades indicate bismuth.

When both antimony and bismuth are presemt the subordinate element combines with part of the predominant element to form isomorphous crystals. The remainder of the predominant element 

forms the crystals characteristic of that element. The isomorphous 

crystals have characteristics foreign to both of the constituent elements. Solutions of antimony chloride in 1 : 5 HC1 and bismuth 

chloride in 1 : 5 HC1 were mixed in different proportions and tested 

with CsCl, with the following results:

0.5% SbCl3 and 0.01% BiCl3 . Antimony hexagons and formless grains. No 

signs of bismuth.

0.5% SbCU and 0.02% BiCl3 . Same as preceding.

0.5% SbCl3 and 0.03% BiCl3 . Antimony stars, hexagons, and formless grains. 

No signs of bismuth.

0.5% SbCl3 and 0.06% Bids. Star-shaped crystals, some of considerable size. 

These undoubtedly contain both antimony and bismuth, as they are not 

characteristic of either.

0.5% SbCl3 and 0.12% BiCl3 . Beautiful star-shaped crystals with branching 

arms.

0.5% SbCl3 and 0.25% BiCl3 . Same as preceding; also some large white hexagons with ribs.

0.5% SbCl3 and 0.5% Bids. Complex star-shaped and treelike crystals.

0.25% SbCl3 and 0.5% BiCl3 . Same as preceding; also characteristic bismuth 

rhombs.

0.12% SbCl3 and 0.5% BiCl3 . Rhombs very abundant; stars less so. Many of 

the crystals are seedlike, irregular grains.

0.06% SbCl3 and 0.5% BiCl3 . Rhombs abundant; "seeds" less so. The "seeds" 

are highly refracting and have a heavy black line around them. Some of 

them are better formed and seem to be flat-lying hexagonal prisms. Irregular forms are abundant.

0.03% SbCl3 and 0.5% BiCl3 . Rhombs greatly predominate, but "seeds" also 

abundant.

0.015% SbCls and 0.5% BiCl3 . Abundant bismuth rhombs; no sign of antimony.

The student should make up one or more of the above-described 

mixed solutions and test with CsCl in order to familiarize himself 

with the appearance of the mixed crystals.

Double iodide test. 

Reagents: Solid potassium iodide (KI) and solid cesium chloride (CsCl). 

Product: Antimony (or bismuth) cesium double iodide (SbI3 .3CsI) in orangecolored to red hexagons and stars. 

Limit: 0.01% SbCl3 solution; 0.01% BiCl3 solution.

The mineral is broken by placing it in a drop of 1:1 HN03 

and evaporating to dryness. The residue is leached in a drop of
MICROCHEMICAL METHODS 215

1:5 HC1, and the drop is transferred to another place on the slide. 

A tiny fragment of potassium iodide is poked into one side of the 

drop. If antimony or bismuth chloride is present in 0.10 percent or 

greater concentration, that part of the solution adjacent to the fragment is colored light yellow. Bismuth gives the solution a deeper 

yellow than antimony. If either element is absent or present in 

concentrations weaker than 0.10 percent of the chloride, no coloration 

of the solution takes place.

When the fragment of KI is dissolved, a fragment of CsCl is 

poked into the opposite side of the drop. Where the solutions come 

into contact the yellow color disappears and the double iodide of 

antimony or bismuth is precipitated in hexagons or stars. The 

antimony compound is orange-colored and the bismuth compound 

rose-red. (See pi. 10 and pi. 11, A, B.} Hexagons are most abundant, but stars are very common with antimony. Hexagons, some 

of them elongated, are the usual forms precipitated by bismuth. 

This presents a marked contrast to the corresponding double chloride 

of cesium and bismuth. Stars are seldom seen where bismuth is 

present to the exclusion of antimony.

If the solution is concentrated with respect to antimony, or if 

some compounds that interfere with the test are present, notably 

arsenic, tellurium, or H2S04 , the first antimony precipitate is likely 

to be in the form of irregular rounded grains. Usually some of these 

grains will act as nuclei around which well-formed hexagonal plates 

will later crystallize. The nuclei are of a darker color than the 

material added later. If the quantity of the antimony precipitate is 

great the color of the aggregate is distinctly reddish and is almost 

identical with that of the bismuth compound. If the antimony 

crystals themselves are thick they may be red and be mistaken for 

bismuth. This is the greatest objection to the double iodide test. 

In oblique reflected light the antimony crystals are likewise red. 

Beginners are likely to make mistakes in judging the color of the 

crystals. With experience this difficulty tends to be minimized, final 

determination on thick, more or less equidimensional crystals being 

avoided and thinner and more platy hexagons and stars sought. It 

is recommended that the student have on hand stock solutions of 

SbCl3 and BiCl3 in 1: 5 HC1 and that he occasionally check his ability 

to distinguish colors.

The bismuth crystals are more constant in character, and if sufficient iodide is present the bismuth hexagons are always red. A few 

of the ore minerals contain both antimony and arsenic. The mineral 

is usually broken down with 1:1 HN03 and then leached with three 

successive drops of 1:7 HN03 . Nearly all the arsenic is removed by 

this treatment, also nearly all the copper and other more basic min-
216 MICROSCOPIC DETERMINATION OF ORE MINERALS

erals. Nevertheless it is very difficult to remove the final traces of 

arsenic.

Compounds containing both arsenic and bismuth are rare in nature.

The double iodide test was performed on solutions of SbCl3 in 1:5 

HC1 and BiCl3 in 1:5 HC1 mixed in various proportions. The following results were obtained:

0.5% SbCl3 and 0.01% BiCl3 . The usual orange-colored hexagons and stars

characteristic of Sb; no sign of Bi. 

0.5% SbCl3 and 0.02% Bids. Orange-colored hexagons predominate, but a few

red hexagons are also present. 

0.5% SbCl3 and 0.03% BiCl3 . Orange-colored stars and hexagons predominate,

but some red stars and hexagons are also present. It is believed that red

stars are indicative of the presence of both Sb and Bi. 

0.5% SbCl3 and 0.06% BiCl3 . Same as preceding. Orange-colored hexagons

predominate over red. 

0.5% SbCl3 and 0.12% BiCl3 . Same as preceding, but red hexagons more

numerous. 

0.5% SbCl3 and 0.25% BiCl3 . Red bismuth hexagons alone seen. No sign

of Sb.

As the proportion of bismuth in the solution increases, the effect 

of the antimony on the characteristics of the mixed crystals decreases and finally becomes zero.

The folio whig conclusions seem justified:

1. If either antimony or bismuth is present to the exclusion of 

the other, either the double iodide or double chloride test is satisfactory. However, the color effect of the double iodide would seem 

to make that test preferable.

2. If both antimony and bismuth are present, with bismuth predominant, only the double chloride test will indicate the presence of 

antimony.

3. If both antimony and bismuth are present, with antimony predominant, the double iodide test is preferable.

4. A small proportion of bismuth is detectable in the presence of 

a large proportion of antimony, but a small proportion of antimony 

is not detectable in the presence of a large proportion of bismuth.

5. If both elements are suspected, both the double chloride and 

double iodide tests should be performed.

Arsenic causes a precipitate identical in character with antimonycesium double iodide. Means for distinguishing between the arsenic 

and antimony compounds are discussed on page 211.

Chemical separation oj antimony and bismuth by means of Na2S. 

The difficulty of distinguishing beween the cesium-antimony double 

chloride and the cesium-bismuth double chloride is largely removed 

by the fact that cesium-antimony double chloride is readily soluble 

in sodium sulphide solution, whereas the cesium-bismuth double 

chloride is converted into black insoluble bismuth sulphide, Bi2S3 .

33 This test was originated by S. B. Riley (Ore-body zoning: Econ. Geology, vol. 31,p. 177,1936).
M1CKOCHEMICAL METHODS 217

The test is carried out as follows: The 1:5 HC1 solution from 

which the antimony or bismuth has been largely precipitated by means 

of cesium chloride is allowed to evaporate until nearly dry. A drop 

of saturated sodium sulphide solution is added directly to the residue. 

If antimony is present, an amorphous yellow precipitate of Sb2S3 is 

first thrown down, but this redissolves in an excess of sodium sulphide. 

Crystals of antimony-cesium double chloride are quickly destroyed. 

If no other metallic elements than antimony are present, the antimony-cesium double chloride crystals are completely dissolved. This 

is the result when this test is carried out with metallic antimony or 

stibnite. If other elements such as lead or copper are present, traces 

of these elements are occluded during the formation of antimonycesium double iodide. As the crystals are dissolved, the extraneous 

element is converted into a black sulphide which remains undissolved 

near the centers of the former crystals. These residues are formless 

rounded grains much smaller than the original crystals. In order to 

dissolve completely all the antimony present, one or more additional 

drops of sodium sulphide may be needed.

With bismuth, the test is carried out in the same manner. The 

1:5 HC1 solution containing bismuth-cesium double chloride crystals 

is allowed to evaporate almost to dryness. A drop of saturated sodium sulphide solution is applied to the residue. If any bismuth 

remains in solution in addition to that present as bismuth-cesium 

double chloride, it is precipitated immediately as a cloud of minute 

black amorphous grains. The bismuth-cesium double chloride 

crystals are very thin and fragile plates, and these are usually broken 

up with the formation of finely divided black amorphous bismuth 

sulphide grains. More rarely black pseudomorphs of the rhomb- or 

blade-shaped plates are formed.

The objection to this test is that a small amount of bismuth completely obscures a larger amount of antimony present. Crystals 

containing both elements behave in every respect like those containing 

bismuth alone.

The sodium sulphide test works still better in distinguishing between the cesium double iodides of antimony and bismuth. The test 

is carried out in the manner already described. The 1:5 HC1 

solution containing antimony or bismuth partly or wholly precipitated 

by cesium iodide is allowed to evaporate until nearly dry. A drop of 

saturated sodium sulphide solution is added to the residue. If 

antimony is present to the exclusion of bismuth, the orange hexagons 

of antimony-cesium double iodide are quickly dissolved. An amorphous precipitate of antimony sulphide is thrown down which dissolves in an excess of sodium sulphide. One or more drops of the 

reagent may be required to bring about complete solution. If extraneous metallic elements such as copper or lead are absent, the anti129151 40 15
218 MICROSCOPIC DETERMINATION OF ORE MINERALS

mony-cesium double iodide crystals are completely dissolved. This 

is the result when metallic antimony or stibnite is tested by this 

method. When extraneous metallic elements are present, the orange 

hexagons are destroyed and black insoluble residues remain. Usually 

these residues are found near the centers of the original crystals, but 

more rarely they concentrate on the peripheries of the hexagons, each 

of which appears to be outlined by a thin black line. Within the 

periphery the crystal is colorless or nearly so.

When bismuth is tested for by this method, the test works much 

better than with the bismuth-cesium double chloride crystals as 

described above. The double iodide crystals are red hexagons instead 

of colorless rhornb- or blade-shaped plates, and the hexagons are 

thicker and stronger. Hence, perfect pseudomorphs of black bismuth 

sulphide are formed from the red double iodide crystals. If some 

bismuth is carried in solution in addition to that precipitated as double 

iodide, an amorphous cloud of black grains is also thrown down which 

may obscure observation of the test. In this event, it is preferable 

to repeat the test on a solution less concentrated with respect to bismuth. The original residue from solution of the mineral powder is 

leached with a second drop of 1:5 HC1, and the bismuth-cesium 

double iodide is precipitated from this solution. The solution is 

allowed to evaporate until nearly dry, and the sodium sulphide is 

added. The red hexagons are immediately stained black.

The objection to this test is the same as that to the corresponding 

test on the double chlorides. The presence of even a large amount of 

antimony is obscured by that of a small amount of bismuth. For 

instance, in testing ullmanite (NiSbS) from Siegen, Germany, enough 

bismuth was present to prevent destruction of the orange hexagons. 

The outlines were preserved and the color was changed from an orange 

to a steely gray. In some places the crystals were opaque and in 

other places partly transparent.

The sodium sulphide test cannot be used to distinguish between the 

double iodides of arsenic and antimony, as they behave exactly alike 

in a sodium sulphide solution.

TIN

Cesium (or rubidium} chloride test. 

Reagent: Solid cesium chloride.

Product: Cesium chlorostannate (Cs2SnClfl), or rubidium chlorostannate (Rb2 -

SnCl6), in colorless highly refracting octahedrons. 

Limit: 0. 01 % SnCl2 .

The test for tin is similar to the cesium chloride test for antimony. 

The mineral is broken down by 1:1 HN03 . The residue is leached 

by three successive drops of 1: 7 HN03 to remove the soluble nitrates 

and finally with a drop of water to remove the remaining traces of
MICROCHEMICAL METHODS 219

HN03 . The tin remains in the residue. This is leached with a 

drop of 1: 5 HC1, and the drop is transferred to another place on 

the slide and tested with a fragment of CsCl (method II). A precipitate of colorless highly refracting octahedrons proves tin. (See pi.

11,0.) 

Usually one drop of 1:5 HC1 will not dissolve all of the tin in

the residue. Three or four successive drops can be applied to the 

residue, each giving a good tin test.

Tin compounds behave similarly to those of antimony in that the 

tin does not dissolve in nitric acid but is oxidized to metastannic 

acid (H2Sn03 ). This is a white amorphous powder which dissolves 

in HC1, forming stannic chloride (SnCl4 ). When a fragment of cesium 

(or rubidium) chloride is placed in a stannic chloride solution the 

insoluble compound cesium (or rubidium) chlorostannate is precipitated in colorless isometric crystals, which are almost invariably 

octahedrons. The tin must be in the tetravalent condition in order 

to give this test. In testing the SnCl2 solution cited in the outline, 

a few drops of H202 were added to oxidize the tin to SnCl4 .

If tin alone is to be tested for, the writer prefers rubidium to 

cesium chloride as a reagent. The rubidium chlorostannate octahedrons are always larger and better-formed than those of the corresponding cesium compound. The sensitivity of the rubidium chloride 

test seems to be as great as that of the cesium chloride test.34 The 

cesium salt is more insoluble than the rubidium salt and forms 

smaller crystals, frequently requiring a 4-millimeter objective for 

their discernment.

On the other hand, rubidium chloride is not a satisfactory reagent 

for identifying antimony and bismuth. Consequently in making a 

systematic series of tests on an unknown mineral it is better to use 

cesium chloride. If tin is present, a precipitate will form, and if 

this is unsatisfactory as far as the forms of the individual crystals 

are concerned, the test can be repeated with rubidium chloride.

The precipitate is sometimes so fine-grained that the individual 

crystal forms cannot be recognized. This indicates that the tin was 

not completely oxidized to the stannic condition. This is remedied 

by replacing a drop of aqua regia on the original residue and evaporating to dryness. The residue is leached by a drop of 1:5 HC1, 

and the drop is transferred to another place on the slide and tested 

with a fragment of rubidium chloride as before.

Antimony does not form an isomorphous compound with tin. 

When a mineral containing both antimony and tin is tested antimony 

forms the characteristic orange-colored hexagons of the double 

iodide, and tin forms colorless octahedrons side by side with the anti-

" The writer differs with Chamot in this regard.
220 MICROSCOPIC DETERMINATION OF ORE MINERALS

mony hexagons. (See detailed description of tests for franckeite 

and cylindrite, p. 276.)

The textbooks of Chamot and Mason and of Behrens and Kley state 

that if a fragment of potassium iodide is dissolved in the tin-bearing 

solution the cesium precipitate will be colored yellow. The present 

writer is unable to agree with this statement. Under the conditions 

described here the octahedrons remain colorless.

Lead forms a double salt with cesium similar to the tin salt. 

Most of the lead is removed by leaching the original residue with 

1:7 HN03 and transferring the drop. By the addition of HC1 the 

remaining lead is converted to PbCl2 , which is almost insoluble in 

the HC1. However, if lead is present in the mineral, it is best to add 

a fragment of KI to the HC1 solution before adding the CsCl. The 

double iodide of cesium and lead will form orange-colored needles, 

which will not interfere with the formation of the tin salt.

The tin test above described is highly satisfactory in every respect. 

The octahedrons are sometimes almost spectacular in appearance. 

All the tin sulphosalts occurring in nature are readily broken down by 

HNO3 . Cassiterite (SnO2 ) is insoluble and must first be fused in a 

sodium carbonate bead in order to be converted to a soluble form. 

The bead is then crushed and dissolved in a few drops of 1:5 HC1 

in a small capsule or annealing cup like that described by Chamot and 

Mason.35 A drop of the solution is drawn up into a capillary tube and 

blown out on a glass slide. A fragment of CsCl is added to the 

drop. The presence of NaCl caused by dissolving in HC1 the excess 

of Na2C03 does not interfere with the formation of the tin salt. Of 

course, the amount of cassiterite required to give a satisfactory tin 

test is several times that of the sulphosalts.

LEAD

Lead iodide test. 

Reagent: Solid KI.

Product: PbI2 ; hexagonal lemon-yellow pleochroic plates and disks.

Limit: 0.005% lead acetate.

This is the most useful test for lead. The mineral is taken into 

solution with nitric acid. Chlorine ions should not be introduced 

into the solution. The acid is carefully evaporated off with a minimum quantity of heat, otherwise a relatively insoluble basic nitrate 

or even the oxide will form. It is preferable not to use strong acid, 

as even with 1 : 1 HN03 part of the sulphur is oxidized and the lead 

precipitated as the insoluble sulphate. Repeated evaporations with 

weak acid have the same effect. The residue frequently shows a boxwork structure, which is one of the indications of the presence of 

lead. (See pi. 12, D.) Mercury compounds leave a similar residue

35 Chamot, E. M., and Mason, C. W., op. cit., vol. 2, p, IGfl.
MICBOCHEMICAL METHODS 221

when broken down by means of aqua regia. Practically all the lead 

minerals are easily attacked by nitric acid, whereas mercury minerals 

are almost completely inert to it.

The residue is leached with water, and the drop is transferred and 

tested with a tiny fragment of KI, using method II. A lemon-yellow 

precipitate of normal lead iodide is thrown down.36 If the concentration of lead ions is relatively large (0.1 percent Pb(N03 ) 2 or greater) 

well-formed hexagonal plates will usually appear. (See pi. 12, A, B.) 

They may, however, be star-shaped or irregular. If the solution is 

very dilute the crystals will be disk-shaped. Chamot recommends 

that a weak nitric acid solution be used. It is the writer's experience 

that better-formed crystals are precipitated from a neutral solution. 

The plates are greenish yellow in transverse illumination. They 

apparently show no double refraction with the analyzer inserted but 

are strongly pleochroic when turned up on edge, and the effect greatly 

resembles that of biotite plates. The best test, however, is to view 

them in oblique light with a 16-millimeter objective. The plates are 

highly reflecting and glitter like the tinsel on a Christmas tree.

As lead iodide is very soluble in potassium iodide solution, the 

size of the KI fragment must be kept at a minimum. (See p. 182.) 

As the drop dries, colorless needles of the double iodide 2KI.PbI2 

usually appear at the edge of the drop. These needles usually form a 

boxwork pattern, which is very characteristic and diagnostic of lead. 

The investigator will soon become familiar with this pattern and will 

not assume that it represents another element in addition to lead.

The double iodide of potassium and lead usually forms as a product 

of the destruction of the normal lead iodide. As the drop evaporates, 

the destruction of the yellow lead iodide plates begins at the periphery, 

and the colorless double iodide needles take the place of the plates. 

As evaporation continues, the wave of destruction advances toward 

the center until all the normal lead iodide plates have been destroyed 

and a mat of the double iodide needles has taken its place. Thus the 

normal iodide is a transitory product, and the observer should have 

his eye to the microscope when he introduces the KI fragment into 

the drop; otherwise the normal lead iodide may disappear before being 

observed. Once destroyed the normal lead iodide cannot be reprecipitated from a solution of the residue. However, if the concentration of lead with respect to KI is high, the normal yellow iodide plates 

may persist even after the drop has completely evaporated.

Copper and bismuth interfere with this test, and double iodides are 

formed. Copper can be removed by ammonia, as described on page 

191, and the residue afterward tested for lead. In the presence of 

bismuth it is best to prove the lead by the chloride test described

» This compound is referred to in this bulletin as "normal" lead iodide to distinguish it from the basic 

iodide and double iodides of lead with potassium, bismuth, and other elements.
222 MICROSCOPIC DETERMINATION OF ORE MINERALS

below. Chamot and Mason 37 say that the normal lead iodide can be 

recrystallized from the double iodide of lead and bismuth by boiling 

the mixed product with hot water. The writer has never had much 

success with this procedure. The drop is so small that trying to 

heat it over a flame will usually result in the entire disappearance of 

the liquid. Adding water already heated is equally futile, as when 

taken from the flame the drop cools too rapidly to do much good. 

The same remarks apply to recrystallizing the normal iodide from 

hot water. In testing ore minerals containing other metallic elements, it is usually preferable to precipitate the lead as chloride and 

then make the iodide test. (See below.)

Chloride test. 

Reagent: 1 : 5 HC1.

Product: PbCl2 , white orthorhombic needles.

Limit: 0.1% lead acetate.

If hydrochloric acid is added to a solution containing lead or a 

soluble salt of lead, characteristic long white acicular crystallites of 

lead chloride separate out. According to Chamot and Mason,38 there 

are also seen feathery dendritic X's and long irregular ragged prisms, 

but under the conditions here described the prism is the only form 

that has been observed by the writer. The investigator should 

familiarize himself with these forms, as they are likely to appear when 

he is testing for other elements involving the use of HC1, notably 

antimony and bismuth. It is usually advisable to make an additional 

test on the lead chloride. This can be done by converting the chloride 

into the more characteristic normal iodide.

Although generally regarded as insoluble in water, lead chloride is 

far more soluble than the iodide. Hence if the dried chloride residue 

is leached with water and the drop transferred, lead iodide in the 

usual hexagonal plates can be precipitated by adding a fragment of 

KI to the drop. This procedure is of far greater value in tests with 

ore minerals than the direct iodide test described on page 221. This 

procedure is particularly valuable if bismuth is present, because the 

bismuth will hydrolyze, forming insoluble BiOCl, when water is added 

to a BiCl3 residue. This is the only method by which the writer has 

been able to obtain a lead solution free from bismuth if any bismuth is 

present in the unknown mineral.

Cesium chloride. Lead chloride has only a slight solubility in 1 : 5 

HC1. However, a fragment of cesium chloride placed in such a solution will precipitate small but well-formed crystals of the double 

chlpride PbCl2 .2CsCl. These are usually short colorless prisms of 

rectangular outline. They give a low interference color in polarized 

light and parallel extinction. The quantity of such a precipitate is

a7 Chamot, E. M., and Mason, C. W., op. cit, vol. 2, p. 182. 

38 Idem, p. 144.
MICROCHEMICAL METHODS 223

small, and it does not interfere with tests for antimony, bismuth, tin, 

and tellurium.

This reaction is not recommended as a test for lead, but the student 

should be familiar with it, as it is likely to appear when he is testing 

for other elements.

If successive fragments of KI and CsCl are placed in a 1:5 HC1 

solution containing lead ions, the double chloride just described usually 

forms. In addition long, thin yellow to orange-colored needles are 

usually observed at the edge of the drop. These may be crystals of the 

double iodide, but this is not certain.

Ammonium bichromate. A fragment of ammonium bicrhomate 

placed in a 1 percent HN03 solution containing lead ions precipitates a finely divided amorphous yellow powder, which is lead bichromate. This reaction often appears in testing for silver. It is best 

to check the presence of lead in the original residue by the potassium 

iodide test. Usually 1 percent HN03 will remove only a small proportion of the lead. A drop of 1:5 HC1 is applied to the original residue 

and then removed. The residue is then leached with two or more 

drops of H20, each of which is transferred and tested with a fragment 

ofKL

GOLD

Gold minerals in nature are few. The most common is the native 

metal itself. It is readily recognized in polished surfaces by its 

color and sectility.

Tellurium is the element most commonly found in chemical combination with gold in fact it is almost the only one in which such a 

combination occurs in nature. A selenide of gold is known but is exceedingly rare. There are a number of tellurides of gold, most of them 

containing more or less silver. These minerals are not characteristic 

in appearance, and it is usually desirable to confirm their identity by 

microchemical tests.

The tellurides are easily broken down by 1:1 HN03 . The dense 

white amorphous tellurous acid and metallic gold remain. The gold 

is usually covered and obscured by the tellurous acid. This acid is 

readily soluble in 1:5 HC1, however, and the gold can then be recognized by its color by observing it in obliquely reflected light. A 16- 

millimeter objective is most satisfactory for this purpose. This suffices 

to identify gold in the majority of tests.

Where a confirmatory test is desirable, the gold can be taken into 

solution by means of aqua regia. Before doing this it is best to 

make sure that all the tellurium is removed. The original residue 

should be leached by successive drops of 1:5 HC1 and each drop tested 

by means of a fragment of cesium chloride until a precipitate is not 

immediately thrown down. (See p. 207.) A drop of aqua regia is then 

applied and the gold dissolved. The residue is gold chloride (HAuCl4 )
224 MICROSCOPIC DETERMINATION OF ORE MINERALS

and is orange-colored and very soluble in water. A drop of H20 is 

applied to this residue. When the residue is dissolved the drop is 

transferred to another place on the slide and tested by one of the two 

following methods. 

Pyridine double bromide test. 

Reagent: 1 volume pyridine in 9 volumes of 40% HBr. 

Product: (ClsH5NH)AuBr4; pleochroic crystals. 

Limit: 0.02% HAuCl4 .4H2 0 solution.

The most satisfactory test for gold is the formation of the double 

bromide of gold and pyridine. Credit for this test is due to Putnam, 

Roberts, and Selchow, who discovered and described it. (See p. 174.) 

The gold is taken into solution by one or more applications of aqua 

regia, and the excess acid is evaporated off. The residue is leached 

with a drop of water, and the drop is transferred to another place on the 

slide and joined to a drop of the reagent by method I. A well-crystallized precipitate forms if gold is present in sufficient concentration.

The crystal form varies somewhat with the concentration. With 

moderately concentrated solutions prisms predominate. With dilute 

solutions the crystals are leaflike and feathery, often radiating inward 

from the periphery of the drop. (See pi. 13.)

The crystals are extraordinarily pleochroic, varying from orangecolored to maroon when the elongation is perpendicular to the vibration direction of the lower nicol and from colorless to pale yellow for 

the parallel orientation. This extreme pleochroism is diagnostic of 

the compound. The crystals are monoclinic or triclinic, and the 

maximum extinction is about 10° on the prism.

The authors cited state that the following elements give no reactions themselves within 10 minutes from 0.02 molal solution, nor 

does their presence singly or together appreciably affect the formation of the gold compound: Al, As, Ba, Be, Bi, Ca, Cd, Cl, Co, Cr, Cs, 

Cu, Fe, Ga, Ge, Hg (mercuric), I, In, Ir, La, Mg, Mn, Mo, Ni, Os, 

P04) Pd, Pt, Ru, Se, Sn (stannic), Te, Th, Ti, U02 , V, W, Zn, and, by 

analogy based on the periodic system, B, Li, Na, K, Rb, Sr, Sb.

Under ordinary conditions the analogous lead compound may 

also be precipitated. This compound is characterized by long, 

grasslike, colorless, nonpleochroic crystals and hence differs widely 

from the corresponding gold compound. Most of the lead remains 

behind as chloride when the residue from the original aqua regia 

treatment is leached with water. The authors state that the lead 

can be kept in solution by adding NaBr to the test drop belore 

joining the reagent drop to it.

This gold test is particularly valuable in that the presence of 

tellurious acid does not interfere with it. The limit of dilution as 

determined by the present writer is about 0.02 percent of gold chloride.

Experiments carried out on lead nitrate solutions of various con-
MICKOCHEMICAL METHODS 225

centrations indicate that the pyridine and HBr solution will cause 

precipitate from a 0.05 percent Pb(N03 ) 2 or more concentrated 

solution. The precipitate is not sufficiently characteristic in appearance and properties to constitute a good lead test, but the student 

should familiarize himself with this compound, as it may appear 

when he is testing for gold. 

Stannous chloride test. 

Reagent: 2% SnCl2 in 1:5 HC1.

Product: Metallic gold (purple of Cassius).

Limit: 0.1% gold chloride.

The mineral is dissolved in 1:1 HN03 . Usually metallic gold 

separates out and may be recognized by its color. If necessary, 

however, a confirmatory test can be made as follows: Dissolve the 

residue in aqua regia. The reddish-brown trichloride is formed. 

Dissolve this in a drop of water and transfer the drop. Test the 

drop with stannous chloride solution, using method I. A precipitate of finely divided metallic gold forms where the solutions come 

into contact. This spreads out toward the periphery of the test 

drop in a widening band. The precipitate is amorphous and ranges 

in color from reddish to bluish tinges. Purple is the most distinctive 

color (purple of Cassius).

Under similar conditions selenious acid yields a red precipitate and 

tellurious acid a dull-black powder. It must be admitted that this 

test leaves much to be desired as regards delicacy and uniformity in 

appearance of the precipitate. It works in the presence of other 

elements, however, and is given here because the reaction may appear 

in testing for selenium. All traces of tellurious acid must be removed 

before testing for gold.

The thallous nitrate reaction described by Behrens and Kley is 

unreliable and should be avoided.

MERCURY

Double thiocyanate test. 

Reagents: Solid cobalt nitrate (Co(N03)2.6H2 0) and solid potassium thiocyanate

(KCNS).

Product: Cobalt mercuric thiocyanate in blue prisms and branching forms. 

Limit: 0.005 HgCl2 in 1% HN02 .

The test for mercury is the formation of cobalt mercuric thiocyanate in other words, this test is the reverse of the cobalt test above 

described. Mercury compounds are distinctive in that they are 

almost insoluble in HN03 and HC1 but are readily soluble in aqua 

regia. The residue (HgCl2 ) when examined under the microscope 

is seen to be decidedly crystalline. It shows dendrites with rightangle branches, which often form a well-defined grating resembling 

that of lead nitrate. (See pi. 14, C, D.}
226 MICROSCOPIC DETERMINATION OF ORE MINERALS

The mineral is broken down with aqua regia, the residue leached 

with 1 percent HNO3 , and the solution transferred. A small fragment of cobalt nitrate is added; when this is dissolved, a fragment 

of potassium thiocyanate is added. Blue prisms and dendrites prove 

the presence of mercury. (See pi. 14, A, B.) If the solution is very 

weak with respect to mercury the formation of the blue crystals is 

delayed. In very dilute solutions less than 0.02 percent HgCl2 , 

for instance blue spherulites will predominate over well-defined 

crystals. Sometimes the usually insoluble cobalt mercuric thiocyanate precipitate will not form at all, but when the solution is 

nearly dry, it will turn a deep-blue color and will precipitate soluble 

light-blue feathery crystals of unknown composition. If a drop of 

water is added to this residue, the dark-blue insoluble cobalt mercuric 

thiocyanate will usually form.

This test is very sensitive, and a 0.005 percent mercuric chloride 

solution will give a satisfactory reaction. The solution must be 

slightly acidified with nitric acid before adding the solid reagents. 

Ordinarily the precipitate will not come down at once, as the solution 

tends to be supersaturated.

In taking the mineral into solution, the evaporation of the aqua 

regia should be carried out slowly by passing the slide in and out of 

the flame. The reason for this precaution is that mercury is very 

volatile, and this tendency to volatilize is increased by boiling the 

drop.

The fragments of cobalt nitrate and potassium thiocyanate should 

be somewhat larger than is usual with method II. However, they 

must not be too large, or the excess of reagents will crystallize out 

ahead of the desired precipitate and obscure it. This test is more 

difficult to obtain than any of the others hitherto described in this 

bulletin. It is recommended that the student practice the test by 

making it on cinnabar until he acquires confidence in his skill.

MANGANESE

Sodium bismuthate test. 

Reagent: Solid sodium bismuthate (NaBiOs).

Product: Sodium permanganate (NaMn04), which colors solution pink to

purple. 

Limit: 0.02% MnCl2 .4H2 0 in 1: 7 HN03 .

The oxides of manganese are only sparingly soluble in nitric acid 

but are readily soluble in 1:1 HC1 and in aqua regia. The powder 

is taken into solution by means of three or four successive drops of 

either of these reagents. The residue, MnCl2 .4H2O, collects as a 

ring around the periphery of the drop. It usually has a pinkish 

tinge and is distinctively crystalline, but the nature of the crystals 

cannot be determined, owing to their small size.
MIOBOCHEMIOAL METHODS 227

The residue is taken into solution in a drop of 1:7 HN03 . A 

fragment of sodium bismuthate is added. This is a strong oxidizing 

agent and oxidizes the bivalent manganese to the permanganate 

state. The Mn04 ions color the solution pink to purple, the color 

depending on the concentration of Mn04 . If the concentration is 

very low the color may not appear immediately but will be apparent 

near the edge of the drop when it begins to dry up.

The permanganate solution thus formed is not permanent under 

the conditions just described. In a short time, usually about a quarter of a minute, the permanganate ions break down and deposit 

manganese dioxide as a brown powder. The deposition of this 

powder begins at the edge of the drop and proceeds toward the 

center. The solution loses its color as the powder is deposited.

It is advisable to use a fairly large quantity of sodium bismuthate, 

much more than is usual in method II. Only a part of the reagent 

is dissolved, the amount depending on the proportion of manganese 

present.

The sodium bismuthate test is described by Chamot but is not 

given the prominence which it merits. It is sensitive and highly 

distinctive, and other elements do not interfere with it. In addition 

it works in a strongly acid solution.

Some minerals, such as huebnerite (MnW04 ) and wolframite 

((Fe,Mn)W04 ), are difficultly soluble in nitric or hydrochloric acids or 

in aqua regia. With these minerals, the sodium bismuthate test can 

be made, after fusion in a sodium carbonate bead. The bead is then 

dissolved in a few drops of 1:7 HN03 in a porcelain crucible, and the 

sodium bismuthate reagent is added to the solution.

SULPHUR

Reagent: Solid calcium acetate (Ca(C2H3 02 )2)- 

Product: Calcium sulphate (gypsum) crystals. 

Limit: 0.15% H2S04 .

Elemental sulphur and H2S. The element sulphur is known in 

several allotropic modifications, but the form existing in commercial 

deposits is the ordinary orthorhombic form, usually with crystal 

faces. It is soluble in carbon disulphide (CS2 ), benzene (C6H8 ), 

and several other organic solvents and can be recrystallized from 

these reagents, thus affording a quick and accurate test. Carbon disulphide is the most effective solvent, but it slowly decomposes, liberating elemental sulphur. If this reagent is employed 

it should be tested by evaporating a drop on a glass slide. If a 

residue is observed the reagent should be rejected. Benzene forms 

a satisfactory solvent, but it evaporates with extreme rapidity. The 

powder to be tested is placed on a glass slide, and a cover glass 

about 3 millimeters in diameter is placed over it. A drop of ben-
228 MICROSCOPIC DETERMINATION OF ORE MINERALS

zene is brought to the edge of the cover glass by means of a platinum wire. When liberated from the wire, it immediately flows 

under the cover glass. Although partly protected by the cover glass 

the drop evaporates in about a minute, but meanwhile some sulphur 

has been dissolved, and as the drop evaporates the sulphur is precipitated in crystals. The crystals are large and tend to agglomerate. Recognizable forms are not common, but straight edges are 

seen at the boundaries of the mass. The best indication of crystalline sulphur is the very high birefringence of the mass. The interference color is white of a high order, similar to that of carbonate 

crystals. In oblique reflected light crystalline sulphur is yellow.

Some simple sulphides, such as stibnite, sphalerite, and bismuthinite, and all or nearly all of the sulphosalts of bismuth yield a 

yellow globule of sulphur when decomposed by nitric acid. This is 

identical in appearance with ordinary fused sulphur but does not 

dissolve in carbon disulphide or benzene. However, the appearance 

of the globule itself is sufficiently indicative, and no other test for 

sulphur is necessary.

A few simple sulphides when treated with nitric acid evolve H2S 

gas. The odor of this gas is very distinctive, and minute quantities 

are easily detectable. The test is carried out directly on the polished 

section, thus obviating the necessity of transferring powder to a glass 

slide. The minerals liberating H2S are alabandite (MnS), troilite 

(FeS), and a small proportion of galena specimens.

By far the greater proportion of the sulphides and sulphosalts 

when decomposed by nitric acid deposit sulphur as a white amorphous 

powder. This powder is not sufficiently distinctive in appearance to 

constitute a test for sulphur. It is one of the allotropic forms of 

sulphur and like the yellow globule above described does not dissolve 

in carbon disulphide or benzene.

Sulphate test. Attempts to take the amorphous white sulphur 

residue into solution on a glass slide have resulted in failure. The 

reagents tried were 1 : 1 HN03 , fuming HN03 , and aqua regia. It is 

possible to take this sulphur into solution by treatment hi a beaker 

with fuming HN03 . The sulphur is oxidized to H2S04 and in quantitative analysis is precipitated by barium chloride and weighed as 

barium sulphate. Oxidation of the sulphur in this manner requires 

continued boiling of the reagent and hence is not possible on a glass 

slide. A possible means of escaping this difficulty is to fuse the 

sulphide in a sodium carbonate or potassium nitrate bead. This requires much more material than has been used in tests already 

described, and high concentrations of sodium or potassium are introduced into the test drop. Hence this means is employed only as a 

last resort.
M1CBOCHEMICAL METHODS 229

Fortunately in treating most sulphur-bearing minerals with 1 : 1 

HN03 , some sulphur is oxidized to H2S04 in addition to that deposited as the solid element. With some sulphides high in sulphur, 

such as pyrite, marcasite, and hauerite (MnS2 ), a relatively large 

proportion of the sulphur is oxidized." The same is true of minerals 

readily soluble in HN03 , such as chalcocite and sphalerite. Sulphides 

difficultly soluble in HNO3 and many sulphosalts yield a relatively 

lower proportion of H2S04 . In evaporating a mineral to dryness 

care should be taken not to raise the temperature so high that the 

H2S04 begins to evaporate. Fortunately HaS04 is not very volatile, 

and to evaporate it requires a higher temperature and more time 

than is usually employed in making microchemical tests. With reasonable care very little is lost by evaporation.

The common elements yielding insoluble sulphates are barium, 

strontium, calcium, and lead. The most insoluble sulphate and theoretically the one which should yield the most sensitive test is barium. 

However, under the conditions here described the precipitate is 

always amorphous and can be recrystallized only in strong sulphuric 

acid. Strontium acetate forms with sulphate ions an amorphous 

precipitate in neutral solutions and weak acids. With high magnifications a few tiny orthorhombic crystals can usually be observed 

at the edge of the amorphous mass. In strong HC1 strontium sulphate is more soluble, and more of the precipitate is crystalline. This 

precipitate should also be observed with high magnification. The 

crystals are colorless and highly refracting but too small to show 

birefringence with crossed nicols. The most common forms are 

orthorhombic bipyramids resembling octahedrons, blunt prisms with 

rounded terminations, and oval spherulites. Like barium sulphate 

the strontium compound can be recrystallized in hot, concentrated 

H2S04 , but this is, of course, prohibited in testing for sulphates. It 

is useful in testing for strontium with H2S04 . The crystals so obtained are larger, more uniform in character, and hence more distinctive than those obtained from strong HC1. The strontium reaction can be used as a test for the presence of sulphate ions, but the 

corresponding calcium test is better, for it yields more distinctive 

crystals. The limit of dilution with the strontium reaction is about 

0.05 percent of H2S04 , or three times that of the calcium test.

Lead acetate furnishes a much more satisfactory test for sulphate 

ions than the strontium salt. Lead sulphate has a stronger crystallizing tendency than strontium sulphate, and a larger percentage of 

the precipitate is crystalline, although most of it is amorphous. 

The most characteristic forms of lead sulphate are tiny colorless 

diamond-shaped or boat-shaped plates and slender prisms with 

sharp-pointed terminations. Many of these prisms have enlargements or protuberances, oval or diamond-shaped in cross section, at
230 MICROSCOPIC DETERMINATION OF ORE MINERALS

the center of the prisms. The limit of possible dilution with this 

test is about 0.02 percent of H2S04 . The reagent is used in solid form 

and is applied to the center of the drop. The lead sulphate test would 

be very satisfactory were it not for the fact that other ions, notably 

those of arsenic acid, also precipitate lead. Lead arsenate resembles 

lead sulphate, and if arsenic is present in a substance, sulphur cannot 

be tested for in this manner.

Calcium acetate precipitates sulphates from a neutral or weak 

hydrochloric acid solution. The reaction is less satisfactory in a 

nitric acid solution. Calcium sulphate is more soluble than the 

sulphates of strontium and lead, and the tendency to crystallize is 

strong, hence the precipitate is entirely crystalline. From neutral 

or weak acid solutions the product is gypsum (CaS04 .2H2O); from 

strong acid solutions anhydrite (CaS04 ) predominates. Under the 

conditions here described gypsum alone will form. Gypsum ordinarily is not precipitated immediately from solutions low in S04 

ions. The first crystals are usually precipitated near the edge of 

the drop, and precipitation proceeds toward the center. The crystals are colorless and very characteristic (pi. 5, Z>). Long, slender 

needles predominate. The terminations show various forms. Usually 

they are very irregular, and many resemble the frayed ends of wood 

splinters. In somewhat more concentrated solutions the ends may 

show regular crystal faces inclined at an angle to the prism, indicating that the crystal form is monoclinic. A highly characteristic 

form is a reentrant angle in either or both ends, giving a fishtail or 

swallowtail appearance to the crystals. Arrowhead twins, although 

occasionally encountered, are not common. The birefringence is 

low, and the crystals are so thin that polarization colors are usually 

not observed. The prisms are often so slender that they resemble 

fibers. These tend to interlock and form a mat.

Arsenic acid precipitates calcium as a white amorphous calcium 

arsenate powder from neutral solutions. Calcium arsenate is kept 

in solution by a 5 percent or stronger HC1 solution. The best gypsum crystals are obtained with as weak an acid solution as can be 

used. Consequently, the 5 percent HC1 solution is recommended 

for the sulphate test.

Aqua regia is the most effective reagent for oxidizing sulphur. 

The mineral is broketo down by two or more applications of aqua 

regia. The residue is cooled and then leached with a drop of 5 percent HC1. This drop is transferred to another place on the slide, 

and a fragment of calcium acetate is added at its center. The characteristic gypsum crystals should appear in half a minute or less. 

The amount of material required to give a good sulphate test is 

about double that used with most tests. An area on the polished 

section about 0.35 millimeter in diameter is sufficient.
MICEOCHEMICAL METHODS 231

Arsenic acid and ferric chloride both delay the crystallization of 

gypsum but do not prevent it entirely. Arsenic acid is a gelatinous 

substance and if present in relatively concentrated proportion prevents the diffusion of the elements through the solution. Ferric 

chloride has practically the same effect. Consequently if too much 

material containing arsenic and iron is used in too small a drop, 

the resulting sulphur test will be unsatisfactory. If the calcium 

acetate dissolves very slowly or fails to dissolve, it indicates that 

either or both of these detrimental compounds are present. The 

student should practice with arsenopyrite until he can get a good 

sulphate test at every attempt. The following modification of the 

procedure above described is suggested:

Break down the mineral with two successive drops of aqua regia, 

evaporating each to dryness. Leach the residue with a drop of 5 

percent HC1, transfer the drop, and add a fragment of calcium acetate. If the fragment dissolves slowly or fails to dissolve, the concentration of arsenic acid or ferric chloride or both is probably too high. 

It is then necessary to wait until the drop is nearly dry before gypsum crystals begin to form, and they may not form at all. If the 

expected gypsum crystals fail to appear add a drop of aqua regia 

to the original residue, evaporate to dryness, leach with 5 percent 

HC1, and test for sulphates with calcium acetates as before. The reason for this modification is that arsenic is much more readily oxidized 

by aqua regia than sulphur. The first attack on the mineral will oxidize most of the arsenic and take into solution most of the iron. 

When these are removed in the first drop of 5 percent HC1, the proportion of arsenic acid and iron with respect to sulphur in the residue is much lower. After the second attack the effect of the detrimental compounds is much less, and the sulphate test can be carried 

out successfully.

In conclusion, the writer does not advocate that the sulphur test 

be made except where absolutely necessary. The great majority of 

ore minerals contain sulphur, and identification of the metallic 

elements usually suffices to determine the mineral. The sulphate test 

is necessary, however, to distinguish between such minerals as arsenopyrite (FeAsS) and loellingite (FeAs2 ) or between chloanthite (NiAs2 ) 

and gersdorffite (NiAsS). Earthy minerals containing suphur, such 

as hauerite (MnS2 ), should be tested for sulphur in this manner.

TESTS AFTER FUSION IN LOOP OF PLATINUM WIRE

Many metallic minerals are only slightly attacked when treated on 

a glass slide in the manner previously described that is, when placed 

in a drop of HN03 , HC1, or aqua regia followed by evaporation of the 

drop to dryness over a microflame. Among such resistant minerals 

are a few sulphides, such as molybdenite, most oxides of iron, chromite,
232 MICROSCOPIC DETERMINATION OF ORE MINERALS

franklinite, most of the tungstates, vanadates, and titanium minerals, 

and many of the rare-earth minerals, such as uranmite and monazite. 

Few if any of these minerals can withstand fusion in some flux such as 

sodium carbonate.

Therefore, this method has been relied upon for many years in ordinary chemical mineralogy to prepare these minerals for chemical tests 

or analysis. The objections to such a fusion in microchemical work 

are that (1) it dilutes the material to be tested, as a volume of flux 

several tunes the volume of mineral to be tested is introduced into the 

test drop; (2) it introduces a high concentration of extraneous elements 

such as sodium; (3) it requires more time and work than is required 

for minerals directly soluble in acids.

On the other hand, the method offers some valuable advantages, of 

which the most valuable is the fact that the flux combines with 

certain unwanted elements to form insoluble compounds that can be 

filtered off.

Sodium carbonate is the flux most commonly used. It fuses readily 

in the ordinary Bunsen flame. Some oxidation accompanies the fluxing action. This is not provided by the sodium carbonate itself but 

is effected by atmospheric oxygen. The oxidizing action is greatly 

increased by adding a small proportion of potassium nitrate; the flux 

most commonly used by the writer is made by mixing 19 parts of 

Na2C03 with 1 of KN03 .

Most of the qualitative tests which are made after fusion are color 

reactions; the reagent does not give a precipitate with the test drop 

but changes its color. The tests for iron and manganese already 

described are of this type. The quantity of liquid concerned is much 

greater than that of reactions on a glass slide, but on the other hand 

many of the color reactions have a sensitivity comparable with those 

which involve precipitation. Usually the total volume of liquid at 

the end of the test is about 0.5 cubic centimeter. This is held in a 

small white porcelain capsule or crucible. The porcelain makes an 

excellent background for observing the color.

Where nitrations are called for, they are made with a funnel and 

filter paper rather than .by means of a capillary tube. The funnel 

used by the writer is 2.0 centimeters in diameter, ,and the filter paper 

is No. 590 Schleicher & Schull.

The fusion takes place in a loop of a platinum wire which has an 

interior diameter of 2 to 2.5 millimeters. The loop is moistened and 

dipped into the bottle containing the flux. It is then withdrawn and 

heated in the flame until the flux fuses, forming a bead. This is 

cooled and moistened. The mineral powder is collected by a moistened 

match in the manner described on page 174. The end of the match 

with its adhering powder is brought to the moistened bead, and on 

touching it, the powder transfers itself to the bead. These manipu-
MICROCHEMICAL METHODS 233

lations are best performed under a binocular microscope, as the lowpower objective of the petrographic microscope lacks sufficient depth 

of focus. After fusion the bead is ready for chemical treatment. 

When the amount of material to be fused is comparatively great for 

instance, the amount collected from an area on the specimen of 1 

millimeter or more in diameter it may be difficult to get all the 

powder to stick to the bead long enough to be incorporated by fusion. 

This can be obviated by making the fusion in two stages. Approximately half of the powder is transferred to the bead and fused in it. 

Then the bead is cooled and moistened, and the remainder of the 

powder is transferred to the bead in the same manner.

It is more than a coincidence that most of the elements that are 

best tested by color reactions are those which form salts with sodium 

that are soluble in water. Some of the elements that form soluble 

sodium salts are phosphorus, arsenic, vanadium, molybdenum, tungsten, chromium, and manganese.

MOLYBDENUM

Potassium thiocyanate test. 

Reagents: 10% solution KCNS in H 2 0 and granulated zinc metal. 

Product: A complex ion of orange to red color. 

Limit: 0.003 milligram of molybdenum.

When thiocyanate ions are added to a molybdate solution, in a 

reducing environment, the solution is colored orange to red, the color 

depending on the concentration of molybdenum. The fact that a 

reducing environment is essential serves to distinguish molybdenum 

from iron. When H2 02 is added to the solution, the red color disappears. With iron the reverse is true, the red color being apparent 

only when the iron is in the ferric condition. Hence when H202 is 

added to a solution containing ferric and thiocyanate ions, the red 

color persists. Molybdenite is the only metallic mineral containing 

molybdenum described in this bulletin, and the following description 

applies to it, although the test works equally well on any molybdenum 

compound.

A flake of molybdenite measuring 0.2 to 0.5 millimeter in diameter 

is fused in a platinum wire loop with Na2C03 , KN03 flux. After 

fusion, the wire is lowered until the bead is immersed in a porcelain 

capsule containing 0.5 cubic centimeter of 1:5 HC1. When the bead 

has dissolved, 0.2 cubic centimeter of 10 percent KCNS is added to 

the solution. This is colored light yellow, but the color may not be 

apparent if the concentration in molybdenum is low. If iron is present 

the solution is colored pink to red. From 5 to 10 fragments of 20- 

mesh metallic zinc are added to the solution. Nascent hydrogen is 

liberated as the zinc is attacked by HC1. This has a powerful reduc129151 40 16
234 MICROSCOPIC DETERMINATION OF ORE MINERALS

ing action, which colors the solution from orange to red. According 

to Feigl,39 the compound formed has the composition K3 [Mo(CNS)6]. 

Several drops of H202 are now added. Unless iron is also present in 

solution, the color disappears immediately. Most textbooks on 

microchemistry advocate adding phosphoric acid to the solution. 

This prevents the reddish iron coloration from forming. However, 

this precaution is unnecessary because the iron is reduced by nascent 

hydrogen to the ferrous condition, in the presence of which the 

thiocyanate is uncolored.

 If other metallic elements besides molybdenum are present in the 

mineral, it is better to immerse the bead in hot water instead of 1: 5 

HC1. The reason for this variation in procedure is that most metallic 

elements form carbonates that are insoluble in water when fused in a 

sodium carbonate bead, but molybdenum forms sodium molybdate, 

which is soluble. Hence, when the bead is immersed in hot water the 

sodium molybdate dissolves, but lead or other insoluble carbonates 

do not. The insoluble residue is filtered off and the filtrate evaporated to dryness in the porcelain capsule. After cooling, 0.5 cubic 

centimeter of 1:5 HC1 is added to the capsule, and from then on the 

test is carried out in the same way as with molybdenite.

Tungsten interferes with the test, producing an insoluble blue oxide 

when the zinc is added to the acid, This should be filtered off, or 

most of the liquid can be withdrawn from the precipitate by means 

of a pipette. Titanium, vanadium, and uranium do not influence the 

reaction.

Sulphuric acid test. 

Reagent: 1:3 H2S04 .

Product: Deep-blue molybdenum oxide of unknown composition.

Limit: 0.03 milligram of molybdenum.

This is a modification of the familiar test described in most textbooks of mineralogy. Although it does not involve a fusion, it is 

included here for convenience.

The test is most applicable to molybdenite, MoS2 , and is the best 

one for distinguishing this mineral from graphite. The minimum 

amount of material required to give a satisfactory test is a flake of 

molybdenite about 0.5 millimeter in diameter. The weight of molybdenum in this flake, on the assumption that its thickness is one-tenth 

its diameter, is 0.03 milligram. Molybdenum must be in the molybdic 

state (valence 6), hence the sulphur of molybdenite must be roasted 

off before making the test. The flake is placed in a small porcelain 

crucible and the crucible tilted at an angle from the vertical. A low 

Bunsen flame is turned on the crucible. It must not be high enough 

to heat the crucible to redness; otherwise the molybdic oxide formed 

may volatilize. If the heat is too low the mineral will not oxidize.

« Feigl, F., Qualitative Analyse mit Hilfe von Tiipfelreaktionen, p. 175, Leipzig, 1931.
MICEOCHEMICAL METHODS 235

Some experience is needed to determine how much heat should be 

used. Usually it requires from 10 to 20 minutes to complete the 

oxidation. The flake should be observed from time to time, if necessary with a hand lens. As oxidation proceeds, the flake loses its 

luster and becomes coated with white (pale yellow when hot) crystals 

of Mo03 . When the oxidation is complete the black core disappears, 

and nothing but the white crystals is visible. This oxidation is the 

most important part of the test. If the fragment is comparatively 

large, say 2 millimeters or more in maximum diameter, the oxidation 

need not be carried to completion. In any event, it is the white oxide 

and not the sulphide which gives the test.

The crucible is cooled and a drop of 1:3 H2S04 is added to the Mo03 

residue. The crucible is again heated over a low flame. As fumes of 

S03 appear, they are blown away with short puffs. This is continued 

until the evaporation is nearly complete. When this stage seems to 

have been reached the crucible is removed with tongs and righted. 

When the proper stage of evaporation is reached no fluid will flow to 

the bottom of the crucible, but the sides will be moist. The crucible 

is then cooled. If the test has been carried out correctly a beautiful 

blue coating will appear in places on the inside wall of the crucible. 

Its exact composition is unknown, but it consists of an oxide or 

mixture of oxides of molybdenum of lower valence than 6. The 

reduction is accomplished by means of the human breath. According 

to some mineralogists it is not the breath itself which accomplishes 

the reduction, but the breath carries reducing dust to the mineral. 

According to others, the saliva carried by the breath is the reducing 

agent. Still others state that moisture is the active agent.

When the molybdenum is already in the molybdate state (as in 

wulfenite, PbMo04 ) no roasting is required.

TUNGSTEN 

Nascent hydrogen test. 

Reagents: 1:5 HC1 and metallic zinc.

Product: Turbid slate-gray to gray-blue solution.

Limit: 0.2 milligram of W03 .

Tungstates can be broken down by strong acids when boiled in a 

test tube, but on a glass slide the action is too feeble to yield a 

satisfactory microchemical test. Hence, fusion in a platinum wire is 

necessary. The sodium carbonate-niter mixture is effective. The 

tungsten of the mineral forms sodium tungstate, Na2W04 , which is 

soluble in water, and if either calcium or iron is present it will remain 

insoluble. If the test is made on scheelite, calcium carbonate is formed 

and can be separated from sodium tungstate by leaching the fused 

mass with hot water and filtering. Similarly, when wolframite 

(Fe,Mn)W04 , is fused in the soda-niter flux, iron oxide is formed and 

can be removed by filtration. Part of the manganese combines with
236 MICROSCOPIC DETERMINATION OF ORE MINERALS

the sodium to form, soluble sodium manganate, but most of it forms 

a brown flocculent precipitate, insoluble in water. The minerals on 

which the test for tungsten is most often made are scheelite, CaWO4 , 

and the isomorphous series ferberite-huebnerite. The end members, 

ferberite, FeWO4 , and huebnerite, MnWO4 , and the intermediate 

compound wolframite, (Fe,Mn) W04 , are fairly widely distributed in 

nature, but the wolframite is the most abundant.

The most satisfactory test for tungsten is the reduction by nascent 

hydrogen, which is described in most textbooks of mineralogy. The 

sodium tungstate produced by fusion in a soda-niter bead is dissolved 

in 0.5 cubic centimeter of water in a porcelain capsule. It is then 

withdrawn from the insoluble iron and manganese residues by means 

of a medicine dropper and transferred to another porcelain capsule. 

1:5 HC1 is added until effervescence ceases. Some small fragments of 

metallic zinc are then placed in the solution. If HC1 is in excess, 

hydrogen will be evolved as minute bubbles. If necessary 1:5 HC1 

is added drop by drop to maintain the effervescence. After 2 to 3 

minutes the solution slowly turns through slate gray to grayish blue, 

owing to a precipitate of W03 +WO2 held in suspension. If the reduction process is continued the blue color disappears, and a brown 

precipitate collects at the bottom of the capsule.

The test is similar to that described for molybdenum except that 

KCNS is not added. Its presence does not affect the test other 

than to dilute the solution. Hence tungsten is sometimes detected 

in testing for molybdenum.

In practice more material is needed for the tungsten than for the 

molybdenum test. The amount of mineral recommended is that 

supplied by a craterlike cavity on the polished section 0.5 to 1.0 

millimeter in diameter. The lower limit will give a slate gray suspension, whereas a crater 1 millimeter in diameter will give a dark 

grayish-blue suspension.

The precipitate remaining in the first capsule contains iron and 

manganese oxides. If the amount of manganese in the mineral is 

comparatively large, the soda-niter bead will be colored bluish green 

when cold. If the percentage of manganese is small, as in some 

wolframite specimens, no coloration of the bead is observed. The 

chemical test for manganese is mucli more sensitive than the bead 

test and will detect the presence of manganese in many soda-niter 

beads that show no greenish coloration.

The precipitate containing iron and manganese oxides is now dissolved in 0.5 to 1.0 cubic centimeter of 1:7 HNO3 . Some undissolved brown particles are ignored. The solution is divided into two 

approximately equal parts by withdrawing about one-half by means 

of a medicine dropper and transferring it to another porcelain capsule. 

To one part is added a few drops of KCNS. If the solution is colored
MIOBOCHBMICAL METHODS 237

red, iron is present. The K4Fe(CN) 8 test is not satisfactory, for some 

reason not exactly understood. The solution turns green instead of 

the expected Prussian blue.

The other portion is tested for manganese by adding fragments of 

sodium bismuthate. The insoluble material is allowed to settle. 

If the solution is colored violet or purple, manganese is present.

Cinchonine test. 

Reagent: Cinchonine solution in HC1.

Product: Flocculent white amorphous precipitate.

Limit: 0.05 milligram of W03 .

Cinchonine is an organic compound which has the property of precipitating tungsten from an acid solution. No other element is pi-ecipitated.41 The reagent is made by dissolving 1 gram of cin.chonine 

in 8 cubic centimeters of 1:1 HC1.

The mineral is fused in a platinum wire with soda-niter flux. If 

manganese is present the bead will be bluish green when cold, provided that the oxidizing flame was used. Dissolve the fused mass in 

a small crucible containing about 0.5 cubic centimeter of warm water. 

If manganese is present as indicated by the bead add 0.2 cubic centimeter of H2O2 and heat on a steam bath until the manganese is precipitated. Filter the residue and collect the filtrate in another porcelain crucible.

The filtrate must now be acidified with 1:1 HC1. The acid is added 

drop by drop until an acid reaction is shown by litmus paper. Three 

drops of cinchonine reagent are now added and the crucible placed on 

a steam bath for an hour. A cover is placed over the crucible to 

prevent evaporation to dryness. If tungsten is present a heavy 

yellowish-white flocculent precipitate collects in the bottom of the 

crucible.

If the presence of molybdenum is suspected the tungsten precipitate 

is filtered off and the filtrate collected in another porcelain crucible. 

A few drops of 5 percent thiocyanate solution and a few grains of 

metallic zinc are added. If molybdenum is present the solution turns 

orange to red.

In practice the cinchonine test will rarely be needed. It finds its 

chief use on minerals which are to be tested for both molybdenum

and tungsten.

VANADIUM

Hydrogen 'peroxide test. 

Reagents: H2 02 solution and HNOa. 

Product: Orange to red solution. 

Limit: 0.6 milligram of V2 06 .

Compounds of vanadium are broken down by means of concentrated HN03 ; the vanadium separates from the solution as V2O5 ,

41 Hillebrand, W. F., and Lundel), O. E. F., Applied inorganic analysis, p. 553, New York, 1929.
238 MICROSCOPIC DETERMINATION OF ORE MINERALS

which is a reddish-brown amorphous precipitate. The V2 05 precipitate is soluble in 1:1 HNO3 or more dilute solutions. If the 

mineral is dissolved in HN03 and the solution is evaporated to dryness a dense red precipitate appears in the residue. This furnishes a 

good indication of the presence of vanadium but not an absolute proof.

A good test for vanadium, which can be made on a glass slide and 

which yields characteristic crystals, unfortunately has not been 

described. Moreover, some compounds of vanadium are only with 

great difficulty broken down on a glass slide. Hence it is best to take 

the mineral into solution in a small porcelain crucible.

All compounds of vanadium are soluble in cold 1:1 HN03 except 

sulvanite, Cu3VS4 , which requires heating in concentrated HNO3 . 

It has been found best to test for the cations Cu, Pb, and Zn by breaking down the mineral on a glass slide with 1:1 HNO3 . The test for 

vanadium is made on additional mineral powder in a porcelain 

crucible.

The hydrogen peroxide test for vanadium has been used in ordinary 

qualitative analysis for many years, and when a sufficient quantity 

of material is available and the conditions of the test are carefully 

controlled, it leaves little to be desired. It is carried out microchemically as follows:

The mineral is taken into solution in a porcelain crucible with about 

0.3 cubic centimeter of . 1 : 1 HNO3 , heating if necessary. Lead, 

copper, zinc, and iron go into solution but do not interfere with the 

test in such a dilute solution. Sulvanite leaves a globule of sulphur 

which is likewise ignored. When solution is complete, the crucible 

and contents are cooled if heat has been applied. An equal quantity 

of water is added, followed by a few drops of H2O2 . If vanadium is 

present the solution is colored orange. If the solution is very dilute 

with respect to vanadium, the color may not appear for 1 or 2 minutes. 

According to Feigl,42 the solution turns rose red to red. This is true 

of fairly concentiated solutions, but under the conditions here described the color is always some shade of orange. The test is not as 

sensitive as those for molybdenum and for tungsten described in this 

bulletin. According to Prescott and Johnson,43 the concentration of 

vanadium in the solution should be 1 to 5,000 or greater. Also unfavorable is the fact that the concentration of vanadium is low in the 

minerals containing it, which are described in this bulletin (mottramite, ±20 percent V205 ; sulvanite ±12 percent V2O5 ). Therefore 

the diameter of the cavity scraped on the polished section should be 

at least 1 millimeter.

Molybdenum and tungsten when tested in the manner above described give pale-yellow solutions. Hence some experience is

n Feigl, F., Qualitative Analyse niit Hilfe von Tiipfelreaktionen, p. 179, Leipzig, 1931. 

43 Prescott, A. B., and Johnson, O. C., Qualitative chemical analysis, revised by McAlpine, R. K., and 

Soule, B. A., p. 393, New York. 1933.
MICROCHEMICAL METHODS 239

necessary is distinguishing between the vanadium solution, which is 

distinctly orange, and the yellow solutions of molybdenum and tungsten. Fortunately the metallic minerals containing molybdenum, 

tungsten, and vanadium are widely different in their physical properties. Chromium when tested in the same manner as vanadium yields 

a greenish-yellow solution when the residue from the evaporation is 

dissolved in 20 percent H2S04 . The color does not change when H2 02 

is added.

The hydrogen peroxide test for vanadium, it must be admitted, is 

not entirely satisfactory, but it has some excellent features and is 

probably the best available at present.

TITANIUM

Hydrogen peroxide test. 

Reagents: H202 solution and 1:1 H2S04 . 

Product: Yellow solution. 

Limit: 0.3 milligram of Ti.

This test is the familiar wet test for titanium used in qualitative 

analysis. Its adaptation to microchemistry involves no new principles. The mineral is fused in a platinum loop with soda-niter flux. 

The loop is then immersed in a small porcelain crucible containing 0.5 

cubic centimeter of warm water. When the flux is dissolved the loop 

is withdrawn. The titanium collects at the bottom of the crucible 

as reddish-brown, somewhat flocculent sodium titanate, Na 2Ti03 . 

This is insoluble in water, in contrast with the corresponding vanadate, 

tungstate, molybdate, and chromate.

The liquid is withdrawn by means of a medicine dropper and rejected. The crucible is then tilted and most of the remaining liquid 

is withdrawn by inserting a strip of filter paper into the side of the 

drop. A little remaining liquid does no harm. The precipitate is 

then dissolved in about 0.3 cubic centimeter of 1 : 1 HaSO^ If necessary, heat until solution of the precipitate is complete. Cool and 

add 0.3 cubic centimeter of water, then a few drops of hydrogen 

peroxide. The solution turns yellow if titanium is present. Iron and 

calcium, the elements most frequently associated with titanium, do 

not interfere with the test.

No other element that forms an insoluble carbonate gives a similar 

test. If desired the test can be confirmed by adding a small quantity 

of powdered sodium fluoride. The titanium forms complex ions with 

fluorine, and the yellow color disappears from the solution.

It is well to have a liberal amount of material fused in the bead. 

The sensitivity given above is based on a cavity 1 millimeter in diameter scraped on a polished section of ilmenite. With rutile a smaller 

amount can be used.
240 MICROSCOPIC DETERMINATION OF ORE MINERALS

CHROMIUM

Diphenyl carbazide test. 

Reagent: 1% diphenyl carbazide in alcohol.

Product: Soluble violet compound of unknown composition.

Limit: 0.015 milligram of chromium.44

To obtain this test the chromium must be in the chromate state. 

This is most conveniently attained by fusing the mineral in the loop 

of a platinum wire with soda-niter flux. The fused mass is dissolved 

in about 0.5 cubic centimeter of warm water in a porcelain crucible. 

If there is an insoluble residue, as when chromite is tested in this 

manner, the filtrate can be withdrawn from the insoluble portion by 

means of a medicine dropper. The filtrate, which contains the chromium as sodium chromate, is transferred to another porcelain crucible and acidified by adding 20 percent H2SO4 drop by drop until effervescence ceases and a litmus paper gives an acid reaction.

A drop of the dimethyl carbazide reagent is added. The solution 

turns a violet color if chromium is present.

The chromate test is very sensitive. An amount of material obtained by gouging a cavity 0.3 millimeter in diameter in chromite gives 

a good test, but if iron or other elements are to be tested for in the 

same material, the cavity should be 0.5 millimeter in diameter.

The diphenyl carbazide solution is red. If the reagent is added to 

the sodium carbonate filtrate before it is acidified, the red color is 

intensified. The red color disappears and the solution becomes colorless when it is acidified. If chromate ions are present in the acidified 

solution, they will give the solution the characteristic violet color, 

which is not influenced by the color of the reagent itself.

TESTS REQUIRING SPECIAL APPARATUS

GERMANIUM

Reagents: Solid fluorite, CaF2 ; solid sodium chloride,, NaCl; concentrated

H2S04 ; l:7HN03 . 

Results: Sodium fluogermanate, NajGeFe, in small transparent six-sided plates or

rosettes. 

Limit: 0.4 milligram of germanium.

This test is a modification of the corresponding test for silicon, which 

was discovered and described by Boficky 45 and which is one of the 

earliest known in microchemistry. Silicon and germanium are closely 

related elements in the periodic table, and their fluosalts of sodium 

are isomorphous and form under similar conditions. Consequently, 

when testing for germanium, care must be taken to prevent contamination of the sample by silica or silicates.

«* This figure was obtained by calculating the weight of chromium contained in a hemisphere of chromite 

0.3 millimeter in diameter.

«» Bofick#, E., Elemente einer neuen chemisch-mikroskopisuhen Mineral- und Qosteinsanalyse: Naturwiss. Landesdurchf. BShmen Archiv, Band 3, Prague, 1877. Translation by Winchell, N. H., Minnesota 

Geol. and Nat. Hist. Survey 19th Ann. Kept., for 1890, p. 18,1892.
MICROCHEMICAL METHODS 241

The procedure described below is identical with that devised by 

Staples 46 for silicon and germanium, and his description is essentially 

as given below with a few small changes.

The mineral, sulphuric acid, and fluorite are mixed in a platinum 

spoon with a bowl approximately 1 centimeter in diameter. The 

spoon weighs 1.33 grams and costs about $3.47 An ordinary charcoal 

block of the type used in blowpipe work is hollowed out so that the 

platinum spoon sits in it. A hole of a slightly smaller diameter than 

the bowl of the spoon is cut through the charcoal directly under the 

bowl (fig. 32).

The sulphuric acid attacks the fluorite, with the production of hydrofluoric acid, HF, and this in turn attacks the germanium-bearing

a

FIGURE 32. Apparatus for germanium test, a, Charcoal block; b, platinum spoon; c, drop of dilute nitric 

acid; rf, celluloid slide. About natural size. (After Staples.)

mineral, with the production of hydrofluo-germanic acid, H2GeF6 .' 

When the mixture is heated at a temperature of about 75° C. this 

distills off in gaseous form. Enough of the gas can be caught in a drop 

of 1:7 nitric acid to give a satisfactory test. A drop about 3 millimeters in diameter is placed on a slide, and the slide is inverted so that 

the drop hangs directly over the spoon. The slide should not be made 

of glass or any material containing silica, as the fumes would attack the 

slide, forming a compound identical in appearance with the germanium 

compound. Transparent celluloid, sometimes called "xylonite," is 

used. A slide 25 by 45 millimeters by 0.5 millimeter thick costs only 

about one-third of a cent.

The charcoal is supported by a ring stand at a distance of about 5 

centimeters over an alcohol lamp like that described on page 179. 

The distillation is carried on for 10 minutes. The distance of the charcoal above the flame depends on the height of the flame. If the distance is too great, the distillation takes place too slowly to give a good 

microchemical test. If the distance is too small, the slide may get so 

warm that the drop wiU dry up in less than 10 minutes. Hence some 

experience is necessary in adapting the height of the charcoal so as to 

give the best temperature. There is no danger of the slide taking fire, 

as the flame does not come into direct contact with it.

44 Staples, L. W., A simple microchemical test for silicon: Ain. Mineralogist, vol. 21, No. 6, pp. 379-383, 

1936. 

« For sale by Elmer & Amend, New York.
242 MICROSCOPIC DETERMINATION OF ORE MINERALS

The slide is then placed right side up on the microscope stage. The 

drop is usually reduced in volume but is still somewhat larger than 

with most other microchemical tests. A fragment of sodium chloride 

is then introduced into the drop. Some experience is necessary to 

judge the right size of the fragment, but it should be several times the 

size of the KI fragments used, in the lead and antimony tests. If 

H2GeF6 is present in appreciable amounts in the HN03 drop, a crystalline precipitate of sodium fluogermanate, Na2GeF6 , is formed immediately. The crystals may assume several habits, but the two most 

characteristic and common are 6-sided tabular plates and 6-pointed 

stars or rosettes (pi. 12, O). The crystals are pseudohexagonal and are 

unusual in that all the indices of refraction are less than that of dilute 

nitric acid. This forms the most valuable diagnositc feature of the 

compound, for even if the crystals do not show good euhedral forms, 

the Becke line on each will move into the liquid when the tube of the 

microscope is raised.

The mode of formation of the crystals is decidedly different from 

that of most other microchemical tests. The fluogermanate crystals 

seem to shoot out in a stream from the sodium chloride fragment 

like bees emerging from a hive.

The amount of material necessary to give a satisfactory test is 

somewhat more than in most other microchemical tests; in part this 

is due to the relatively low percentage of germanium in the two 

minerals containing it. The crater on the surface of argyrodite 

(6.9 percent Ge) should be about 1.5 millimeters in diameter. On 

germanite with a somewhat higher percentage of germanium the 

crater can be smaller. The sensitivity of the test as given above is 

based on the amount of the element in a hemisphere of argyrodite 

1.5 millimeters in diameter. The amount of fluorite does not need 

.to be rigidly controlled: Staples recommends 2 milligrams, whereas 

the present writer uses about three tunes the volume of the mineral 

tested. Likewise, the amount of sulphuric acid is not fixed. The 

writer uses three drops about 2 millimeters in diameter, or enough to 

form a mixture which, when stirred with a platinum wire, is about 

the consistency of thick soup.

The chief precaution necessary is to use pure reagents. Fluorite, 

especially, is liable to be contaminated with silica, and a blank test 

should be run on fluorite alone according to the preceding directions.

The test for germanium seems more complicated than it really is. 

After making a few tests on a known germanium mineral, one quickly 

acquires skill with and confidence in the method. According to 

Staples, no other element except silicon gives a similar test under the 

same conditions.
MICEOCHEMICAL METHODS 243 

THE PLATINUM METALS

The following description of the microchemistry of the platinum 

metals is condensed from a recent paper by Eraser. 48 The platinum 

metals (hereafter designated the platinum group) can be divided 

into two subgroups on the basis of their atomic weight and chemical 

behavior. One subgroup consists of palladium, rhodium, and ruthenium; the atomic weights of these elements are 106.7, 102.9, and 

101.7, respectively. The other subgroup consists of platinum, iridium, 

and osmium; the atomic weights of these elements are 195.2, 193.1, 

and 191.5, respectively.

It is difficult to distinguish between individual members of a subgroup such as rhodium and ruthenium, but less difficult to distinguish 

between two elements belonging to different subgroups, such as 

platinum and palladium.

Characteristic tests for all six elements are available, and when 

any one of them occurs alone in a solution, no difficulty is encountered 

in identifying it. Unfortunately, these elements are closely associated in nature. Sperrylite (PtAs2 ) and cooperite (PtS) are almost 

pure, but nearly all other natural occurrences of the platinum group 

are alloys of two or more native metals. Platinum alloys with gold, 

with iron, and with copper have also been found in nature.

The solubility of platinum alloys varies greatly, depending on their 

composition. Those containing substantial amounts of iron or copper 

are fairly soluble in aqua regia. Those containing much osmium or 

iridium are not soluble in aqua regia. Sperrylite is only slightly 

soluble in aqua regia. On the other hand, palladium differs from the 

other five elements in that alloys containing a substantial percentage 

of palladium are readily decomposed by nitric acid, and the palladium 

is taken into solution. This procedure is similar to the parting of 

gold and silver in fire assaying. Finely divided palladium is also 

soluble in concentrated hydrochloric acid. Hence, in testing a specimen for unkown platinum metals, a first separation can be made by 

treating the specimen in a small porcelain crucible with concentrated 

HN03 . The acid is evaporated to dryness, and the residue is taken 

up with concentrated HC1 and tested for palladium with dimethyl 

glyoxime.

A specimen (ordinarily not larger than a pin head) insoluble in concentrated HN03 or aqua-regia or an insoluble residue from such an 

acid attack requires a special apparatus and procedure. Fusion with 

Nad in a chlorine atmosphere is the treatment recommended by 

Fraser.

« Fraser, H. J., Microchemistry of the precious-metal elements: Am. Mineralogist, vol. 22, pp. 1016-1034, 

1937.
244 MICROSCOPIC DETERMINATION OF ORE MINERALS

The specimen is ground in an agate mortar with about four times 

its weight of fused sodium chloride and the mixture placed in a small 

hard open glass tube. (See fig. 33.) The tube is connected with a 

chlorine generator and is heated carefully until the mass gently glows. 

The heating is maintained for 3 minutes; the tube is then disconnected

and its contents allowed to cool. The 

contents of the tube are dissolved 

in distilled water and filtered. Any 

undissolved residue is again treated 

in a similar manner. Ordinarily, two 

fusions are sufficient. Avoidance of 

overheating is necessary, particularly 

with platinum, the chloride of which is 

readily decomposed by excess heat. 

All six of the platinum metals are converted to water-soluble chlorides. The 

color of the solution may be of some 

assistance in determining the dominant 

elements present. Thus, a 1 percent 

solution of gold is yellow; platinum and 

osmium, yellow with a faint tinge of 

orange; iridium, brown; palladium, 

brownish red; rhodium, cherry red; 

ruthenium, dark cherry red.

The tests for the six platinum metals 

are made in a neutral solution of the 

chloride of sodium and the metals, except as otherwise mentioned 

for example, for palladium.

Figure 33. Apparatus for decomposition of 

platinum minerals, a, Acid chamber, 

concentrated HC1; b, chlorine chamber, 

containing KClOd c, cork with hole in 

end to take tube d; d, hard glass tube containing mixture of crushed mineral and 

fused NaCl; e, Bunsen burner.

PALLADIUM

Dimethyl glyoxime test*9

Reagent: 2% dimethyl glyoxime in alcohol.

Product: Heavy precipitate of felted needles, yellow in direct light, pale green

with a tinge of yellow in reflected light. 

Limit: Not determined, but less than 0.1% Pd.

This test is the most satisfactory of those described for the six elements of the platinum group, as it is specific for palladium, and if the 

test is made in a strong HC1 solution, it is not interfered with by any 

other element.

The powder is treated in a crucible with strong nitric acid. The 

residue is evaporated to dryness, the crucible and contents cooled, 

and the residue leached with a drop of concentrated HC1. The drop 

is taken up with a capillary tube and blown out on a glass slide. A

48 Putnam, P. C., Roberts, E. J., and Selchow, D. H., Contributions to determinate mineralogy: Am. 

Jour. Sci., 5th ser., vol. 15, p. 424,1928. Chamot, E. M., and Mason, C. W., Handbook of chemical microscopy, vol. 2. p. 265, New York, 1931.
MICROCHEMICAL METHODS 245

drop of dimethyl glyoxime solution is added to the test drop. An 

immediate heavy precipitate of felted needles, yellow in direct light 

and pale green with a tinge of yellow in reflected light, is thrown down. 

After standing, the precipitate recrystallizes into larger needles that 

are darker and greener in direct light and almost colorless in reflected 

light. The dried drop is pinkish.

If the test is not acidified with strong HC1, the palladium precipitate 

will be identical in appearance with that tormed by palladium in 

concentrated HC1, but rhodium or gold if present will also be precipitated. The palladium and rhodium precipitates are almost identical 

in appearance; hence the test is not distinctive; rhodium is precipitated in a 1:5 HC1 solution. For this reason the dimethyl glyoxime 

test for palladium must always be made in a strong HC1 solution.

PLATINUM

Benzidine test. 

Reagent: Benzidine in acetic acid.

Product: Long, thin, hairlike needles, green to yellowish green in direct light.

Limit: Not determined, but less than 0.1% Pt.

Benzidine reagent is made up by dissolving 0.05 gram of the powder 

in 10 cubic centimeters of glacial acetic acid, and diluting this solution 

to 100 cubic centimeters with distilled water. A drop of reagent is 

merged with the neutral test drop (method I). If platinum is present, 

long, thin, hairlike needles immediately form. These are green to 

yellowish green in direct light and yellowish white in reflected light. 

The needles have parallel extinction, and the thicker ones show high 

birefringence. The needles are usually single units, but in places 

several will radiate from a common center.

Benzidine reactions with other elements of the platinum group are 

as follows:

Palladium: As the drop evaporates, radiating needles and branching blades, brownish yellow in direct light and yellow in reflected 

light, form. The crystals grow from the edge of the drop. The test 

is slow but strong before the drop dries.

Rhodium: No immediate precipitate. Spherules and rounded 

grams slowly form. These are pink to yellowish black in direct light 

and rose pink in reflected light. The test is slow but strong.

Ruthenium: No immediate precipitate. As the drop dries, rounded 

grains and spherules or a few needles (sometimes both) form. These 

are faintly yellowish in direct light. The forms are not very distinctive.

Osmium: Long brilliant yellow blades form after about 15 seconds. 

These are also yellow in reflected light. The blades give parallel 

extinction and are faintly pleochroic between shades of yellow.

Iridium: A precipitate immediately forms, consisting of small isometric grains, faint yellow to black in direct light and light yellow in
246 MICROSCOPIC DETERMINATION OF ORE MINERALS

reflected light. The test is strong, but the forms are not very 

distinctive.

The benzidine test for platinum is not distinctive in the presence of 

iridium but is not seriously influenced by palladium, rhodium, ruthenium, or osmium. If much iridium is present with platinum, a precipitate is immediately obtained, but often it is an aggregate of small 

shapeless grains with no diagnostic features. If this appears, the 

thiocyanate test for platinum should be made on a fresh sample.

Potassium mercuric thiocyanate test. 

Reagent: 3% K2Hg(CNS) 4 in H2 0.

Product: Lemon-yellow octahedrons and cubes, also some feathery crosses.

Limit: Not determined, but less than 0.1% Pt.

The reagent drop is merged into the neutral test drop (method I). 

A precipitate of lemon-yellow octahedrons and crystals showing 

cubic and octahedral faces rapidly forms. In places feathery crosses 

are also present. The crystals are isometric. The color ranges 

from greenish yellow in thin crystals to black in thick crystals. All 

are canary yellow in reflected light. °

Thiocyanate tests for other elements of the platinum group are as 

follows:

Palladium: An immediate color change from yellow to yellowish 

brown takes place, at the junction of the drop, followed by precipitation of chocolate-brown, rather acicular crystals. These are 

greenish yellow to orange in reflected light. As the drop dries, pale 

greenish-yellow radiating branches grow.

Rhodium: Immediate precipitation of tiny black grains, white in 

reflected light. As the solution dries, branching spines develop with 

a pale pink color and strong birefringence.

Ruthenium: Tiny black grains, white in reflected light, slowly 

precipitate. Small dark crosses, reddish brown in reflected light, 

form at the edge as the drop dries.

Iridium: A few small black grains form, but no characteristic 

precipitate. As the drop dries, small pale-yellow rectangular needles 

and crosses form at the edge.

Osmium: A few small black cubes, yellowish in reflected light, 

form first. Then cubes, feathery crosses, dendrites, and blades, 

ranging from yellow in thin blades to reddish or dark brown in thick 

crystals, slowly grow at the edge of the drop. These later forms are all 

reddish brown in reflected light and have no birefringence.

Although the thiocyanate test for platinum is not as distinctive 

as the benzidine test, it is of special value when platinum and iridium 

are both present, as iridium will not prevent the formation of the 

typical platinum test.
Benzidine test. 

MICROCHEMICAL METHODS 247 

IRIDIUM

Reagent: Benzidine in acetic acid.

Product: Small yellow isometric cyrstals and spherulites.

Limit: Not determined but less than 0.1% Ir.

Iridium is the most difficult element of the -platinum group to 

identify, as, with most reagents, it gives a poorly formed precipitate, 

or no precipitate. The benzidine test for iridium is described under 

the corresponding test for platinum. In spite of the fact that benzidine 

gives an immediate precipitate with both platinum and iridium, it 

is the best test for iridium in the presence of platinum. The platinum 

is first removed with potassium mercuric thiocyanate; the drop is 

filtered and the filtrate transferred to another place on the slide and 

tested with benzidine. Iridium will yield small yellow isometric 

crystals in the absence of platinum, and small yellow spherules if 

platinum is present. Enough platinum will be carried over from the 

thiocyanate precipitation to influence the shape of the iridium compound precipitated by benzidine.

Ammonium bichromate test. 

Reagent: Ammonium bichromate solution in H2 0.

Product: Black cubes, cubo-octahedral forms, rhombs, and crosses.

Limit: Not determined, but less than 0.1% Ir.

The regent is prepared by dissolving approximately 0.1 gram of 

solid ammonium bichromate in 30 cubic centimeters of water. A 

drop of the reagent is introduced into the test drop (method I).

Iridium yields an immediate precipitate of black isometric forms. 

Among these are cubes, combinations of cube and octahedron, rhombs, 

and crosses. They are black in reflected light. Ammonium bichromate reactions with other elements of the platinum group are as 

follows:

Platinum: Immediate precipitation of feathery crosses, cubes, and 

octahedrons. These are dark in direct light and yellow in reflected 

light.

Osmium: Immediate precipitation of orange to black rhombs and 

hexagons.

Palladium: Strong precipitation of feathery crosses and jagged threepointed stars, brown to black in direct light and reddish brown in 

reflected light. The test is slow but strong.

Rhodium: There is no precipitate until the drop evaporates.

Ruthenium: Fairly rapid precipitation of very small red grains, 

red in reflected light.

Of the above-named elements, platinum and osmium must be 

removed before testing for iridium. This is accomplished as follows:

A fragment of solid KI is introduced into the neutral test drop. 

Reactions of elements of the platinum group are:
248 MICROSCOPIC DETERMINATION OF ORE MINERALS

Platinum: Immediate change in color of solution to rich brown, 

followed by rapid precipitation of small black grains, which grow to 

hexagons and rhombs with accompanying bleaching of solution to 

pink or colorless. All crystals are black in reflected light.

Palladium: Immediate precipitation of a dense, very fine-grained 

chocolate-brown to black powder, which spreads as a ring from the 

KI. The. color is black to golden brown in reflected light.

Rhodium: Rapid precipitation of needles, radiating clusters, 

blades, and stumpy prisms. The solution turns yellow. All forms 

are highly birefringent, pink in both direct and reflected light.

Ruthenium: No precipitation until the drop dries; then small 

yellowish needles form at the edge. They are golden yellow in reflected light. The drop dries brown.

Iridium: No immediate precipitation. As the drop dries, colorless 

hexagons grow and slowly stain yellow. The drop dries colorless.

The reactions produced by KI are not distinctive; hence it is not 

recommended as a reagent. Its value lies in the fact that platinum, 

osmium, palladium, and rhodium produce immediate precipitates, 

whereas the corresponding iridium and ruthenium compounds do not 

form until the drop is nearly dry. It is obvious that in order to make 

a good chemical separation the capillary tube must not be introduced 

too soon ; otherwise the bulk of the platinum or osmium will not have 

precipitated; likewise, the filtration cannot be delayed until the drop 

is about to dry up, or iridium will be deposited as well. A compromise is necessary, and 30 seconds after the union of reagent and test 

drop is recommended as the time to withdraw the filtrate. The bulk 

of the platinum and osmium will have deposited from solution, and 

the remaining portion will not materially interfere with the subsequent 

ammonium bichromate test for iridium.

OSMIUM

Thiourea test. 

Reagent: 10% solution of thiourea, SCCNE^K in H20.

Product: Drop, gently heated after addition of thiourea, develops reddishpurple color. 

Limit: Not determined but less than 0.1% Os.

A drop of reagent is merged with the test drop (method I) . Osmium 

gives no immediate precipitate. If the drop is allowed to evaporate 

to dryness without heat treatment, thiourea blades form which are 

stained yellow. If the drop is gently heated after the addition of 

thiourea, the solution immediately turns to a reddish purple.

The character of reaction of thiourea with each of the six elements 

of the platinum group is given in the "Outline of microchemical 

tests" (p. 257).

Platinum and iridium do not interfere with the thiourea test for 

osmium, but palladium, rhodium, and ruthenium prevent a satis-
MICKOCHEMICAL METHODS 249

factory test on 0.1 percent osmium solution, if they are present in 

proportions of 1 percent or greater. If the concentration of osmium 

is equal to that of these elements, the test is satisfactory. Palladium 

can be removed by precipitation with dimethyl glyoxime in a strong 

HC1 solution. The filtrate is transferred to a glass slide and the drop 

evaporated to dryness to remove the excess of HC1. The residue is 

then taken up with a drop of water and tested for osmium with 

thiourea as already described.

The ammonium chloride test, a description of which follows, will 

detect osmium in the presence of rhodium or ruthenium.

Ammonium chloride test. 

Reagent: Solid NH4C1.

Product: Reddish-brown to black cubes, three- and four-pointed stars, and

octahedrons. 

Limit: Not determined but less than 0.1% Os.

A fragment of the reagent is introduced into the neutral test drop 

(method II).

Osmium gives an immediate and copious precipitation of mediumsized reddish-brown to black cubes, three- and four-pointed stars, 

and octahedrons. The crystals are reddish brown in reflected light.

Reactions with other elements of the platinum group are as follows:

Platinum: Immediate precipitation of feathery crosses, cubes, 

feathery three-, four-, and five-pointed stars, dark in direct light and 

faint yellow in reflected light.

Iridium: Immediate and abundant precipitation of tiny cubes 

around the reagent. The cubes are black in both direct and reflected 

light.

Palladium: Slow formation of long yellow needles which in some 

tests grow across the drop. The needles are pleochroic, from golden 

yellow to grass green; they have parallel extinction and high 

birefringence.

Rhodium: No precipitate until drop is partly evaporated; then tiny 

dark cubes with high index of refraction form first, followed by large 

pink hexagons and petals. These are usually thin and have a moderate index. The test is very characteristic but slow.

Ruthenium: No precipitate until drop is partly evaporated; then, 

slowly, tiny yellow cubes form and become dark as they grow. Excess 

reagent becomes stained dark yellow. Later a few crosses may form, 

and in some tests, just as the drop dries, pinkish-brown hexagons 

may appear. The test is not distinctive or reliable.

The ammonium chloride test for osmium is riot as distinctive as 

the thiourea test. It is unreliable in the presence of platinum and 

iridium but is not seriously interfered with by palladium, rhodium, 

or ruthenium.

129101 40 17
250 . MICKOSCOPIC DETERMINATION OF ORE MINERALS

RHODIUM

Dimethyl glyoxime test. 

Reagent: 2% dimethyl glyoxime solution in alcohol.

Product: Precipitate of small needles, dark in direct light, greenish yellow in

reflected light. 

Limit: Not determined but less than 0.1% Rh.

The reagent drop is added directly to the test drop. Rhodium 

produces in a few seconds a very fine-grained mass of small needles, 

sometimes felted, dark in direct light and greenish yellow in reflected 

light. As the drop dries, blades and feathery crosses, faintly pink in 

direct light, form. The needles become greenish. The test is similar 

to that for palladium, except that needles lack yellow color in direct 

light. The dried residue is pink.

Rhodium can be identified in the presence of platinum, iridium, 

osmium, and ruthenium, by dimethyl glyoxime added to a neutral 

or weakly acidic solution. Palladium and gold will also be precipitated. A drop of concentrated HC1 added to the precipitate will 

dissolve the rhodium glyoxime readily, the gold slowly, and the 

palladium glyoxime not at all.

Benzidine will give a distinctive test for rhodium in the presence of 

gold and iridium. The palladium is first precipitated with dimethyl 

glyoxime in concentrated HC1. The filtrate is transferred to another 

place on the slide and tested for rhodium with benzidine.

Benzidine test. 

Reagent: Benzidine solution in acetic acid.

Product: Slow precipitation of pink to black rounded grains.

Limit: Not determined but less than 0.1% Rh.

The preparation of the reagent and the reactions with other elements of the platinum group are described under the heading 

"Platinum."

Rhodium does not yield an immediate precipitate. Spherules and 

rounded grains, pink to yellowish black in direct light and rose pink 

in reflected light, slowly form. The test is slow but strong. It is 

not as good as the dimethyl glyoxime test for rhodium but is of 

special value where rhodium and palladium are associated in the same 

sample.

RUTHENIUM

Thiourea test. 

Reagent: 10% solution of thiourea, SC(NH2) 2 , in H2 0. 

Product: Warmed drop turns inky blue. 

Limit: Not determined but less than 0.1% Ru.

The reagent drop is merged with the test drop (method I). Ruthenium gives no immediate precipitate. The cold drop very slowly 

develops a deep inky-blue color. As the drop dries, blue-black 

spherules form at the edge of the drop. If the drop is gently heated 

after the thiourea is added, it immediately turns inky blue.
MICKOCHEMICAL METHODS 251

Reactions of thiourea with other elements of the platinum group 

are described in the outline of microchemical tests (p. 265).

Thiourea gives a satisfactory test for ruthenium in the presence of 

any other elements of the platinum group except palladium and 

possibly osmium. An experienced observer can recognize the ruthenium test in the presence of osmium, but the blue ruthenium color 

is quickly replaced by the red osmium color. The rhodanine test, a 

description of which follows, gives a satisfactory ruthenium test in 

the presence of osmium. If palladium is present, it can be removed 

with dimethyl glyoxime, the solution filtered, and ruthenium tested 

for with either thiourea or rhodanine.

Rhodanine test. 

Reagent: 0.03% dimethyl-amino-benzal-rhodanine in acetone. 

Product: Colorless to yellowish-brown needles and radiating clusters. 

Limit: Not determined but less than 0.1% Rh.

The reagent drop is merged with the test drop (method I).

Ruthenium produces an immediate precipitation of colorless to 

yellowish-brown needles and radiating clusters. The crystals have 

low birefringence. They are yellowish in reflected light.

Reactions of the rhodanine reagent with other elements of the 

platinum group and gold are as follows:

Platinum: No precipitate.

Palladium: Strong pink color in the solution. Pink needles and 

clusters are precipitated immediately. The presence of 1:5 HC1 

prevents coloration of the solution but not the formation of the 

precipitate.

Rhodium: The color of the test drop changes from normal faint 

pink to yellow, with precipitation of tiny needles.

Iridium: No definite precipitate.

Osmium: No definite precipitate.

Gold: Immediate precipitation of a purplish fine-grained aggregate which slowly changes in color to yellow green. A mass of 

small yellow-green needles forms; the needles are white in reflected 

light. Still later, red hairy clusters, red in reflected light, may also 

appear. The test is strong.

The rhodanine test for ruthenium is not satisfactory in the presence 

of palladium, rhodium, and gold. Palladium can be removed before 

testing, as described under the thiourea test for ruthenium. If 

rhodium or gold is associated with ruthenium, the thiourea test is 

preferable to the rhodanine test. On the other hand, if ruthenium is 

associated with osmium, the rhodanine test is preferable.
252 MICROSCOPIC DETERMINATION OF ORE MINERALS

A SYSTEMATIC SCHEME OF MICROCHEMICAL ANALYSIS 5°

Microchemical tests are applicable to any mineral provided it can 

be taken into solution. However, it is usually most convenient to 

determine transparent minerals by optical methods. The best of 

these methods consists in determination of the indices of refraction 

of the mineral by immersing the powdered mineral in liquids of 

known indices of refraction.

The scheme of analysis described here is almost entirely confined 

to the opaque ore minerals. Under the reflecting microscope the 

opaque minerals are easily distinguished from the nonopaque minerals 

by their color in vertically reflected light. This is discussed more 

fully on pages 69 and 70.

As already stated (p. 63), it is best to identify the opaque ore 

mineral by means of hardness, polarization, and etching tests and 

then confirm the identification by microchemical tests for one or 

more elements. However, this scheme may fall down because of 

errors in the etch tables, mistakes in interpreting polarization or 

etching effects, or errors in the formulas of the minerals as given in 

the standard texts, or because the minerals may be "new" that is, 

not yet described in the literature. Any one of these factors would 

cause a failure in the scheme. The next resource will then be to 

identify the elements of the mineral by a systematic scheme similar 

to that of qualitative chemical analysis.

Any systematic scheme of this kind is greatly simplified by making the hardness test at the outset. This is described on page 63 

but is outlined here for convenience. The mineral is tested for hardness by means of a sharp needle. Minerals that scratch readily are 

classified as soft, and those that scratch with difficulty or not at all 

are classified as hard. The advantage of making this classification 

at the outset is that certain elements are rarely if ever found in the 

hard group and others are rarely if ever found in the soft group.

HARD MINERALS

The hard opaque minerals may be divided into three groups, as 

follows:

A. Insoluble or very difficultly soluble in concentrated HN03 , 

1 : 1 HN03 , or aqua regia. This group consists mostly of the ironbearing oxide minerals, also cassiterite, tungstates, and rare-earth 

minerals.

B. Readily soluble in 1 : 1 HNO3 or aqua regia. This group consists mostly of the nickel-cobalt-iron arsenides, antimonides, sulpharsenites, and sulphantimonites; also pyrite and marcasite.

w This scheme is based on one originated by the writer's former students, Drs. H. J. Fraser and H. A. 

Powers.
MICROCHEMICAL METHODS 253

C. Insoluble or slightly soluble in HN03 but readily soluble in 

aqua regia and 1 : 1 HC1. This group consists of the hard manganese 

oxides.

Group A. The No. 9 needle ordinarily used will not scratch, a 

hard mineral. A powder may usually be obtained by using a No. 

6 or stronger needle, the point of which is ground back by means 

of the emery wheel so as to give a relatively large cutting edge. 

By working it back and forth in a pit on the surface of the minera], 

powder will gather in front of the needle. A nonmetallic red powder indicates hematite; brick red to light brown indicates the hydrous iron oxides of the "limonite" group, also chromite; dark brown 

indicates franklinite or braunite; a black metallic powder is probably 

magnetite. All these minerals except braunite are almost insoluble 

in strong nitric acid and aqua regia. However, by many repeated 

applications of aqua regia, each evaporated over the microflame, 

enough material can be taken into solution to give an iron test. The 

residue is colored orange to brown. It is taken into solution with a 

drop of 1 : 7 HN03 and tested for iron with a fragment of K4Fe(CN)8 .

If the mineral is too hard to yield any powder by the method 

just described it is probably a rare-earth mineral. As already stated, 

these minerals are not considered in this investigation.

Group B. The minerals of group B are readily broken down by 

means of 1:1 HNO3 or aqua regia. Minerals of the nickel-cobaltarsenic-antimony group tend to be associated together. Niccolite 

is recognized at sight from its color. Its neighbors can be assumed 

to belong to that group until proved otherwise. This group is large, 

and nearly all the minerals in it look almost exactly alike. In reflected light they are ashy gray and "hard" (in the sense already 

mentioned). With experience one can always tell from color and 

hardness that a mineral with the above description is chloanthite, 

loellingite, gersdorffite, skutterudite, rammelsbergite, smaltite, kallilite, willyamite, safflorite, or ullmanite. Arsenopyrite and loellingite cannot be told apart by etching, polarization, or microchemical tests. They are slightly different in color, however, arsenopyrite being galena white and loellingite ashy gray. Arsenopyrite 

tends to be associated with pyrrhotite, sphalerite, galena, and chalcopyrite; and loellingite with the other arsenides in the group just 

described.

A hard white mineral soluble in nitric acid will contain only nickel, 

cobalt, iron, arsenic antimony, bismuth, or sulphur. Dissolve the 

mineral in either HN03 or aqua regia. Notice the residue:

A white amorphous powder indicates sulphur or antimony.

A gelatinous ring around the drop indicates arsenic.

If the residue is green look for nickel; if yellow or orange, iron; 

if pink, cobalt.
254 MICROSCOPIC DETERMINATION OF ORE MINERALS

Tests for all the elements of this group can be made from the 

above-mentioned residue. Leach the residue with 1 percent HN03 .

1. Iron, nickel, cobalt, and arsenic go into the solution. Transfer 

1 percent HN03 drop and evaporate almost to dryness. Add drop 

of ammonium molybdate solution and heat very slowly till dry. Add 

drop of 1 : 7 HN03 , which dissolves the excess of ammonium molybdate, leaving minute yellow octahedrons or twinned isometric crystals if arsenic is present. Filter the solution from these crystals, 

transfer to another place on the slide, and add potassium mercuric 

thiocyanate (method I). If iron is present, the solution turns red. 

If cobalt is present, it forms acicular blue crystals, which usually are 

grouped as crosses or rosettes. If nickel is present, minute brown 

spherulites form slowly near the edge of the drop.

2. Antimony, bismuth, and sulphur remain undissolved in the original residue. The formation of a flocculent precipitate at this stage 

indicates bismuth. Leach original residue with at least three successive drops of 1 percent HN03 to remove remaining traces of 

arsenic, then a drop of H20 to remove the nitric acid. Then leach 

original residue with a drop of 1:5 HC1. Transfer drop and add 

fragment of KI. If an amorphous yellow or orange-colored precipitate or yellow hexagonal plates form, some arsenic is still present. If 

solution turns light yellow, it contains antimony or bismuth, or both. 

Add fragment of CsCl. Orange-colored hexagons prove antimony; 

red hexagons prove bismuth.

There are only three hard minerals that contain antimony 

kallilite (Ni(Bi,Sb)S), ullmanite (Ni(As,Sb)S), and willyamite 

((Co,Ni)SbS).

Group C. If a hard mineral is almost insoluble in 1:1 HN03 but 

readily soluble in aqua regia, the presence of manganese should be 

tested for by leaching the residue with a drop of 1:7 HNO3 , transferring the drop, and adding solid sodium bismuthate. If the drop 

turns pink or purple, manganese is present.

SOFT MINERALS

The soft minerals are divided into three groups (A) soluble in 

1:1 HN03 , (B) insoluble in 1:1 HN03 but soluble in aqua regia, 

(C) insoluble in both 1:1 HN03 and aqua regia. Group C is not considered in the analytical scheme here proposed.

Group A. A soft mineral soluble in nitric acid will not contain 

mercury. Examine the residue for the following hints:

A white amorphous powder indicates sulphur or antimony.

A gelatinous ring indicates either arsenic or silver.

A curdy white structureless mass suggests bismuth or tellurium.

A white lattice or boxlike structure indicates lead: more rarely an 

excess of silver may give a similar structure.
MICROCHEMICAL METHODS 255

Sulphur sometimes leaves a light-yellow agglutinated mass instead 

of a white powder.

An agglutinated brick-red mass proves selenium.

If the residue is green, suspect copper or nickel; if yellow or orangecolored, iron.

Selenium occurs only in combination with silver, gold, bismuth, 

copper, mercury, lead, tellurium, and sulphur.

Step 1, for iron, copper, zinc, cobalt, nickel, and silver: Leach the 

residue from the original mineral solution with a drop of 1 percent 

HN03 . Transfer the drop, and merge with it a drop of potassium 

mercuric thiocyanate. If iron is present, the test solution will turn 

red; copper forms a crystalline precipitate consisting of greenishyellow mosslike clumps, prisms, and crosses; zinc forms white or 

gray feathery crosses; silver forms an amorphous white powder; 

cobalt forms indigo-blue prisms; nickel forms small brown spherulites near the edge of the drop.

Step 2, for copper, arsenic, selenium, and tellurium: Leach the 

original residue with a drop of H20 to remove traces of HN03 . 

Kemove the drop. Then leach the original residue with a drop of 1 '5 

HC1. Any silver remaining from the preceding test will come down 

here as curdy white chloride. Lead compounds are changed to the 

chloride, which often appears as white needles. Transfer the filtrate 

to another place on the slide. Add a fragment of KI. Any copper 

remaining from the preceding test will come down as an amorphous 

white, gray, or light-yellow powder; arsenic will come down as an 

amorphous yellow to orange-colored powder; if the solution is concentrated with respect to arsenic, yellow hexagonal plates may 

appear either with or without the amorphous precipitate; tellurium 

or selenium will come down as a fine-grained amorphous chocolatebrown powder; if antimony or bismuth is present in appreciable 

amount (0.1 percent or more of SbCl3 ), the solution will be colored 

yellow in the neighborhood of the KI fragment. The selenium and 

tellurium precipitates come down instantly. . The arsenic and copper 

precipitates are sometimes slow in forming. Therefore, it is best to 

observe the drop until it has nearly evaporated, then add sufficient 1:5 

HC1 to give the drop its original volume. Once formed, the copper 

and arsenic precipitates are not soluble in the additional acid. All the 

precipitates thrown down by KI are amorphous and are best observed 

in oblique reflected light. The tellurium and selenium precipitates 

will not be confused with the arsenic and copper precipitates, owing 

to the peculiar manner in which the tellurium and selenium reactions 

proceed. (See detailed description, p. 207.) When a tellurium or 

selenium precipitate is obtained, confirmatory tests by the cesium 

chloride and stannous chloride reactions should be made to determine 

whether the precipitate is tellurium or selenium. The confirmatory
256 MICROSCOPIC DETERMINATION OF ORE MINERALS

tests can usually be made by leaching the original residue with 

another drop of 1:5 HC1 and testing the resulting solution. If not 

enough material remains in the original residue to yield these tests, 

it may be necessary to make them on additional mineral powder. 

The arsenic and copper precipitates are very much alike a fact 

which may cause some confusion in identification. The arsenic precipitate is yellower than the copper precipitate and sometimes is distinctly orange-colored. If there is any doubt as to the identity of 

the precipitate, it is advisable to make the ammonium molybdate test 

for arsenic on fresh mineral powder. It should be remembered that a 

large part, possibly most of the arsenic has been removed from the 

original residue by leaching the residue with 1 percent HNO3 and 

the subsequent drop of water and removal of both drops. This 

arsenic is lost in making the thiocyanate test for other elements 

(step 1); hence failure to get the iodide test for arsenic at this stage 

does not indicate that no arsenic was present in the mineral. This 

is the weakest point in the entire analytical scheme. However, in 

nine cases out of ten there is enough arsenic acid remaining to give 

a good indication for arsenic when tested with KL As already 

indicated, if there is any reason to believe that arsenic is present in 

the mineral and the iodide test fails to show it, the molybdate test 

for arsenic should be made on additional powdered mineral.

Step 3, for arsenic, antimony, bismuth, and tin: A fragment of 

cesium chloride is added to the same drop in which the KI test has 

just been made. The drop should first be restored to its original 

size by adding more 1:5 HC1. If arsenic is present, part of the 

iodide precipitate formed by the preceding test is converted to the 

cesium arsenic double iodide, which forms as an orange-colored amorphous precipitate near the border of the drop. Usually the concentration in arsenic is not great enough to cause the formation of the 

orange-colored hexagons described on page 211. The precipitate is 

best observed in oblique reflected light. Antimony will form orangecolored hexagons; bismuth will form red hexagons; tin will form 

colorless octahedrons.

Step 4, for lead, gold, sulphur, and selenium: Examine the original 

residue in obliquely reflected light, using a 16-millimeter objective.-, 

Gold, if present, will be seen as yellow metallic grains; any selenium 

not converted to the soluble acid and removed in the preceding operation remains behind in the original residue as the native element, 

which is observed as a reddish powder or agglutinated mass. When 

HC1 was added in step 2, the lead was converted into the chloride 

(PbCl2 ). Usually the lead chloride needles can be recognized at 

sight under the microscope. It is always advisable, however, to 

make the iodide test for lead, whether the chloride crystals are recognizable or not. Leach residue with two or three successive drops of
MICROCHEMICAL METHODS 257

water. Transfer each drop to another place on the slide and add a 

fragment of KL Lead, if present, will form lemon-yellow hexagons 

and disk-shaped plates of the normal iodide. To confirm the presence 

of gold, add a drop of aqua regia to the residue and evaporate to dryness over the micron1 ame. Gold is converted to an orange-colored 

to brown residue that is very soluble in water. Leach this residue 

with a drop of water, transfer the drop, and merge with it a drop of 

pyridine-HBr solution. Pleochroic red crystals prove gold.

The following observations on the soft minerals attacked by HN03 

are of value: The presence of arsenic excludes bismuth, tellurium, 

mercury, selenium, and zinc; the presence of nickel excludes copper, 

gold, lead, mercury, selenium, silver, and zinc.

Group B. A soft mineral insoluble in HN03 but easily soluble 

in aqua regia probably contains mercury or manganese. If Hg is 

present, the HgCl2 residue will usually show a lattice pattern. (See 

pi. 13, D.) A manganese residue forms as a circular ring with a 

faint tinge of pink.

Test for mercury: Dissolve in aqua regia. Leach the residue with 

1 percent nitric acid. Transfer the drop, and add a small fragment of Co(N03 ) 2 . When dissolved, add a fragment of KCNS. 

Blue branching or acicular crystals, the same as seen in the test 

for cobalt, prove the presence of mercury. M'ercury is associated 

only with antimony, selenium, tellurium, and sulphur.

Test for manganese: Dissolve in aqua regia. Leach the residue 

with a drop of 1:7 HN03 , transfer the drop, and add solid sodium 

bismuthate. If the solution turns pink or purple, manganese is 

present.

The following outline of the systematic analytic procedure was 

kindly prepared for the writer by F. M. Chace, of Harvard University.

OUTLINE OF SYSTEMATIC ANALYSIS 

HARD MINERALS

Group A. Minerals insoluble or very difficultly soluble in concentrated HNOs, 

1:1 HNOs, or aqua regia. Group consists of iron-bearing oxides, cassiterite, 

tungstates, and rare earths.

Procedure: Remove powder of mineral from polished section with chiselpointed No. 6 needle. Nonmetallic red powder=hematite. Brick-red to lightbrown powder=limonite or chromite. Dark-brown powder=franklinite or 

braunite. All these minerals are almost insoluble in aqua regia, but by several 

applications and evaporations enough material taken into solution to test for Fe. 

Residue orange to red. Take residue into solution with 1:7 HNOs. Add fragment of K4Fe(CN)6- Prussian blue precipitate=Fe.

Group B. Minerals soluble in 1:1 HNOs or aqua regia. Group consists of 

cobalt-nickel-iron arsenides, antimonides, sulpharsenides, sulphantimonites, 

pyrite, and marcasite.

Procedure: Hard white minerals soluble in HNOs contain only Ni, Co, Fe, As, 

Sb, Bi, S. Dissolve mineral in HNO3 . Note residue. White amorphous powder
258 " MICROSCOPIC DETERMINATION OF ORE MINERALS

= Sb or S. Gelatinous ring around drop = As. Green residue = Ni. Yellow or 

orange residue = Fe. Pink residue = Co. Leach residue with 1 percent HNOs.

Solution: Fe, Ni, Co, and As go into solution. Transfer drop. Add 1 percent 

HNOs. Evaporate almost to dryness. Add (NH4 )Mo04 and slowly evaporate 

to dryness, then add drop of 1 percent HN03 .. Precipitate, yellow octahedrons 

or twins=As. Filter and transfer drop. Add K2Hg(CNS) 4 . Precipitate, blue 

acicular crystals=Co; solution turns red=Fe; brown spherulites=Ni.

Residue: Sb, Bi, S remain in original residue. Leach original residue with 

three drops 1 percent HNOs to remove traces of As. Then leach with H2 0 to 

 remove HN03 . Addl:5HCl. Transfer drop. Add fragment KI. Precipitate, 

amorphous yellow to orange=As; solution turns light yellow=Sb or Bi. Add 

fragment CsCl. Precipitate, orange hexagons = Sb; red hexagons = Bi.

Group C. Minerals insoluble or slightly soluble in HNOs but readily soluble in 

aqua regia and 1:1 HCl. Group includes hard manganese oxides. Test for 

Mn, use sodium bismuthate test.

SOFT MINERALS

Group A. Minerals soluble in 1:1 HNOs. These minerals will not contain Hg. 

Presence of As excludes Bi, Te, Hg, Se, Zn; presence of Ni excludes Cu, Au, Pb, 

Hg, Se, Ag, Zn. Powder mineral. Place in 1:1 HN03 . Warm gently. Examine 

residue.

Residue: White amorphous powder=S or Sb. Gelatinous ring=As or Sb. 

Curdy white structureless mass = Bi or Te. White lattice or boxwork structure = 

Pb, rarely Ag. Light-yellow agglutinated mass, may be S. Agglutinated brickred mass = Se. Residue green = Cu or Ni. Residue yellow or orange=Fe.

Solution: Step 1 (test for Ag, Co, Cu, Fe, Ni, Zn). Leach residue from original 

mineral solution with 1 percent HNOs. Transfer drop. Add drop of K2Hg- 

(CNS)4. Amorphous white powder=Ag. Indigo-blue prisms=Co. Precipitate of green-yellow moss, prisms and crosses=Cu. Solution turns red = Fe. 

Small brown spherulites at margin of drop = Ni. White to gray feathery crosses 

= Zn.

Step 2 (test for Cu, As, Se, Te). Leach original residue with H2 0 to remove 

HNOs. Add drop 1:5 HCl. Ag present will precipitate as a curdy white mass. 

Pb may precipitate as needles. Transfer drop. Add fragment KI. Amorphous 

yellow to orange powder=As. Amorphous white, gray, or yellow powder=Cu. 

Solution turns yellow near fragment= Sb or Bi. Fine amorphous chocolatebrown precipitate=Te or Se. All precipitates by KI are amorphous. Observe 

in reflected light. Te and Se precipitate will not be confused with As and Cu, 

because of peculiar way in which Te and Se precipitate. When Te and Se are 

present distinguish the two by CsCl and SnCl2 tests made by leaching original 

drop with 1:5 HCl, transferring drop and adding reagent. Cu and As precipitates 

very similar; arsenic yellower and sometimes orange. To confirm, use ammonium 

molybdate test on new mineral powder.

Step 3 (test for As, Bi, Sb, Sn). Fragment of CsCl is added to same drop in 

which KI test was made in step 2. May restore drop to original size with 1:5 

HCl. Iodide precipitate of step 2 will form orange amorphous precipitate at 

border of drop=As. Red hexagons=Bi. Orange hexagons=Sb. Colorless 

octahedrons = Sn. Milky white crosses = Pb.

Step 4 (test for Au, Pb, Se, S). Examine residue from original solution in 

reflected light. Yellow metallic grains = Au. White PbCl2 needles = Pb (if any 

remains). Metallic red powder=Se (if any remains). Pb confirmatory test. 

Leach original residue with two or three successive drops of H2 0, transfer and add 

to each a fragment of KI. Pb will form lemon-yellow hexagons and disklike 

plates. Au confirmatory test: Add drop of aqua regia to residue, and evaporate
MICROCHEMICAL METHODS 259

to dryness. Au is converted to orange-brown residue very soluble in H20. 

Transfer drop, merge with pyridine-HBr solution. Pleochroic red crystals = Au.

Group B. Soft minerals insoluble in HNOs but soluble in aqua regia. Probably 

contain Hg or Mn. HgCl2 residue forms lattice pattern. Mn forms a pink circular 

ring.

Test for Hg: Dissolve in aqua regia. Leach residue with 1 percent HNOs. 

Transfer drop. Add fragment Co(NOs)2- When dissolved add fragment ofKCNS. Blue branching or acicular crystals prove presence of Hg. Hg occurs 

only with Sb, Se, Te, and S.

Test for Mn: Dissolve in aqua regia. Leach residue with 1:7 HNOs. Transfer ' 

drop and add a fragment of sodium bismuthate. Solution turns pink or purple if 

Mn is present.

Group C. Soft minerals insoluble in both 1:1 HNOs and aqua regia. No procedure given.

OUTLINE OP MICROCHEMICAL TESTS

In this outline is given in condensed form a description of the 

precipitates formed by the standard microchemical reagents. The 

elements to be identified are present in either 1 percent HN03 or 1 : 5 

HC1. Except as otherwise described, precipitates appear within 30 

seconds after the reagent is applied. Exceptions to this rule are the 

potassium mercuric thiocyanate tests for nickel and cobalt and the 

cesium chloride test for cobalt. These compounds are slow in forming, 

not because they are soluble but because they tend to supersaturate. 

Once formed they will not dissolve by restoring the drop to its original 

size.

The data on mutual interference of elements are taken from the 

paper of Fraser and Dreyer.51

Numbers in parentheses refer to concentration in percentage of the 

element in solution. Thus, Cu (0.1) signifies 0.1 percent of copper.

POTASSIUM MERCURIC THIOCYANATE

A drop of a 3 percent solution of the reagent is merged with a drop 

of the element in a 1 percent HN03 solution.

Antimony: No precipitate.

Arsenic: No precipitate.

Bismuth: No precipitate.

Cadmium: Blunt white hemimorphic prisms which usually have cavities at each

end.

Cobalt: Blue prisms and spherulites. 

Co (0.1) Cu(l): No Co test.

Co (1.0) Cu (0.1): Crystals of Cu form, but all colored blue. A mineral with 

concentrations approximately equal, as in carrollite (Co,Cu)sS4, gives 

typical tests for both elements. 

Co (0.1) Fe (1): All Co crystals dark, irregular peripheral aggregates. Test

poor.

Co (1.0) Fe (1): All Co crystals dark but well formed.

Co (1.0) Ni (1): No evidence of Co until evaporation is far advanced, and 

then only blue globules form.

81 Fraser, H. J., and Dreyer, R. M., Mutual interference in the microchemical determination of ore 

minerals: Am. Mineralogist, vol. 22, pp. 949-976,1937.
260 MICROSCOPIC DETERMINATION OF ORE MINERALS

Co (1) Ni (0.1): No nickel test. Most specimens 'of siegenite, (Ni,Co)aS4, 

smaltite-chloanthite, (Co,Ni)As2 , and other minerals containing both 

nickel and cobalt give satisfactory tests for both elements. The cobalt 

compound does not precipitate until the drop begins to dry.

Copper: Greenish-yellow mosslike aggregates and prisms.

Gold: Dilute solutions of AuCU in 1 percent HNOs give white or creamy amorphous 

precipitate. Solutions containing more than 0.1 percent AuCls give 

yellowish-brown mosslike aggregates.

Iron: Solution turns red. Faint in solutions containing 0.1 percent Fe.

Lead: White granular amorphous precipitate. Comes down only in concentrated 

solution.

Manganese: No precipitate.

Mercury: No precipitate.

Nickel: White to brown spherulites.

Selenium: Numerous peripheral greenish-yellow globules, very late forming. As 

drop dries up, elemental selenium is deposited.

Silver: Curdy white, exceedingly fine-grained precipitate.

Tellurium: No precipitate.

Tin: Not soluble in HN03 solution.

Zinc: White feathery crosses. 

Zn (0.1) Cu (1): No Zn test. 

Zn (1.0) Cu (1): Some Zn crosses, all purple.

The reagent is applied in solid form to the element in a 1 : 5 HC1 

solution.

Antimony: White hexagons and stars. Most of the precipitate tends to be

amorphous.

Sb (0.1) Te (0.1): Sb crystals small and indistinct. 

Sb (1) Te (0.1): Te precipitate largely masked by Sb. 

Sb (0.1) Sn (1): Sb masked by Sn precipitate.

Arsenic: Numerous small, formless, late-forming peripheral grains and hexagonal 

plates.

Bismuth: Very thin colorless rhombs and knifelike prisms. 

Bi (0.1) As (1): Bi largely masked by As.

Cadmium: Tiny colorless hexagonal bipyramids. Best observed with an 8-millimeter objective. A 0.5 percent solution of CdCl2 gives a good test, but a 

0.1 percent solution gives a poor test.

Cobalt: Soluble light-blue blades and prisms form around the periphery of the 

drop very late in period of evaporation.

Copper: Golden-yellow needles appear at edge of drop after evaporation is somewhat advanced. Does not appear in dilute solutions.

Co (1) Cu (1): Residue colored light green by Cu, so that Co cannot be 

recognized.

Gold: Small brown octahedrons and skeletal isometric crystals.

Iron: From relatively concentrated solutions tiny golden-yellow crystals appear 

at edge of drop, at a late stage in evaporation. Bipyramids resembling 

octahedrons, pseudohexagons, and diamond-shaped plates predominate. 

They are probably orthorhombic.

Lead: Small colorless rhombs, crosses, and plates of irregular shape. Crystals 

have a high refractive index and low double refraction. Precipitate appears 

only in relatively concentrated solutions.

Manganese: No precipitate.
MICEOCHEMICAL METHODS 261

Mercury: In a 0.5 percent solution of HgCl2 in water highly refracting colorless 

cubes and octahedrons are precipitated in a few seconds. These resemble 

the AgCl precipitate (see p. 202), but the crystals of the mercuric compound 

are much larger. In a 0.1 percent solution of HgCl2 in water no precipitate 

appears until the evaporation is advanced. The presence of HCl greatly 

reduces the sensitivity of this reaction.

Nickel: No precipitate until drop is almost dry, when tiny formless grains appear. 

These are white to light yellow.

Selenium: Colorless plates and formless grains. Weak in 0.1 percent solution.

Silver: Not soluble in an HCl solution; hence no precipitate.

Tellurium: Honey-yellow isometric crystals, chiefly octahedrons, also hexagonal 

and triangular plates.

Tin: Small white octahedrons and Y-shaped crystals. 

Sn (0.1) Cu (1): No Sn test.

POTASSIUM IODIDE

A small fragment of the reagent is applied to a drop of 1:5 solution 

containing the element.

Antimony: Solution turns yellow. Color very faint in 0.1 percent Sb solution.

Arsenic: Yellow amorphous precipitate. Concentrated solutions sometimes give

small disks. Reaction delayed in 0.1 percent As solution. 

As (0.1) Sb (1): Arsenic precipitate alone gives very few disks at this concentration, but in the presence of Sb the solution is colored yellow, and 

yellow disks appear which are very similar to those formed in the Sb 

double iodide test.

Bismuth: Solution turns yellow.

Cadmium: No precipitate.

Cobalt: No precipitate.

Copper: Light-yellow amorphous precipitate.

Cu, Se, Te, Au: It is very difficult to distinguish the KI precipitates of these 

elements. In the systematic scheme Au will not go into solution when the 

mineral is broken down with 1:1 HNOs. Most of the Cu is removed by 

leaching original residue with 1 percent HNOs.

Cold: Yellow amorphous ring surrounds KI fragment. Ring widens as KI dissolves. Liquid inside of ring colors yellow. Reaction resembles that of 

selenium, but precipitate is a lighter yellow, and some metallic gold (goldcolored in reflected light), as well as light-green rectangular to irregular 

plates, is precipitated.

Iron: No precipitate. Solution turns yellow.

Lead: Greenish-yellow hexagons. HCl reduces solubility of Pb; hence test is 

better in neutral H2 0 solution.

Manganese: No precipitate, but solution turns yellow near KI fragment.

Mercury: From a solution of HgCl2 in water, KI precipitates a fine-grained 

amorphous precipitate which spreads in a widening solution away from 

the KI precipitate as it dissolves. This precipitate is light yellow in oblique 

reflected light. Within the yellow ring, and adjacent to it, is another ring 

composed of red particles. These are amorphous but larger in grain size 

than those of the yellow ring. The red ring follows the yellow ring as it 

expands. Within the red ring the solution is colorless. This test was satisfactory in a 0.1 percent solution but unsatisfactory in a 0.05 percent solution of HgCl2 . In a 1:5 HCl solution, small red and orange cubes appear. 

Satisfactory in a 0.5 percent solution but unsatisfactory in a 0.1 percent 

solution of HgCl2 .
262 MICROSCOPIC DETERMINATION OF ORE MINERALS

Nickel: No precipitate.

Selenium: Chocolate-brown amorphous precipitate.

Silver: Not soluble in HCl solution.

Tellurium: Chocolate-brown precipitate.

Tin: No precipitate.

Zinc: No precipitate.

CESIUM CHLORIDE AND POTASSIUM IODIDE

Small fragments "of the reagents are applied to a 1:5 HCl solution 

containing the element.

Antimony: Orange to red hexagons and stars.

Sb-As: Arsenic gives a precipitate which, in all concentrations, is very much 

like the Sb precipitate. Arsenic, however, unlike Sb, gives a precipitate 

after the KI has been added and before the CsCl has been added. 

Sb (0.1) Cu (1): No Sb test.

. Sb (1) Cu (1): Sb somewhat masked by Cu precipitate. 

Sb (0.1) Sn (1): Sb crystals small, poorly formed, and somewhat delayed. 

No difficulty is met in getting both Sb and Sn from the same drop when 

testing cylindrite (6PbS.Sb2S3 .6SnS2) and franckeite (5PbS.Sb2S3 .2SnS2). 

Arsenic: Brown amorphous precipitate near edge of drop. Concentrated solutions may give orange hexagons indistinguishable from the antimony 

precipitate.

As (0.1) Cu (1): Arsenic test masked by Cu. 

As (1) Cu (1): Many As crystals, but many of the crystals are poorly formed

and more brown than usual.

As-Fe: If the Fe concentration is relatively high, there may be some difficulty in noting As.

Bismuth: Red hexagonal plates. Very thin plates may be orange and indistinguishable from those of the antimony test. 

Bi-Sb: Except where one of the elements is greatly in excess of the other,

Sb and Bi can usually be separated. The Bi test is more sensitive. 

Bi (0.1) Cu (1): Bi largely masked by Cu. 

Bi (0.1) Se (1): No Bi test. Satisfactory tests for both elements are obtained

without difficulty from guanajuatite, Bi2 (Se,S) 3 .

Bi (0.1) Te (1): Bi masked by Te. The double iodide test for Bi in tetradymite, Bi2Te3 , is unsatisfactory. The cesium chloride test should be used 

instead.

Cadmium: Colorless cubes. Precipitate appears only in concentrated solutions. 

Cobalt: Soluble bluish-green prisms and rhombs appear at edge of drop. Precipitate appears only in relatively concentrated solutions. Late forming. 

Copper: Long, ragged, slender colorless needles appear in addition to normal copper

iodide.

Gold: Same as cesium chloride test.

Iron: Aggregates of splintery colorless needles slowly appear at edge of drop. 

These resemble calcium sulphate needles but tend to assume hexagonal 

star shapes.

Lead: In addition to normal lead iodide, there is precipitated a mat of slender white 

needles, probably the double iodide of cesium and lead. This test is valuable in that it often appears in tests for other elements, especially antimony, bismuth, and tin. 

Manganese: No precipitate. 

Mercury: Same as KI test. 

Nickel: No precipitate.
MICROCHEMICAL METHODS 263

Selenium: Same as normal potassium iodide and cesium chloride tests, plus irregular reddish-brown to opaque crystals.

Silver: Not soluble in HC1.

Tellurium: Same as KI test.

Tin: Same as CsCl test.

Zinc: Colorless rhombs and branching forms at edge of drop. Forms show loworder interference colors.

POTASSIUM BICHROMATE

A small fragment of the reagent is applied to a 1 percent HN03 

solution containing the element.

Antimony: No precipitate.

Arsenic: Slow formation of good octahedrons and tetrahedrons. Some colorless,

some dark. Form only at a late stage in evaporation of drop. 

Bismuth: Immediate fine-grained dark precipitate, yellow in reflected light.

Soluble in 1:7HN03 . 

Cadmium: No precipitate. 

Cobalt: No precipitate. 

Copper: No precipitate. 

Gold: No precipitate. 

Iron: No precipitate. 

Lead: Yellow amorphous precipitate predominates, but minute rhombs and

needles sometimes appear. 

Manganese: No precipitate. 

Mercury: Mercuric ions give no precipitate. 

Nickel: No precipitate. 

Selenium: No precipitate.

Silver: Ruby-red crystals. Concentrated solutions give rectangular or coffinshaped plates. Dilute solutions give needles at edge of drop. 

As (1) Ag (1): Slow formation of good octahedrons and tetrahedrons, colorless to dark in transmitted light, white in reflected light. No silver bichromate or chromate blades.

As (1) Ag (0.1): Octahedrons and tetrahedrons. No silver test. 

As (0.1) Ag (1.0): Strong silver test.

As (0.1) Ag (0.1): Slow formation of bladed and feathery dark-red to black

forms at edge of drop. Forms sometimes very feathery. Good bichromate

tests for silver are obtained from pearceite, 8Ag2S.As2S3, and proustite,

3Ag2S.As2S3 .

Cu (1) Ag (0.1): Silver blades are small and often colored blue, but test is

satisfactory.

Pb (1) Ag (0.1): Heavy yellow lead precipitate masks later silver test. 

Pb (0.1) Ag (0.1): Immediate precipitation of lead chromate; later good

silver precipitate.

Se (1) Ag (0.1): Immediate fine-grained precipitate; no silver test. 

Se (1) Ag (1): Black grains, clumps of needles, crosses, white in reflected light.

Later a few red spherules are formed. No silver blades. 

Se (0.1) Ag (0.1): Black grains and needles, then red grains and spherules

around edge of drop. No silver bichromate blades.

Se (0.1) Ag (1): Black grains, colorless to black feathery blades, feathery 

clumps and crosses; later formation of blades that are red when they 

begin to form but turn black as they grow. Owing to the slight solubility 

of selenious acid in 1 percent HNOs, satisfactory bichromate tests for 

silver are obtained from aguilarite, Ag2 (S,Se), and eucairite, Cu2Se.Ag2Se.
264 MICROSCOPIC DETERMINATION OF ORE MINERALS

Te (0.1) Ag (0.1): Fine-grained precipitate. Weak silver test of red to black

blades.

Te (0.1) Ag (1): Fine-grained precipitate, then good silver test. Satisfactory 

bichromate tests for silver are obtained from hessite, Ag2Te, and petzite 

(Ag,Au)2Te. The chloride test for silver is preferable for sylvanite, 

AuAgTe4 , krennerite, (Au,Ag)Te2, and calaverite, (Au,Ag)Te2 . 

Tellurium: No precipitate. 

Tin: Not soluble in HNOs solution. 

Zinc: No precipitate.

THIOUREA M

A drop of a 10 percent solution of thiourea, SC(NH2 ), was added to a 

drop of a 1 percent solution of the element in 1:5 HC1, unless otherwise 

stated.

Ag: Reaction in 1 percent nitric acid. Ag precipitates as a dark ring at junction of

drops, soluble in excess of thiourea. 

Au: No reaction.

Bi: Solution turns canary yellow; no precipitate. Reaction succeeds best in HNOs 

solution. Less sensitive in HC1. The interference tests for bismuth were 

run in a nitric acid solution.

Ag (1) Bi (0.1): Silver precipitate partly masks Bi color. 

Cr (1) Bi (0.1): Yellow drop, slightly greenish. 

Fe (1) Bi (0.1): Yellow drop, slightly orange. 

Hg (1) Bi (0.1): Mercury precipitate partly masks Bi color. 

Se (1) Bi (0.1): Bi color not visible unless drop is filtered. 

Cd: No reaction. 

Co: No reaction.

Cr: Solution turns faint blue; no precipitate.

Cu: Immediate heavy dark fine-grained precipitate, white in reflected light. 

Fe: Solution turns faint pink; no precipitate. 

Hg: Immediate precipitate of colorless needles and dark grains. 

Mn: No reaction. 

Ni: No reaction. 

Pb: No reaction.

Sb: Solution turns faint yellow; no precipitate 

Se: Immediate precipitation of reddish to blue-black small grains that spread as

streamers through the drop. Brilliant red in reflected light. 

Bi (1) Se (0.1): The strong yellow color of Bi appears first. Se precipitates, 

but color is obscured by the Bi. In strong reflected light the red selenium 

precipitate is sometimes visible. Test not always satisfactory. 

Bi (1) Se (1): Satisfactory test. 

Cu (1) Se (0.1): Copper precipitate, dark in transmitted light, brownish in

reflected light; selenium obscured. 

Cu (1) Se (1): Satisfactory selenium test.

Hg (1) Se (0.1): Heavy Hg precipitate partly obscures red Se precipitate. 

Sb (1) Se (0.1): Part of the Sb may precipitate by hydrolysis and partly

obscure the Se precipitate. 

Sn: No reaction. 

Te: No reaction. 

Ti: No reaction. 

Zn: No reaction.

*' Eraser, H. J., and Dreyer, R. M., op. cit.
MICROCHEMICAL METHODS 265

Platinum group and gold. 53 Concentration of element in solution, 1 percent

Platinum: Thin yellow petals and rhombs, moderate index of refraction, low 

birefringence, slowly form. Then jagged prisms grow, dark in direct 

light, brick red in reflected light. Still later, long spines grow across 

drop, dark in direct light, faint yellow in reflected light. These usually 

form just before drop dries.

Palladium: Immediate strong precipitation of fine-grained chocolate-brown to 

black aggregate, followed by very feathery blades and needles that range 

from light yellow green to deep yellow, high index of refraction, high 

birefringence. Yellow to pinkish brown in reflected light. Strong test. 

Color of solution changes from brown to yellow on addition of thiourea. 

Heating the drop dissolves all the precipitate, but it re-forms with slow 

cooling. Rapid cooling gives radiating clusters of (thiourea?) needles 

stained yellow.

Rhodium: Fairly rapid precipitation of feathery crosses and rosettes of faint to 

strong yellow color. Very low index of refraction, isotropic. On drying, 

pink blades form at edge of drop. Fairly strong test. Gentle heating 

changes the solution from pinkish to yellowish orange.

Ruthenium: No precipitate. Cold drop very slowly develops a deep inky-blue 

color. As drop dries, blue-black spherules form at edge of drop. If drop 

is gently heated after thiourea is added, it immediately turns inky blue. 

Strong, positive test.

Iridium: No precipitate. No color change in cold or hot solution.

Osmium: No precipitate. Cold drop dries to give thiourea blades stained 

yellow. Gently heated drop after addition of thiourea immediately changes 

to a reddish purple. Very distinctive test.

Gold: Immediate precipitate of blue-black grains, soluble in excess thiourea. As 

drop evaporates, colorless rhombs and petals form at edge, low birefringence. 

Still later prisms appear with a high birefringence. .

SPECIFIC TESTS FOR MINERALS

The systematic scheme of microchemical analysis described above 

is designed to identify in as few operations as possible alJ the principal 

elements occurring in ore minerals. The tests given in the scheme 

are those which are applicable to the greatest number of elements. 

The test for any particular element as described in the outline is not 

necessarily the best test for that element. For instance, the tests 

with potassium mercuric thiocyanate for both silver and nickel are 

given preference in the scheme because that reagent will indicate the 

presence of a considerable number of additional elements; but the 

ammonium bichromate test for silver and the dimethyl glyoxime test 

for nickel are more characteristic for the respective elements than 

the thiocyanate tests.

As a rule, the investigator, by means of polarization, color, hardness, 

and etch tests, has a definite clue to the identity of the mineral under 

investigation. He then desires to confirm his observation by tests 

for the elements he believes are contained in the mineral. In the 

following pages the procedure believed to be the best for identifying

53 Fraser, H. J., Microchemistry of the precious-metal elements: Am. Mineralogist, vol. 22, pp. 1016-1034, 

1937.

129151 40 ]8
266 ' MICROSCOPIC DETERMINATION OF ORE MINERALS

the elements in each mineral is described in detail. In using this 

procedure many short cuts are possible, in comparison with the 

systematic scheme. For instance, there is no particular need of 

making a thiocyanate test on galena. It is hoped that the use of these 

short cuts will result in much saving of time.

Aguilarite (Ag2 (S,Se)). Decompose mineral with 1:1 HNOs. Usually several 

applications of acid are necessary. The residue is gelatinous and may be tinged 

yellow and reddish. Leach residue with drop of 1 percent HNOs, transfer drop 

and add fragment of ammonium bichromate. The presence of selenious acid 

will delay but not prevent the formation of silver bichromate. Red crystals 

prove silver. Leach original residue with drop of 1:5 HC1. Any silver remaining 

is converted into white flocculent chloride. Transfer filtrate and merge with it 

a drop of stannous chloride. A brick-red amorphous precipitate proves selenium. 

If desired the silver chloride in the residue can be tested by the ammonia method. 

Remove all traces of selenium by several successive drops of 1:5 HC1; remove 

traces of HC1 with a drop of water. Then place a drop of concentrated NH4OH 

on residue. After a few seconds colorless highly refracting isometric crystals 

will separate out of the solution. (See description of naumannite, p. 283.)

Aikinite (Cu2S.2PbS.Bi2S3). Decompose mineral with one or two applications 

of 1:1 HNOs. Aikinite is very easily attacked by HNOs. The residue is white 

and curdy. Carefully examine residue for lattice structure (lead nitrate). Leach 

residue with drop of 1 percent HNO3 , transfer drop, and merge with a drop of 

K2Hg(CNS)4 solution. Greenish-yellow mosslike aggregates and needles prove 

copper. Leach original residue with successive drops of water to remove remaining traces of copper and HN03 . Test each drop with a drop of K2Hg(CNS)4 

until the copper precipitate does not immediately appear. Usually three or 

four drops of water are needed. The original residue is now sufficiently free from 

copper to permit the subsequent tests for bismuth and lead . Leach original residue 

with a drop of 1:5 HC1. Lead is precipitated in shreddy white prisms. Usually 

this is a sufficiently diagnostic test for lead. Bismuth goes into solution. Transfer drop and add fragment of KI. If any copper has persisted after the preceding 

efforts to remove it, an amorphous yellow precipitate of copper iodide will appear. 

The solution turns yellow from the presence of bismuth. Add fragment of CsCl. 

Red hexagons prove bismuth. The double iodide test for bismuth is not as satisfactory with aikinite as with most other bismuth minerals. For some unknown 

reason orange hexagons appear in addition to the red. The double chloride test 

for bismuth is more satisfactory. Leach the original residue with another drop 

of 1:5 HC1, transfer drop and make the double chloride test by adding fragment 

of CsCl. The colorless rhombs of the double chloride precipitate are of normal 

shape. The chloride test for lead can now be confirmed by the iodide test. 

The original residue should be freed from remaining traces of bismuth by washing 

with additional drops of 1:5 HCl, transferring each drop and adding to each a 

small fragment of KI. As long as bismuth is present in the original residue, the 

drop will be stained yellow near the fragment. Usually one or two drops of HCl 

will suffice. Leach original residue with successive drops of water, transfer each 

and test for lead with a fragment of KI. The first drop may not give a satisfactory test, owing to the presence of traces of HCl or of bismuth, but the second or 

third drop will give yellow hexagons of the characteristic normal lead iodide.

Alabandite (MnS). Take mineral into solution with 1:1 HNOs. H2 S gas is 

usually evolved. Evaporate, cool, and leach residue with a drop of 1:7 HNO3 . 

Add fragment of sodium bismuthate. Solution turns pink or purple, proving 

manganese.
MICKOCHEMICAL METHODS 267

Alaskaite ((Pb,Ag2)S.Bi2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HN03 . Transfer drop and add fragment of ammonium bichromate. A yellow amorphous precipitate indicates lead, and red 

crystals indicate silver. The original residue contains bismuth and most of the 

lead. Add drop of 1:5 HCl, transfer drop, and add fragment of KI. Solution 

turns yellow near fragment. Then add fragment of CsCl. Red hexagons prove 

bismuth. Leach original residue with three more drops of 1:5 HCl to remove 

last traces of bismuth. Then add three successive drops of water to original 

residue. Transfer each drop to another place on slide and add fragment of KI. 

The first drop may not give a satisfactory test for lead, owing to the presence of 

remaining traces of HCl and Bi. The second or third drop will yield yellow 

hexagons, which prove lead.

Frequently the presence of lead masks the bichromate test for silver. In case 

there'is reason to suspect its presence from etch tests on the polished section or 

otherwise, it is best to start with fresh mineral powder. Take mineral into solution with 1:1 HN03 . Leach original residue with at least five successive drops 

of 1:5 HCl to remove bismuth, followed by five drops of water to remove some of 

the lead. Reject these drops. Part of the lead and all the silver remain in the 

original residue as chlorides. Place a diop of concentrated NH4OH on the original 

residue. After a few seconds, tiny colorless highly refracting isometric crystals 

of AgCl will separate out of the solution, beginning hear the edge of the drop. 

Lead is changed to the insoluble hydroxide and does not mask the silver test.

Algodonite (CiieAs). Take mineral into solution with aqua regia. Residue is 

gelatinous with tinge of brown. Leach residue with drop of 1:7 HN03 , transfer 

drop, and heat slowly by passing in and out of flame. When nearly dry add 

direc.tly a drop of ammonium molybdate solution. Again heat slowly over flame 

till dry. Add drop of 1:7 HNOs solution to redissolve excess of ammonium 

molybdate and examine. Tiny yellow isometric crystals prove arsenic. Transfer 

filtrate (containing copper nitrate and dissolved ammonium molybdate) and 

merge with it a drop of potassium mercuric thiocyanate solution. A yellowishgreen precipitate proves copper.

Altaite (PbTe). Decompose mineral with 1:1 HN03 . Examine residue for 

lattice structure. Leach residue with drop of 1:5 HCl. Transfer drop and add 

fragment of CsCl. Honey-yellow isometric crystals prove tellurium. Add to 

original residue two or three successive drops of H20, transfer each, and add fragment of KI. The first or second drop may not give a satisfactory test for lead, 

owing to remaining traces of HCl, but the second or third drop will yield the usual 

yellow hexagons of the normal lead iodide.

Andorite (Ag2S.2PbS.3Sb2 S3 ). Decompose mineral with 1:1 HN03 . The mineral is somewhat resistant to attack, but with a liberal supply of finely divided 

material and at least 10 successive drops of liquid, enough can be taken into 

solution to yield to tests. Leach residue with a drop of 1: 5 HCl, transfer drop, 

and test for Sb by means of KI and CsCl. Orange-colored hexagons prove antimony. Leach original residue with two additional drops of HCl to remove remaining traces of Sb and reject drops. Then leach original residue with three successive drops of H2 0, transfer each drop, and test with a small fragment of KI. 

Yellow glistening hexagons prove lead. Finally leach original residue with drop 

of concentrated NH.iOH. Observe edge of drop with an 8-millimeter objective. 

Tiny highly refracting isometric crystals of AgCl separate from solution after a 

few moments. The bichromate test for silver has been successfully used on andorite, but it requires about three times the amount of material ordinarily used for 

microchemical tests and, in addition, more than an average degree of technical 

skill.
268 MICROSCOPIC DETERMINATION OF ORE MINERALS

Antimony (Sb). Decompose mineral with 1:1 HN03 . Residue is white and 

powdery. Leach residue with drop of 1: 5 HC1, transfer drop, and add fragment 

of KI. Solution turns yellow near fragment. Add fragment of CsCl. Orangecolored hexagons and stars prove antimony.

Aramayoite (Ag2S.(Sb,Bi) 2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HNOs, transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Leach original residue with drop 

of 1:5 HC1. Transfer drop and add fragment of KI. Solution turns yellow 

near fragment. Add fragment of CsCl. Both orange-colored and red hexagons 

form, proving antimony and bismuth.

Argentite (Ag2S). Decompose mineral with 1 :1 HN03 . Leach residue with 1 

percent HN03 , transfer drop, and add fragment of ammonium bichromate. Red 

crystals prove silver.

Argyrodite (4Ag2S.GeS2). Argyrodite is very resistant to attack by 1:1 HN03 , 

but by repeated applications (usually about ten suffice) enough material can be 

taken into solution to give the silver test. Leach residue with drop of 1 percent 

HNOs, transfer drop, and add fragment of ammonium bichromate. Red crystals 

prove silver. Test for germanium by procedure described on page 240.

Arsenargentile (Ag3As). Take a mineral into solution with 1:1 HN03 . Leach 

residue with two or three successive drops of 1 percent HN03 , and add to each 

a fragment of ammonium bichromate. Red crystals prove silver. The second 

or third drop will probably give a better test than the first, as the concentration 

of gelatinous arsenic acid in the first drop is usually high. Take more mineral 

into solution with aqua regia. Leach original residue with a drop of 1:7 HNO3 , 

transfer drop, and slowly evaporate over microflame. When nearly dry add drop 

of ammonium molybdate solution and again evaporate slowly over microflame. 

When dry, cool and add drop of 1:7 HNO3 to residue. Tiny yellow isometric 

crystals prove arsenic.

Arsenic (As). Take mineral into solution with two or three drops of aqua 

regia. Residue forms a white gelatinous ring around periphery. Leach residue 

with drop of 1:7 HN03 , transfer drop, and evaporate almost to dryness. Leach 

residue with drop of ammonium molybdate reagent and evaporate slowly to dryness over flame. Cool and leach with a drop of 1:7 HN03 . Tiny yellow isometric crystals prove arsenic.

Arsenopyrite (FeAsS). Decompose mineral with aqua regia. The residue usually has a tinge of yellow or orange. Leach residue with drop of 1:7 HN03 , 

transfer drop, and evaporate slowly by passing slide in and out of microflame. 

When nearly dry add directly a drop of ammonium molybdate solution and again 

heat slowly over microflame. When dry, cool and add drop of 1:7 HN03 to redissolve excess ammonium molybdate. Examine under microscope. Tiny yellow 

isometric crystals prove arsenic. Take up filtrate and transfer to another place 

on slide. Merge with it a drop of potassium mercuric thiocyanate solution. A 

red coloration of test solution proves iron.

Baumhauerite (4PbS.3As2S3). Decompose mineral with aqua regia. Leach residue with drop of 1:7 HN03 , transfer drop, and evaporate almost to dryness. 

Add a drop of ammonium molybdate solution directly to the residue from the 

evaporation of the 1:7 HNOa drop. Evaporate slowly to dryness. Cool and add 

drop of 1:7 HN03 . Tiny yellow isometric crystals prove arsenic. Leach original 

residue with three successive drops of H2 0, transfer drops, and add to each a 

fragment of KI. Yellow glistening hexagonal plates prove lead.
MICROCHEMICAL METHODS 269

Benjaminite ((Cu,Ag) 2S.2PbS.2Bi2Ss). Decompose mineral with 1:1 HNOs. 

Leach residue with drop of 1 percent HNO3 , transfer drop, and merge with it a 

drop of potassium mercuric thiocyanate solution. Greenish-yellow mosslike 

clumps and prisms prove copper, and an amorphous white powder proves silver. 

Leach original residue with drop of 1:5 HCl, transfer drop, and add fragments 

of KI and CsCl. Red hexagonal plates prove bismuth. Leach original residue 

with successive drops of 1:5 HCl,and test each with KI and CsCl until no test 

for bismuth appears. Leach original residue with three successive drops of H2 0, 

transfer each drop, and test with a fragment of KI. The first drop may not 

give a satisfactory test, owing to presence of remaining traces of HCl. This 

keeps a trace of bismuth in solution, which tends to modify the lead iodide precipitate. The second or third drop will give good yellow hexagons, which prove 

lead.

Berthierite (FeS.Sb2S3). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1:7 HN03 , transfer drop, and add fragment of KiFeCCNV A blue 

amorphous precipitate proves iron. Leach original residue with drop of 1:5 

HCl, transfer drop, and add fragment of KI. Solution is colored yellow near 

fragment. Then add fragment of cesium chloride. Yellow hexagons and stars 

prove antimony.

Berthonite (2(Pb,Cu2)S.Sb2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HN03 . Transfer drop and merge with drop of 

K2Hg(CNS)4 solution. Greenish-yellow mosslike aggregates and needles prove 

copper. Leach original residue with successive drops of 1 percent HN03 to remove 

most of the remaining traces of copper and HNOs. Test each drop with a drop 

of K2Hg(CNS)4 until the yellow copper precipitate does not immediately appear. 

The original residue is then reasonably free from copper. Leach original residue 

with drop of 1:5 HCl. Transfer drop arid add fragment of KI. If any copper 

has persisted after the preceding efforts to remove it, an amorphous yellow precipitate of copper iodide will appear. The solution turns yellow near the fragment of KI. Add fragment of CsCl. Orange-colored hexagons prove antimony. Leach original residue with two or three successive drops of water, transfer each drop, and test each with a fragment of KI. The first drop may not 

give a satisfactory test for lead, owing to traces of HCl, but the second or third 

drop will yield yellow hexagons of the characteristic normal lead iodide.

Bismuth (Bi). Dissolve mineral in 1:1 HN03 . All goes into solution. Evaporation leaves white curdy residue. Take this into solution with 1:5 HCl. Add 

fragment of KI. Solution turns vivid yellow. Then add fragment of CsCl. 

Red hexagons prove bismuth.

Bismuthinite (Bi2S3). Decompose mineral with 1:1 HN03 . Add drop of 1:5 

HCl to residue. The bismuth goes into solution as chloride; the sulphur remains 

as an agglutinated yellow mass. Transfer filtrate, then add successive fragments 

of KI and CsCl. Red hexagons prove bismuth. Most specimens of bismuthinite 

give a distinct copper reaction.

Bismutoplagionite (5PbS.4Bi2S3). Decompose mineral with 1:1 HNOs. Leach 

residue with drop of 1:5 HCl, transfer drop, and add to it fragments of KI and 

CsCl. Red hexagonal plates prove Bi. Leach original residue with two successive drops of 1:5 HCl to remove most of the remaining bismuth; reject both drops. 

Then leach original residue with three successive drops of H2 0. Add to each a 

fragment of KI. The first drop may not give a satisfactory lead test, owing to 

remaining traces of HCl. This keeps a trace of bismuth in solution, which tends 

to modify the lead iodide precipitate. The second or third drop will give good 

yellow hexagons, proving lead.
270 MICROSCOPIC DETERMINATION OF ORE MINERALS

Bornite (CusFeS^. Decompose mineral with 1:1 HNOs. Leach residue with 

1:7 HNOs. Transfer drop and merge with it a drop of potassium mercuric 

thiocyanate solution (method I). Part of the drop turns pink, indicating iron. 

A greenish-yellow crystalline precipitate indicates copper. The iron test may be 

faint unless a relatively large amount of mineral is used. A microchemical test 

for bornite is rarely needed.

Boulangerite (5PbS.2Sb2S3). Decompose mineral with 1:1 HN03 . Examine 

residue for lattice structure, indicating lead. Leach residue with drop of 1:5 

HC1. Transfer drop and add fragment of KI. Solution turns yellow near fragment as it dissolves. Then add fragment of cesium chloride. Orange-colored 

hexagons and stars prove antimony. Leach original residue with two or three 

drops of water and test each with a fragment of KI. The first drop may not give 

a satisfactory test for lead, owing to traces of HCl, but the second or third drop 

will yield yellow hexagons of the characteristic normal lead iodide.

Bournonite (Cu2S.2PbS.Sb2S3). Decompose mineral with 1:1 HNOs. Examine residue for lead nitrate lattice. Leach residue with drop of 1 percent HNOs. 

Transfer drop and merge with drop of K2Hg(CNS)4 solution. Greenish-yellow 

mosslike aggregates and needles prove copper. Wash original residue with three 

or more successive drops of 1 percent HNOs, followed by one of H2 0 to remove 

most of the remaining traces of copper and HNOs. Test each with a drop of 

K2Hg(CNS)i until the yellow copper precipitate does not immediately appear. 

The original residue is then reasonably free from copper. Leach original residue 

with drop of 1:5 HCl. Transfer drop and add fragment of KI. If any copper 

has persisted after the preceding efforts to remove it, an amorphous yellow precipitate of copper iodide will appear. The solution turns yellow near the fragment of KI. Add fragment of CsCl. Orange-colored hexagons prove antimony. 

Leach original residue with two or three successive drops of water, transfer each drop, and test each with a fragment of KI. The first drop may not give 

a satisfactory test for lead, owing to traces of HCl, but the second or third drop 

will yield yellow hexagons of the characteristic normal lead iodide.

Braunite (4Mn0.3Mn02 .Si02 ). Decompose mineral with several successive 

applications of aqua regia. Leach residue with drop of 1:7 HNOs, transfer drop, 

and add solid sodium bismuthate. Solution turns purple near fragments, proving 

manganese.

Bravoite ((Fe,Ni)S2 ). Decompose mineral with 1:1 HNOs. Leach residue 

with 1 percent HNOs, transfer drop, and evaporate to dryness. To residue from 

this evaporation add directly a drop of 20 percent NH40H. A gelatinous yellow 

or orange-colored precipitate proves iron. Add directly drop of dimethyl glyoxime solution to the ammonia drop (method I-A). A pink precipitate proves 

nickel.

Breithauptite (NiSb). Decompose mineral with 1:1 HNOs. Leach residue 

with 1:7 HNOs, transfer drop, and evaporate to dryness. To residue from this 

evaporation add directly a drop of 20 percent NH4OH. Add drop of dimethyl 

glyoxime solution directly to the ammonia drop. A pink precipitate proves 

nickel. Leach original residue with a drop of 1:5 HCl, transfer drop, and add 

fragment of KI. Solution turns yellow near fragment as it dissolves. Then add 

fragment of CsCl. Orange-colored hexagons and stars prove antimony.

Calaverite ((Au,Ag)Te2 ). Decompose mineral with 1:1 HNOs. One or two 

applications of acid suffice. The residue is a white curdy powder. Leach residue with 1:5 HCl. The white curdy residue goes into solution at once. Transfer 

drop and add a fragment of CsCl. Honey-yellow isometric crystals prove
MICKOCHEMICAL METHODS 271

tellurium. Gold remains in the original residue in small metallic grains and lumps. 

Usually examination in oblique reflected light, with a 16-millimeter objective, 

suffices to identify it. If the appearance of the residue is not sufficiently characteristic, add drop of aqua regia to residue and evaporate to dryness. An orangecolored residue of gold chloride remains. Dissolve this in water, transfer drop, 

and merge with it a drop of pyridine-HBr reagent. Strongly pleochroic (colorless to deep-red) prisms prove gold. Silver remains in the original residue as the 

curdy white chloride. Leach it with two or three successive drops of 1:5 HCl to 

remove remaining traces of gold and tellurium, then one of water to remove HCl. 

Then add to original residue a drop of concentrated NH4OH. Observe with an 

8-millimeter objective. In a few seconds tiny colorless isometric crystals of 

AgCl will be seen floating in the solution.

Canfieldite (4Ag2S.(Sn,Ge)S2). Decompose mineral with several applications 

of aqua regia. Leach residue with drop of 1:5 HCl, transfer drop, and add fragment of rubidium chloride. Highly refracting colorless octahedrons prove tin. 

Germanium is not tested for. Add drop of water to original residue and remove 

drop to remove last traces of HCl. Then add to original residue a drop of concentrated NH4OH. Observe drop with 8-millimeter objective. After a few 

seconds tiny colorless, highly refracting isometric crystals of AgCl will be seen 

floating in the solution.

Carrollite ((Co,Cu)3S4). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1:7 HN03 , transfer drop, and merge with it a drop of potassium 

mercuric thiocyanate solution. Copper precipitates as yellowish-green prisms 

and mosslike clumps, and cobalt precipitates separately as indigo-blue prisms.

Cassiterite (Sn02 ). Fuse mineral in sodium carbonate bead in loop of platinum 

wire. Dissolve residue in a small capsule containing a few drops of 1:5 HCl. 

Pick up a drop by means of capillary tube and transfer to slide. Add to the drop 

a fragment of rubidium chloride. Highly refracting colorless octahedrons prove 

tin.

Cerargyrite (AgCl). Place fragment of mineral in a drop of concentrated 

NH4OH. In a few seconds crystals of AgCl will separate out of the solution. 

Frequently these are skeletal and opaque. They are white in obliquely reflected 

light. The prevalent forms are right-angle crosses and stars in which three arms 

120° apart radiate from a center. A careful search and repeated drops of ammonia 

will almost always yield well-formed transparent cubes and other isometric forms.

Chalcocite (Cu2S). Decompose mineral by means of 1:1 HN03 . One application suffices. Leach residue with drop of 1:7 HNOs, transfer drop, and merge 

with it a drop of potassium mercuric thiocyanate solution. Greenish-yellow 

mosslike clumps and needlelike prisms prove copper.

Chalcophanite ((Mn,Zn)0.2Mn02 .2H2 0). Take mineral into solution with 

aqua regia. Leach residue with 1:7 HN03 . Transfer drop and merge with a 

drop of potassium mercuric thiocyanate. A precipitate of violet or purple prisms 

proves zinc. Some crosses are usually to be seen, but most of the crystals are 

arrow-shaped or short prisms with ragged ends. Take fresh material into solution 

with aqua regia, leach residue with drop of 1:7 HNOa, transfer drop, and add 

solid sodium bismuthate fragments. Solution turns purple near fragments, 

proving manganese.

Chalcopyrite (CuFeSj). Decompose mineral by several applications of 1:1 

HNO3 . Residue is orange-colored or yellow. Leach residue with drop of 1:7 

HN03) transfer drop, and merge with it a drop of potassium mercuric thiocyanate.
272 MICROSCOPIC DETERMINATION OF ORE MINERALS

Greenish-yellow mosslike clumps and needlelike prisms prove copper. The test 

drop turns reddish, proving iron. A microchemical test for chalcopyrite is rarely 

needed.

Chalcostibite (Cu2S.80283). Decompose mineral by means of 1:1 HNO3 . 

Leach residue with drop of 1:7 HNOs, transfer drop, and merge with it a drop of 

potassium mercuric thiocyanate solution. Greenish-yellow mosslike clumps and 

prisms prove copper. Leach residue with successive drops of 1 percent HNOs 

to remove copper; test each drop with potassium mercuric thiocyanate until no 

test for copper is obtained. Then leach residue with a drop of 1:5 HC1, transfer 

drop, and add fragment of KI. The solution near the fragment turns yellow as 

the KI dissolves. If copper ions are present, an amorphous yellow precipitate 

will appear. Add fragment of CsCl. Orange-colored hexagons and stars prove 

antimony.

Chloanthite-smaltite ((Ni,Co)As2). Take mineral into solution with aqua regia. 

Leach residue with drop of 1:7 HNOs, transfer drop, and slowly evaporate till 

nearly dry. Then add drop of ammonium molybdate solution and again slowly 

evaporate by passing the slide over the microflame several times. When dry, 

cool and add drop of 1:7 HNOs to redissolve the excess of ammonium molybdate. 

Examine with 8-millimeter objective. Tiny yellow isometric crystals prove 

arsenic. Transfer filtrate to another place on slide and merge with it a drop of 

potassium mercuric thiocyanate solution. Small brown spherulites prove nickel. 

If desired this test may be confirmed by drying drop and adding first a drop of 

20 percent NH4OH and then one of dimethyl glyoxime solution (method I-A). 

A pink precipitate will form if nickel is present. The reason the thiocyanate 

test is given precedence here is that iron and cobalt are usually contained isomorphously in chloanthite, and these are immediately precipitated by the thiocyanate 

solution. If the ratio of nickel to cobalt is relatively high, the blue spherulites 

may not appear. In this ev.ent it is best to make the cesium chloride test for 

cobalt as a confirmatory test. Take new material into solution with aqua regia. 

Leach residue with a drop of 1:5 HC1, transfer drop, and add fragment of CsCl. 

Iron and nickel come down first in small brown isometric crystals. Cobalt comes 

down late in the evaporation as long soluble blue prisms.

Chromite (FeO.Cr2 03). Fuse mineral in loop of platinum wire with soda-niter 

flux. Dissolve melt in 0.5 cubic centimeter of warm water in a porcelain crucible. 

Withdraw nearly all of solution by means of a medicine dropper and transfer to a 

second porcelain crucible. Acidify solution by adding 20 percent H2S04 drop 

by drop until effervescence ceases and the solution gives an acid test with litmus 

paper. Add one or two drops of a 1 percent solution of diphenyl carbazide in 

alcohol. Solution turns violet, proving chromium. The undissolved residue in 

the first crucible consists of iron carbonate. Take this into solution with about 

0.5 cubic centimeter of 1:5 HC1, warming if necessary. Add a small amount of 

powdered K4Fe(CN) 6 . A deep-blue precipitate proves iron.

Cinnabar (HgS). Decompose mineral by means of aqua regia. Two successive drops are sufficient. The evaporation should be carried out slowly. Examine 

residue for mercuric chloride lattice structure. Leach residue with drop of 1 

percent HN03 , transfer drop, and add fragment of Co(NOs)2. When dissolved 

add fragment of potassium thiocyanate. Indigo-blue prisms and branching 

forms of cobalt mercuric thiocyanate prove mercury.

Clausthalite (PbSe). Decompose mineral by means of 1:1 HNOs. Examine 

residue for lead nitrate lattice structure and a red tinge, which indicates selenium. 

Leach residue with drop of 1:5 HC1, transfer drop, and merge with it a drop of
MICROCHEMICAL METHODS 273

stannous chloride solution. A brick-red amorphous precipitate proves selenium. 

Leach original residue with two or three successive drops of water, transfer each 

drop, and test each with a fragment of KI. The first drop may not give a satisfactory test for lead, owing to traces of HC1, but the second or third drop will 

yield yellow hexagons of the characteristic normal lead iodide.

Cobaltite (CoAsS). Decompose mineral by means of 1:1 HNOs. Leach residue 

with drop of 1:7 HN03 , transfer drop, and slowly evaporate by placing slide in 

and out of microflame several times. When nearly but not entirely dry add 

drop of ammonium molybdate solution and again slowly evaporate over flame. 

When dry cool and add drop of 1:7 HNOs to redissolve excess of ammonium 

molybdate. Examine microscopically with 8-millimeter objective. Tiny yellow 

isometric crystals prove arsenic. Transfer filtrate to another place on slide and 

merge with it a drop of potassium mercuric thiocyanate solution (method I). 

Indigo-blue prisms prove cobalt. If the solution turns pinkish, iron is also present.

Coloradoile (HgTe). Decompose mineral by means of aqua regia. Leach residue with drop of 1 percent HN03 , transfer drop, and add to it a fragment of 

cobalt nitrate. When dissolved, add a fragment of potassium thiocyanate 

(KCNS) to the solution. Indigo-blue branching forms and prisms prove mercury. 

Leach original residue with drop of 1:5 HC1. Transfer drop and add fragment 

of CsCl. Honey-yellow isometric crystals prove tellurium.

Copper (Cu). Dissolve mineral in 1:1 HNQs. One application is sufficient. 

Leach residue with drop of 1:7 HNOs, transfer drop, and merge with it a drop of 

potassium mercuric thiocyanate solution. Greenish-yellow mosslike clumps and 

prisms prove copper.

Corynite (Ni(As,Sb)S). Decompose mineral with 1:1 HNOs. Leach residue 

with 1:7 HNOs, transfer drop, and evaporate slowly. When nearly dry add drop 

of ammonium molybdate solution and again evaporate slowly till completely dry. 

Add drop of 1:7 HNOs to residue in order to redissolve excess of ammonium 

molybdate. Tiny yellow isometric crystals prove arsenic. Transfer filtrate and 

merge with it a drop of potassium mercuric thiocyanate. Small brown spherulites 

prove nickel. Leach original residue with two successive drops of 1:7 HNOs and 

one of H2 0 in order to remove remaining traces of arsenic. Then leach residue 

with drop of 1:5 HC1, transfer drop, and add fragment of CsCl. Colorless hexagons prove antimony.

Cosalite (2PbS.Bi2Ss). Decompose mineral with 1:1 HNOs. Examine residue 

for lead nitrate lattice. Leach residue with 1:5 HCl. Transfer drop and add 

fragment of KI. Solution turns yellow near fragment as it dissolves. Then add 

fragment of CsCl. The yellow color disappears near fragment, and red hexagons 

form, proving bismuth. Leach original residue with successive drops of 1:5 HCl 

and test each with KI and CsCl fragments until no test for bismuth appears. 

Then add three successive drops of water to original residue. Transfer each drop 

to another place on slide and test with fragment of KI. The first drop may not 

give a satisfactory test for lead, owing to the presence of remaining traces of HCl 

and bismuth, but the second or third drop will yield yellow hexagonal plates, 

which prove lead.

Covellite (CuS). Decompose mineral with 1:1 HNOs. Leach residue with 

drop of 1:7 HNOs, transfer drop, and merge with it a drop of potassium mercuric 

thiocyanate solution. Greenish-yellow mosslike clumps and prisms prove copper. 

A microchemical test for covellite is rarely needed.
274 MICKOSCOPIC DETERMINATION OF ORE MINERALS

Cubanite (Cu2S.Fe4S6). Decompose mineral by several applications of 1:1 

HN03 . Residue is orange-colored or yellow. Leach residue with drop of 1:7 

HN03 , transfer drop, and merge with it a drop of potassium mercuric thiocyanate. 

Greenish-yellow mosslike clumps and prisms prove copper. A reddish color in the 

test drop proves iron.

Cuprite (Cu2 0). Dissolve mineral with 1:1 HN03 . Leach residue with drop 

of 1:7 HNO3 , transfer drop, and merge with it a drop of potassium mercuric thiocyanate solution. Greenish-yellow mosslike clumps and prisms prove copper.

Cylindrite (6PbS.Sb2S3 .6SnS2). Decompose mineral with 1:1 HN03 . Leach 

residue with 1:5 HC1, transfer drop, and add fragment of KI. Solution turns yellow near fragment as it dissolves. Add fragment of CsCl to solution. The yellow 

color of the solution disappears, and orange-colored hexagons and colorless octahedrons form, proving antimony and tin. Leach original residue with two or three 

successive drops of water, transfer each drop, and test with a fragment of KI. The 

first drop may not give a satisfactory test for lead, owing to traces of HC1, but the 

second or third drop will yield yellow hexagons of the characteristic normal lead 

iodide.

Delafossite (Cu20.Fe2 03). Dissolve mineral in 1:1 HN03 . Leach residue with 

drop of 1:7 HN03 , transfer drop, and merge with it a drop of potassium mercuric 

thiocyanate solution. Greenish-yellow mosslike clumps and prisms prove copper. 

Test solution is tinged red, proving iron.

Domeykite (Cu3As). Take mineral into solution with aqua regia. Add drop 

of ammonium molybdate solution to residue 'and slowly evaporate to dryness. 

Cool and add drop of 1:7 HNOs to residue to redissolve excess of ammonium 

molybdate. Tiny yellow isometric crystals prove arsenic. Transfer filtrate and 

merge with it a drop of potassium mercuric thiocyanate solution. Greenishyellow mosslike clumps and prisms prove copper.

Dufrenoysite (2PbS.As2S3). Decompose mineral with 1:1 HN03 . Examine 

residue for lead nitrate lattice. Leach residue with drop of 1:5 H.Cl, transfer 

drop, and evaporate almost to dryness. Leach residue from this evaporation 

with drop of ammonium molybdate solution. Evaporate slowly to dryness. 

Cool and add drop of 1:7 HNOs. Tiny yellow isometric crystals prove arsenic. 

Leach original residue with three successive drops of H2O, transfer drops, and add 

to each a fragment of KI. Yellow glistening hexagonal plates prove lead.

Dyscrasite (Ag3Sb). Decompose mineral with 1:1 HN03 . Leach residue with 

drop of 1 percent HN03 , transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Leach original residue with 1:5 HC1, transfer 

drop, and add fragment of KI. Solution is colored yellow near fragment as it dissolves. Add fragment of CsCl. Orange-colored hexagons and stars prove 

antimony.

Electrum (alloy of Au and Ag). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HN03 , transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Examine original residue in 

obliquely reflected light. Usually the gold color of the particles of the residue is 

sufficient to identify the element. If not, dissolve residue in aqua regia and 

evaporate to dryneso. Leach residue with drop of water, transfer drop, and 

merge with it a drop of pyridine-HBr solution. Strongly pleochroic red prismis 

prove gold.
MICKOCHEMICAL METHODS 275

Emplectite (Cu2S.Bi2S3). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1 percent HN03 , transfer drop, and merge with it a drop of potassium 

mercuric thiocyanate solution. Greenish-yellow mosslike clumps and prisms 

prove copper. Leach original residue with two additional drops of 1 percent 

HN03 and one of water in order to remove as much of the remaining traces of 

copper as possible. Reject the three drops. Leach original residue with drop 

of 1:5 HC1, transfer drop, and add fragment of KI. If any copper was carried 

over in the drop, it will precipitate as a light-yellow or gray amorphous powder. 

The solution turns yellow near the fragment as it dissolves. Then add fragment 

of CsCl. Red hexagons prove bismuth.

Enargite (Cu2S.4CuS.As2S3). Decompose mineral with aqua regia. Leach residue with 1:7 HN03 , transfer drop, and evaporate slowly over microflame. When 

nearly dry add drop of ammonium molybdate solution and again slowly evaporate. 

When dry, cool and add drop of 1:7 HN03 to redissolve excess of ammonium 

molybdate. Tiny yellow isometric crystals prove arsenic. Transfer filtrate and 

merge with it a drop of potassium mercuric thiocyanate solution. Greenishyellow mosslike clumps and prisms prove copper.

Eucairite (Cu2Se.Ag2Se). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1 percent HN03 , transfer drop, and merge with it a drop of potassium 

mercuric thiocyanate solution. Greenish-yellow mosslike clumps and prisms 

prove copper, and an amorphous milky-white powder proves silver. Leach 

original residue with drop of 1:5 HCl, transfer drop, and merge with it a drop of 

stannous chloride solution. A brick-red amorphous precipitate proves selenium. 

Some elemental selenium will probably remain behind in the original residue as a 

red powder and can be best observed in oblique reflected light, using a 16-millimeter objective.

Famatiniie (Cu2S.4CuS.Sb2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1:7 HN03 , transfer drop, and evaporate slowly over microflame. When nearly dry, add directly a drop of ammonium molybdate solution. 

.Again evaporate over microflame. When dry, cool and add drop of 1:7 HN03 

to redissolve excess of ammonium molybdate. If arsenic is present in the mineral 

tiny yellow isometric crystals will be seen near the periphery of the drop. Most 

specimens of famatinite contain more or less arsenic, and some idea of the proportion can be gained from the abundance of the ammonium-arsenic molybdate 

precipitate. Transfer filtrate and test it for copper with potassium mercuric 

thiocyanite. Leach original residue with two drops of 1 percent HN03 and one 

of water to remove final traces of copper. Then leach original residue with drop 

of 1:5 HCl, transfer drop, and add fragment of KI. If any copper is present in 

the drop it will be precipitated as a light-yellow amorphous powder. The solution 

stains yellow near the KI fragment as it dissolves. Add fragment of CsCl. The 

yellow color of the solution disappears, and orange-colored hexagons and stars 

form, proving antimony. Examine edge of drop in obliquely reflected light for 

amorphous orange-colored cesium-arsenic iodide precipitate.

Ferberite (FeW04 ). Fuse mineral in a platinum wire with soda-niter flux. 

Dissolve fused mass in about 0.5 cubic centimeter of water in a porcelain crucible, 

warming on a hot plate to hasten solution. Withdraw filtrate from undissolved 

material by means of a medicine dropper and transfer to another porcelain 

crucible. Add about 0.2 cubic centimeter of 1:5 HCl. This is usually sufficient 

to acidify the solution. Add some fragments of metallic zinc. Hydrogen should 

be evolved vigorously if acid is in excess. If not, add more 1:5 HCl drop by drop 

until abundant bubbles are evolved from the zinc fragments. After 2 or 3 minutes 

the solution turns slate gray to grayish blue, the color depending on the concentra-
276 MICROSCOPIC DETERMINATION OF ORE MINERALS

tion in tungsten. Add about 0.6 centimeter of 1:7 HN03 to the precipitate in 

the other crucible. Ignore undissolved material. Divide the solution into two 

parts by withdrawing part by means of a medicine dropper and transferring to a 

third crucible. To one part add a few drops of KCNS. A red coloration proves 

iron. To the other solution add some fragments of sodium bismuthate, allowing 

excess reagent to settle. A violet solution proves manganese. Ferberite is 

distinguished from wolframite only by the absence of a manganese test.

Franckeite (5PbS.Sb2S3.2SnS2). Decompose mineral with 1:1 HNO3 . Leach 

residue with 1:5 HCl, transfer drop, and add fragment of KI. Solution turns 

yellow near fragment as it dissolves. Add fragment of CsCl to solution. The 

yellow color of the solution disappears, and orange-colored hexagons and colorless 

octahedrons form, proving antimony and tin. Leach original residue with two 

or three successive drops of water, transfer each drop, and test each with a fragment of KI. The first drop may not give a satisfactory test for lead, owing to 

traces of HCl, but the second or third drop will yield yeJlow hexagons of the 

characteristic normal lead iodide.

Franklinite (ZnO.Fe2O3 ). Take mineral into solution with repeated applications of aqua regia. Leach residue with a drop of 1:7 HN03 , transfer drop, and 

merge with it a drop of potassium mercuric thiocyanate solution. Solution turns 

red, and black prisms of indefinite form are precipitated. The composition of 

these prisms is unknown, but both manganese and zinc may possibly be present. 

The test solution turns deep red, proving iron. Take additional fresh material 

into solution by means of aqua regia, leach residue with a drop of 1:7 HNO3 , 

transfer drop, and test for manganese with sodium bismuthate fragments. A 

liberal amount of franklinite powder should be used in making the manganese 

test. Franklinite is extremely resistant to acid attack, but with persistence 

enough material can be taken into solution to give satisfactory tests.

Freibergite (5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3). Decompose mineral with 1:1 

HNO3 . Leach residue with 1 percent HNO3 , transfer drop, and merge with it a 

drop of potassium mercuric thiocyanate solution. A white curdy precipitate 

proves silver, and greenish-yellow mosslike clumps and prisms prove copper. 

Leach original residue with successive drops of 1 percent HNO3 , transfer each, and 

test with a drop of K2Hg(CNS)4 until no copper precipitate appears. Then 

leach original residue with drop of 1:5 HCl, transfer drop, and add fragment of 

KI. Solution turns yellow near drop as it dissolves. If copper is present in drop 

it is precipitated here as an amorphous light-yellow or gray powder. Add fragment of CsCl. Orange-colored hexagons and stars prove antimony.

Freieslebenite (2Ag2S.3PbS.2Sb2S3). Decompose mineral with 1:1 HN03 . 

Leach residue with drop of 1 percent HNO3 . Transfer drop and add fragment of 

ammonium bichromate. Red crystals, usually needle-shaped, prove silver, and 

am amorphous yellow precipitate proves lead. Leach original residue with a 

drop of 1:5 HCl. Transfer drop and add fragment of KI. Solution turns yellow 

near fragment. Then add fragment of CsCl to opposite side of drop. Orangecolored hexagonal plates prove antimony. If necessary, lead can be confirmed 

by adding a drop of H2 O to the original residue, transferring drop, and adding a 

fragment of KI. Yellow glistening hexagonal plates prove lead. Silver can now 

be confirmed by placing a drop of concentrated NH4OH on original residue. Tiny 

colorless isometric crystals appear near edge of drop.

Galena (PbS). Decompose mineral with 1:1 HNO3 . Examine residue for lead 

nitrate lattice structure. Add drop of 1:5 HCl to residue. Remove and reject 

HCl drop. Leach original residue with two or three drops of water, transfer each
MICROCHEMICAL METHODS 277

drop, and test with a fragment of KI. The first drop may not give a satisfactory 

test for lead, owing to traces of HC1, but the second or third drop will yield yellow 

hexagons of the characteristic normal lead iodide.

Galenobismutite (PbS.Bi2S3). Decompose mineral with 1:1 HN03 . Examine 

residue for lead nitrate lattice. Leach residue with 1:5 HC1. Transfer drop and 

add fragment of KI. Solution turns yellow near fragment as it dissolves. Then 

add fragment of CsCl. The yellow color disappears near fragment, and red 

hexagons form, proving bismuth. Leach original residue with two successive 

drops of 1:5 HC1, rejecting both. This will remove most of the remaining trace 

of bismuth. Then add three successive drops of water to original residue. Transfer 

each drop to another place on slide and add fragment of KI. The first drop may 

not give a satisfactory test, owing to presence of remaining traces of bismuth. 

The second or third drop will give good yellow hexagons, which prove lead.

Geocronite (5PbS.Sb2S3). Decompose mineral with 1:1 HNOs. Examine 

residue for lattice structure, indicating lead. Leach residue with drop of 1:5 HC1. 

Transfer drop and add fragment of KI. Solution turns yellow near fragment as 

it dissolves. Then add fragment of cesium chloride. Orange-colored hexagons 

and stars prove antimony. Leach original residue with two or three drops of 

water and test each with a fragment of KI. The first drop may not give a satisfactory test for lead, owing to traces of HC1, but the second or third drop will yield 

yellow hexagons of the characteristic normal lead iodide.

Germanite (Cu3(Fe,Ge)S4). Mineral very resistant to acid attack, but by 

repeated efforts with aqua regia enough can be broken down to give a copper test. 

Leach residue with 1:7 HN03 , transfer drop, and test for copper by merging with 

drop of potassium mercuric thiocyanate solution. Test for germanium by procedure described on page 240.

Gersdorffite (NiAsS). Decompose mineral with aqua regia. Leach residue 

with 1:7 HN03 , transfer drop, and slowly evaporate over microflame. When 

nearly dry, add a drop of ammonium molybdate and again slowly evaporate over 

niicroflame to dryness. Cool and add drop of 1:7 HN03 to redissolve excess of 

ammonium molybdate. Tiny yellow isometric crystals prove arsenic. Transfer filtrate and merge with drop of potassium mercuric thiocyanate (method I). 

Small brown spherulites prove nickel. Cobalt and iron, if present, will be indicated at the same time. If desired, the dimethyl glyoxime test for nickel can be 

made by adding directly to the dried residue from the thiocyanate test a drop of 

20 percent NH4OH followed by a drop of dimethyl glyoxime solution.

Glaucodot ((Co,Fe)AsS). Decompose mineral with aqua regia. Leach residue 

with 1:7 HN03 , transfer drop, and evaporate slowly over microflame. When 

nearly dry, add drop of ammonium molybdate solution and again evaporate 

slowly to dryness over microflame. Cool and add drop of 1:7 HN03 to redissolve 

excess of ammonium molybdate. Tiny yellow isometric crystals prove arsenic. 

Transfer filtrate and merge with drop of potassium mercuric thiocyanate solution. 

Test drop turns pink, proving iron, and dark indigo-colored prisms are precipitated, proving cobalt. Sometimes the prisms are so dark that the blue color is 

not apparent. In this event cobalt should be confirmed by the cesium chloride 

test. Take new material into solution with aqua regia. Leach residue with drop 

of 1:5 HC1, transfer drop, and add fragment of CsCl. Iron and nickel come down 

first in small brown isometric grains. Cobalt comes down late in the evaporation 

as long soluble blue prisms.

Goethite (Fe2 03.H2 0). Take mineral into solution in aqua regia. Repeated 

applications are necessary, as the mineral is resistant to chemical attack. Leach
278 MICROSCOPIC DETERMINATION OF ORE MINERALS

residue with a drop of 1:5 HC1 and add K4Fe(CN) 6 fragments. A blue amorphous 

powder proves iron.

Gold (Au). The color and sectility of gold are usually sufficient to identify it 

in polished section. If desired, however, a microchemical test can be made by 

dissolving mineral in aqua regia. The residue is dissolved in water, and the 

drop transferred and merged with a drop of pyridine-HBr solution. Strongly 

pleochroic (colorless to deep red) prisms prove gold.

Guanajuatite (Bi2 (Se, S) 3). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1:5 HC1, transfer drop, and add fragment of CsCl. Thin iridescent 

colorless rhombs and knifeblade-shaped plates prove bismuth. Examine residue 

in obliquely reflected light. An agglutinated deep-red mass proves selenium. If 

necessary a microchemical test for selenium can be made by decomposing more 

mineral with 1:1 HN03 . The residue is leached with a drop of 1:5 HC1, and the 

drop is transferred and merged with a drop of stannous chloride solution (method I). 

A brick-red powder proves selenium.

Guitermanite (3PbS.As2S3). Decompose mineral with 1:1 HN03 . Examine 

residue for lead nitrate lattice. Leach residue with 1:5 HC1, transfer drop, and 

evaporate slowly over microflame. When nearly dry add drop of ammonium 

molybdate solution and again evaporate slowly over microfiame. When dry, 

cool and add drop of 1:7 HN03 to redissolve excess of ammonium molybdate. 

Tiny yellow isometric crystals prove arsenic. Leach original residue with two 

or three successive drops of water and test each with a fragment of KI. The first 

drop may not give a satisfactory test for lead, owing to traces of HC1 and arsenic 

acid, but the second or third drop will yield yellow hexagons of the characteristic 

normal lead iodide.

Hauerite (MnS2). Take mineral into solution in aqua regia. Take up residue 

in drop of 1:7 HN03 and add fragments of sodium bismuthate. Solution turns 

pink or purple near fragments, proving manganese.

Hausmannite (MnO.Mn2 03). Take mineral into solution in aqua regia. Leach 

residue with a drop of 1:7 HN03 , transfer drop, and add fragments of sodium 

bismuthate. Solution turns pink or purple near fragments, proving manganese.

Hematite (Fe2 03 ). This mineral is very resistant to chemical attack, but by 

repeated applications of aqua regia enough material can be taken into solution to 

give an iron test. Leach residue with a drop of 1:5 HCl, transfer drop, and add 

fragment of K4 Fe(CN) 6 . A blue amorphous precipitate proves iron.

Hessite (Ag2Te). Decompose mineral with 1:1 HN03 . Leach residue with 

drop of 1 percent HNO3 , transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Leach original residue with drop of 1:5 

HCl. Any silver not already removed will be precipitated as a curdy white mass. 

Transfer filtrate and add fragment of CsCl. Honey-yellow isometric crystals 

prove tellurium.

Huebnerite (MnW04 ). Fuse mineral in a platinum wire with soda-niter flux. 

Bead turns green when cold, proving manganese. Dissolve fused mass in about 

0.5 cubic centimeter of water in a porcelain crucible, warming on a hot plate to 

hasten solution. Withdraw filtrate from undissolved material by means of a 

medicine dropper and transfer to another porcelain crucible. Add about 0.2 

cubic centimeter of 1:5 HCl. This is usually sufficient to acidify the solution. 

Add some fragments of metallic zinc. Hydrogen should be evolved vigorously 

if acid is in excess. If not, add more 1:5 HCl drop by drop until abundant bubbles
MICROCHEMICAL METHODS 279

are evolved from the zinc fragments. After 2 or 3 minutes the solution turns 

slate gray to grayish blue, the color depending on the concentration in tungsten.

llmenite (FeO.Ti02). Fuse mineral in soda-niter bead in loop of platinum wire. 

Leach residue in 0.5 cubic centimeter of warm water. Withdraw water by means 

of medicine dropper and reject it. Brown insoluble residue contains both Ti and 

Fe. Dissolve residue in 0.3 cubic centimeter of 1:1 HNO3 , warming if necessary. 

Cool, add an equal quantity of water. By means of a medicine dropper withdraw part of the solution and transfer to another porcelain crucible. Add fragments of K4Fe (CN) 8 . Solution turns deep blue, proving Fe. To other crucible 

add two to four drops of H2 02 . Solution turns yellow, proving Ti. Confirm by 

adding fragments of NaF. Yellow color disappears.

Jamesonite (4PbS.FeS.3Sb2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HNOs, transfer drop, and merge with drop of potassium mercuric thiocyanate solution. Test solution is tinged pink, proving 

presence of iron. Leach original residue with drop of 1:5 HC1. Transfer filtrate 

and add fragment of KI. Solution is colored light yellow near fragment as it 

dissolves. Add fragment of CsCl. Orange-colored hexagons and stars prove 

antimony. Leach original residue with two or three drops of water, transfer 

each, and test with a fragment of KI. The first drop may not give a satisfactory 

test for lead, owing to traces of HC1, but the second or third drop will yield yellow 

hexagons of the characteristic normal lead iodide.

Jordanite (4PbS.As2S3). Decompose mineral with 1:1 HN03 . Examine 

residue for lead nitrate lattice. Leach residue with 1:5 HC1, transfer drop, and 

evaporate slowly over microflame. When nearly dry, add drop of ammonium 

molybdate solution and again evaporate slowly over microflame. When dry, 

cool and add drop of 1:7 HNOs to redissolve excess of ammonium molybdate. 

Tiny yellow isometric crystals prove arsenic. Leach original residue with two or 

three drops of water, transfer each drop, and test with a fragment of KI. The 

first drop may not give a satisfactory test for lead, owing to traces of HC1 and 

arsenic acid, but the second or third drop will yield yellow hexagons of the characteristic normal lead iodide.

Kallilile (Ni(Sb,Bi)S). Decompose mineral with 1:1 HN03 . Leach residue 

with 1 percent HNOs, transfer drop, evaporate to dryness. Add to residue from 

this evaporation first a drop of 20 percent NH4OH followed by a drop of dimethyl 

glyoxime solution (method I-A, p. 182). A pink precipitate proves nickel. Leach 

original residue with drop of 1:5 HC1, transfer drop, and add fragment of KI. 

Solution turns yellow. Add fragment of CsCl. Red hexagons prove bismuth. 

Leach original residue with another drop of 1:5 HC1 and repeat cesium double 

iodide test. Both red and orange-colored hexagons will form, proving that 

antimony is also present.

Keeleyite (PbS.Sb2S3 ). Decompose mineral with 1:1 HN03 . Examine residue 

for lattice structure, indicating lead. Leach residue with drop of 1:5 HC1. 

Transfer drop and add fragment of KI. Solution turns yellow near fragment as 

it dissolves. Then add fragment of cesium chloride. Orange-colored hexagons 

and stars prove antimony. Leach original residue with two or three drops of 

water, transfer each drop, and test with a fragment of KI. The first drop may not 

give a satisfactory test for lead, owing to traces of HC1, but the second or third 

drop will yield yellow hexagons of the characteristic normal lead iodide.

Kermesite (2Sb2S3 .Sb2 03). The KOH etch test is very characteristic. The 

presence of antimony can be confirmed microchemically as follows: Decompose 

mineral with aqua regia. Leach residue with drop of 1:5 HC1, transfer drop, and
280 MICROSCOPIC DETERMINATION OF ORE MINERALS

add fragment of KI. Solution turns yellow near fragment as it dissolves. Add 

fragment of CsCl to solution. Orange-colored stars and hexagons, the stars 

predominating, prove antimony.

Klaprothite (3Cu2S.2Bi2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HNOs, transfer drop, and merge with it a drop of 

potassium mercuric thiocyanate solution. Greenish-yellow mosslike clumps and 

prisms prove copper. Leach original residue with two additional drops of 1 

percent HNOs and one of water in order to remove as much of the remaining 

traces of copper as possible. Reject the three drops. Leach original residue with 

drop of 1:5 HCl, transfer drop, and add fragment of KI. If any copper was 

carried over in the drop, it will be precipitated as a light-yellow or gray amorphous 

powder. The solution turns yellow near the fragment as it dissolves. Then add 

fragment of CsCl. Red hexagons prove bismuth.

Klockmannite (CuSe). Take mineral into solution in 1 : 1 HNOs. Leach 

residue with a drop of 1 percent HN03 , transfer drop, and test for copper by merging with a drop of potassium mercuric thiocyanate reagent. Greenish-yellow 

mosslike clumps and prisms prove copper. Leach original residue with drop of 

1 : 5 HCl, transfer drop, and merge with drop of stannous chloride reagent. An 

amorphous brick-red precipitate proves selenium.

Krennerite ((Au,Ag)Te2). Decompose mineral with 1 : 1 HNO3 . One or two 

applications of acid suffice. The residue is a white curdy powder. Leach residue 

with 1:5 HCl. The white curdy residue goes into solution at once. Transfer 

drop and add a fragment of CsCl. Honey-yellow isometric crystals prove tellurium. Gold remains in the original residue in small metallic grains and lumps. 

Usually examination in oblique reflected light with a 16-millimeter objective suffices 

to identify the gold. If the appearance of the residue is not sufficiently characteristic, add drop of aqua regia to residue and evaporate to dryness. An orangecolored residue of gold chloride remains. Dissolve this in water, transfer drop, and 

merge with it a drop of pyridine-HBr reagent. Strongly pleochroic (colorless to 

deep red) prisms prove gold. Silver remains in the original residue as a curdy 

white chloride. Leach it with two or three successive drops of 1:5 HCl to remove 

remaining traces of gold and tellurium, then one of water to remove HCl. Then 

add to original residue a drop of concentrated NH4OH. Observe with an 8-millimeter objective. In a few seconds tiny colorless isometric crystals of AgCl will 

be seen floating in the ammonia.

Lengenbachite (6PbS.(Ag,Cu) 2S.2AsS3). Decompose mineral with 1:1 HNOs. 

Leach residue with drop of 1 percent HN03 , transfer drop, and merge with drop 

of potassium mercuric thiocyanate reagent. An amorphous finely divided milkwhite precipitate proves silver, and greenish-yellow mosslike clumps and prisms 

prove copper. Leach original residue with drop of 1:5 HCl, take up drop, and 

reject. Leach original residue with three successive drops of H2 0, transfer each 

drop, and test with a tiny fragment of KI. Yellow glistening hexagonal plates 

prove lead. Some arsenic and copper may get into the first drop and hinder the 

formation of characteristic PbI2 plates, but the second or third drop should give 

a good test for lead. To test for arsenic take additional fresh mineral into solution in 1:1 HN03 . Leach residue with drop of 1:7 HN03 , transfer drop, and 

slowly evaporate. When nearly dry, add drop of ammonium molybdate reagent 

and slowly evaporate to dryness. Cool and add a drop of 1:7 HNOs to residue. 

Tiny yellow isometric crystals prove arsenic.

Leucopyrite (Fe3As4). Decompose mineral with aqua regia. The residue 

usually has a tinge of yellow or orange. Leach residue with drop of 1:7 HNOj,
MICKOCHEMICAL METHODS 281

transfer drop, and evaporate slowly by passing in and out of microflame. When 

nearly dry add directly a drop of ammonium molybdate solution and again heat 

slowly over microflame. When dry, cool and add drop of 1:7 HN03 to redissolve 

excess ammonium molybdate. Examine under microscope. Tiny yellow isometric crystals prove arsenic. Take up filtrate and transfer to another place on 

slide. Merge with it a drop of potassium mercuric thiocyanate solution. A red 

coloration of test solution proves iron.

Limonite (Fe203.nH2 0). Take mineral into solution in aqua regia. Repeated 

applications of acid are necessary, as limonite is resistant to chemical attack. Add 

drop of 1:5 HCl to residue and add fragment of K4Fe(CN)6 to drop. An amorphous blue precipitate proves iron.

Linnaeile (Co3 S4 ). Decompose mineral with 1:1 HNOs. Leach residue with 

1:7 HN03 , transfer drop, and merge with it a drop of potassium mercuric thiocyanate solution. Indigo-blue prisms prove cobalt. Brownish nickel spherulites 

will usually come down in addition to the cobalt precipitate.

Livingstonile (HgS.2Sb2S3). Decompose mineral with a drop of aqua regia. 

Examine residue for mercuric chloride lattice structure. Leach residue with drop 

of 1 percent HN03 , transfer drop, and add fragment of cobalt nitrate. When 

dissolved add fragment of KCNS. Indigo-blue arborescent forms and prisms 

prove mercury. Leach original residue with drop of 1:5 HCl. Transfer drop and 

add fragment of KI. Solution turns yellow near fragment as it dissolves. Add 

fragment of CsCl. Orange-colored hexagons and stars prove antimony.

Loellingile (FeAs2). Dissolve mineral in aqua regia. The residue usually lias 

a tinge of yellow or orange. Leach residue with drop of 1:7 HN03 , transfer drop, 

and evaporate slowly by passing in and out of microflame. When nearly dry, add 

directly a drop of ammonium molybdate solution and again heat slowly over microflame. When dry, cool and add drop of 1:7 HN03 to redissolve excess ammonium 

molybdate. Examine under microscope. Tiny yellow isometric crystals prove 

arsenic. Take up filtrate and transfer to another place on slide. Merge with it 

a drop of potassium mercuric thiocyanate solution. A red coloration of test 

solution proves iron.

Lorandite (T^S.AsaSs). A fragment of mineral tested in a Bunsen flame gives 

a vivid green (thallium) coloration to the flame. Fragment was held in a very 

small loop on a platinum wire that also held a small drop of HCl. Under the 

same conditions copper gives a blue flame. To test for arsenic decompose mineral 

with aqua regia. Leach residue with drop of 1:7 HN03 , transfer drop, and slowly 

evaporate over microflame. When nearly dry, add drop of ammonium molybdate solution and again slowly evaporate, this time to dryness. Cool and add 

drop of 1:7 HN03 to redissolve excess of ammonium molybdate. Tiny yellow 

isometric crystals prove arsenic.

Magnetite (FeO.Fe203). This mineral is very resistant to chemical attack, but 

by repeated applications of aqua regia enough material can be taken into solution to give an iron test. Leach residue with a drop of 1:5 HCl, transfer drop, 

and add fragment of K4 Fe(CN)fl . A blue amorphous precipitate proves iron.

Manganite (MnO.Mn2 03). Take mineral into solution in aqua regia. Take 

up residue in drop of 1:7 HN03 , transfer drop, and add fragments of sodium 

bismuthate. Solution turns pink or purple near fragments, proving manganese.

Marcasile (FeS2). Decompose mineral with 1:1 HN03 . Residue is colored 

yellow or orange. Leach residue with drop of 1:5 HCl, transfer drop, and add 

fragment of K4Fe(CN) 6 . A blue amorphous precipitate proves iron.

129101 40 19
282 MICROSCOPIC DETERMINATION OF ORE MINERALS

Matildite ((Ag2 ,Pb)S.Bi2S3). Decompose mineral with 1:1 HN03 . Leach residue with drop of 1 percent HN03 , transfer drop, and add fragment of ammonium 

bichromate. A yellow amorphous precipitate proves lead, and red crystals 

prove silver. Leach original residue with drop of 1:5 HC1. Transfer drop and 

add fragment of KI. Solution stains yellow near fragment as it dissolves. Then . 

add fragment of CsCl. Red hexagons prove bismuth. Leach original residue 

with two successive drops of 1 : 5 HC1 and reject both. This will remove nearly all 

the remainder of the bismuth. Then leach original residue with three successive 

drops of water. Transfer each drop to another place on slide and test with 

fragment of KI. The first drop may not give a satisfactory test for lead, owing 

to the presence of remaining traces of lead and bismuth, but the second or third 

drop will yield hexagonal plates, which prove lead. The silver can now be confirmed by the chloride test. Apply a drop of concentrated NEUOH to the original 

residue and observe edge of drop. Tiny colorless isometric crystals prove silver.

Maucherite (Ni3As2). Take mineral into solution with aqua regia. Leach 

residue with drop of 1:7 HN03 . Transfer drop and evaporate slowly over 

microflame. When nearly dry, add drop of ammonium molybdate and again 

evaporate slowly over microflame, this time to dryness. Cool and add drop of 

1:7 HN03 to redissolve excess of ammonium molybdate. Tiny yellow isometric 

crystals prove arsenic. Transfer filtrate and evaporate to dryness. Cool and 

add directly to the residue from this evaporation a drop of 20 percent NH4OH, 

followed by a drop of dimethyl glyoxime solution (p. 192). A pink precipitate 

proves nickel.

Melaconite (CuO). Dissolve mineral in 1:1 HNOs. Take up residue with 

drop of 1 : 7 HN03 , transfer drop, and merge with drop of potassium mercuric 

thiocyanate reagent. Greenish-yellow mosslike clumps and prisms prove copper.

Melonite (NiTe2). Dissolve mineral in 1:1 HN03 . Take up residue in drop 

of 1:5 HC1. By means of capillary tube divide this drop into two equal parts. 

Test one part by adding fragment of CsCl. Honey-yellow isometric crystals 

prove tellurium. Evaporate other part to dryness and add a drop of 20 percent 

NEUOH, followed by a drop of dimethyl glyoxime reagent. A pink precipitate 

proves nickel.

Meneghinite ^PbS.SbaSs). Decompose mineral with 1:1 HNOs. Examine 

residue for lattice structure, indicating lead. Leach residue with drop of 1 percent 

HN03 , transfer and merge with drop of potassium mercuric thiocyanate reagent. 

Meneghinite differs from other lead sulphantimonites in that it contains enough 

copper to give a positive test. Leach original residue with drop of 1:5 HC1. 

Transfer drop and add fragment of KI. Solution turns yellow near fragment as it 

dissolves. Then add fragment of cesium chloride. Orange-colored hexagons and 

stars prove antimony. Leach original residue with two or three drops of water, 

transfer each drop, and test with a fragment of KI. The first drop may not give 

a satisfactory test for lead, owing to traces of HC1, but the second or third drop 

will yield yellow hexagons of the characteristic normal lead iodide.

Metacinnabar (HgS). Decompose mineral by means of aqua regia. Examine 

residue for mercuric chloride lattice structure. Leach residue with drop of 

1 percent HN03 and transfer drop. Add fragment of Co(N03) 2 . When dissolved add fragment of potassium thiocyanate. Indigo-blue prisms and branching forms of cobalt mercuric thiocyanate prove mercury.

Miargyrite (Ag2S.Sb2S3). Decompose mineral with 1:1 HN03 . Leach residue 

with 1 percent HN03 , transfer drop, and add fragment of ammonium bichromate. 

Red crystals prove silver. Leach original residue with a drop of 1 : 5 HC1, transfer
MICRO CHEMICAL METHODS 283

drop, and add fragment of KI. Solution turns yellow near the fragment as it 

dissolves. Then add a fragment of CsCl. Orange-colored hexagons and stars 

prove antimony.

Millerite (NiS). Decompose mineral with 1:1 HN03 . Leach residue with 

drop of 1:7 HN03 , transfer drop, and evaporate to dryness. To the residue from 

this evaporation add a drop of 20 percent NH4OH followed by a drop of dimethy 

glyoxime solution (p. 192). A pink precipitate proves nickel.

Molybdenite (MoS2). Fuse mineral in a platinum wire with soda-niter flux. 

Cool and dissolve fused mass in about 0.5 cubic centimeter of 1:5 HCI in a small 

porcelain crucible. Add 0.2 cubic centimeter of 10 percent KCNS. Solution 

may remain colorless or may change to a light yellow. Add five to ten fragments 

of 20-mesh zinc dust. Solution is colored orange to red. Add a few drops of 

H2 02 . The solution becomes colorless.

Mottramite (cuprodescloizite) (Pb2 (Cu, Zn) 2V2 09 .H20). Test for Cu and Pb 

on a glass slide. Dissolve mineral in 1:1 HN03 . Residue is gelatinous and reddish brown, resembling iron residue. Leach residue with drop of 1 percent HN03 , 

transfer drop, and merge with it a drop of potassium mercuric thiocyanate solution. Greenish-yellow mosslike clumps and prisms prove copper. If zinc is 

present, the typical zinc precipitate should appear. (See description of zinccopper isomorphous precipitate, p. 188.) Two specimens of mottramite from different locations gave negative zinc tests. Leach original residue with drop of 

1:5 HCI. remove drop from residue and reject it. Leach original residue with 

two or three successive drops of water, transfer each drop, and test with fragment 

of KI. The first drop may not give a satisfactory test for lead owing to traces of 

HCI, but the second or third drop will yield glistening yellow hexagons and plates 

of the normal lead iodide. Test for V in a porcelain crucible. Dissolve additional 

mineral powder (diameter of crater should be about 1 millimeter) in 0.3 cubic 

centimeter of 1:1 HN03 . Add an equal amount of water, then two to four drops of 

H2 02 . If solution turns orange, vanadium is present.

Nagyagite (Pb,Au,Te,S?). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1:5 HCI. Transfer drop and add fragment of CsCl. Honey-yellow 

isometric crystals prove tellurium. The crystals are smaller with nagyagite than 

with other tellurides, for some unknown reason. Leach original residue with 

two or three drops of water, transfer each drop, and test with a fragment of KI. 

The first drop may not give a satisfactory test for lead, owing to traces of HCI, but 

the second or third drop will yield yellow hexagons of the characteristic normal 

lead iodide. The writer was unable to get a test for gold by the pyridine-HBr 

method.

Naumannite ((Ag2 ,Pb)Se). Decompose mineral with 1:1 HN03 . Leach residue 

with 1 percent HN03 , transfer drop, and add a fragment of ammonium bichromate. 

A yellow amorphous precipitate proves lead, and red crystals prove silver. Add 

drop of 1:5 HCI to original residue. Any silver remaining is converted into the 

white flocculent chloride. Transfer filtrate and merge it with a drop of stannous 

chloride solution. A brick-red amorphous precipitate proves selenium. Leach 

original residue with two or three drops of water, transfer each drop, and test with 

a fragment of KI. The first drop may not give a satisfactory test for lead, owing 

to traces of HCI, but the second or third drop will yield yellow hexagons of the 

characteristic normal lead iodide. Naumanite and aguilarite can be distinguished 

only by a microchemical test for lead.

Niccolile (NiAs). Take mineral into solution with aqua regia. Leach residue 

with drop of 1:7 HN03 . Transfer drop and evaporate slowly over microflame.
284 MICROSCOPIC DETERMINATION OF ORE MINERALS

When nearly dry, add drop of ammonium molybdate and again evaporate slowly 

over microflame, this time to dryness. Cool and add drop of 1:7 HN03 to redissolve excess of ammonium molybdate. Tiny yellow isometric crystals prove 

arsenic. Transfer filtrate and evaporate to dryness. Cool and add a drop of 20 

percent NH4OH, followed by a drop of dimethyl glyoxime solution (method I-A). 

A pink precipitate proves nickel.

Orpiment (As2S3). Decompose mineral with five drops of aqua regia. Leach 

residue with 1:7 HN03 , transfer drop, and evaporate slowly over microflame. 

When nearly dry, add drop of ammonium molybdate solution and again evaporate 

slowly over microflame, this time to dryness. Cool and add drop of 1:7 HNOs to 

dissolve excess of ammonium molybdate. Tiny yellow isometric crystals prove 

arsenic.

Owyheeite (8PbS.2Ag2 S.5Sb2 S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HN03 , transfer drop, and add tiny fragment of 

ammonium bichromate. An amorphous yellow precipitate proves lead, and red 

needlelike crystals prove silver. Leach original residue with drop of 1:5 HCl. 

Transfer drop and add tiny fragment of KI to one side of drop. Solution turns 

yellow near fragment. Then add fragment of CsCl to other side of drop. Orangecolored hexagons and stars prove antimony. If necessary the presence of lead 

can be confirmed by leaching original residue with two or three drops of water, 

transferring each drop, and testing for lead with fragment of KI.

Pearceite (8Ag2S.As2S3 ). Decompose mineral with 1:1 HNO3 . Leach residue 

with 1 percent HNO3 , transfer drop, and merge with a drop of potassium mercuric 

thiocyanate solution (method I). An amorphous white precipitate proves silver. 

Usually there is also a significant proportion of copper in pearceite, which is the 

reason for choosing the thiocyanate test in preference to the bichromate test. 

Much of the arsenic acid was also carried from the original residue by the 1 percent 

HN03 drop. Usually there is enough arsenic remaining to give a satisfactory test, 

Leach original residue with aqua regia, and evaporate to dryness. Cool, then 

leach original residue with drop of 1:7 HN03 , transfer drop, and evaporate slowly 

over microflame. When nearly dry, add drop of ammonium molybdate solution 

and again evaporate slowly over microflame, this time to dryness. Cool and add 

drop of 1:7 HN03 to redissolve excess of ammonium molybdate. Tiny yellow 

isometric crystals prove arsenic.

Pentlandile ((Fe,Ni)S). Decompose mineral with 1:1 HNO. Leach residue 

with 1:7 HN03 , transfer drop, and evaporate to dryness. Add drop of 20 percent 

NH4OH to residue. A yellow gelatinous precipitate of Fe(OH) 3 proves iron. 

Add directly to the ammonia drop a drop of dimethyl glyoxime solution. A pink 

(nickel) precipitate will form on top of the gelatinous ferric hydroxide coating.

Petzite ((Ag,Au) 2Te). Decompose mineral with 1:1 HN03 . Leach residue 

with 1 percent HNO3 , transfer drop, and add a fragment of ammonium bichromate. 

Red crystals prove silver. Leach original residue with a drop of 1:5 HCl, transfer 

drop, and add a fragment of CsCl. Honey-yellow isometric crystals prove 

tellurium. Examine original residue in oblique illumination with a 16-millimeter 

objective. Usually metallic gold particles in the residue are sufficiently distinctive in appearance to make a confirmatory test unnecessary. If necessary it 

can be made as follows: The original residue is dissolved in aqua regia and the 

solution evaporated to dryness. The residue, which is orange-colored to light 

brown, is dissolved in water, the drop transferred and merged with a drop of 

pyridine-HBr solution. Strongly pleochroic (colorless to deep red) prisms prove 

gold.
M1CROCHEMICAL METHODS 285

Plagionite (5PbS.4Sb2S3). Decompose mineral with 1:1 HNO3 . Examine 

residue for lattice structure, indicating lead. Leach residue with drop of 1:5 

HCl. Transfer drop and add fragment of KI. Solution turns yellow near 

fragment as it dissolves. Then add fragment of cesium chloride. Orangecolored hexagons and stars prove antimony. Leach original residue with two or 

three drops of water, transfer each drop, and test with a fragment of KI. The 

first drop may not give a satisfactory test for lead, owing to traces of HCl, but the 

second or third drop will yield yellow hexagons of the characteristic normal lead 

iodide.

Plessite ((Ni,Fe)AsS). Decompose mineral with aqua regia. Leach residue 

with a drop of 1 : 7 HNOs, transfer drop, and evaporate it slowly over microflame. 

When nearly dry add drop of ammonium molybdate solution and again evaporate 

slowly, this time to dryness. Cool and add drop of 1 : 7 HN03 . Tiny yellow 

isometric crystals prove arsenic. Transfer filtrate and merge with drop of potass'ium mercuric thiocyanate. Test solution turns red, proving iron, and small 

brown spherulites are precipitated, proving nickel. If desired confirm nickel 

test by allowing the drop containing the nickel spherulites to evaporate to dryness 

and then adding directly a drop of 20 percent NH4OH, followed by a drop of 

dimethyl glyoxime solution. A pink precipitate confirms the presence of nickel.

Polybasite (8Ag2 S.Sb2 S3 ). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1:7 HN03 , transfer drop, and merge with drop of potassium 

mercuric thiocyanate. A white curdy amorphous precipitate proves silver, and 

just behind it a copper precipitate usually forms when testing polybasite. Leach 

original residue with drop of 1:5 HCl, transfer drop, and add fragment of KI. 

The solution turns yellow near the fragment as it dissolves. Then add fragment 

of CsCl. Orange-colored hexagons and stars prove antimony.

Polydymite (Ni 3S,i). Decompose mineral with 1:1 HN03 . Leach residue with 

drop of 1:7 HN03 , transfer drop, and evaporate to dryness. To the residue from 

this evaporation add a drop of 20 percent NH4OH. If any iron is present it 

will be precipitated as a brown gelatinous coating at the bottom of the drop. 

To the ammonia drop add directly a drop of dimethyl glyoxime solution. A pink 

precipitate proves nickel.

Proustite (3Ag2 S.As2S3). Decompose mineral with 1:1 HN03 . Leach residue 

with drop of 1 percent HNO3 , transfer drop, and add fragment of ammonium 

bichromate. Red crystals prove silver. Some of the arsenic was removed from 

the original residue in the drop of 1 percent HN03 . Enough is usually left in the 

original residue to give a satisfactory arsenic test. Leach original residue with 

drop of 1:7 HN03 , transfer drop, and evaporate slowly over microflame. When 

nearly dry, add drop of ammonium molybdate solution and again evaporate slowly 

over microflame, this time to dryness. Cool and add drop of 1:7 HN03 to dissolve 

excess ammonium molybdate. Tiny yellow isometric crystals prove arsenic.

Psilomelane (Mn02 .Mn2 03 ). Take mineral into solution in aqua regia. Take 

up residue in drop of 1:7 HN03 and add fragment of sodium bismuthate. Solution turns pink or purple near fragments, proving manganese.

Pyrargyrite (3Ag2 S.Sb2 S3 ). Decompose mineral with 1:1 HN03 . Leach 

residue with 1 percent HN03 , transfer drop, and add fragment of ammonium 

bichromate. Red crystals prove silver. Leach original residue with a drop of 

1:5 HCl, transfer drop, and add fragment of KI. Solution turns yellow near the 

fragment as it dissolves. Then add a fragment of CsCl. Orange-colored hexagons 

and stars prove antimony.
286 MICROSCOPIC DETERMINATION OF ORE MINERALS

Pyrolusite (MnO2). Take mineral into solution in aqua regia. Take up residue 

in drop of 1:7 HNO3 and add fragments of sodium bismuthate. Solution turns 

pink or purple near fragments, proving manganese.

Pyrrhotite (FeSi+). Decompose mineral with 1:1 HN03 . Residue is colored 

yellow or orange. Leach residue with drop of 1:5 HC1, transfer drop, and add 

fragment of K4Fe(CN) 6 . A blue amorphous precipitate proves iron.

Pyrite (FeS2). Decompose mineral with 1:1 HNOs. Residue is colored yellow 

or orange. Leach residue with drop of 1:5 HCl, transfer drop, and add fragment 

of K4Fe(CN) 8 . A blue amorphous precipitate proves iron.

Rammelsbergite (NiAs2). Take mineral into solution with aqua regia. Leach 

residue with drop of 1:7 HNOs. Transfer drop and evaporate slowly over microflame. When nearly dry, add drop of ammonium molybdate and again evaporate 

slowly over microflame, this time to dryness. Cool and add drop of 1:7 HN03 

to redissolve excess of ammonium molybdate. Tiny yellow isometric crystals 

prove arsenic. Transfer filtrate and evaporate to dryness. Cool and add a 

drop of 20 percent NH4OH to residue. Add directly to the drop of ammonia a 

drop of dimethyl glyoxime solution. A pink precipitate proves nickel.

Rathite (3PbS.2As2S3). Decompose mineral with 1:1 HN03 . Examine 

residue for lead nitrate lattice. Leach residue with 1:5 HCl, transfer drop, and 

evaporate slowly over microflame. When nearly dry, add drop of ammonium 

molybdate solution and again evaporate slowly over microflame, When dry 

cool and add drop of 1:7 HN03 to redissolve excess of ammonium molybdate. 

Tiny yellow isometric crystals prove arsenic. Add three successive drops of 

water to original residue. Transfer each drop to another place on slide and add 

fragment of KI. The first drop may not give a satisfactory test for lead, owing 

to the presence of remaining traces of arsenic, but the second or third drop should 

give yellow hexagons, possibly somewhat distorted, proving lead.

Realgar (AsS). Decompose mineral with five drops of 1:1 HNOs, followed by 

one of aqua regia. Leach residue with 1:7 HNOs, transfer drop, and evaporate 

slowly over microflame. When nearly dry, add drop of ammonium molybdate 

solution arid again evaporate slowly over microflame, this time to dryness. Cool 

and add drop of 1:7 HNO3 to dissolve excess of ammonium molybdate. Tiny 

yellow isometric crystals prove arsenic.

Rezbanyile (Cu2S.3PbS.5Bi2S3). Decompose mineral with 1:1 HNOs. Examine residue for lead nitrate lattice. Leach residue with 1:5 HCl. Transfer drop 

and add fragment of KI. Solution turns yellow near fragment as it dissolves. 

Then add fragment of CsCl. The yellow color disappears near fragment, and 

red hexagons form, proving bismuth. Leach original residue with two successive 

drops of 1:5 HCl, rejecting both. This will remove most of the remaining trace 

of bismuth. Then add three successive drops of water to original residue, transfer 

each drop to another place on slide, and add fragment of KI. The first drop may 

not give a satisfactory test for lead, owing to the presence of remaining traces of 

Bi and HCl, but the second or third drop should yield yellow hexagons, proving 

lead.

Rickardite (Cu3Te2 ). Take mineral into solution with 1:1 HN03 . Leach 

residue with 1 percent HNO3 , transfer drop, and merge with drop of potassium 

mercuric thiocyanate solution. Greenish-yellow mosslike aggregates and prisms 

prove copper. Leach original residue with 1:5 HCl, transfer drop, and add 

fragment of CsCl. Honey-yellow isometric crystals prove tellurium.
MICROCHEMICAL METHODS 287

Rutile (Ti02). Most specimens of rutile contain appreciable amounts of iron. 

Fuse mineral in soda-niter bead in loop of platinum wire. Leach residue in 0.5 

cubic centimeter of warm water. Withdraw water by means of medicine dropper 

and reject. Brown insoluble residue contains both Ti and Fe. Dissolve residue 

in 0.3 cubic centimeter of 1:1 HN03, warming if necessary. Cool, add an equal 

quantity of water. By means of a medicine dropper withdraw part of the solution 

and transfer to another porcelain crucible. Add fragments of K4Fe(CN) 6 . 

Solution turns deep blue, proving Fe. To other crucible add two to four drops of 

H2 02 . Solution turns yellow, proving Ti. Confirm by adding fragments of 

NaF. Yellow color disappears.

Safflorite-loellingite ((Co,Fe)AsS). Decompose mineral with aqua regia. Leach 

residue with 1:7 HN03, transfer drop, and evaporate slowly over microflame. 

When nearly dry, add drop of ammonium molybdate solution and evaporate 

slowly to dryness. Cool and add drop of 1:7 HN08 to redissolve excess of ammonium molybdate. Tiny yellow isometric crystals prove arsenic. Transfer 

filtrate and merge with drop of potassium mercuric thiocyanate solution. Test 

drop turns pink, proving iron, and dark indigo-colored prisms are precipitated, 

proving cobalt. Sometimes the prisms are so dark that the blue color is not 

apparent. In this event cobalt should be confirmed by the cesium chloride 

test. Take new material into solution with aqua regia. Leach residue with 

drop ofj[.l:5 HC1, transfer drop, and add fragment of CsCl. Iron and nickel 

come down first in small isometric grains. Cobalt comes down late in the 

evaporation as long soluble blue prisms.

Schapbachite (Ag2S.PbS.Bi2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HN03 , transfer drop, and add fragment of ammonium bichromate. A yellow amorphous precipitate proves lead, and red crystals 

prove silver. Leach original residue with drop of 1:5 HCl, transfer drop, and 

add fragment of KI to one side of drop. Solution turns yellow near drop. Then 

add fragment of CsCl to opposite side of drop. Red hexagons prove Bi. If 

necessary the presence of lead can be confirmed as follows: Leach original residue 

with three successive drops of 1:5 HCl, rejecting each. This removes most of 

remaining traces of bismuth. Then leach original residue with successive drops 

of H2 0, transfer each drop, and test with a fragment of KI. The first drop may 

give an indefinite result, owing to a trace of bismuth, but this tends to hydrolyze 

in the water, and the second or third drop will give good PbI2 hexagonal plates.

Semseyite (9PbS.4Sb2S3). Decompose mineral with 1:1 HNOs. Examine 

residue for lattice structure, indicating lead. Leach residue with drop of 1:5 

HCl. Transfer drop and add fragment of KI. Solution turns yellow near 

fragment as it dissolves. Then add fragment of cesium chloride. Orangecolored hexagons and stars prove antimony. Leach original residue with two 

or three drops of water, transfer each drop, and test with a fragment of KI. 

The first drop may not give a satisfactory test for lead, owing to traces of HCl, 

but the second or third drop will yield hexagons of the characteristic normal 

lead iodide.

Siegenite ((Ni,Co) 3S4). Decompose mineral with 1:1 HN03 . Leach residue 

in drop of 1:7 HN03 , transfer drop, and merge with drop of potassium mercuric 

thiocyanate reagent. If solution turns red, iron is present. Brown spherulites 

prove nickel, and blue spherulites prove cobalt. If the ratio of cobalt to nickel 

is low, the blue cobalt spherulites may not appear. Siegenite is much more 

abundant than the nickel end member, polydymite, Ni3S4 . Hence, if a mineral 

has been determined by etch tests or otherwise as a member of the linnaeite 

group (linnaeite, Co3S4 , carrollite, (Co,Cu) 3S4) of'polydymite, the cesium chloride
288 MICROSCOPIC DETERMINATION OF ORE MINERALS

test for cobalt should u be made jn addition to the thiocyanate test. Take new 

material into solution with aqua.regia. Leach residue with drop of 1:5 HC1, 

transfer drop, and add fragment of CsCl. Iron and-'nickel .come down first in 

small brown isometric grains. Cobalt comes down late in the evaporation as 

long soluble blue prisms.

Silver (Ag). Take into solution with 1:1 HNOs. Add drop of 1 percent 

HNOs to residue. Transfer drop and add fragment of ammonium bichromate. 

Red crystals prove silver.

Skutterudite ((Co.Ni) Ass). Take mineral into solution with aqua regia. Leach 

residue with drop of 1:7 HNOs, transfer drop, and evaporate slowly over microflame. When nearly dry, add drop of ammonium molybdate solution and again 

slowly evaporate. When dry, cool and add drop of 1:7 HNOs solution to redissolve excess of ammonium molybdate. Tiny yellow isometric crystals prove 

arsenic. Transfer filtrate and merge with drop of potassium mercuric thiocyanate solution. Brown spherulites prove nickel and blue prisms cobalt.

Smaltite-chloanthtie ((Co,Ni) As2). ;Take mineral into solution in aqua regia. 

Take up residue in a drop of 1:7 HNOs, transfer, and slowly evaporate drop. 

When nearly dry, add drop of ammonium molybdate reagent and slowly evaporate 

to dryness. Cool, add drop of 1:7 HNOs to residue, and examine with an 8- 

millimeter objective. Tiny yellow isometric crystals prove arsenic. Transfer 

filtrate and merge with drop of potassium mercuric thiocyanate reagent. If 

solution turns red, iron is present. Brown spherulites prove nickel; blue spherulites, cobalt.

Sphalerite (ZnS). Decompose mineral with aqua regia. Leach residue with 

drop of 1:7 HNOs, transfer drop, and merge with drop of potassium mercuric 

fchiocyanate. White feathery crosses prove zinc. If iron is present the crosses 

will be stained pink.

Stannite (Cu2S.FeS.SnS2 ). Decompose mineral with 1:1 HNOs. Leach residue 

with drop of 1:7 HN03 , transfer drop, and merge with drop of potassium mercuric 

thiocyanate solution. Greenish-yellow mosslike aggregates and isolated prisms 

prove copper. Test solution turns pink, proving iron. Remove last traces of 

copper in original residue by leaching it with two successive drops of 1:7 HNOs 

and one of H2O; reject all three drops. Leach residue with drop of 1:5 HCl, 

transfer drop, and add fragment of rubidium chloride. Colorless highly refracting 

octahedrons prove tin.

Sternbergite (Ag2S.Fe4S5). Decompose mineral with 1:1 HNOs. Leach residue 

with 1 percent HN03 , transfer drop, and add fragment of ammonium bichromate. 

Red crystals prove silver. Leach original residue with a drop of 1:7 HN03 , 

transfer drop, and add fragment of K4Fe(CN) 8 . An amorphous blue precipitate 

proves iron. Most of the iron was carried over with the silver in the first drop, 

but there is usually enough left in the original residue to yield a satisfactory test.

Stephanile (5Ag2S.Sb2Ss). Decompose mineral with 1:1 HNOs. Leach 

residue with 1 percent HNOs, transfer drop, and add fragment of ammonium 

bichromate. Red crystals prove silver. Leach original residue with a drop of 

1:5 HCl, transfer drop, and add fragment of KI. Solution turns yellow near the 

fragment as it dissolves. Then add a fragment of CsCl. Orange-colored hexagons 

and stars prove antimony.

Stibnite (Sb2S3). The KOH etch test is very characteristic. The presence of 

antimony can be confirmed microchemically as follows: Decompose mineral with 

aqua regia. Leach residue with drop of 1:5 HCl, transfer drop, and add fragment
MICKOCHEMICAL METHODS 289

of KI. Solution turns yellow near fragment as it dissolves. Add fragment of 

CsCl to solution. Orange-colored stars and hexagons, the stars predominating, 

prove antimony. With some specimens of stibnite the double iodide crystals 

are redder than with most antimony compounds.

Stromeyerile ((Ag,Cu) 2S). Decompose mineral with 1:1 HN03 , leach residue 

with drop of 1 percent HN03 , transfer drop, and add fragment of ammonium 

bichromate. Red crystals prove silver. Leach original residue with drop of 

1:7 HNOs, transfer drop, and merge with drop of potassium mercuric thiocyanate 

solution. Greenish-yellow mosslike aggregates and prisms prove copper.

Sulvanite (CusVS^. Copper is tested for on a glass slide. Decompose mineral with 1:1 HNOs. Leach residue with drop of 1:7 HNOs, transfer drop, and 

merge with drop of potassium mercuric thiocyanate reagent. Greenish-yellow 

inosslike clumps and prisms prove copper. Vanadium is tested for with additional mineral powder in porcelain crucible without fusion in soda-niter flux. 

Scrape surface until a crater about 1 millimeter in diameter is formed. Transfer 

powder to crucible containing about 0.3 cubic centimeter of 1 : 1 HN03 . Heat on 

hot plate or over small Bunsen flame with slow boiling until solution is evaporated. 

Ignore undissolved material, which is chiefly sulphur. Residue is orange-colored 

to brown. Cool and take up residue in about 0.5 cubic centimeter of 1 : 1 HN03 . 

Add two to four drops of H2 02 . Solution is colored orange with a suggestion of 

red.

Sylvanite (AuAgTe^. Decompose mineral with 1:1 HN03 . One or two 

applications of acid suffice. The residue is a white curdy powder. Leach 

residue with 1:5 HC1. The white curdy residue goes into solution at once. 

Transfer drop and add a fragment of CsCl. Honey-yellow isometric crystals 

prove tellurium. Gold remains in the original residue in small metallic grains 

and lumps. Usually examination in oblique reflected light with a 16-millimeter 

objective suffices to identify the gold. If the appearance of the residue is not 

sufficiently characteristic, add drop of aqua regia to residue and evaporate to 

dryness. An orange-colored residue of gold chloride remains. Dissolve this in 

water, transferdrop, and merge with it a drop of pyridine-HBr reagent. Strongly 

pleochroic (colorless to deep red) prisms prove gold. Silver remains in the 

original residue as a curdy white chloride. Leach it with two or three successive 

drops of 1:5 HCl to remove remaining traces of gold and tellurium, then one of 

water to remove HCl. Then add to original residue a drop of concentrated 

NH4OH. Observe with an 8-millimeter objective. In a few seconds tiny colorless isometric crystals of AgCl will be seen floating in the ammonia solution.

Teallite (PbS.SnS). Decompose mineral with aqua regia. Leach residue 

with drop of 1:5 HCl, transfer drop, and add fragment of rubidium chloride. 

Colorless, highly refracting octahedrons prove tin. Leach original residue with 

two or three drops of water, transfer each drop, and test with a fragment of KI. 

The first drop may not give a satisfactory test for lead, owing to traces of HCl, 

but the second or third drop will yield yellow hexagons of the characteristic normal 

lead iodide.

Tellurium (Te). Dissolve in 1:1 HN03 . Leach residue with drop of 1:5 

HCl, transfer drop, and add fragment of CsCl. Honey-yellow isometric crystals 

prove tellurium. Metallic tellurium is most frequently mistaken for tetradymite. 

The precautions for obtaining the bismuth test described under tetradymite should 

be carried out; the mineral cannot be identified as tellurium until bismuth is 

definitely proved to be absent.
290 MICROSCOPIC DETERMINATION OF ORE MINERALS

Tennantite (5Cu2S.2(Cu,Fe)S.2As2S3). Decompose mineral with aqua regia. 

Leach residue with 1:7 HNOa, transfer drop, and evaporate slowly over microflame. When nearly dry, add drop of ammonium molybdate solution and again 

slowly evaporate. When dry, cool and add drop of 1:7 HNOs to redissolve excess 

of ammonium molybdate. Tiny yellow isometric crystals prove arsenic. Transfer filtrate and merge with it a drop of potassium mercuric thiocyanate solution. 

Greenish-yellow mosslike clumps and prisms prove copper. The molybdate test 

for arsenic is more difficult than with enargite. The crystals tend to be very 

small and the distinctive yellow color not so apparent.

Tetradymite (Bi2 (Te,S) 3). Dissolve mineral in 1:1 HNOs. Dissolve residue 

in drop of 1:5 HC1. Add fragment of CsCl. Colorless iridescent rhomb-shaped 

and blade-shaped plates prove bismuth. In addition honey-yellow isometric 

crystals also form, proving tellurium. All the tellurium must be precipitated as 

cesium chlorotellurite before bismuth cesium double iodide can form. Hence 

if the reagent is not in excess, the bismuth compound may not appear in the drop. 

The beginner usually gets the first drop too concentrated. For this reason the 

second or third drop of 1:5 HC1 taken from the original residue usually gives a 

better bismuth test than the first drop. The cesium double iodide test for 

bismuth will not work in the presence of tellurium.

Tetrahedrite (5Cu2S.2(Cu,Fe)S.2Sb2S3). Decompose mineral by means of 

1:1 HN03 . Leach residue with drop of 1:7 HNOs, transfer drop, and merge with 

it a drop of potassium mercuric thiocyanate solution. Greenish-yellow mosslike 

clumps and prisms prove copper. Leach residue with successive drops of 1 percent HNOs, transfer each drop, and test with drop of K2Hg(CNS)4 until no test 

for copper appears. Then leach original residue with a drop of 1:5 HC1, transfer 

drop, and add fragment of KI. If copper is present in the drop, an amorphous 

precipitate of yellow copper iodide will appear. The solution near the fragment 

turns yellow as the KI dissolves. Then add fragment of CsCl. Orange-colored 

hexagons and stars prove antimony.

Tiemanmte (HgSe). Decompose mineral with aqua regia. Examine residue 

in oblique reflected light for mercuric chloride lattice and red tinge of elemental 

selenium. Leach residue with 1 percent HNOs, transfer drop, and add a small 

fragment of cobalt nitrate. When dissolved add a fragment of KCNS. Indigoblue arborescent forms and prisms prove mercury. Leach original residue with 

drop of 1:5 HC1, transfer drop, and merge with a drop of stannous chloride solution. A brick-red amorphous precipitate proves selenium.

Troilite (FeS). Dissolve mineral in 1:1 HN03. Leach residue with drop oi: 

1:5 HC1, transfer drop, and add fragment of K^FeCCNV Amorphous blue precipitate proves iron.

Tungstenite (WS2). Fuse mineral in a platinum wire with soda-niter flux. 

Dissolve fused mass in about 0.5 cubic centimeter of water in a small porcelain 

crucible. Make solution acid with 1:1 HCl and evaporate to dryness. Add drop 

of concentrated H2S04 and heat over low Bunsen flame until fumes appear. Cool 

and examine residue. If it is liquid add a few crystals of hydroquinone. Residue 

turns violet. After a few minutes violet color disappears and residue turns gray 

to black.

Ullmanite (NiSbS). Decompose mineral with 1:1 HNOs. Leach residue with 

drop of 1:7 HNOs, transfer drop, and evaporate to dryness. To the residue from 

this evaporation add a drop of 20 percent NH^H, followed by a drop of dimethyl 

glyoxime solution (p. 192). A pink precipitate proves nickel. Leach original residue 

with a drop of 1:5 HCl, transfer drop, and add fragments of KI and CsCl. 

Orange-colored hexagons and stars prove antimony.
MICKOCHEMICAL METHODS 291

Umangite (Cu3 Se2). Decompose mineral with 1:1 HN03 . Examine residue 

in oblique reflected light for a reddish tinge, indicating selenium. Leach residue 

with a drop of 1 percent HN03 , transfer drop, and merge with drop of potassium 

mercuric thiocyanate solution. Greenish-yellow mosslike aggregates and isolated 

prisms prove copper. Leach original residue with drop of 1:5 HC1, transfer 

drop, and merge with drop of stannous chloride solution. An amorphous brickred precipitate proves selenium.

Violarite ((Ni,Fe) 3S4 ). Decompose mineral with 1:1 HN03 . Leach residue 

with 1:7 HN03 , transfer drop, and evaporate to dryness. Add drop of 20 percent 

NH4OH to residue. A yellow gelatinous precipitate of Fe(OH) 3 proves iron. 

Add directly to the ammonia a drop of dimethyl glyoxime solution. A pink precipitate will form on top of the gelatinous ferric hydroxide coating, proving nickel.

Vrbaite (Tl2S.3(As,Sb) 2S3). Thallium tested for by flame coloration. A small 

loop at the end of the platinum wire is filled with a drop of 1:1 HC1, and a small 

fragment of mineral is placed in this loop. This is placed in the hottest part of a 

Bunsen flame. A vivid green color identifies thallium. Test for arsenic and 

antimony microchemically as follows: Decompose mineral with 1:1 HN03 . 

Leach residue with drop of 1:7 HN03 and slowly evaporate over microflame. 

When nearly dry add drop of ammonium molybdate solution and again evaporate 

slowly over microflame. When dry add a drop of 1:7 HN03 to redissolve excess 

of ammonium molybdate. Tiny yellow isometric crystals prove arsenic. Leach 

original residue with drop of 1:5 HC1, transfer drop, and add fragment of CsCl. 

Colorless hexagons prove antimony.

Weissite (Cu2Te). Take mineral into solution with 1:1 HN03 . Leach 

residue with 1 percent HISOs, transfer drop, and merge with drop of potassium 

mercuric thiocyanate solution. Greenish-yellow mosslike aggregates and prisms 

prove copper. Leach original residue with 1:5 HC1, transfer drop, and add fragment of CsCl. Honey-yellow isometric crystals prove tellurium.

Whitneyite (Cu9As). Take mineral into solution with aqua regia. Residue 

is gelatinous with a tinge of brown. Leach residue with drop of 1:7 HN03 , transfer drop, and heat slowly by passing in and out of flame. When nearly dry, add 

directly a drop of ammonium molybdate solution. Again heat slowly over flame 

until dry. Add drop of 1:7 HN03 solution to redissolve excess of ammonium 

molybdate and examine. Tiny yellow isometric crystals prove arsenic. Transfer filtrate (containing copper nitrate and dissolved ammonium molybdate) and 

merge with it a drop of potassium mercuric thiocyanate solution. A yellowishgreen precipitate, usually mosslike, proves copper.

Willyamite ((Co,Ni)SbS). Decompose mineral with drop of 1:1 HN03 . 

Leach residue with drop of 1:7 HNOs, transfer drop, and merge with it a drop of 

potassium mercuric thiocyanate. Blue prisms prove cobalt, and brown spherulites, nickel. (See description on p. 193.) Leach origi'nal residue with a drop 

of 1:5 HC1. Transfer drop and add fragment of KI. The solution turns yellow 

near the drop as it dissolves. Then add fragment of CsCl. Orange-colored 

hexagons and stars prove antimony.

Wittichenite (3Cu2S.Bi2S3). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1 percent HN03 , transfer drop, and merge with drop of potassium mercuric thiocyanate solution. Greenish-yellow mosslike clumps and 

prisms prove copper. Leach original residue with drop of 1:5 HCl, transfer drop, 

and add fragment of CsCl. Thin colorless rhombohedral plates prove bismuth. 

The double iodide test for bismuth does not work well, as more or less copper is 

carried over with the bismuth, and this is precipitated by KI, masking the bismuth 

test.
292 MICROSCOPIC DETERMINATION OF ORE MINERALS

Wolframite (Fe, Mn) W04 . Fuse mineral in a platinum wire with soda-niter 

flux. Dissolve fused mass in about 0.5 cubic centimeter of water in a porcelain 

crucible, warming on a hot plate to hasten solution. Withdraw filtrate from 

undissolved material by means of a medicine dropper and transfer to another 

porcelain crucible. Add about 0.2 cubic centimeter of 1:5 HCI. This is usually 

sufficient to acidify the solution. Add some fragments of metallic zinc. Hydrogen should be evolved vigorously if acid is in excess. If not, add more 1:5 

HCI drop by drop until abundant bubbles are evolved from the zinc fragments. 

After 2 or 3 minutes the solution turns slate gray to grayish blue, the color depending on the concentration in tungsten. Add about 0.6 cubic centimeter of 

1:7 HNOs to the precipitate in the other crucible. Ignore undissolved material. 

Divide the solution into two parts by withdrawing part by means of a medicine 

dropper and transferring to a third crucible. To one part add a few drops of 

KCNS. 'A red coloration proves iron. To the other solution add some fragments of sodium bismuthate, allowing excess reagent to settle. A violet solution 

proves manganese. Wolframite is distinguished from ferberite only by giving a 

definite manganese test.

Wurlziie (ZnS). Decompose mineral with aqua regia. Leach residue with 

drop of 1:7 HNO3 , transfer drop, and merge with drop of potassium mercuric 

thiocyanate reagent. White feathery crosses prove zinc.

Zinkemte (PbS.Sb2S3 .) Decompose mineral with 1:1 HN03 . Examine 

residue for lattice structure, indicating lead. Leach residue with drop of 1:5 

HCI. Transfer drop and add fragment of KI. Solution turns yellow near fragment as it dissolves. Then add fragment of cesium chloride. Orange-colored 

hexagons and stars prove antimony. Leach original residue with two or three 

drops of water, transfer each drop, and add a fragment of KI. The first drop may 

not give a satisfactory test for lead, owing to traces of HCI, but the second or third 

drop will yield yellow hexagons of the characteristic normal lead iodide.
TABLES

TABLE 1. Colored minerals

Color

Do-- 

Do 

Do 

Do.. 

Do 

Do. ... ... ... ... 

Do.... -

Do 

Do 

Do..-.-,':..-...-....------..-.-...-.

Do.. 

Do -

Do 

Bufflsh pink.........- . -.

Do.. -__ ......

Do .

Do. ... ... 

Mineral

Rickardite..---_ . 

Millerite... ........... ....................

Gold 

Cobaltite ...... ....-----. . -------------

Famatinite... __ . _________

Coloradoite... ... _____ . __ . ___ 

Cubacite ___________ . __ .. _

Emplectite ___ . ___________ ...

Composition

(Ni,Fe) 3S4

(Ni,Fe)Ss

Cu»FeS<

Cu3 (Fe,ae)S4

CuS

FeSs

NiS

FeSj

(Fe,Ni)S

CuFe32

An

Ni3Asj

NiTej

Cu

NiAs

NiSb

Bi

CnjS.4ruS.A<!jSj

Ho-Tn

AgjS.Fe^Ss

CujS.Fe^Ss

eu2S.Bi2S3

(Au,Ag)Te3

TABLE 2. Minerals that show distinctive internal reflections

Color of internal reflection

Do 

Do.. . -.. ...... 

Yellow.... 

Do. .... . ...........

Rod 

Do ... .... ... 

Do ~ .

Do........ ..... . 

Do.. 

Do .

Do.... .... . ...... ...... 

Do.. .... ... .....

. Do. ...... ... . ... ....

Do . 

Do 

Mineral

Rutile... ....... ........... .... ..

Orpiment _____ . ___ : ...............

Composition

AsS

2Sb2S3 .Sb2 Or

TiOj

MnS

ZnS

8Ag2S.8b2Ss

SAgzS.AszSs

3AgsS.SbsS3

4PbS.3As2S!

HgS.2Sb2S3

Mn3 04

293
294 MICROSCOPIC DETERMINATION OF ORE .MINERALS 

TABLE 3. Minerals that give characteristic colored powders

Color of powder

Red-- -

Do 

Do. 

Do... -

Do... -

Do.. ... 

Do.... .

Do... 

Do 

OrtvniTA

Do.... ...

White to resin brown. 

Do... - ... 

Do-. ... ... ... ... ...

Mineral

Polybasite 1 -------

Proustite.. . ....

Pearceite 1 -. ... 

Hematite- . ...__

Tetrahedrite 2 -

Tennautite J. ___

Sphalerite. .

Franklinite . 

Composition

HgS.... 

SAgzS.SbzSs- ---.-----...

3Ag20.As2S3 _ -.- ... . 

AgjS.SbaSj 

SAgjS.AsaSs. ------------- ...

HgS.2SbiSj..._ -----.

Tho.AsjSs ___________ -

VetOt.. .......................

MnO.MnjOj 

AsS - 

5Cu2S.2(Cu,Fe)S.2Sb2 33 

5Cu2S.2(Cu,Fe)S.2As2S3.. -

MnS.- 

ZnS . 

ZnO.FejOs- ----- _ -

4MnO.3MnOj.Si02- 

Color of mineral in vertically reflected light

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Bluish gray. 

Do.

Do.

Do.

1 Most specimens give black powder. J Some specimens give black powder. 

TABLE 4. Reflectivity of minerals

Name

Aiklnite 

Alabandite.. ___ .-. ... 

Altaite -------- _ -.-.-..-.-------.--.

Andorite _ _ _ -------------------

Aramayoite. _ ------------- ..- ...

Berthierite... . . . - .

Berthonite __ ----------------------- ...

Calaverite. ___ _ . 

Canfleldite- _ -._- -.- 

Carrollite _____ ..---..-. -.-. ... 

Chalcostibite. --------------------------

Chloanthite-smaltite _ ..-..---------.--

Cobaltite _ -.... _ ....--..------------

Covellite _ .. .. . .

Cnnrite _________________ .

Symbol

Aga (S,Se)-- ... ... -----

CujS.2PbS.Bi»S3 . ----------------- 

MnS... .... -- .. ..-.-.-....... ....

(Pb,Ag2)S.Bi2S3 

PbTe. - - -

AgjS.2PbS.3SbsS3 ---------------- 

Sb . . ... ... .

AgjS.(Sb,Bi)jSs- -- - 

AgjS. ... . .. -..

4AgjS.aeSj.. .. 

As.-.--.--- __ ---..-. -.-

FeAsS..... ... .... 

(Fe.Nij). - 

(Cu,Ag,)S.2PbS.2BijS3 

FeS.SbjSs. .. . ------- -

2(Pb,Cu3)S.Sb8S3. .. -------------- 

Bi .-.... . . -- 

Bi2 S3 ...... ------------------

Cu5FeS4 -------------------

5PbS.2Sb2S3 ------------------

CujS.2PbS.SbjS8 

4MnO,3MnOs.SiO2 - ------------- -----

NiSb... - . . .

(Au,Ag)Te8...- . ------------------- 

4Ag2S.(Sn,Ge)S2... --. ------------- . 

(Co,Cu) 3S4 .... .... - 

Sn02 ... ... _ ... _ -.-.----.--..--.-.-

CujS . . ... ....

(Mn,Zn)0.2Mn02 .2H2 0. ----...-.. -

CuFeSj. ......... ....... ...-..-..-....

Cu2S.Sb2 S3. __ . .- .-- --.--

(Ni,Co)Asj-. ... . . 

FeO.Cr2Os, __. .. .- -..--.-

HgS.... ...... ..--..-

PbSe -.. ------------

CoAsS. . ..... ....... ------------

HgTe __ . __. .. . --. .-.-

Cu- - ----------

Ni(As,Sb)S_... --------- -----

CuS .. -..

Cu2 0... . .-.. ....

Photometric 

measurement '

 - 

43

61

31

68

34 

36

25

62

57

27-38 

68

18

37

33

18

35-45

56 

 .....

29

42

"""""14""

29

50

52

85

18-26

41

29

Photoelectric cell 

measurement 8

31.6

43.5 

24.8

66.5

66.5

37.8 

72.6

38.5 

328.6

55.5

55.5

60.9

45.8

35.7

35.7 

68.5

43.0

25.0

41.5

36.2

21.3

50.2

64.1 

25.2 

47.2 

13.8

33.4

19.9-21.1

44.9

41.6

59.1 

4 12.8

27.7

55.5

36.2

68.3-71.3

50.2

42.1

10. 5-16. 3

44.0

28.6

See footnotes at end of table.
TABLES 295

TABLE 4. Reflectivity of minerals Continued

Name

Cylindrite.............................

Famatinite ____ . _ ... _ ....... ...

Franklinite ..........................

Freibergite... _ . _ ..-.---.. _ ........

Goethite. ....... .. .._... ........ ..

Gold................ ....................

Jamesonite. ______________

ICallilite. ............................

Lengenbachite. ___ .. ___ ..-. __ ..

LoelHnglto.......... ...... ... .....

Moneghinite. ___ .. __ .... __ .....

Millerite............... ....... ..........

Mcttramite ______ . ________

Peti'.ite... ..............................

Plaiiionite.... _ .......-...--.-.....-.

Polianite ............ ............ .....

Rathite...... ___ .....-----.-...

Rezbmyite. ____ .. . ......

Symbol

6PbS.Sb2Ss.6SnSj. ............ ..... 

CujO.FejOs ______ ........ .......

2PbS.AsjS3 .......... ... ...........

AgsSb..... ............. ...._._.__.....

Cu2S.Bi2S3-... ...................... ..

CusS.4CuS.As2S3.~_ ..-.....-...--....

Cu2Se.Ag8Se. .........................

Cu2S.4CuS.SbjS3 .......... .... ........ 

FeO.WOs... ..........................

SPbS.SbjSj^SnSj.. ............. ......

ZnO.FeaOs ....................

5(Cu,Ag)jS.2(Cu,Fe)S.2Sb2S3-.- ... 

2Ag2S.3PbS.2Sb2S3 .................

PbS...................................

PbS.BizSs ... .......................

5PbS.Sb2Ss. ...... ................

Cu3 (Fe,Ge)S«.. .. ........_. ...

NiAsS. ...............................

Fej03.H20. ..... .............. ....

Au............. .......................

C..... .....

MnSi.'......... ................. ......

MnO.Mn203 . ___ .. __ . _ ........

Fes03 ... ..............................

AgjTe ..... .........................

m(Ba,Mn)2Mn05+nFe<(Mn05)3 .. .

MnO.W03 . _____ . __ ...........

FeO.TiOj.. _ . __ ... _ . __ ......

MnO.Fe20j __ . __ .............. ..

4PbS.FeS.3Sb2S3 ..... ....... .......... 

4PbS.AsjS3 ..... ....... ................

Ni(Sb,Bi)S_. .........................

PbS.SbjSj................ ......... ..

2SbiS8 .Sbj03 .. ........... ............

3Cu8S.2BisS3.. .. ...... ............

CuSe....... __ . ___ ...............

(Au.AfOTes.......... ... ...... ........

6PbS.(Ag,Cu) 2S.2As2S3 -. - 

HgS.2Sb2S3.... _____ .-.....--......

FeAsj.......... ........ ...............

ThS.AsjSj.... ........................

Cu2S.4CnS.As2S3......... ............

FeO.FejOj. .. _ .....................

FeSt.. ................... . .........

NiiAsj... _________ . _____ .

4PbS.Sb2S3..... .......... . ......... 

HgS............ .......................

NiS

MoSj.. ........ ....

2CuO.2PbO.V2 03 .H20. ...............

(Agj,Pb)Se .. .......................

NiAs............. ..... .. .........

AsjS3 __________ . .........

3CuSe.2PbSe2.5(Ni,Co)Se2... ..... .

(Ag.AuijTe...... ......................

5PbS.4Sb8S3 ....... .......... .......... 

Pt._ ..................................

Mn02.. _____ .........--.-.--......

3Ap2S.As2S3__._ .... ... . ...

MnOs.Mn203.. ... . ..... . ..

FeSa.. ................................

MnOs... _____ ......................

NiAs2

3PbS.2AszS3 -.-.-. -... . 

AsS........... ........................

Cii2S.3PbS.5Bi 8S3. ............... .....

Photometric 

measurement

40

66

37

23

32.5

25

37

16

43.3

36

22

50

21

83 5-22

25

21

25

43

28

17

21

38 

38

34-38

19-31

............

36

56

29

25

21

16-18

52

59

40

32.5

53

15-35

............

35

43-50

26-31

51

32.5 

70

33

29

27

32.5

54

37

............

Photoelectric cell 

measurement

33. 0-35. 5

23.1

65.4

46.5

42.6

29.6

48.6

27.9 

19.0

36.6

10 7

32.5 

36. 8-38. 0

43.8

28.2

43.0

24.3

66.6

16.9

 60.0

48.8

25.2

19.7-21.5

28.4

41.2

17.4

20.1

39.6 

40.3

49.7

38.2

27.9

61.8

40.4 

50.2

56.5

30.2

28.2

21.7

«34. 1

53. 3-54. 5

39.6 

28.8

35.5

55.4

24. 0-26. 4

16.5 

40. 0-41. 2

54.3

23.1

33.2

42. 8-52. 0

51.0

38.9

40. 7-42. 8

32.2

32.1

29. 7-34. 4

32.5

57.2

34.8

43.0

63.0

36.2 

19.7

45.6

See footnotes at end of table.
296 MICROSCOPIC DETERMINATION OF ORE MINERALS 

TABLE 4. Reflectivity of minerals Continued

Name

Semseyite __ _ . .... ..... -------- 

Silver... ....... ........................

Stibnito.... .. -

Teallite. ._.. . ... -

Tenorite... ___ .... ___ ... ____ .

Tetrahedrite.... ....... -.-.-.--.-.-..-..

Troilite...... ........ ...... ...-....--.

Vlolarite............... ..............

Symbol

TiOi .-.- ....

(Co,lS7i,Fe)As2. ........................

2Ag2S.MnS.Sb2S3 ..... .............. ..

PbS.AssSj... ....... ................. ..

9PbS.4Sb2 ?3 ...................... .. 

AR.. .......... ........................

9Mn203.4Fe203.Mn02.3CuO... .......

(Co,Ni)As3- .. -- .-. --.--

(Co,Ni)As2~. .

PtAsj. ................... ..........

ZnS..... ........................... ..

CujS.FeS.SnS2. ................ ....

AgsS.Fe<S6 .. ..........................

SbsSj..... ...... ................. ..

(Ag,Cu)sS._ ...........................

3Cu8S.V2S5--. ------------------------ .

AuAgTe4.- ---------------- --------

RnS.PbS..... .........................

Te.......... ..........................

5Cu?S.2(Cu,Fe)S.2As2S3 ------ 

CuO. ................. ...... ........ ..

Bi2(Te,SVi -_ 

5Cu2S.2(Cu,Fe)S.2Sb2S.i . ......... ..

HgSe. ............. ...... ........... -

FeS-. . .. ... ........ ....

NiSToS... .............................

Uranate of TJ,Pb, etc. _ . .............

(Ni,Fe)sS4 -. 

Cu2Te. _ . ________ .....

(Co,Ni)SbS... ..................... ...

(Fe,Mn)O.W03.~ ~

ZnS....... ............................

PbS.Sb2S3 ........... ............

Photometric 

measurement

21

57

32.5

39 

95

21

61

56

18

22

28

30-43

27

27

56

40

63

48

29

29

48

16-18

16

18

37

Photoelectric cell 

measurement

24.0

Ofi A

44.0

38. 7-40. 5 

S6. 0-97. 0

58.2

58 2

cc o

18.5

28. 0-29. S

31.4

39. 6-42. 2

32. 1-34. 0

31.2

58.6

36.2

59. 3

30. 0-32. 1

22.2

44.0

32.8

27.1

39.8

49.7

16. 1

44.2

33. 5

49.7

1Q r,

18.3

i Schneiderhohn, Hans, and Ramdohr, Paul, op. cit., Band 1, pp. 162-163.

> Moses, J. H., op. cit.

> Etches too rapidly for positive reading.

4 On one specimen; 16.5 on another.

8 Polish was poor.

6 On one specimen; 19.9 on another.

TABLE 5. Summary of tests for precious-metal elements

Element

Gold.............

Test

plus concentrated 

HC1.

thiocyanate.

mate.

Distinctive in 10 times 

as much of 

Pt, Rh, Ru, Ir, Os, Au..

Pt, Ir, Rh, Ru, Os, Au..

Pd, Rh, Ru, Os, An....

Ir, Rh, Ru.... ..........

Ir, Rh.Ru.. ............

Pd, Rh, Ru, Au........

Au, Rh...... ...........

Pt, Ir, Au .. .... ......

Ru, Au._. _ ...........

Pt, Ir, Os, Ru.-.-._. .

Ir, Au..... .........

Pt, Rh, Ir, Os, Au......

Pt, Ir, Os...... .........

Pt, Pd, Rh, Ru. Ir, Os..

Distinctive in 

equal proportions of 

Pt, Os-.. ...... .

Pt, Ru.... ......

Pd.Rh, Ru.....

Pt, Pd, Os......

Pd..............

Au.. ............

Not distinctive 

in presence of 

Ir.

Pd, Os, Au.

Pd, Os, Au.

Os, Pd.

Pt, Pd, Ir, Rh.

Pd, Au.

Ru.

Pd. Rh.
PLATE 2 

PHOTOMICROGRAPHS OF ORES POLISHED BY HARVARD PROCESS

A. General field of pyrite in quartz gangue and containing inclusions of gold.

Hollinger mine, Porcupine district, Ontario. X 1,000. 

13. General field of pyrite with composite inclusion containing gold and three

other soft minerals (probably tellurides) and gangue. Ferreira deep mine,

Rand district, South Africa. X 860. 

C. Dendrites of bornite in field of bornite-chalcocite, and fractured, corroded

pyrite. 2,000-foot level, .Magma mine, Superior district, Ariz. X 108.

Dark-gray cigar-shaped areas, bornite; light-gray mineral in groundmass,

chalcocite. 

/). Highly porous area of oolitic marcasite (supergene), derived from pyrrhotite;

sphalerite and chalcopyrite have resisted attack. Mary mine, Ducktown

district, Tenn. X 43.

bn, bornite; cc, chalcocite; py, pyrite; ms, marcasitc; cp, chalcopyrite; si, sphalerite.
GEOLOGICAL SURVEY

c
PLATE 3 

PHOTOMICROGRAPHS OF ORES POLISHED BY WATERTOWN PROCESS

A. Supergene native silver replacing supergene chalcocite. Aspen, Colo. X 150. 

B. Native copper, cuprite, and malachite. All are supergene. The order of

deposition is cuprite, copper, malachite. Morenci, Ariz. X 150. 

C. Covellite and chalcocite replacing pyrite. Butte, Mont. X 150. 

D. Intergrowth of magnetite, chalcopyrite, pyrrhotite, and pentlandite. Sudbury, Ontario. X 150.

Ag, native silver; cc, chalcocite; Cu, native copper; Cu2 0, cuprite; mal, malachite; 

cv, covellite; py, pyrite; mag, magnetite; cp, chalcopyrite; po, pyrrhotite; 

pnt, pentlandite; g, galena.
GEOLOGICAL SURVEY
BULLETIN 914 PLATE 3
PLATE 4

PHOTOMICROGRAPHS OF POLISHED SECTION OF SPECIMEN FROM MAGMA 

MINE, SUPERIOR, ARIZ.

Pyrite (py), chalcopyrite (cp), bornite (bn), and quartz (qtz). Same field taken 

with different objectives to illustrate effect of relief. White rectangles on 

A, B, and C show areas covered by B, C, and D, respectively. B (green) 

filter, Bausch & Lomb inverted photomicrographic camera.

A. 32-millimeter objective, 3-minute exposure. X 47. The black borders at the 

peripheries of pyrite grains are very pronounced. They are due to reflection from the beveled sides of the pyrite. (See p. 56.)

B. 16-millimeter objective, 2-minute exposure. X 112. The black borders are 

much less pronounced than in A, owing to the fact that the cone of rays 

striking the surface is flatter than it is with the 32-millimeter objective, 

and more oblique rays strike the beveled edge of the pyrite. The 16- 

millimeter objective is the most satisfactory for most purposes.

C. 9-millimeter (No. 4 Leitz) objective, 1.5-minute exposure. X 200. This picture 

is likewise satisfactory, but the objective was focused on the softer minerals, and the imperfections (small pits) on the surface of the pyrite are 

slightly out of focus. The black borders are very inconspicuous.

D. 4-millimeter objective, 2-minute exposure. X 455. Pyrite is decidedly out of 

focus.
GEOLOGICAL SURVEY
BULLETIN 014 PLATE 4
PLATE 5

MlCROCHEMICAL TESTS FOR ZINC AND SULPHUR

A, B, C, Zinc mercuric thiocyanate crystals obtained from a 0.5-percent solution 

of zinc nitrate. X 175. D, Calcium sulphate (gypsum) crystals obtained 

from a 1.0-percent solution of H2S04 . X 210.
GEOLOGICAL SURVEY

 *

£P* MUBfc(tehihiaii>ife»
BULLETIN 9H PLATE 5

D
GEOLOGICAL SURVEY BULLETIN 914 PLATE 6 

*» \,

\

MICROCHEMICAL TESTS FOR COPPER.

A, Copper mercuric thiocyanate crystals. Obtained from a 0.5 percent solution of copper nitrate. X130. 

B, Copper mercuric thiocyanate crystals. Obtained from a 0.1 percent solution of copper nitrate. X130. 

C, D, From same solution as B.
GEOLOGICAL SURVEY

sir

BULLETIN 914 PLATE 7

C D

MICROCHEMICAL TESTS FOR COBALT AND NICKEL.

A, Cobalt mercuric thiocyanate prisms. From a 1 percent solution of cobalt nitrate. X130. B, Cobalt 

mercuric thiocyanate prisms and spherulites. From a solution containing 0.5 percent of cobalt nitrate 

and 0.1 percent of nickel nitrate. X260. C, Cobalt and nickel mercuric thiocyanate spherulites. From 

a solution containing 0.5 percent of cobalt nitrate and 0.5 percent of nickel nitrate. X210. D, Nickel 

mercuric thiocyanate spherulites. From a solution containing 0.5 percent of nickel nitrate. X210.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 8

: .*r:

'^»J»

' 54

\

 ^ *

- » ^^

x -" *- /

i '_ ^ /%

V.

*Ki>«'

MICROCHEMICAL TESTS FOR SILVER.

A, Silver bichromate crystals. Obtained from a 0.5 percent solution of silver nitrate. X70. B, C, From 

same solution as A. D, Silver bichromate crystals. Obtained from a 0.1 percent solution of silver 

nitrate. X70.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 9

&

Q 9« £ ' o

ct-' 

V <;?

>«M

<#

W 

<**<,*»

<5 

o

O

°4^>a jp*J? ??3 0 . <?~a ft

^

*CS

f~\

0

/rM:

SQfc oP1

4 * *o» ^

B

K1

V

; a *f ! Q*?; 

-,«+»<0

0^

oo fl>

^' o

Q

* * 

*J

Q

«i

,5 ?WQ»

po a,

cQ

a

&

QtlC^ * ^ Lt

'<** *

^

-^ <& dP3^ 

- jP

'<?

o OQ<? v

flb 

V

~V 9j&

* £e .

S<

a **o

,..»!,_»-.,

?t.

!*

^<Sv o 

%

MICROCHEMICAL TESTS FOR ARSENIC AND TELLURIUM.

^4, Cesium chloro-tellurite crystals. Obtained from a 1:5 HCI solution containing 1 percent of Te. X70. 

B, From same solution as A. C, Ammonium arseno-molybdate crystals. Obtained from a 1:7 HNOa 

solution containing 0.5 percent of ammonium arsenate. X215. D, From same solution as C.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 10 * * ' 

'***: >

MICROCHEMICAL TESTS FOB ANTIMONY.

, B, C, D, Antimony-cesium iodide crystals. Obtained from a 1:5 HCI solution containing 0.5 percent of

antimony iodide. X130.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 11

*«

 f?. > i :-**t «

1 V * a

v-» «-. ?l

It

 

t

®# 

et

»* 

/I

«?r,-v./ f^ay^Vvr -v»-»- - ---

«""> r . N* '^ - ' 

^,v;^:^.^>:- :-

^»:>, C^-|1 .A.

,^s. ^/i*T

o o

O

'A^^

 fJtL/

a

B , 

O a

D

MICROCHEMICAL TESTS FOR BISMUTH AND TIN.

A, B, Bismuth-cesium iodide crystals. X250. C, Bismuth-cesium chloride crystals. X75. D, Tinrubidium chloride crystals. X145.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 12

MICROCHEMICAL TESTS FOR LEAD AND GERMANIUM.

A, Lead iodide crystals. Obtained from a 0.5 percent solution of lead acetate. X 70. B, From same solution as A. C, Sodium fluogermanate crystals obtained from argyrodite, 4Ag2S.GeS2. X 60. D, Lead 

nitrate residue. Obtained by evaporating to dryness a drop of 1:1 HNOs containing galena powder. 

X 70.
GEOLOGICAL SURVEY

/ v j v

BULLETIN 914 PLATE 13 

%

7

\

^ -,

MICEOCHEMICAL TESTS FOR GOLD.

Crystals of gold pyridine double bromide. X250. Agfa color plate. Halftone by The Colorplate Engraving Co., New York, N. Y. Courtesy of G. P. Putnam, E. J. Roberts, and D. Selehow.
GEOLOGICAL SURVEY BULLETIN 914 PLATE 14

C D

MICROCHEMICAL TESTS FOR MERCURY.

A, B, Cobalt mercuric thiocyanate. From a 0.5 percent solution of mercuric chloride. X210. C, Mercuric chloride residue. Obtained by evaporating cinnabar (HgS) powder to dryness with aqua regia. 

X23. D, Mercuric chloride residue. Obtained by evaporating cinnabar powder to dryness on a glass 

slide with aqua regia. X130.
INDEX OF MINERALS GROUPED BY ELEMENTS

Antimony (Sb)

Page

Andorite, Ag2S.2PbS.3Sb2 S3 --------------------------- 130, 144, 149, 156, 267

Antimony (native), Sb.___.._.. 125, 126, 133, 134, 139, 141, 212-218, 268, pi. 10 

Aramayoite,Ag2S.(Sb,Bi) 2S3---.------------------------------ 140,144,268

: Berthierite, FeS.Sb2 S3 -------------------------------_-----.--- 144,156,269

Berthonite, 2(Pb,Cua)S.Sb2S3 -~-- ------------------------------- 145, 269

* Boulangerite, 5PbS.2Sb2 S3 -_---_ ______ _-___________..____-_.__ 116, 124,270

Bournonite, Cu2S.2PbS.Sb2 S3 ------------------------------- 146, 157, 270

Breithauptite, NiSb__------.____._-_-.-...-......_.. ------- 141, 163, 270

Chalcostibite, Cu2 S.Sb2S3 ---------------_-. --._._-___-_--_ 138, 146, 154,272

Corynite, Ni(As,Sb)S_.-.--------..--- __---__.____._-_-_-_- 160, 164, 273

Cylindrite, 6PbS.Sb2 S3 .6SnS2 ------------------------- 129, 130, 141, 145, 274

Dyscrasite, Ag3Sb. ______.__- 118, 119, 120, 121, 125, 126, 133, 134, 139, 141,274

v Fainatinite, Cu2S.4CuS.Sb2 S3 ___-_-_-_-.......__._._.___.-__._--.-_ 139, 275

Franckeite, 5PbS.Sb2S3 .2SnS2 _________________________________ 131, 145, 276

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2 S3 ____.-__________-___._- 139, 143, 276

Freieslebenite, 2Ag2S.3PbS.2Sb2S3 __-_--------------------- 142, 155, 158, 276

Geocronite, 5PbS.Sb,S3 --------.---------------------------------- 117,277

Goldfieldite,6CuS.Sb2 (S,Te) 3 ---_---_--..--_...-._.-.._-.---.---.- 136 

Jamesonite, 4PbS.FeS.3Sb2S3~ ----------------------------- 116, 123,279

 Kaffllite, Ni(Sb,Bi)S__---i.__----.---_-..-.-____.__ --------- 140,163,279

Keeleyite, PbS.Sb2 S3 - ------r ---- ---. ------------------------ 155,279

Kermesite, 2Sb2S3.Sb2 03 . _... T ---_-_-._..._-...__..__..._^ 128, 137, 145, 279 

Livingstonite, HgS.2Sb2S3 _ _----------.---.-_._-.-__.--_ 137, 153, 156, 281

Meneghinite^PbS-SbaSs--.---------------------------------- 117,124,282

Miargyrite, Ag2 S.Sb2S3 .___.___--__ --_..__-_._.,.._ 138, 14.8, 149, 153, 156,282 

Owyheeite, 8PbS.2Ag2S.5Sb2S3 __ ----..----..____-_____---_------- 132,284

, . Plagioriite, 5PbS.4Sb2S3 ------------------------.-------------- 116,123,285

Polybasite, 8Ag2 S.Sb2S3 ------------ 126, 128, 135, 137, 147, 148, 151, 154, 285

Pyrargyrite, 3Ag2 S.Sb2S3 - ---------- 127, 132, 134, 135, 137, 148, 151, 153, 285

Ramdohrite, 3PbS.AgaS.3Sb2SS ----_----- -.___--.---.-_----.._--__ 131

: Samsonite, 2Ag2S.MnS.Sb2 S3 ___--_------...--___._ ._.. ------------ 154

Semseyite, 9PbS.4Sb2S3 -------------------..--__------------------ 116,287

r Stephanite, 5Ag2 S.Sb2S3 . ___-.----_-.-_...... 126, 127, 134,147, 148, 153, 288

Stibnite, Sb2S3 ______-___-____-___.._._____.______..__ 114, 120, 128, 138,288

Tetrahedrite, 5Cu2S.2(Cu,Fe)S.2Sb2S3 -._.. ....__._..__._ .. 136,144,156,290

Ullmanite, NiSbS.._._.----------_.-_...--........._._._._.._ 140, 163, 290

Vrbaite> Tl2S.3(As,Sb) 2S3 -------------- .. .---..--...-_.----- .. 145,291

Willyamite, (Co,Ni)SbS---- _--- --------._.-__..__ 140,144,163,164,291

Zinkenite, PbS.Sb2S3 .-_--...--___-__--......._.__..__..__..__._.._ 116, 292

 i'

Arsenic (As) 

' Algodonite, Cu6As.---_--_---___-__--....._..___..------- 112, 117, 120, 267

Arsenargentite, Ag3As__________-___--__. .__.._.._.._.___________. 113, 268

Arsenic (native), As .--_-------_---.--.--....._.._ 121, 208-212, 268, pi. 9

Arsenopyrite, FeAsS-_-___ .________.__._ _.____.__.___.-._.- 161, 164, 268

Baumhauerite, 4PbS.3As2S8 ---------------.---------.------------- 169, 268

129151 40 20 297
298 MICKOSCOPIC DETERMINATION OF ORE MINERALS

Page

Chloanthite-smaltite, (Ni,Co)Asj--____._____---____... 159, 160,272 

Cobaltite, CoAaS-_ --------- ------ - ----- 164, 166, 167, 273

Colusite, (Cu,Fe,Mo,Sn) 4 (S,As,Te)3-4 ----------------------------- 127

Corynite, Ni(As,Sb)S_.._ I--- 160, 164, 273

Domeykite, Cu8As_---__-----_--_--_-____--__.__._._._.._ 112, 117, 274

Dufrenoysite, 2PbS.As2S8 -- ----------- -- .- 169,274

Enargite, Cu2S.4CuS.AsaSj.___-_ -- __-._ __ ___-_ __ 138,154, 275 

Gersdorffite, NiAsS- - _.--.- -.--.--- --_ _.-__- 161, 164, 277 

Glaucodot, (Co,Fe)AsS__ --_--- ----------_---_---_ __ 161,164,277

Gratonite, Pb9As4 Si5 ----------------------------------------------- 147

Guitermanite, 3PbS.As2S3 _._._---_..__-.._.____._____-_.__ 124, 147, 278

Huntilite, Ag3As (identical with arsenargentite) ________________________ 113

Jordanite, 4PbS.As2S8 - --_-.- ---- -.__ 124, 279

Lengenbachite, 6PbS.(Ag,Cu) 2S.2As2S3-_____..._____..__.____._.__. 169,280

Leucopyrite, Fe3As4_-_____-- -----------------______ 161,164,280

Loellingite, FeAs2 ________________________________________ 161, 281

Lorandite, TlaS.Ae^Ss- -_. -_-__ _ . 147, 281

Luzonite, Cu2S.4CuS.As2S3 (variety of enargite)____-_-___-______._._ 138,154

Maucherite, Ni3As2 --_-_______--_ ._-____-____._______._ 121, 282

Niccolite, NiAs__________________________________________ 121,160, 283

Orpiment, As2 S3 --_-._-_----- ---------------_________ 153, 284

Pearceite, 8Ag_S.As_Sa ----_--------_----_----_---_---___-__ 135, 284

Plessite, (Ni,Fe)AsS-___.________________________________________ 160,285

Proustite, 3Ag2S.Aa2S8 -_-------------------------__ 133, 137, 151,152, 285

Rammelsbergite, NiAs2 ____._---__--_-_-_-__________________ 121, 160, 286

Rathite, 3PbS.2As2S3 ____.___-----_---_---______-_____________ 169, 286

Realgar, AsS____ -_ ----_--------------------_- 152, 158, 286

Safflorite-loellingite, (Co,Pe)As2_--.-.---.---------------------__ 160,287

Seligmannite, Cu2S.2PbS.As2 S3 _-------------------------------___ 169

Skutterudite, (Co,Ni)A8s---------- ---- -- ------ _- 160, 162, 288

Smaltite-chloanthite, (Co,Ni)As2 ________________________ 159,160,288

Sperrylite, PtAs2------..------------------------------------_--_-__ 165

Tennantite, 5Cu2S.2(Cu,Fe)S.2As2 S3 _._____________________ 136,144,290

Vrbaite, T_aS.3(A8,Sb) 8Ss___-_-__--____-__-__ ___________________ 145,291

Whitneyite, Cu9As-_-___ - _------- -_ _ _ 112,117, 291

Bismuth (Bi) 

Aikinite, Cu2S.2PbS.BiaS8 -__---------_ _-__--- 122, 123, 142,145, 266

Alaskaite (Pb.AgOS.BijSs...-- ______ _~-__-~_--__-_ ____ 130,142,267

Aramayoite, Ag2S.(Sb,Bi) 2S3 ------------------_-----_--_____ 140, 144, 268

Benjaminite (Cu,Ag) 2S.2PbS.2Bi2 S8 ____ _.____________________ 115,122,269

Bismuth (native), Bi________ _ -_-__________ 115, 212-218, 269, pi. 11

Bismuthinite, Bi2S3 ._______________________________________ 116,123, 269

Bismutoplagionite, 5PbS.4Bi2S3 _________________________ 116,131,146, 269

Cosalite, 2PbS.Bi2Sn_________ ______ ___________________________ 130, 273

Emplectite, Cu2S.BiaS8 ___________--_----_____-___-___________ _ 124,146, 275

Galenobismutite, PbS.Bi2S8 ---_-_----_____ _--_--_____-______ 122, 142,277

Guanajuatite, Bi2(Se,S)3--------------------------------_---_- 124,146, 278

Kallilite, Ni(Sb,Bi)S_____.___ _-----_- _. _-_- __-____ 140, 163,279

Klaprothite, 3Cu2S.2Bi2Sa____ _- _----_ __----_____- 120, 139, 152, 280

Matildite (Ag2 ,Pb)S.Bi2S8 __-__ __ _ __ _ ______________________ 170,282

Rezbanyite, Cu2S.3PbS.5Bi2S8 ___ _ _ ____________________________ 124,286

Schapbachite, Ag2S.PbS.Bi2S8____---_ ___-_______________"_ 130, 287

Tetradymite, Bi2 (Te,S) 3 ._____________________________________ 115,121, 290

Wittichenite, 3Cu2S.Bi2S3 ____-----_-_______________ 142, 291
INDEX OF MINERALS GROUPED BY ELEMENTS 299

Cadmium (Cd) Page 

. Cadmium, Cd_. ___________________________________________________ 190

Carbon (C) 

Graphite, C ------------- .....__......_._....._.----........_-.. 159

Chromium (Cr) 

Chromite, FeO.Cr2 03 _--_---------- _______________________________ 166,272

Chromium, Cr.. ___________________________________________________ 240

Cobalt (Co) 

Carrollite (Co,Cu) 3 S4 --------------------------------------------- 142, 271

Cobalt, Co-_-.-.-.. --------------------------------------- 193-198, pi. 7

Cobaltite, CoAsS-___--___-_-_--------_-_-__--_-_--_-_--- 164, 166, 167,273

Glaucodot (Co,Fe)AsS--------------------------------------- 161, 164, 277

Linnaeite, Co3S4 - ________________________________________________ 143, 281

v Safflorite-loellingite (Co,Fe)As2 ----------------------.-------_-_--- 160,287

Siegenite (Ni.CoJsS^ -- ---.------_---_--.------_---_._____- 143,287

Skutterudite (Co.NiJAss--. ----------------------------------. 160, 162, 288

Smaltite-chloanthite (Co,Ni) As2 _ __ _ _________________________ 159, 160, 288

Willyamite (Co.NiJSbS. -------------------.--..--- 140, 144, 163, 164, 291

Copper (Cu) 

Aikinite, Cu2S.2PbS.Bi2S3 -------------------------- 122, 123, 142, 145, 266

Algodonite, CueAs___-___-___---_----_-__________________ 112, 117, 120, 267

Benjaminite (Cu > Ag)jS.2PbS.2Bi2Ss--------_-___-____--_______. 115, 122,269

Berthonite, 2(Pbl Cuj)S.Sb2S8 --- ---------- .__-_______-_-___-_.____ 145,269

Bornite, Cu5FeS4 -_ _ ___________________________________________ 119,270

Bournonite, Cu2S.2PbS.SbaS3 -_-_--_ ------------------------ 146,157,270

Carrollite (Co,Cu) 3S4 -----------------------------------_-----_-_- 142,271

Chalcocite, CuS --------------------------------- 113, 114, 118, 119, 270

Chalcopyrite, CuFeS2______-_-_-___-________-____ 143, 146, 155, 157, 159, 271

Chalcostibite, Cu2S.Sb2S3 -_---------.-.-. _________________ 138,146,154,272

Chalmersite. See Cubanite.

Colusite, (Cu,Fe,Mo,Sn) 4 (S,As,Te) 3 _4 -. _______________________________ 127

Copper (native), Cu__ _ __ _ __________________ 113, 118, 190- 192, 273, pi. 6

Covellite, CuS___-__-_------_-______-______________..________ 138, 154, 273

Cubanite, Cu2S.Fe4S8 _-__-_-_-_-____-______-__________________ 146, 157, 274

Cuprite, Cu2 0 ________________________________________________ 114,274

Delafossite, Cu2O.Fea03--------------------. -.-_-_---_-_--._-___. 150, 274

Domeykite, Cu3A8_____---_----_____-_-__.____________..______ 112, 117,274

Emplectite, CujS.BijSa.----.---------_----__-_-_-_-___-_-____ 124, 146, 275

Elnargite, Cu_S.4CuS.As2Ss~-- ------------ ____________________ -138, 154, 275

Elucairite, Cu2Se.Ag2Se-------------------_-----_-... -.-.--... 135,151,275

Famatinite, Cu2S.4CuS.Sb2S3 . _ __________________________________ 139,275

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 ---------------------.- 139,143,276

Germanite, Cu3 (Fe,Ge)S4 ---------_-._--. ________________ _________ 155, 277

Goldfieldite, 6CuS.Sbj(S,Te) 8 ----. _________________ _________________ 136

Eilaprothite, 3Cu2S.2Bi2S3 ______________________________ 120, 139, 152, 280

Klockmannite, CuSe_ _____________________________________________ 136, 280

Lengenbachite, 6PbS. (Ag,Cu) 2S. As2S3 _ _____________________________ 169, 280

Luzonite, Cu2S.4CuS.As2S3 (variety of enargite)_---_-_____ ___________ 138,154

Melaconite, CuO (variety of tenorite) _____._______'________.____ 132, 150, 282

Mottramite, 2Cu0.2PbO.V2 05 .H2 0 ________________________________ 129, 283

Bezbanyite, 3PbS.Cu_S.5BiaS3 _______________________ _ _________ 124, 286
300 MICROSCOPIC DETERMINATION OF ORE MINERALS

Page 

Rickardite, Cu8Te2 - --------------1 _------..---_..-------__------ 113, 286

Seligmannite, (5u2S.2PbS. As2S3 - ------------------------------------- 169

Stannite, Cu2S.FeS.SnS2 -------------------------------------- 144,147,288

Stromeyerite (Ag,Cu) 2S------------------------------------------- 126, 289

Sulvanite, BC^S-VtSt---- ------------------------ 152, 154, 156, 158,289

Tennantite, 5Cu2S.2(Cu,Fe)S.2As2S3 - __-_-------.--.._--.------ 136, 144, 290

Tenorite, CuO.------------------------------------------------- 132, 150

Tetrahedrite, 5Cu2S.2(Cu,Fe)S.2Sb2 S3 - ._..-_-- -_-_ -_--- 136, 144, 156, 290 

Umangite, Cu3Se2 -..__--.--------._...-.---..-._..___._-__._..___ 127, 291

Weissite, Cu2Te._...___..._...._......._. _'e ..................... 113, 291

Whitneyite, Cu9As ..A ...:__.__---_-__- 112, 117,291

Wittichenite, 3Cu2S.Bi2S3----------------------.---_--------.----- 142, 291

Germanium (Ge) 

Argyrodite, 4Ag2S.GeS2 . .__.-_-.--...._......__..__... 136, 138, 152, 154, 268

Canfieldite, 4Ag2S.(Sn,Ge)S2 _.....-.....__....._..__._. 132, 133, 135, 136, 271

Germanite, Cu3 (Fe,Ge)S4---.-----..-.-.------------_..-------..-- 155, 277

Germanium, Ge___--_-_._-_--___.___...._._._..._.... ----- 240-242, pi. 12

Gold (Au) 

Calaverite, (Au,Ag)Te2 --~---------------------- ---------- 122,124,270

Electrum, Ag, Au-_-_________ _____ _ -V - ----------------- 119, 274

Gold (native), Au__ ____.._.._....__-_..-_. 152, 155, 223-225, 278, 296, pi. 13

Krennerite, (Au,Ag)Te2 --_---_---_-_____-__-__-___,.__-__----__'___ 122, 280

Nagyagite, Pb,Au,Te,S -_-_-_ __.___.*___ ,._ _ 147, 158,283 

Petzite, (Ag,Au) 2Te-- - --__ -_1_ _ __. _ 128, 133, 140,284 

Sylvanite, AuAgTe4 --------------------------------- 122, 125, 142, 147,289

Iridium (Ir) 

Iridium, Ir_..__-_-.____-__-_--_______._____-_.____....-..-______ 247, 296

Iron (Fe) 

Arsenopyrite, FeAs8....__.....---....-.._._....-__-_._.____. 161, 164, 268

Berthierite, FeS.Sb2S3 --------- ----------------------- ------- 144, 156, 269

Bornite, Cu5FeS4 -------- ----------------._-_-__.--_ ----------- 119,270

Bravoite, (Fe,Ni)S2 ...._-.---_-_-........_._____._._. ----------- 163,270

Chalcopyrite, CuFeS2 ---...-----------...-..----_- 143, 146, 155, 157, 159, 271

Chalmersite. See Cubanite.

Chromite, FeO.Crg03 ---------------------------- ----------- 166, 272

Colusite, (Cu,Fe,Mo,Sn)<(S,AF,TO:'-<------------------- - --- ------- 127

Cubanite, Cu2S.Fe4S5 ---.--------- --- -- ------- 146, 157, 274

Delafossite, CuO.Fe2 03 - -- - - - -. ---.--_-.___ 150, 274

Ferberite, FeO.WOs--.-.------------------------------ ----------- 167, 275

Franklinite, ZnO.Fe2 03 .-----------_------------------- ------- 165, 166, 276

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 --------------- ------- 139, 143, 276

Germanite, Cu3 (Fe,Ge) 84-------------- -.-- ----_.-.-..----.--- -- 155,277

Glaucodot, (Co,Fe)AsS ----------.-------_ - ------------ 161, 164,277

Goethite, FejO3 .H2 0_...--..._----_----------- -..... ----------- 167,277

Hematite, Fe2O3 . _____. --_-_ ..__.._ ._._ '166, 167,278

Hollandite, m(Ba,Mn) 2Mn08 + nFe4 (MnOj) 3 - --__-.-.__-.--_____.-__ 166, 167

Ilmenite, FeO.TiO2 ._...__.........-.......___. __. .._..___ ___ 168,279

Iron, Fe -_-------..------------_-_-_ _-__-.-.__. .,.-_-_-.._._._. 198

Jacobsite, MnO.Fe2 03 .-------------------------------- ---------- 166, 168

Jamesonite, 4PbS.FeS.3Sb2 S3 . --.- _ . - 116, 123,279 

Leucopyrite, Fe3As4 .-. ----_ .___- __ ..__ . - 161, 164, 280
INDEX OF MINERALS GROUPED BY ELEMENTS 301

Page 

Doellingite, FeAs2 _..._....._... ....___.__....___.._._...._._.____ 161, 281

Magnetite, FeO.Fe2 08 _-_--___---_-_-------_--____---_ 149, 158, 165, 167,281

Marcasite, FeS2 __-.._._.....__.._...-._.....____._.....__._. 161, 164, 281

Pentlandite, (Fe,Ni)S..........-........._.--.__...__--.....___._ 143,284

Plessite, (Ni.Fe)AsS_...._...._.._....._......____...___..._.____ 160, 285

Pyrite, FeS,._______.._______________._____.__________ 161, 162, 164, 165,286

Pyrrhotite, FeSi+________________________________________ 131, 148, 157,286

Safflorite-loellingite (Cof Fe)As2 -_____---______-.___________________ 160,287

Sitaparite, 9Mn2 08 .4Fe_08 .Mn02 .3CaO---__- ..___--______-._____._. 167, 168

Stannite, Cu2S.FeS.SnS2 ________--______----'____-_-_-_-___-_._ 144, 147, 288

Sternbergite, Ag.S.Fe4S5 __._______________________________ 129, 131, 149, 288

Tennantite, 5Cu2S.2(Cu,Fe)S.2As2S3_----_---_-__-----_---_--__ 136, 144,290

Tetrahedrite, 5Cu2S.2(Cu,Fe)S.2Sb2S3 --_-__________________ 136, 144, 156, 290

Troilite, FeS_.______________________________________________ 116, 131, 290

Violarite, (Ni,Fe) 3S4 _-_ ---------------------------------------- 123, 291

Wolframite, (Fe,Mn)O.WOs ___.___________________________________ 168, 292

Lead (Pb) 

Aikinite, Cu_S.2PbS.Bi_S8 ______._____________-________ 122, 123, 142, 145, 266

Alaskaite, (Pb,Ag_)S.Bi2S3------------------------------------ 130, 142, 267

Altaite, PbTe.._______________------------------------------ 114, 121,267

Andorite, Ag2S.2PbS.3Sb2S3 ._-._______________________ 130, 144.149, 156, 267

Baumhauerite, 4PbS.3As2S3 __- __---------------_--_-------__ -- 169,268

Benjaminite, (Cu,Ag) 2S.2PbS.2Bi2S3 - __________________________ 115, 122, 269

Berthonite, 2(Pb,Cu_)S.Sb,S8 ----------------------------------- 145,269

Bismutoplagionite, 5PbS.4Bi_S3 -_-_----------------------- 116, 131, 146,269

Boulangerite, 5PbS.2Sb2S3 - ________________________________ 1 _. 116, 124, 270

Bournonite, Cu8S.2PbS.Sb2S3 ------------------------------- 146, 157, 270

Clausthalite, PbSe_.____-_____-_____-_-___-_--_-__-___-_-_-._ 129, 140, 272

Cosalite, 2PbS.Bi.S8 ___------------------------------------------- 130, 273

Cylindrite, 6PbS.Sb2S3 .6SnS2 _ _________________________ 129, 130, 141, 145, 274

Dufrenoysite, 2PbS. As2S3 .________________________________________ 169, 274

Franckeite, 5PbS.Sb2S3 .2SnS2 .____________________ . ____________ 131, 145,276

Freieslebenite, 2Ag2S.3PbS.2Sb2 S3 ..________________________ 142, 155, 158,276

Galena, PbS______.._______________-__-______________________ 115, 129,276

Galenobismutite, PbS.Bi2 S3 -__________________________________ 122, 142, 277

Geocronite, 5PbS.Sb.Sj_-_-___-__-___---__--___-______-_-_________ 117, 277

Gratonite, PbjAs^Sis __________._____-_____-______..__._ .__-___.___. 147

Guitermanite, 3PbS.As.S8 ---------------------------------- 124, 147,278

Jamesonite, 4PbS.FeS.3Sb2S3-__-_-_---__---__-___--_----______ 116, 123, 279

Jordanite, 4PbS.As2S8------_-_---_---------------..----_-----__-_- 124, 279

Keeleyite, PbS.Sb2S8 __--_________________-____________._______-__ 155, 279

Lead, Pb_______________________ -------------------------- 128,220-223

Lengenbachite, 6PbS.(Ag,Cu) 2S.2As2S3 ---------------------------- 169, 280

Matildite, (Ag2 ,Pb)S.Bi2S3 -_________________________________________ 282

Meneghinite, 4PbS.Sb2S3 -_----------------------------------- 117, 124,282

Mottramite, 2Cu0.2PbO.V.08 .H2 O ________________________________ 129,283

Nagyagite, Pb.Au.Te.S....................................... 147, 158, 283

Naumannite, (Ag2 ,Pb)Se--__-___..____-_____..-._-_-_.__-_-_-l_____ 135,283

Owyheeite, 8PbS.2Ag2S.5Sb2S3 _.__. __________________________.__ ___ 132,284

Plagionite, 5PbS.4Sb2S8 __ _----__------- ---.. --_---_--___ 116, 123,285

IRamdohrite, 3PbS.Ag2S.3Sb2S3 ___________________________.__.__- 131

Rathite, 3PbS.2As2S3 -_-.----_.__._--_.__-.-._-_._-._____ 169, 286

Rezbanyite, Cu2S.3PbS.5Bi2S3 _______.____________.----.___- 124, 286
302 MICROSCOPIC DETERMINATION OF ORE MINERALS

Page 

Schapbachite, Ag2S.PbS.Bi2S3 ______ ._ __.__ ____________ _ 130, 287

Seligmannite, Cu2S.2PbS.As2S3 __.________.._.._._._......._________. 169

Semseyite, 9PbS.4Sb2 S3 ---------------------------------------- 116, 287

Teallite, SnS.PbS. ------ - --------_-----.--- 132,150,289

Uraninite, uranate of U, Pb, etc______________-_--__----------------- 163

Zinkenite, PbS.Sb2S3 .-------------------------------------------- 116,292

Manganese (Mn) 

Alabandite, MnS.__._..____._-__-_______ __ ____________ _ 115, 266

Braunite, 4MnO.3Mn02 .Si02 ______________________________ 166, 167,270

Chalcophanite, (Mn,Zn)0.2Mn02 .2H2 0 __________.__._-__--_._._-__ 157,271

Hauerite, MnSa __._ _--------------_-__---------------- 130, 149, 278

Hausmannite, MnO.Mn2 03 ____-_.._..._____.._-.. 132, 150, 162, 165, 278

Hollandite, m(Bat Mn)jMn05+nFe4(Mn08)8----------------------- 166, 167

Huebnerite, MnO.WOi- _--------------------- 159, 168, 278

Jacobsite, MnO.Fe2 03 -._-_._._-_____._.._.__..____-------._. _ 166,168

Manganese, Mn_________..___-.__-_-___--__-_-_-____---__.-------_ 226

Manganite, Mn2 03 ._.__________.________..__._..____-____._.. 150, 281

Polianite, Mn02 -___-__.________..._........_.-_._---.__-____ 168

Psilomelane, Mn02.Mn_0___ _ -__ _ 162, 285

Pyrolusite, MnO____________---------------------------__ 150, 159, 286

Samsonite, 2Ag2S.MnS.As2S3 _____________._-._..__.._-____________._ 154

Sitaparite, 9Mn2O3 .4Fe2O3 .MnO2 .3CaO_-----____-___--______.___.__ 167, 168

Wolframite, (Fe,Mn)O.W03 _ _ _______-_ -__ _-___-_-____ 168,292

Mercury (Hg) 

Cinnabar, HgS--_-_-...-_-_-___._._-._..___-_..__-___--__.-_.____ 158,272

Coloradoite, HgTe.._ .- - -----.- -_- ____ 129, 140, 155, 273

Livingstonite, HgS.2Sba S8~ ---------------------------- 137, 153, 156, 281

Mercury, Hg__-_----_-_---_-__------.-_----.._-_--__---___-__ 225, pi. 14

Metacinnabar, HgS_------_-___--_-_-__--..--------________-_- 155, 158, 282

Tiemannite, HgSe__-______-___._--___.---_-__--__.___-______ 152, 156, 290

Molybdenum (Mo)

Colusite, (Cu,Fe,Mo,Sn) 4 (S,As,Te)3_4 ------------------------------ 127

Molybdenite, MoS2 -- - --------------------------- _ - 159, 283

Molybdenum, Mo__________._---__-________-__--___-_______. ____ 233-235

Nickel (Ni) 

Bravoite, (Fe,Ni)S2 _-__ 163, 270

Breithauptite, NiSb______________________________________ 141, 163, 270

Chloanthite-smaltite, (Ni,Co)As2 -______-_-_-------_---____-_-- 159, 160, 272

Corynite, Ni(As,Sb)S_____.____--_-_-_-___._---_-_-- __-__ 160, 164, 273

Gersdorffite, NiAsS____-______. _ . _______-_ -_-_ _______ 161, 164, 277

Kallilite, Ni(Sb,Bi)S_-_--__-_---_-_-_._-_-___--_-______-____ 140, 163, 279

Loellingite (nickeliferous), (Fe,Ni)As2 -------.-----_-_-_---_--------- 161,281

Maucherite, Ni3As2 ___________..--_-___-__.__-____.-..____________ 121, 282

Melonite, NiTe2 _____________________________ _ _ ________ 115, 122, 282

Millerite, NiS__________________________ ._._--.-----.________ 124, 145,283

Niccolite, NiAs____-_________________-_-_---_-___-________ 121, 160, 283

Nickel, Ni______ ___ _ _______ __ _ _______ 192, pi. 7

Pentlandite, (Fe,Ni)S__-_-__--__-_-_-----------.--_--_-___-___:__ 143, 284

Plessite, (Ni,Fe)AsS_____ _____ _ ______-_ __________________ 160,285

Polydymite, Ni3S4-- _ _ _----- 123, 143, 285

Rammelsbergite, NiAs2 _.___________.-_.---__-...___________ 121, 160, 286
INDEX OF MINERALS GROUPED BY ELEMENTS 303

Page 

Siegenite, (Ni,Co) 3S4 ------------------------------------------ 143, 287

Skutterudite, (Co,Ni)As8 -__ __ 160, 162, 288

Ullmanite, NiSbS ---------------------------------------- 140, 163, 290

Violarite, (Ni,Fe) 8S4_---_ - --.------- ---- --- - -- - 123, 291

Willyamite, (Co,Ni)SbS __-._... . 140, 144, 163, 164, 291

Osmium (Os) 

Osmium, Os_..-.___-._-_______-__--_--__-___-______-__.____ 248,284

Palladium (Pd) 

Palladium, Pd... ----_.--_-._-__-----_--_-------_-----__---__- 244,296

Platinum (Pt) 

Platinum, Pt ___________________-__- _-__ __-__--_____ __ 245, 296

Sperrylite, PtAs2 ------------_---------___..____________ 165

Rhodium (Rh) 

Rhodium, Rh...._.__..__.._._____________________ 250,296

Ruthenium (Ru) 

Ruthenium, Ru__-___.___________.____________ 250, 296

Selenium (Se) 

Aguilarite,Ag2 (S,Se)--.---------------------------------- 128,134,141,266

Clausthalite, PbSe...__._.__.___.._._..._.._..._ ..._....... 129,140,272

Eucairite, Cu2Se.Ag2Se___-._--__-_--------_--------------_. 135,151, 275

Guanajuatite, Bi_(Se,S) 8 ____ _ 124, 146, 278

Klockmannite, CuSe_-___-----__-_---_--_--_------.--_-.____--- 136, 280

Naumannite, (Ag2,Pb)Se_-_-._----------------------_______._.. 135, 283

Selenium, Se_._._._.--__._ _ __ - - - 204-206

Tiemannite, HgSe________________-----------________ 152,156, 290

Umangite, Cu3Se2 __-____-__-__-----_--______--- 127, 291

Silver (^g) 

Aguilarite, Ag_(S,Se) __ _ 128, 134, 141,266

Alaskaite, (Pb,Ag_)S.Bi_S_-- 130, 142, 267

Andorite, Ag2S.2PbS.3Sb2Sj-__________________________ 130, 144, 149, 156, 267

Aramayoite, Ag2S.(SblBi) 8S8 _ - __-_-------- 140, 144, 268

Argentite, AgaS -------------------------- 125, 126, 147, 148,150, 151, 268

Argyroditel 4Ag_S.GeSj -------------------------- 136,138,152,154,268

"Arcenargentite, Ag3As_______,.__----_-__-_-_--___--_-__ ____-_ 113, 268

Benjaminite, (Cu,Ag) 2S.2PbS.2Bi2S3 - ----------------------- 115, 122,269

Calaverite, (Au,Ag)Tea_____..________________________________ 122, 124, 270

Canfieldite, 4Ag2S.(Sn,Ge)S2 __________________________ 132,133,135,136, 271

Cerargyrite, AgCl._____________________________________________ 152, 271

Dyscrasite, Ag3Sb.__________ 118, 119,120, 121, 125, 126, 133, 134, 139, 141, 274

Electrum, AgAu_______________________________________________ 119, 274

Eucairite, Cu_Se.Ag_Se______-_______-________________________ 135, 151, 275

Freibergite, 5(Cu > Ag)_S.2(Cu,Fe)S.2SbaS8----_ ------__-_-_ 139, 143, 276

Freieslebenite, 2Ag2S.3PbS.2Sb2S3 ----------------------- 142, 155, 158, 276

Hessite, Ag2Te___________--------_________________ 126, 129, 142, 278

Huntilite, Ag3As (identical with arsenargentite)_____-__-__--_-_____.__ 113

Krennerite (Au,Ag)Te2 ---_----r ------___-_____--____-_.________ 122, 280

Lengenbachite, 6PbS.(Ag,Cu) 2S.As2 S3 _-----------_-_-______ 169, 280

Matildite (Ag2 ,Pb)S.Bi2S3 ------------------------------------- 170, 282.

Miargyrite, Ag2S.Sb«S8 ___ _____-----_----- 138, 148, 149, 153, 156, 282
304 MICEOSCOPIC DETERMINATION OF ORE MINERALS

Page 

Naumannite (Ag2 ,Pb)Se___.___._.____..___..._.__...._ __....._. 135,283

Owyheeite, 8PbS.2Ag2S.5Sb2 S3 --------------------------------- 132, 284

Pearceite, 8AgaS.As2S8 -_~-~---- ------------------------- 135, 284

Petzite (Agf Au)_Te_____-__ __ _ 128, 133, 140, 284

Polybasite, 8Ag2S.Sb2S3 ------------ 126, 128, 135, 137, 147, 148, 151, 154, 285

Proustite, 3AgaS.AsaS3 -- ------- -- 133, 137, 151, 152, 285

Pyrargyrite, 3Ag2S.SbjS3 ----------- 127, 132, 134, 135, 137, 148, 151, 153, 285

Ramdohrite, 3PbS.Ag2 S.3Sb2S3 ------------------------------------- 131

Samsonite, 2Ag2S.MnS.Sb2S3 -_ ___ - _ _,-__-- _--- __ 154

Schapbachite, AgaS.PbS.BiaS3 __--.--------_------.--.-_--------- 130,287

Silver (native), Ag.__ - --_. ----. -_- 113, 114, 119, 199-204, 288, pi.. 8

Stephanite, 5Ag2S.Sb2S3 _ ____-_ _- --- -._ - 126, 127, 134, 147, 148, 153, 288

Sternbergite, Ag_S.Fe4S5--------------_ -- 129, 131, 149, 288

Stromeyerite (Cuf Ag)2S____-_ -_- - 126, 289

Sylvanite, AuAgTe4___ -- _ _-_ _ _ _ 122, 125, 142, 147,289

Sulphur (S) 

Sulphur, S __ -_ ____ - --_- --------- 227-231, pi. 5

Tellurium (Te) 

Altaite, PbTe_- _ _ -- - _ - - 114, 121,267

Calaverite (Au,Ag)Te2------ -- - - - - 122, 124, 270

Coloradoite, HgTe_._---_--__ --------_-__-------_--_-_- 129, 140, 155, 273

Colusite, (Cu,Fe,Mo,Sn) 4 (S,As,Te) 3 _4-------------------------------- 127

Goldfieldite, 6CuS.Sb2 (S,Te) 3 ___.--_- _ _-_.-_. --.- _ 136 

Hessite, Ag_Te._ _- - --__ -_ - __ ------- 126, 129, 142, 278

Krennerite(Au,Ag)Te2 ------------------------------------------ 122,280

Melonite, NiTea ___----------------------------- 115, 122, 282

Nagyagite, Pb,Au,Te,S -- _.__ _. _. .- -_ _- __ 147, 158, 283 

Petzite (Ag,Au) 2Te__-_-._._____.__ ___._._.. __.._ __ 128, 133, 140, 284

Rickardite, Cu8Tea- _--.-----_ --_- ---- ----- --- 113,286

Sylvanite, AuAgTe*-----.---------------------..----- 122, 125, 142, 147, 289

Tellurium (native), Te__-_ -. - __. 115, 117, 121, 125, 206-208, 289, pi. 9 

Tetradymite, Bi2 (Te,S) 3 ._-.--.__-._ _ .__..__._._.-__ -__- 115, 121, 290 

Weissite, CuaTe_-_-_ - --_ -- ----- 113,291

Thallium (Tl) 

Lorandite, T12 S.As2S3 ._ .-___________ __ __ ___ _ ______ 147, 281

Vrbaite, Tl2S.3(As,Sb) 2S3 . ___---__..-__ - -.___ .--._-_-. 145, 291

Tin (Sn) 

Canfieldite, 4Ag2S.(Sn,Ge)S2 .------------------------- 122, 133, 135, 136, 271

Cassiterite, SnOa __.._________________.____ ___________________.. 167,271

Colusite, (Cu,re,Mo,Sn) 4 (S,As,Te) 3 -4 ------------------------------ 127

Cylindrite, 6PbS.Sb2 S3 .6SnS2 - .__________..___ __._____ 129, 130, 141,145,274

Franckeite, 5PbS.Sb2 S3 .2SnS2 _____ __________.________________ 131, 145,276

Stannite, Cu2S.FeS.SnS2 ___-___._ _--____.____________________ 144, 147, 288

Teallite, SnS.PbS_ . _ __ _.__. _____ _ ___________ 132,150, 289

Tin, Sn__ - - .-_ 218-220.pl. 11

Titanium (Ti) 

Ilmenite, FeO.Ti02 --------------------_______--_____..____... 168, 279

Rutile, TiOa .._.._ __________ -___--__ ________________________ 168,287

Titanium, Ti___ _--_ --_ _.__ _ _ _ __ _ 239
INDEX OF MINERALS GROUPED BY ELEMENTS 305

Tungsten (W) Page 

Ferberite, FeO.W03 ------------------------------------------- 167, 275

Huebnerite, MnO.W03 __-------_-------___..____.__._.___ 159, 168, 278

Tungsten, W-------------------------------------------------- 235-237

Tungstenite, WS2 ---------_-----------------_.--_____-_-_-__ 159, 290

Wolframite (Fe.MnJO.WOs- -- - .- -- -- 168, 292

Uranium (U) 

Uraninite, uranate of U, Pb, etc___..______----_-_-_-_-__-__-_-_____. 163

Vanadium (V) 

Mottramite, 2Pb0.2CuO.V2 05 .H2 0->_.______.__.___._._.___._.____ 129, 283

Sulvanite, 3Cu2S.V2S5 ---------------------------- 152, 154, 156, 158, 289

Vanadium, V__-------------- _.__...__..._.___._._......____.____ 237-239

Zinc (Zn) 

Chalcophanite (Mii,Zn)0.2Mn02 .2H8 0._..-..__------..___..._-__.- 157, 271

Franklinite, ZnO.FeaOs ---- - --------------- - ------------ 165, 166, 276

Sphalerite, ZnS..._------.-----------_-_------------- 123, 130, 143, 156, 288

Wurtzite, ZnS___... ..-. - --_ ._-. ... .-.._._.____.. 144, 292

Zinc, Zn__.__._._.___-__.__......-.-..._.--... .....___... 187-190, pi. 5

ALPHABETICAL INDEX OF MINERALS AND ELEMENTS

[Minerals marked * are discredited or doubtful] 

t Page

Aguilarite, Ag2 (S,Se;__._____________________ --- _._ 128, 134, 141, 266

 ., Aikinite, Cu2S.2PbS.Bi2S3 - - ... .......-.--.-_..-- 122, 123, 142, 145, 266

Alabandite, MnS--.-.---.------------------------------------- 115,266

; Alaskaite, (Pb,Ag2)S.Bi2S3 -------------------------------- 130,142,267

Algodonite, Cu9As___ .-_---_ __ -- -------------- 112, 117, 120, 267

-1- Altaite, PbTe.... . ~_---~--~--------_ - 114, 121, 267

Andorite, Ag2S.2PbS.3Sb2S3 . ------------------------- 130, 144, 149, 156, 267

Antimony, Sb..__------------ 125, 126, 133, 134, 139, 141, 212-218, 268, pi. 10

Aramayoite, Ag2S.(Sb,Bi) 2S3 ------------------------------- 140,144,268

Argentite, Ag2S...------------------------- 125,126, 147,148,150,151, 268

Argyrodite, 4Ag_S.GeS. _-_ ----_----_------- 136, 138, 152, 154, 268

Arsenargentite, AgaAs -__----_-________________________________ 113, 268

Arsenic, As..._..._._._.__..-._..__.____.._..-._.. 121, 208-212, 268, pi. 9

*Arsenoferrite, FeAs2 _____________________________________________ 169

Arsenopyrite, FeAsS_______..._._....____..._.._..._.._.._ 161, 164, 268

^ Baumhauerite, 4PbS.3As2 S3 -----__-_-_---_-_-----------_..--_-.--- 169, 268

1 Benjaminite (Cu,Ag) 2S.2PbS.2Bi2S3 -____.__.._...____.__._... 115,122,269

Berthierite, FeS.Sb_S8 -.-------~_--------------------------- 144, 156, 269

Berthonite, 2(Pb,Cu2)S.Sb2Ss- ----~------------~------~~ 145, 269

Bismuth, Bi...__..__.---_--.....-..--_.-.-.-.-_- 115, 212-218, 269, pi. 11

Bismuthinite, EitS,......................................... 116, 123, 269

Bismutoplagionite, 5PbS.4Bi2S3 __.__._.._._._..._._._.___. 116, 131, 146, 269 

Bornite, Cu5FeS4 ----------------------------------------------- 119,270

Boulangerite, 5PbS.2Sb2S3 __-..--_-...._._....__._.___....._. 116, 124, 270

Bournonite, Cu2S.2PbS.Sb2S3 ______._____-___-_._...___..._. 146, 157, 270

2 Braunite, 4Mn0.3Mn02 .Si02 ___..____..__..__.._._._.._.... 166, 167, 270

Bravoite (Fe,Ni)Sa-------..-..-__...__._...._.-.-_-____.-_...._ 163,270

Breithauptite, NiSb_.__....._..__.__._...__._._._..._...._.. 141, 163,270

 Cadmium, Cd_-_--___--_-___-_._--__-_______-____________._____ 190

Calaverite (Au,Ag)Te2 ._.__..._.._.____........__..._.._. 122, 124, 270

' Canfieldite, 4Ag2S.(Sn,Ge)S2 _.____...______._.._._... 132, 133, 135,136,271

Carrollite (Co,Cu)sS4-___________________________________________ 142, 271

 * Cassiterite, Sn02 ______-___---______ ....1....................... 167,271

Cerargyrite, AgCl_____._-_-_..___.._-_-_-_________-_.__.._.._.. 152,271

Chalcocite, Cu2S._.____.____________________________ 113, 114, 118, 119,270

Chalcophanite, (Mn,Zn)0.2Mn02 .2H2 0-_---------------_-._______ 157,271

Chalcopyrite, CuFeS2 _.....___......___._________ 143, 146, 155, 157, 159, 271

Chalcostibite, Cu2S.Sb2 S3 -.--. ___________________________ 138,146,154,272

Chalmersite. See Cubanite.

*Chilenite, Ag6Bi...._.._.___......________________________________ 169

*Chiviatite, 2PbS.3Bi2S3 _________________________________________ 169

H Chloanthite-smaltite (Ni,Co)As2 __. ___________________________ 159,160,272

Chromite, FeO.Cr_08 . _---.--_._____________'__________________ 166, 272

*"* Chromium, Cr___________________________________________________ 240

Cinnabar, HgS__________--____.________________________________ 158, 272

Clausthalite, PbSe._________....._.___._..._________._._..__ 129, 140,272

307
308 MICROSCOPIC DETERMINATION OF ORE MINERALS

i Page 

Cobalt, Co...._--_---------------------.------.---------- 193-198, pi. 7

Cobaltite, CoAsS - -- - 164, 166, 167,273

Coloradoite, HgTe 129, 140, 155, 273

Colusite, (Cul Fe,Mo,Sn)4(Sl As,Tb) 3-4 127

*Comuccite___. ______________.._____________.___--_--____-__-_----- 169

Copper, Cu .._-------.-.-_--------_--------- 113, 118, 190-192, 273,,pl. 6

Copper pitch ore. See Tenorite.

Corynite, Ni(As,Sb)S__ - - -------------.,- 160,164,273

Cosalite,2PbS.Bi2 S3 _ --------------.-_-..--------------------- 130,273

CoveUite, CuS_____---------------------------------------- 138, 154, 273

Cubanite, CuaS.Fe486 -------------- 146, 157, 274

Cuprite, Cu2 0____---------------------------------------------- 114,274

*Cuprobismutite, 3Cu2S.4Bi2S3 ---______-_.____.-_-_-_.___---_.__--__ 169

Cylindrite, 6PbS.Sb2S3 .6SnS2 _ _---.-._...._ ....-.__:_ 129, 130, 141, 145, 274

Delafossite, Cu2O.Fe2 03 -.__'_--_-_-_-__.._-_--._-_-_---_-_-__.--- 150, 274

*Dognacskaite, Cu,Bi,S__-__----____-_______-______-__-_____-___-_- 169

Domeykite, Cu8As _.--_------------------------.--_---___- 112, 117, 274

Dufrenoysite, 2PbS.As2 S3 -. 169, 274

Dyscrasite, Ag3Sb___._-_.__ 118, 119, 120, 121, 125, 126, 133, 134, 139, 141, 274 

Electrum, Ag,Au_ - .. - 119, 274

Emplectite, CuaS.Bi2 S8 -- 124, 146, 275

Enargite, CuaS.4CuS.AsaS3~------------_-- ---------------- 138, 154, 275

*Epiboulangerite, 3PbS.Sb2 S5 _.____-._.___.______.__.__._.___.______ 169

Eucairite, Cu2Se.Ag2Se_ --- _ __ - __ 135, 151, 275

Famatinite, Cu2S.4CuS.SbaS8 _ __ __ _ _--------- 139, 275

Ferberite, FeO.W03 ------------------------------------------ 167, 275

Franckeite, 5PbS.SbaS8.2SnS2- ----------------------- 131, 145,276

Franklinite, ZnO.Fe2 03 -----_ 165, 166, 276

Freibergite, 5(Cu,Ag) aS.2(Cu > Fe)S.2Sb2S3 ------------------- 139, 143, 276

Freieslebenite, 2Ag2S.3PbS.2Sb2S3 _ - _- ____ 142, 155, 158, 276

Galena, PbS -- -- -_--_-------- 115, 129, 276

Galenobismutite, PbS.Bi8S8 __ 122, 142, 277

Geocronite, 5PbS.Sb2S3 117, 277

Germanite, Cu3 (Fe,Ge)S4 _ .-.-. -----.. 155, 277

Germanium,, Ge_ _-----------_---_---_-_---_-_-_---_-___- 240-242, pi. 12

Gersdorffite NiAsS____.-.__-_.__.._.__..__._ ._____...__ 161, 164, 277

Glaucodot (Co,Fe)AsS-_ - 161, 164, 277

Goethite, Fe2 03 .H2 0_-.____--__-_______-.--_----__.___.__ _ 167, 277

Gold, Au __ __ 152, 155, 223-225, 278, 296, pi. 13

Goldfieldite, 6CuS.Sb2 (S,Te) 3 - - 136

Graphite, C_ .._......_ _ . _ _ ._. 159

Gratonite, Pb9As4Si5 __ _ _ _ _______ _ _ 147

Guanajuatite, Bi2 (Se,S) 3 -----------------__----_-_-_____... 124, 146, 278

*Guejarite, Cu8S.2SbaS8 __-_ 169 

Guitermanite, 3PbS.As2S3 _____.-__.._._____.____.._..___ 124, 147, 278

*Hauchecornite (Ni,Co)7(S,Sb,Bi) 8 ______..__._._ __.._.___________ 170

Hauerite, MnS2 .-__ __--_-___---_-_-.____.._._ 130, 149, 278

Hausmannite, MnO.Mn2 03 ---_ _ --------- 132, 150, 162, 165, 278

Hematite, Fe2O3 ___-_______ .._________._..______.____ 166, 167, 278

*Hengleinite (Fe,Ni,Co)S2 __ __________.---__ 170

Hessite, Ag8Te 126, 129, 142, 278

Hollandite, m(Ba,Mn)2Mn05 +nFe(Mn05)3 _..-.._._._____... 166, 167

Huebnerite, MnO.W03 ----------------.--------------------- 159, 168,278

Huntilite, Ag3As (identical with arsenargentite)______________________ 113
ALPHABETICAL INDEX OF MINERALS AND ELEMENTS 309

Pago 

> Ilmenite, FeO.Ti02 -___..._.........--......--_...._--...-_-..._. 168, 279

Iridium, Jr----------------------------------------- --------- 247,296

rroii,Fe--_------------------------------------------------------ 198

Jacobsite, MnO.FeoOs--.------ - ----------------------------- 166, 168

t. Jamesonite, 4PbS.FeS.3Sb2 S3 -------------------------------- 116,123,279

' Jordanite, 4PbS.As2S3 __-_---_-____-__-_-__--__---____-_____--_ 124, 279

 *Kalgoorlite, HgAu2 Ag6Te8 ----------------------------------------- 170

Kallilite, Ni(Sb,Bi)S--__------.-------------- --------__-- 140,163,279

Keeleyite, PbS.Sb2 S8 - ------__----------------- --__ - 155, 279

^ Kermesite, 2Sb2S3.Sb2 03--.------------------------------ 128, 137, 145, 279

Klaprothite (also known as klaprotholite), 3Cu2 S.2Bi2S3--___- 120, 139, 152, 280 

Klockmannite, CuSe--_-----------------------------_-----.-_-._ 136,280

Krennerite, (Ag,Au)Tea -_- -----_-- ------ -.-- 122,280

Lead, Pb - - -----------_----_--- -- -- 128, 220-223, pi. 12

*Lehrbachite, (PbSe+HgSe)------ ----------------_- _-_.--_ 170

Lengenbachite, 6PbS.(Ag,Cu) 2S.2As2 S3 ------------------- _ 169,280

Leucopyrite, Fe8As4----------------------------------------- 161, 164, 280

*Limonite, Fe2O3 .nH2O._ ._._._. ---_--_---__----__--_-____________ 170, 281

Linnaeite, Co3S4--------_-----------. ---------------_---- __. 143, 281

Livingstonite, HgS.2Sb2 S3 ------------------.-..- - 137, 153, 156, 281

Loellingite, FeAB2 _ -_-------_--_----'_-_-----.-.._...._._____ 161,281

Lorandite, Tl2 S.As2 S3 --- __---------- ----.---_.._ ._ 147, 281

Luzonite, Cu2S.4CuS.As2 S3 (variety of enargite).___- _______________ 138, 154

Magnetite, Fe3 04 --,-------------------------------- 149,158,165,167,281

\ Manganese, Mn._____..__-__-____-___----_______.___._______._.__ 226

Manganite, Mn203 --------_--------_----_----_-------__---___-._ 150, 281

Marcasite, FeS2 --_------------------------------.---_------- 161, 164, 281

Matildite, (Ag2 ,Pb)S.Bi 2S3 ----------------------------------------- 282

Maucherite, Ni3As2 _-_--------------------------------_---___-__ 121,282

Melaconite, CuO (variety of tenorite)____--.--__--...____.____ 132, 150,282

Melonite, NiTe2 _______--__------_----------___--__--_______ 115, 122,282

Meneghinite, 4PbS.Sb2 S3 _- ---------------------------------- 117, 124,232

Mercury, Hg_ _----__--__---_-----__-_--'--__---_-____---___- 225, pi. ll

> Metacinnabar, HgS---__-.------__---_____--___. __._._--____ 155, 158, 282

Miargyrite, Ag2 S.Sb2 S3 __----------__ ._ 138, 148, 149, 153, 156,282

Millerite, NiS ------- -.----.--------___-.-_.---_. ____ _ _ 124, 145,283

Molybdenite, MoS2 . ._.._.----_---.._________._..____.__________ 159, 283

Molybdenum, Mo_--__-_---_------___----_-__----_.__-__-__ 233-235

Mottramite, 2Cu0.2PbO.V2 08 .H2 0_--_-_-_____-__-___-_-__-____._ 129, 283

1 Nagyagite, Pb,Au,Te,S.-..--_--.---._ ___._ .___. __ 147, 158,283

^ Naumannite, (Ag2|Pb)Se -. ---------------_---_-.-_--.__._ 135,283

Niccolite, NiAs ------------------____-__^._._._..._.__ 121, 160, 283

* Nickel, Ni._-_._ ___ .__._.___.._ ____ _ .-_ 192, pi. 7

*0nofrite, Hg(S,Se) .---.---...--_._...---..._..__.._...._. .___ 170

Orpiment, As2S3 -_---_---_---.--_.--___--_------..-_._-._._._... 153, 284

Osmium, Os...-________________-__._______.____..._._.__.__..._ 248, 284

»? Osvyheeitef 8PbS.2Ag2S.5Sb2 Sa .-..---.------------.-------_._... 132,284

i Palladium, Pd... _.___..___..___.____.____.___. __._.__.____ 244, 296

^ Poarceite, 8Ag2S.A82S3 ---------------__----_-__----_--_-__-____._ 135, 284

,, Pentlandite, (Fe,Ni)S. . .-.----...______...________ 143,284

Potzite, (Ag,Au) 2Te _--.----------_ _ ._.___ . 128,133,140,284

Plagionite, 5PbS.4Sb2S3 _-------.-.-_-__-_____._______ 116, 123, 285

Platinum, Pt...____.___-_-._.____..__._.._-__.__..__._.____ 245, 296

Plessite, (Ni.FeJAsS....-----------.._._.__..___.__________. 160, 285
310 MICKOSCOPIC DETERMINATION OF ORE MINERALS

Polianite, Mn02 _- ______ ________________________. 168

*Polyargyrite, 12Ag2S.Sb2S3_- ------ _________________________ 170

Polybasite, 8Ag2S.Sb2S3 ----------- 126, 128, 135, 137, 147, 148, 151, 154, 285

Polydymite, Ni3S4____----_---------_____-_-__--______-____- 123, 143, 285

Proustite, 3Ag2S.As2S3 ______._____.____----________ 133, 137, 151, 152,285

Psilomelane, Mn02 .Mn2 03-----------------------------_______ 162,285

Pyrargyrite, 3Ag2 S.Sb2S3 ---------- 127, 132, 134, 135, 137, 148, 151, 153, 285

Pyrite, FeS____ __---------- _ _ 161, 162, 164, 165, 286

Pyrolusite, Mn02 _.__.__________________________________ 150, 159, 286

Pyrrhotite, FeS.*. ------------- - - 131, 145, 157, 286

Ramdohrite, 3PbS.Ag2S.3Sb.S3_----- --------------------_____ 131

Rammelsbergite, NiAsj____.------..---_-------------_____--__ 121, 160, 287

Rathite, 3PbS.2AsjS8 __---._-----.----_--------- -- 169, 286

Realgar, AsS.__ ._- .___--------- ._---- ___-___-_______ 152, 158, 286

*Regnolite, 5(CuS,FeS> ZnS).As8S8 .--------------------_-----_ 170

Rezbanyite, Cu2S.3PbS.5Bi2S3 ____------__-_-_-______-_-______ 124, 286

Rhodium, Rh___________.___--__.__.______-.____.____ 250, 296

Rickardite, Cu3Te2 ____________________ _______________________ 113, 286

Ruthenium, Ru._______.________________________________ 250, 296

Rutile, TiOa ____ _ .-------____ _______ ____ 168, 287

Safflorite-loellingite, (Co,Fe)As2 ____ _ _________________________ 160,287

Samsonite, 2Ag2S.MnS.Sb2S3 ___-------------_______________ 154

Schapbachite, Ag2S.PbS.Bi_S3 -_---_--------_______-_-__________ 130, 287

Selenium, Se.____._____-_-------_---_-_-_-.______________ 204-206

Seligmannite, Cu2S.2PbS.As2S3 -__------_----__.----__________ 169

Semseyite, 9PbS.4Sb2S3 __-_-------------_-______-___________-__ 116, 287

Siegenite, (Ni,Co) 3S4 (?)------- --_-------_- _______-_____.____. 143,287

Silver, Ag____________--___-----_-----____ 113, 114, 119, 199-204, 288, pi. 8

Sitaparite, 9Mn2 03 .4Fe2O3 .Mn02 .3CaO___.._________________ 167, 168

Skutterudite, (Cof Ni)As8 _---~-- --------_--- -------- 160,162,288

Smaltite-chloanthite, (Co,Ni) As2 _ _ _ _ __--_____--_____-____ 159, 160, 288

Sperrylite, PtAs2 _._____--.--------____-________.__________ 165

Sphalerite, ZnS_ - ----------_ --_ 123, 130, 143, 156, 288

Stannite, CujS.FeS.SnSj -------------__________________ 144, 147, 288

Stephanite, 5Ag2S.Sb2S3 _________----__-__-__ 126, 127, 134, 147, 148, 153, 288

Sternbergite, Ag2S.Fe4S5 ____.___--_----____________ 129, 131, 149, 288

Btibnite, SbaSj.... __--------- ____- 114, 120, 128, 138, 288

Stromeyerite, (Ag,Cu)_S__.__---------- ____.___________ 126, 289

Sulphur, S ._.. _-__------_- -_ __-.------ 227-231, pi. 5

Sulvanite, 3Cu2S.V2 S5 _-_____________________-_-_ 152, 154, 156, 158, 289

Sylvanite, AuAgTe4 ___ -___-----_________ 122, 125, 142, 147, 289

*Tapalpite, 3Ag,(S,Te).Bi_(S,Te) 8 _--_-- _ __ 170

Teallite, SnS.PbS_.__ _---_ __.___ 132, 150, 289 

Tellurium, Te._.__._ __ __-_----_ 115, 117, 121, 125, 206-208, 289, pi. 9

Tennantite, 5Cu2S.2(Cu,Fe)S.2As2S3 __________________________ 136, 144, 290

Tenorite, CuO.__-______._ --_._____ ______________ 132, 150

Tetradymite, Bi_(Te,S) 8 - ------- ---- -- 115, 121, 290

Tetrahedrite, 5Cu2S.2(Cu,Fe)S.2Sb2S3 -________..___-___-__ 136, 144, 156, 290

Tiemannite, HgSe__ _----_____-__-- _______ 152, 156,290

Tin, Sn.______--________-_-___-____-__________--____-_-__ 218-220, pi. 11

Titanium, Ti______-___-____--_---_-_-_-_-_-___._______-_-______ 239

Troilite, FeS__ _----- _-_------ _ ----------------- 116, 131, 290

Tungsten, W_________----____-__--____-_____-.__.____..__ 235-237

Tungstenite, WS2 ___---_-____-____. _-_...._-_.____-______--__ 159, 290
ALPHABETICAL INDEX OF MINERALS AND ELEMENTS 311

i Page 

' :N Turgite, 2Fe203 .H2 0--.._------------------------____..._____ 170

y Ullmanite, NiSbS......._...._........_..___...___..__. _.. 140, 163, 290

Umangite, Cu3Se2 -_---____..._._.._.______________------ 127, 291

Uraninite, uranate of U, Pb, etc___-__-_-__-_______________________ 163

 ± Vanadium, V__._______________________________________________ 237-239

" Violarite, (Ni,Fe) 3S4 -.-----------------------------------_----- 123, 291

» Vrbaite, TljS.SCAs.StOjSs--------.. ----------~-_--~~~--~ 145, 291

Weissite, Cu2Te___..___.___...... ..__....._.__.._._..____ 113, 291

\ Whitneyite, Cu0As____________________________________ 112, 117, 291

| Willyamite, (Co,Ni)SbS -------------------------- 140,144,163,164,291

v Wittichenite, 3Cu2S.BijSi .._..----..-._..-___ __ __...__ 142, 291

Wolframite, (Fe,Mn)O.W03--_-_.------_ ._. - -- 168,292

Wurtzite, ZnS__. ...-....._.--.-..__.....____.____._._.., 144, 292

, Zinkenite, PbS.SbaS8 ----------- ---------------------------- 116,292

Zinc, Zn.____.____._.___._._...__...___._.._._.__.._. 187-190, pi. 5

Zorgite____._____.__......__....____.__.____..._.._______ 170

GENERAL INDEX

Page

Abrasives, Rodda method of sizing. . 23 

size of........................_...._.. 26-27

sizing of.-__----.-..-.-.....--- ___ 22-26

Vander wilt classifier of.... _.-...-. _.. 23-26

Ambrose, J. W............................... 136

B 

Bakelite, mounting in............_..__ 11-13

Batenian, A. M.....................__._ 29,39

Behrens, H..................... 173,174,175,197,208

Berek slit microphotometer...___...___ 78-83 

Binuenthal minerals...........__._..... 168-169

Bird, P. H................................ 11,40-42

Bofickjr, Emmanuel............______ 173,240

Broderick, T. M_............................ 112

Buerger, M. J................................ 169

Buttgenbach, H.............................. 145

Cabeen, C. K................................ 76

Camera, improvised......._................ 45-46

Le Chatelier model....................... 51

Leitz "Makam"............__._._.. 47-49

vertical.___.........________.... 49

Canada Department of Mines and Resources,

polishing process at._.......... 38

Chalcography.__............._.__.__ 3

Chamot, E. M......................... 174,178,178,

179,181,182,190,196,208, 220,222,244 

Chance, F. M................................ 257

Color _. . __.......................... 59-63

Colored minerals..........________.. 293-294

Columbia University, polishing process at.... 40-42

Corundum saw.................._.__ _ 15-16

D 

Dake, H. C.................................. 14-15

Davy, W. M.......................... 1,8,13,44,76

Determinative table........................ 103-170

Development of photographic plate...._.... 57-58

Diamond saw...................__.__ 13-15,16

Discredited or doubtful minerals...__.... 169-170

Dreyer, R. M...................... 176,197,259,264

Drude, P...................................... 77

Dunn, J. A......;.......................... vii, 104

E

Eardley-Hatton polishing table............... 18-21

Electroconductivity.......................... 76

Erickson, E. T............................... viii

Erzmikroskopie..................._._._ 3-4

Etch reactions.....................__.__ 95-103

Evans, M. H................................. 205

Exposure formula.....i-..................... 55-57

Page 

Fahey, J. J.................................. viii

Fairbanks, E. E....................... . 67

Farnham, C. M....................... 1,8,13,44,76

Feigl, F.....-................-.-.--.--..... 234,238

Films............-.-.........-....--..-..-..- 51

Filters, light...... . - -. .--.. 51-55

Ford, W. E-. . . . 118,120 

Foshag, W. F................................ 104

Fraser, H. J............ 176,197,243,253,259,264,265

Frebold, Qeorg................. ... 170

G 

Gaudin, A.M....__- .. -- - viii, 170

Geological Survey, U. S., polishing process at. 27-32 

Goranson, E. E......------------------------ 71

Graton, L. C............................... viii, 39

Graton-Vanderwilt polishing machine........ 34-36

Grinding table.....____-_-_--..---.-. 21

H 

Hard minerals . 110-111,159-168

solubility of.. - 252-254

systematic analysis of ------- 257-258

Hardness. . .. --.-..- -.------. 63-66

type minerals classified according to...... 65

Harvard University, polishing process at...-. 32-40 

Harvey, R. 'D................................ 76

Hatton, j:n................................. 19

Haycock, Maurice -.I.-.-----.---.-- viii, 92, 93

Hillebrand, W. F.--.... 185,237

Impregnation of specimen.... . ......... 30-32

J

Johnson, O. C. 238

K

Kerr. P. F................................... 76,101

Kley, P. D. C..... ............ 174,175,197,208

Koenigsberger, J.____...... __ __ 67

Krieger, Philip. . 11,41

Landes, K. K.................... ......... 175

Landon, R. E.___ 127

Lasky, S. G..__.................. ..... 39

Lathe feed..-........................... 16-18, pi. 1

Lindgren, Waldemar._____...___.... 174

Lohman, K. E...___________._ viii 

Lundell, G. E. F . . . 185,237 

Lux, defined................................. 83

313
314 INDEX

M Page 

McAlpine, R. K... .... ............. 238

McKinstry, H. E. _ 174 

Mann, David-.....-...-......-----....---... 34-86

Mason, C. W.......................... 174,176,178,

179,181^ 182,190,196, 208, 220,222,244 

Mergner, J. L...-.......?______-----_.. 29

Merwin, H. E....-....-..-..-----.----------- 170

Metallographic microscope...... ......... 1,4-8

Microburner. .............................. 178-179

Microchemical methods..... ____.... _ 173-296

Microchemical tests.....-....--.-.------. 95-103

outline of..------.....-.-....-....-.-.-- 259-265

Microphotometer_......._____...-- 78-83

Mineragraphy_------------ ---...i.-.---..- 3

Mineralography_____ ... ...- 3 

Minnesota, University of, polishing process at. 42 

Mogilnor, A. ~B................................ 127

Moses, J.H -....-.--.........-.-... viii, 89,90-91

Mounts, brass.........________-----_. 8-10

Murdoch, Joseph.................. 1,18,59,61,63-64

N 

Norton, M. R......................... viii,43^4

O 

Orcel, J---..-.-----------------.---------- vii,84,86

Ore microscopy-.--.---- ... -- 4 

Osborne, F. F__._..-------------------------- 67

Oven, warming..- ...-. -...].. ... 10-11

Patton, L. T............- - ............. 16

Patton lathe feed..............--.------- 16-18, pi. 1

Photoelectric cell, measurement of reflectivity

with..........-....- . 83-92

Photomicrography of polished sections.44-58, pis. 2-4 

Platinum metals, microchemical tests for... 243-251 

Polarized light.-.-------....----- ------.- 66-76

Polishing, Eardley-Hatton table for......-... 18-21

materials used for..-..-.. . . 18,22 

Polishing process at Canada Department of

Mines and Resources__... ..- 38 

at Columbia University-____....-. . 40-42

at Harvard University.---...------------ 32-40

at U. S. Geological Survey-- - --.--- 27-32 

at University of Minnesota_........... 42

at Watertown Arsenal (Mass.)...- 42-44 

Posnjak, E -- 170

Potassium mercuric thiocyanate, preparation

Of 185-186

Powers, H. A.. .-.-.--- -. - 252 

Prescott, A. B.... ......... ...... 238

Putnam, P. C.. ..... . 174,187,244

R

Ramdohr, Paul .. . - vii, 1,

4, 78,82,86,125,133,134,137,139,141,170

Reagents.--.-,---..-.-.. - 179,184-187

etch.._ ------- _------ 99

Reed, F. S._ 29

Keflectivity, measurement of __ 76-92

of minerals.._.......___.___ 293-296

Biley, S. B ........ .. 216

Eoberts, E. J........... 174,187,244

Rodda, J. L .-.---- - - -- 23

1'age

Rosenbusch (Wiilfing). _ . _ . ________ 14 

Ross, C. S...... ....--. ......... .............. 27,31

Sampson, Edward.--.--. _ ..... ..'....... 27,67,73

Saw, corundum.... _ .. .. _ ... ___ ...-.- 15-16

diamond----. . . -- . ____ 13-15,16

silicon carbide--.-.--. .. . _ .. __ .-. _ 15-16 

Schneiderhohn.Hans --. _.... vii, 1,

4, 62, 67, 78, 82, 86, 125, 133, 134, 139, 141 

Schwartz, G. M 42

Selchow, D. K...............^. ........ '174,187,244

Sensitivity of microchemical tests. __ .... 183-184

Shannon, E. V.. ............... 116,123,143,155,163

Shaub, B. M. ...... ............ ... ... _'_ 13

Short, M. N.... ..... 10, 122, 123, 125, 142, 147, 155, 163

Silicon carbide saw.,... ,__ __ .............. 15-16

Slides, glass-.. - _ ...... 178

Slit microphotometer... . -- . --- . 78-83

Smitheringale, W. V.. ....... ...... ........ 99

Soft minerals-...-..-.-..--..... 105-110,112-159

solubility of...-........- .- _ .... 254-257

systematic analysis of .... . 258-259

Soule, B. A.. .. . 238 

Spectroscopy, methods of.. ----------------- 92-94

Spencer, L. J- .............. ............... 140,144

Staining tests....---.------------ 170-172

Staples, L. W ................................ 241

Systematic scheme of microchemical analysis . 252-259

Talmage, S. B..... .59-60,61,63,64

Tests, miciochemical --- -.__.... 95-103

michrochemical, for platinum metals... 243-251 

outline of..........:..........__ 259-265

sensitivity of---..-.-..--.-----. 183-184

Thiel, G. A .............. ............. 99

Thomson, EHis...... ........_. _.... 140,163

Tolman, C. F............. ........... 136

Vander Veen, R. W.......... ..... 1,6,64,67

Vanderwilt, J. W................. 13-14,21,23,33,65

Watertown Arsenal (Mass.), polishing process

at . . 42-44

specimens polished at. pi. 3

Watson, J. A .__ 206

Wax, modeling...............__............ S

sealing ..._.__...... 8-10

Wells, R. C... .............. . _.- 159

Wherry, E. T................................ . 104

Whitehead, W. Ij............................ 3

Winchell, A. N............. . ... ... 67

Winchell. N. H . 67,173,240 

Wolfe, C. W ..... ... 116

Wright, F. E . 67

Young, F. S.

Zieler, H. W.

O