UNITED STATES DEPARTMENT OF THE INTERIOR 

- Ray Lyman Wilbur, Secretary

GEOLOGICAL SURVEY 

George Otis Smith, Director

Bulletin 825

MICROSCOPIC DETERMINATION OF THE 

ORE MINERALS

M. N. SHORT

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON: 1931

For sale by the Superintendent of Documents, Washington, D. C. - Price 60 cents (paper cover)

CONTENTS

	Page 

Preface .________._______________________.___..____._________ __..__ vn

Part 1. Microscopic technique._______________________________________ 1

Introduction. __________________________________________________ 1

The metallographic microscope..____________________ T _________ 3

Mounting the specimen._______________________________________ 7

Mounting in modeling wax_________________________________ 7

Mounting in brass boxes.__.__________________________^____ 8

Warming oven....__...-................__...__..._._..._. 10

Polishing the specimen____________.__________-____---____--__. 10

Polishing process at the United States Geological Survey_._.. 10

Procedure.-.--..-_-__-_.___-_____.__--_-__--_-_---___ 10

General remarks._______u.____________________________ 14

Impregnating the specimen_________---_-__--_-_-_--__- 15

Polishing process at Harvard University______________________ 16

Polishing process at the University of Minnesota._____________ 17

Photography of polished sections-__-_____-_____-_--__---___--___ 17

Photomicrographic camera_________________________._______ 17

Light filters. __ ____.______________________ 23

 Exposure formula___...._._._^_.__________.-_-.._. 26

Developing and printing,__________________________________ 28

References._______-_-_____________-_-_-_______---_-_---__ 29

Part 2. Physical properties-_________._____ ________-___-____-___-__ 30

Color._....._......_..._....______.__________________________ 30

Hardness----_-_---___---_-_----___-__^_--------_--_-_-_-----. 35

Examination of opaque minerals in polarized light--.___-__._---___ 39

Summary _._____________________________________________ 39

References _ __ _-__-__--____-____________-_-__---___--__ 39

Apparatus ________-____-__________--___:.-_______-__--_- 40

Details of the method._____________________________________ 42

Methods of observation._______-_____-___-____--_____-_-_-_ 45

Adjustment of the instrument.,________---_-_-_-_-_____-_-_ 46

Sources of error..__._____-___._-__^-____-_________________ 48

Electroconductivity _ _ _________________________________________ 49

Part 3. Etch reactions and determinative tables______-______-____--__ 50

Etch reactions.,______----__-___________----__--__-_---------- 50

Determinative table.._______-_._._______-..___-_-_.__--_-_--._ 58

Introduction__ __________ j___..-__-_--_--_--__---.__-_._ 58

Index. _._...___-_-_-_-_-----__-____-----_-----------_---_ 61

Outline of table......_____________________________________ 65

Soft minerals__.._______----__-_.-_.----.._-_-_-----..---- 71

Hard minerals..________-_-___________---___-__-_--------- 97

Binnenthal minerals.________________________-___---_------ 103

Discredited or doubtful minerals__-___----------_---_------- 104

Index of minerals grouped by elements._.-___---_-_..-_._-_-_ 107
IV CONTENTS

	Page 

Part 4. Microchemical methods.. _ __ _ . 115

Introduction.._________________________________________________ 115

Procedure and apparatus __ _ _ - _ _ 116

Collecting the material..___________________________________ 118

Glass slides...-....._...___..._..._.______._..-._...___.._ 120

Microburner. ______________________'________.____.______-____ 121

Reagents.________________________________________________ 121

CapiUary tubes.._.__...__....___._.....__...___-.----..._ 122

Methods of applying tests__-_-_________--___-._----------------- 123

Sensitivity of microchemical tests.__._._._.__.________--_-__-_-'_ 126

Reagents.. ___________________________________________________ 127

Details of tests.----_.___._______-____--__.____-___---___---.._ 129

Zinc----.--_---------------------- 129

Copper.-_ _ - _ -_ _ _ 132

Nickel.- ...............................................r .. 133

Cobalt _-----_--_----------_-__- --- 135

Iron.-.--_---_--._-_-.-.._---------_-_----.-----_-------. 139

Mercury. . __ _ __ _ 142

Lead-----. . 143

Silver.._----.------.---------------_ 148

Gold... ----------_--_------------- 152

Arsenic___ _ _ __ _ - _-_-_-___-_--.__ 155

Antimony and bismuth.__._:.'._.__.__._____^_.___._-___.__ 159

Tin.___- . _ - _. - _. 163

Tellurium. _______________________________________________ 165

Selenium.._.___.____.._____._._.___._________..____....__ 167

Manganese.--____________________________________________ 168

Sulphur.---..----.---.-.-.-_...---.-.------_---_--- 169

A systematic scheme of microchemical analysis.___________________ 173

Specific tests for minerals._____________________________________ 179

Index _._...___._._. ___. - ._ _ -_ _ 203

ILLUSTRATIONS

	Page

PLATE 1. Photomicrographs of polished specimen of ore from Magma

	mine, Superior, Ariz__-____________________________ ____ 24

2. Microchemical tests for zinc______________________________ 136

3. Microchemical tests for copper.__-_._.____________________ 136

4. Microchemical tests for cobalt and nickel _________________ 136

5. Microchemical tests for mercury - _-_---_-------.._-__-_ 136

6. Microchemical tests for lead-_____________________________ 152

7. Microchemical tests for silver.____________________ ____ 152

8. Microchemical tests for gold.-_-._--_-___-_----- --__.--_ 152

9. Microchemical tests for arsenic and tellurium. __---___-__._. 168

10. Microchemical tests for antimony. ________________________ 168

11. Microchemical tests for bismuth and tin.. _----_-__._.____ 168
ILLUSTEATIONS V

	Page

1. Metallographic microscope with equipment for investigating

polished sections.____--_________-'--__--_-__-------___ 4

2. a, Prism illuminator; b, glass-disk illuminator______-__-.___ 5

3. Improvised microscope camera.__________________________ 18

4. Leitz''Makam" camera attachment-..----.-------------- 21

5. Bausch & Lomb large metallographic camera.------------- 22

6. Comparison microscope._______ ________.________________ 31

7. Talmage hardness machine._____________________________ 37

8. Microscope arranged for use with polarized reflected light... 41

9. Photomicrographs of ores taken in polarized reflected light.. 47

10. Platinum wire for transferring drop of reagent to polished

surface._ ____________________________________________ 52

11. Enlarged view of needle for scraping polished surface..-.-.. 119

12. Improvised alcohol lamp.-_-_.--_-_----------.---..-___- 121

13. Capillary tube-....___-_-_-----___-.,._----.----_-_.--_- 122

14. Diagram showing Chamot's method I-_---_---_-----_--__- 124

15. Glass-stoppered dropping bottle._________________________ 128

16. Zinc and copper mercuric thiocyanate crystals-___---__-___ 131

PREFACE

The development of improved methods for the determination of 

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

occupied the attention of the writer for more than four years. The 

work has been done mostly in the laboratories of the United States 

Geological Survey except from September, 1927, to June, 1928, while 

he 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 Economic Geology.

It would be impossible to do full justice to all who have contributed to the completion of the present work. The writer wishes 

to acknowledge his especial indebtedness to his former teacher, Prof. 

L. C. Graton, who first aroused his interest in the subject and who 

has on many occasions given his counsel on the various problems encountered. Professor Graton also lent to the writer Murdoch's 

original suite of ore minerals, which is deposited in the Harvard 

Laboratory of Economic Geology.

Much assistance was rendered by the writer's former students, 

Drs. H. J. Eraser and H. A. Powers, who contributed some valuable 

original data as well as advice and criticism.

Dr. William F. Foshag, curator of mineralogy of the United States 

National Museum, has at all times helped the writer in his endeavors 

to procure authentic specimens of ore minerals. Practically all of 

the writer's suite of specimens were obtained from the National 

Museum. The Roebling and Canfield collections of minerals,, recently acquired by the Museum, have proved valuable sources of rare 

minerals.

Dr. George A. Thiel, of the University of Minnesota, made available the results of his admirable research on the manganese minerals 

and gave generously of his time in checking the identity of the manganese minerals in the writer's collection.

Prof. Edward Sampson, of Princeton University, contributed to 

much of the recent improvements in the technique of polishing and 

more than anyone else is responsible for the perfection of methods 

using polarized light in the investigation of opaque minerals. Much 

of the information on the subject contained in this bulletin was 

taken from Professor Sampson's recent article.

Finally the writer desires to thank his colleagues of the Geological 

Survey for the assistance they have given him. George Steiger made 

possible several quantitative analyses of doubtful minerals. Thomas 

S. Lovering read the manuscript and has given valuable advice and 

criticism.

vn

MICROSCOPIC DETERMINATION OF THE ORE

MINERALS

By M. N. SHORT

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 * have all contributed new 

ideas and methods of attack.

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 which 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 Murdoch, J., Microscopical determination of the opaque minerals, New York, 1916.

- Davy, W. M., and Farnham, C. M., Microscopic examination of the ore minerals, New 

York, 1920.

1 Schneiderhohn, Hnns, Mikrokopischen Bestimmung und Untersuchung von Erzen, 

Berlin, 1922.

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

1
2 MICROSCOPIC DETERMINATION OP ORE MINERALS

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 chemical tests on a microscopic scale 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 per cent of 

these minerals were 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 can not, at present, distinguish beegerite, 

Pb6Bi2S9 ; cosalite, Pb2Bi2 S5 ; 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 riot necessary to make analyses, as the
THE METALLOGRAPSIC MICROSCOPE 3

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, a 

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 has 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 5 and used by Davy and 

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

The term " chalcography " used by Schneiderhbhn is unsuitable, as 

it has already been applied to the art of engraving on copper.6 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 

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 behind or through the reflector, and 

after passing through the ocular reaches the eye.

6 Whitehead, W. L., Notes on the technique of minerngraphy: Econ. Geology, vol. 12, 

p. 697, 1917.

 Webster's New International Dictionary. This confusion in terminology was first 

noted by Mr. E. E. Fairbanks in an unpublished paper.
4 MICROSCOPIC DETERMINATION OE ORE MINERALS

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

a glass prism which occupies approximately 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 glassdisk illuminator consists of a thin transparent glass disk which 

occupies practically the entire area of the tube. In both types the 

reflector is rotatable in the tube and is turned until the clearest 

image appears.

FIGURE 1. Metallographic microscope with equipment for investigating polished sections, a, Spencer lamp, No. 374; 6, rubbing block; c, reagents 

for etch tests; d, vertical illuminator; c, polished section on modeling wax: 

f, screw for racking stage up and down ; g, polished section in brass mount; 

h, lead filling; i, sleeve ; /, nipple; k, flange ; I, mounting cup

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 

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
THE METALLOGRAPHIC MICEOSCOPE O

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 

one-fourth, 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 of 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

V

'M

a/

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 Bauscli & Lomb Optical Co.)

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 7 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] illum'nator, 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

* Van der Veen, R. W., op. clt., p. 3.
6 MICROSCOPIC DETERMINATION OF ORE MINERALS

is satisfactory for use with the 16-millimeter and 4-millimeter objectives. With objectives of higher magnification than the 4-niillimeter lens and lower magnification than the 16-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 can 

not 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 

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. 1.)

3. It is the best lens for use with polarized light. (See p. 42.) 

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 glass and do not require special short mounts; the 24 and 32 

millimeter petrographic objectives are entirely satisfactory for 

metallographic work.
MOUNTING THE SPECIMEN 7

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 can not be obtained, artificial illumination is 

required. The writer used a Spencer microscope lamp No. 374, made 

by the Spencer Lens Co., Buffalo, N. Y., equipped with a 115-volt 100- 

watt Mazda bulb, a condenser, and a " daylight" color screen.

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 1^ inches by y8 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.8

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

a sleeve 2i£ 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

8 Davy, W. M., and Farnham, C. M., op. cit., p. 5.
8 MICROSCOPIC DETERMINATION" OF ORE MINERALS

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.

Moimting 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 

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. In the United States Geological Survey laboratory polished sections that are to be examined 

once and then discarded are polished unmounted, and those that are 

to be retained permanently are mounted in brass.

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

before cementing it in 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 li/2 by iys inches outside

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

tubes were furnished already cut by the American Brass Co., Waterbury, Conn., and cost approximately 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 ^-inch dies. The tube is supported while being stamped by a steel bar without the triangular 

slot.
MOUNTING THE SPECIMEN 9

When- a specimen is to be mounted in sealing wax both the specimen and the'tube must be warmed to approximately 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 clone on an electric hot plate but is most 

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

9}/2 by 10 inches by % 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 approximately 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. The 

section is placed face down, and plaster of paris is poured until it 

almost fills, the tube, about ys inch of clearance being left in order 

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

A sealing wax made of 120 grams of flaked shellac, 90 grams of 

resin, and 45 cubic centimeters of turpentine has been found entirely 

.satisfactory and is superior to those 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 by means of 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. 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 five 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.

15122 31 2
10 MICROSCOPIC DETERMINATION". OF OEE MINERALS

Portland cement has been suggested as a mounting "medium,9 but 

experience has shown that it is almost impossible to prevent particles 

of grit from breaking from the surface and scratching the softer 

minerals.

Undoubtedly the best mounting medium yet discovered is solid 

bakelite. This requires a hydraulic press with heating units, and 

as this apparatus costs at least $500, it will probably not come into 

general use. This method has been developed at Harvard University 

but has not yet been described.

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 Wy2 

by 12% by 121/2 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 

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

burning. The temperature maintained was very 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 15.

POLISHING THE SPECIMEN

POLISHING PEOCESS AT THE GEOLOGICAL SUEVEY

Procedwe. 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 65 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.

"Short, M. N., Preparation of polished sections of ores: Econ. Geology, vol. 21, p. G58, 

1026.
POLISHING THE SPECIMEN 11

A. The desired fragment of the specimen is first chipped off 

by means of a cold chisel. Davy and Farnham. 10 advocated the use 

of a diamond saw, which consists of a steel disk with diamonds 

mounted in its periphery. Although this offers decided advantages, 

especially when a definite section through the specimen is required, 

its use is best avoided when possible because of its rapid wear when 

sawing hard minerals. A simple diamond saw, consisting of a 

thin copper disk which is fed with diamond dust, is described by 

Vanderwilt. 11

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 minerals,

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 steel1 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 

brass, as the mounting medium is more or less viscous and would have 

greater 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. If such a wheel is not available it 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

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

n Vanderwijt, j. W., A simple diamond^ saw: $con. Qeology, vol. 25, p. 222,
12 MICROSCOPIC DETERMINATION OF ORE MINERALS

the F carborundum are ground out, and a still narrower and shallower series are 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 65 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 65 F alundum varies with the specimen. It is best to 

polish for one 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 65 F 

alundum accomplishes all it can do in three 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 

65 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. 

Magnesium oxide or magnesia 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 as a polishing agent is due to Dr. Hans Schneiderhohn, of the Technische Hochschule, Aachen, 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
POLISHING THE SPECIMEN 13

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 consists of 99 per cent Fe2 03 . 

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 ihe 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 by F. S. Reed:

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.

Fo. 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, f ranklinite, 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 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 dumped into a tall cylindrical measuring jar 7.2 centimeters in 

diameter and 56 centimeters tall. 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 thoroughly shaken and 

allowed to stand 10 minutes. The top 15 centimeters is again
14 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 over night, and the sludge settles to the 

bottom. The excess of clear water is siphoned off, and the sludge 

is ready for use.

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 from 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 anal 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. The 

bevel should make an angle of about 45° with the sides of the ,tube. 

Mounting in brass slows down the polishing somewhat but gives a 

more nearly plane surface to the specimen and does not affect 

adversely the quality of the polish.

The following table summarizes the characteristics of the abrasives 

as observed under the microscope. The size was measured by means 

of a micrometer ocular.

Characteristics of abraswes used, in polishing mineral specimens

Abrasive

65 alundum ________

Size (millimeters)

Largest 

grain 

observed

0.291 

.26 

.135

.075

Most of the 

material

0.156 

. 052-. 104 

. 02 -. 04

. 004-. 01

Characteristics

Angular and well sized. 

Do. 

Contains considerable proportion of 0.06, also much 

fine material, poorly sized. 

Contains a considerable proportion of 0.025, also much 

fine material, poorly sized.

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. They have been checked and supplemented by the present 

Avriter and further checked by C. S. Ross.
POLISHING THE SPECIMEN 15

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.

The technique of impregnation with bakelite varnish has been 

described by Ross,12 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 syrup. 

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 four to eight 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

12 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.
16 MICROSCOPIC DETERMINATION OF ORE MINERALS

tend to blow out material already in the pores. The excess bakelite is ground off 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 per cent 

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 two and a half days 

for its completion. A method of impregnation in a vacuum with 

Canada balsam and xylol that has been : described by the writer 13 

^wolves much more work than the bakelite method, and the result 

is somewhat less satisfactory, but it can .be accomplished in half an 

hour.

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 

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 three minutes. The specimen is then removed, care being taken to keep the flat surface uppermost so as to prevent the syrupy melted kollolith from running off. 

On cooling the kollolith again hardens.

POLISHING PROCESS AT HABVABD UNIVERSITY

The most effective method°of polishing yet described is the Vanderwilt method,14 devised in the Laboratory of Economic Geology, 

Harvard University. This 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 of the sludge through a series of cylinders. This method is slow 

but gives a much more uniform product than the quick method described on page 13 of this paper.

13 Short, M. N., The preparation of polished sections of ores: Econ. Geology, vol. 21, 

p. 653, 1926.

14 Vanderwilt, J. W., Improvements in the polishing of ores: Econ. Geology, vol. 23. 

p. 292. 1928.
PHOTOMICROGRAPHY OF POLISHED SECTION'S 17

2. Cloth laps are dispensed with, and the polishing is done on 

laps made of lead. Water is excluded in the final stages of the 

process, and the abrasive is carried in oil. The laps run at a slow 

speed, the process from beginning to end requiring from 6 to 12 

hours. The grinding and polishing are all done by machinery; 

there are four machines, each holding six specimens, and all stages 

of the process are carried on at the same time. With allowance for 

stoppage the time per specimen is from 20 to 40 minutes.

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 where it is desired to study 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 more time and a higher degree of technical skill than the 

usual methods, and an expensive apparatus is necessary.

POLISHING PBOCESS 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 one 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,15 who describes 

this process, states that the average amount of time spent for specimens presenting no unusual difficulties is from 6 to 10 minutes.

PHOTOMICROGRAPHY OF POLISHED SECTIONS

Photomicrograpliic camera. Davy and Farnham 10 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 pre15 Schwartz, G. M., Discussion on polishing methods: Econ. Geology, voj. 22, p. 193. 

1927.

18 Davy, W. M., and Farnham, C. M., op. cit., p. 12.
18 MICROSCOPIC DETERMINATION OF ORE MINERALS

paring 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.

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

FIGURE 3. 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

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 of 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 con-
PHOTOMICROGRAPHY OF POLISHED SECTIONS 19

sequently 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 3 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 tl'ipod and 

20 centimeters above the ocular of the microscope, with a 16-millimeter 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 one second to ten 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 on a ground glass 

at the top of the camera; the ground glass is in the position that 

the photographic plate 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 at first. If the minerals 

do hot all show clearly, the effect produced by different color filters 

is then tried out by testing them in succession, as described on 

pages 24-25.

For an accurate focus the ground glass is inadequate, as the 

image produced is not sufficiently sharp, and a transparent plate
20 MICROSCOPIC DETERMINATION OF ORE MINERALS

of glass is required. This is conveniently obtained by soaking a 

photographic plate in hot water and scraping off the emulsion. The 

ground glass is removed from the camera and the transparent 

glass put in its .place. When this is 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 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 

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 inclosed 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 has 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 a 5 x ocular. (The cost of this attachment is approximately $86.) 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
PHOTOMICROGRAPHY OP POLISHED SECTIONS 21

it is only necessary to focus the image in the eyepiece. However, the 

adjustment of the eyepiece varies for each objective, but it can be 

accurately recorded by means of the circular scale. Immediately before the photographic plate is exposed the prism that throws the 

image through the eyepiece is removed from the path of the light rays 

by pressing on a spring release. This arrangement permits an almost

FIUUKE 1. Leitz " ilakam " camera attachment

instantaneous focus before exposure and is of particular advantage in 

photographing growing crystals. All the photographs of microchemical tests shown in part 4 of this bulletin Avere taken with this 

attachment. The size of the photographs is S1/^ by 4^4 inches. As the 

camera attachment is an inverted pyramid, larger sizes would tend to 

make it more unstable and more readily affected by vibration.
22 MICROSCOPIC DETERMINATION OF ORE MINERALS
PHOTOMICEOGEAPHY OF POLISHED SECTION'S 23

Satisfactory pictures with exposures as long as 12 minutes have 

been taken in the Avriter's laboratory with this attachment, though 

the laboratory is by no means free from vibration caused by heavy 

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 near-by 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, 

170; 4-millimeter, 385. The tube length of the microscope with 

vertical illuminator attached is 205 millimeters.

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. The present writer uses the Bausch & Lomb instrument 

shown in Figure 5.

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 and 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 costs 

$482.50 (January, 1930), however, and it is not very often that 

magnifications as high as 800 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. 1, £>.)

Light flters. Photomicrographs of thin sections usually show 

sufficient contrast between, the component minerals if photographed 

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
24 MICROSCOPIC DETERMINATION OF ORE MINERALS

colors of white light. The resulting negatives may appear satisfactory, but the printing paper does not permit as much contrast 

as 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. 

To get good results a panchromatic plate that is, one which is 

sensitive to all colors of the visible spectrum is required. The 

Wratten M plate, made by the Eastman Kodak Co., has been used 

by the writer and fills this requirement admirably. In the following 

descriptions this plate is specified.

A color filter is a sheet of colored emulsion between glass plates. 

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 

between them. If a yellow filter (the K3 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,
PLATE 1

PHOTOMICROGRAPHS OF POLISHED SURFACE 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. 37.)

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 with the 32-millmeter 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.

15122 31 3
TJ. S. GEOLOGICAL SURVEY

B
BULLETIN 825 PLATE 1
PHOTOMICROGRAPHY OF POLISHED SECTIONS 25

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 

round 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; K3 , 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, K3 ; with other filters this mineral appears too dark.

Gangue minerals, K3 or G is best; with other filters transparent minerals 

print too dark.

The A and G filters are good for reducing contrast.

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 1, #, 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 two 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.

The following factors were obtained experimentally with an M 

plate and .a tungsten incandescent lamp and differ somewhat from
26 MICROSCOPIC bETER-MHfTA^IOtt 0$ ORE MINERALS

those given by the manufacturer: No filter, 1; A, 2y2 5 B, 6; K3 , 2^; 

G,5;H,21.

The exposure factors vary with the thickness of the emulsion, 

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 

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. 

Three-fifths 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 

two-fifths 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 aditional 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
PHOTOMICROGRAPHY OF POLISHED SECTIONS 27

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 exposure of the negative varies inversely as the square of the numerical 

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.

24-millimeter (No. 2 Leitz) .....

9-millimeter (No. 4 Leitz) __ ..

. /v.

0.10

.20 

.25

.45 

.85

( l V VN. A.J

100

25 

16

' 5 

1.4

Approximate

factor

6.2

1.7 

1.0

 »-n

086 U8° = _ 11.6L

The exposure of the negative varies directly with the square of the 

magnification. Hence the numerical aperture factor and the magnification factor tend to balance each other. To take a specific 

example: The negative of Plate 1, #, 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 magni4T2

fication of 47 is 2 minutes X 6.2 XJT^= 2.2 minutes. The exposure actually used, which was determined by exposing successive 

strips of the negative, was 3 minutes. (See pi. 1, A) It is apparent 

that the factor above given is not very accurate.

The same procedure was applied to the other lenses, and the total 

results may be tabulated as follows:

Lens

32-millimeter..... __ . ......

24-millimeter..... __ . .....

16-millimeter. ...... .........

9-millimeter..... ............

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
28 MICROSCOPIC DETERMINATION OP ORE MINERALS

It is evident that the objectives of lower magnification are slower 

and the objectives of higher magnification faster than the calculations would indicate. 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.

The pyro or elon-hydrochinon developers, formulas for which are 

given on a card inside the box containing the Wratten M plates, are 

satisfactory. The writer prefers, however, to use Rodinal developer, 

which is prepared by the Agfa-Ansco 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 three 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 it 

than to add to the time of development purely by guesswork. The 

author 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 grains; 

potassium metabisulphite, 25 grams; water, 1,000 cubic centimeters. 

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.
PHOTOMICROGRAPHY OF POLISHED SECTIONS 29

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 remifications. A very 

excellent textbook is Handbook of Photomicrography, by H. Lloyd 

Hind and W. Brough Kandles, published by George Eoutledge & 

Sons (Ltd.), London, 1927. Another is the booklet Photomicrography published by the Eastman Kodak Co.

V
PART 2. PHYSICAL PROPERTIES 

COLOR

The color of a transparent mineral in reflected light depends largely 

on its index of refraction. Minerals of 1'ow index have low reflective 

capacity 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 mineral's 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 17 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.

"Murdoch, J., op. cit., p. 28. 

30
COLOR 31

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,18 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 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.

FIGURE G. Comparison microscope

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 sufficient number of polished surfaces of pyrrhotite, it 

can be recognized immediately. Cubanite (CuFe2S3 ) 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

18 Talmage, S. B., Diagnostic value of color in polished sections: Econ. Geology, vol. 

20, p. 171, 1925.
32 MICROSCOPIC DETERMINATION OP ORE MINERALS

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, chalmersite, pentlandite, and several other minerals. In the writer's opinion, none of these minerals resembles 

cream in color. Pyrite, marcasite, pentlandite, and millerite closely 

resemble one another in color and could be described as pale yellow.

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 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 glass-disk 

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 arid the reflectors 

until the appearance of the two halves of the single field, as viewed 

through the comparison eyepiece, is identical.19

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,20

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 20a says:

19 Talmage, S. B., op. cit, p. 173.

20 Idem, p. 174.

2<m Murdoch, J., Microscopical determination of the opaque minerals, p. 28, New York. 

1916.
COLOR 33

Except in the case of the distinctly colored minerals, the differences in color 

are so slight that 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, 

1? 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.

Schneiderhohn 21 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:

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 Nos. 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."

The following minerals when viewed in vertically reflected light 

exhibit colors other than shades of white or gray:

21 Schneiderhohn, H., Mikroskopischen Bestimmung und Untersu.chung von Erzen, p. 91, 

Berlin, 1922.
34 MICROSCOPIC DETERMINATION OF ORE MINERALS

Colored minerals

Color

Do 

Do. ------

Do. 

Do. 

Do 

Do 

Do. ---------

Do 

Do 

Do 

Do ---------

Do --- --

Do . -.

Do 

Do 

Do 

Mineral

Bornite (tarnished) .... . .

Rickardite. ... 

Millerite ... 

Marcasite _______ .... 

Pentlandite _________ . _ '..

Gold 

Cubanite. __________ . ...

Calaverite.... ....................

Composition

(Ni,Fe)3S4

(Ni,Fe)Sj

Cu8FeS4 , CiuTes

CuaSea

Cu.GeSy

CuS

Cll2S

FeS2

NiS

FeS2

(Fe,Ni)S

CuFeSj

Au

Ni 2Te3

Cu

NiAs

NiSb

Bi

TTfrTp

Cu5 FeS4

AgFeaS3

FeSi +

CuS.2FeS

CuzS.BizSs

(Au,Ag)Tej

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 tJtat shoic distinctive internal reflections

Color of internal reflection

Orange ___ . ________ . .............

Do. ... ......

Do.... ...... ..... ........... ... ... ....

Greenish yellow ...........................

Yellow...... .........................

Do ... ..... ... ... .... ... ... .......

Do ... .... .... ... ... ...... ......

Red .... . ... 

Do ___ .. _ ......... __ 

Do...... .. .........

Do ....... ................... .

Do 

Do....... 

Do.... .... .... ........ .... .... .....

Do.. .

Do ... ..

Do ...........................

Do...... ............ ..........

Do... .. ....... ..........

Mineral

AT i n r P vri fp

TTp.tlsmftnnitfi

Composition

AsS

SbzSO

Ti02

MnS

ZnS

FeaOa

8Ag2S.SbaS3

Ag2S.Sb2S3

4PbS.3As8S8

HgS.2SbzS
HAKDNESS 35

Nonmetallic minerals and some metallic mineral's, such as sphalerite, hiibnerite, 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 

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 poivders

Color of powder

Red........................

Do..... .............

Do.....................

Do.... ..............

Do.....................

Do.....................

Do.....................

Do. ....................

Do......... ............

Do.. ................

Eeddish brown .............

Light orange-yellow. . ......

Brownish red ...............

Do...... ...............

Reddish orange .............

White to resin-brown .......

Olive-green .................

Dark brown ............

Do... ....

Do...... ...............

Mineral

Franklinite.....

Composition

CuiO. ......... .................

HgS.............................

SAgjS.SbjSs ....... ....

3Ag2S.SbzS3 - ....... -----

SAgaS.AsaSs .... - -

AgaS.SbaSs - ......... 

HgS.2Sb2S3~ _ ... _ --- __ ...

ThS.AssSa . ___ ... _ 

(Ag.Cu)jS.......... ............ .

FeaOs.. ......................

AsS . ....... ... ... ....... ...

AsjSs ........ ... 

5CusS.2(Cu,Fe,Zn)S.2Sb2S3.. ..

5Cu2S.2(Cu,Fe,Zn)S.2AsaS3~ -

FeaOj-nHaO....... .... ....... ...

ZnS ....... ..... ..........

MnS. ..................

(Fe,Zn,Mn)0.(Fe,Mn)203 . ...

Mn20.3MnSi02. _ ..........

Color of mineral in 

vertically reflected

light

Do.

Do.

Gray.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Gray, like sphalerite.

Do.

Gray.

Do.

Most specimens give black powder. Some specimens give black powder.

HARDNESS

There is lack of agreement among various authorities as to the 

definition of hardness.22 Methods of measuring the relative hardness of materials may 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

22 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.
36 MICROSCOPIC DETERMINATION OF ORE MINERALS

use on polished -surfaces under the microscope. The scratching 

method most used up to the present time is that described by 

Murdoch.23 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.

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 has 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

"Murdoch, J., op. clt, p. 29. '
HARDNESS 37

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

FIGURE T. Tahmige hardness machine. Detailed view o£ scratching mechanism. 

(After Talmagc. Courtesy of Economic Geology)

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.24

An instrument for measuring the relative hardness of minerals 

in polished sections was recently described by Talmage.25 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

21 Van der Veen, K. W., op. cit., p. 28.

25 Talmagc, S. B., Quantitative standards for hardness of the ore minerals: Kcon, 

Oeology, vol. 20, pp. 535-043, 1925.

13122 31 1
3S MICROSCOPIC DETERMINATION OF ORE MINERALS

scratching point is directly under the objective of the microscope, 

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.

Talmage divides the minerals into seven classes of hardness, as 

follows:

Designation

A...........

B. ..........

C. ..........

D... ........

Type mineral

Position 

of weight 

on beam

0.2

.4

.7

1.0

Actual 

weight on 

scale 

(grams)

0.105

.210

.365

.520

Designation

E. ..........

F. ..........

G. ..........

Type mineral

Position 

of weight 

on beam

1.5

2.0

3.0

Actual 

weight on 

scale 

(grams)

.775

1.035

1.550

According to Vanderwilt,26

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 

can not 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.

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 af26 Vanderwilt, J. W., The nature of polished surfaces (doctor's thesis, Harvard University), p. 70, 192J.
EXAMINATION IN POLARIZED LIGHT 39

ford 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 can not 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 

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.27 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.

27 Koenigsberger, J., Ueber einen Apparat zur Erkennung und Messung optiscber Anisotropie undurchsichtiger Substanzen und dessen Verwendung: Centralbl. Mineralogie, 1908, 

pp. 565-573, 597-605. (Translated in Wincbell, N, H, and A, N,, Elements of optical 

mineralogy, pp. 465-475, 1909,
40 MICROSCOPIC DETERMINATION" OF ORE MINERA.LS

A mathematical discussion of the optical principles involved' in 

Koenigsberger's apparatus is given by Wright,28 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. 29 A discussion of Schneiderhohn's method, together with many original data, 

is given by Sampson. 30 Some of Schneiderhohn's theoretical conclusions are criticized by Van der Veen.31 Excellent descriptions 

of the apparatus and directions for its use are given by Osborne 32 

and by Fairbanks.33 The most recent and in many respects the 

most comprehensive description of the method is that of Sampson.34

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 

the lower nicol 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 nicol (made by E. Leitz) which is 

attached directly to the vertical illuminator and can be rotated. 

(See fig. 8.) A cap analyzer can be used instead of an inside

analyzer. This form is required when the Wright biquartz wedge 

(p. 46) 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 analyzer 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

28 'Wright, F. E., Polarized light in the study of ores and metals: Am. Philos. Soc. Proc., 

vol. 58, pp. 401-447, 1919.

29 Schneiderhohn, H., Anlcitung zur inicroscopischeu Bestimmung und Untersuchung 

von Erzen und Aufbereitungsproduktcn besondcrs im auffallenden Licht, pp. 65-71, Berlin, 

1922.

30 Sampson, Edward, Note on the determination of anisotropism in metallic minerals: 

Econ. Geology, vol. 18, pp. 775-777, 1923.

81 Van der Veen, J. W., Mineragraphy and ore deposition, pp. 32-36, The Hague, 1925.

33 Osborne, F. F., Technique in the investigation of iron ores: Econ. Geology, vol. 23, 

pp. 442-450, 1928.

33 Fairbanks, E. E., The laboratory investigation of ores, pp. 24-27, New York, 1928.

34 Sampson, Edward, The determination of anisotropism in metallic minerals: Econ. 

Geology, vol. 24, pp. 412-423, 1929.
EXAMINATION IN POLAEIZED LIGHT 41

should be parallel or at right angles to the plane of the reflector of 

the vertical illuminator; if these planes arc 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. 8.) It is 

provided -with a clockwork attachment for automatically feeding

KIOUER 8. Microscope arranged for use with polarized reflected light. I'hotngra plied by K. B. Dane, jr. a, Bausch & Lomb petrographic microscope; 

li, vertical illuminator; c, polished section ill brass mount; d, polarizer 

attached to vertical illuminator ; e, analyzer ; f, blue ground glass ; <j, rota table 

disk ; h, small arc lamp

the carbon to the electric arc; this attachment is convenient but not 

necessary. A rheostat giving approximately 13 ohms is necessary 

Avhen 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 8 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
42 MICROSCOPIC DETERMINATION" OP ORE MINERALS

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 United States 

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 approximately $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 16-millimeter short-mounted objective is by far the best for 

observations on opaque minerals in polarized light. The 24-millimeter and 8-millimeter objectives also give fair results but are inferior to the 16-millimeter objective. 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 has 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 microscope, isometric minerals show no change of color or change 

of intensity of light during the revolution. Minerals o,f 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
EXAMINATION IN POLAEIZED LIGHT 43

great, diagnostic value because so small a proportion of the total 

number of opaque anistropic 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 Schneiderhb'hn and Sampson. 

The same is. true of certain varieties of hessite, argentite, stannite, 

ouprite, pyrite, bornite, and perhaps a few others. Most specimens 

of these minerals are isotropic, however. A possible cause of the 

a.nisotropism 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 

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 at 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.
44 MICROSCOPIC DETERMINATION OF ORE MINERALS

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.

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 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 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 Van der Veen's tables.

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 

oven with this a series of observations is required in order to obtain
EXAMINATION IN POLARIZED LIGHT 45

the exact angle by the method of averages. This must be extended 

to a number of grains, as the rotation of the polarization plane 

produced by the mineral depends .on the orientation of the grain 

and thus 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 anisotropisnl is barely discernible, gives a rotation of a small 

fraction of a degree, Avhereas 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.

Methods of observation*5 Anisotropism 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 lie 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 mod8G This description of tbe three ways of detecting anisotropism is taken almost verbatim 

from Sampson's article (Econ. Geology, vol. 24, pp. 415-418. 1929).
46 MICROSCOPIC DETERMINATION OF ORE MINERALS

erate 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 

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 of 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
EXAMINATION" IN TOLAKIZED LIGHT 47

a polished section of chalcopyrite from Sudbury, Ontario, of value 

in getting the best adjustment of nice Is. 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 9 shows chalcopyrite that gives an 

effect identical with that of the Sudbury mineral.

FIGURE 0. 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. li, Twinning in chalcopyrite 

from Bass prospects, Grand Canyon, Ariz. Grain of pyrite in upper 

left corner. Nicols nearly crossed. Specimen unctchcd. X150.

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
48 MICEOSCOPIC DETERMINATION" OP ORE MINERALS

the illuminator, and the same process is repeated. This is continued 

until the chalcopyrite lamellae show the maximum contrast. Then 

the lamp i$ shifted to one side or the other until the illumination 

as seen in the microscopic image is evenly distributed. After some, 

experience a student of average ability can make this adjustment in 

five minutes or less.

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 section 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 

means of the leveling cup (see p. 7) 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 grained 

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.
MICROSCOPIC DETERMINATION OP ORE MINERALS 49

ELECTROCONDUCTIVITY

Methods for determining the electrical conductivity of minerals 

have been described by Farnham,36 by Kerr and Cabeen,37 and by 

Harvey.38 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.

00 Davy, W. M., and Farnham, C. M., op. cit, p. 8.

37 Kerr, P. F., and Cabeen, C. K., Electrical conductivity of the ore minerals: Econ. 

Geology, vol. 20, pp. 729-737, 1925.

38 Harvey, R. D., Electricsil conductivity and polished sections : Econ. Geology, vol. 23, 

p. 778, 1028.
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: HNO3 , 1:1; HN03 , concentrated; 

aqua regia; HC1, 1:1;-HC1, concentrated; KCN, 20 per cent; FeCl3 , 

20 per cent; KOH, saturated solution. In his descriptions of the 

individual minerals, their reactions with the following solutions 

are also given: NH4OH, NaOH, iodine, (NH4 ) 2 SX , K4Fe(CN) c , 

K3Fe(CN) 6 , 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 HN03 , 

1:1; HC1, 1:1; KCN, 20 per cent; and FeCl3 , 20 per cent. In their 

detailed 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

50
ETCH REACTIONS 51.

at other times, even on the same specimen. It is known that the 

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 HN03 " 

groups. Sphalerite is usually negative to 1:1 HCI, but one or two 

specimens that gave faint reactions with HCI have been observed. 

These facts should be noted in describing sphalerite, and it should 

be placed in both the " etched by HCI" and " negative to HCI" 

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 identifying 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. Schneiderhorn immerses the entire polished
52 MICROSCOPIC DETERMINATION OF ORE MINERALS

surface face down in a receptacle containing the reagent. In this 

manner the entire section is acted upon by the etching reagent, and 

electrolytic effects can not 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 10. 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 

one minute, then Avashes 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 three 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 longthe etch structure is entirely destroyed and the surface turns black.

FIGURE 10. Platinuar wire for transferring drop of reagent to polished 

surface. Plane of loop is at an angle 

to rest of wire
ETCH REACTIONS 53

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 contact 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 one minute. If no 

effect can be observed at the end of one minute the test is considered 

negative. If the mineral is not completely etched at the end of one 

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

HNO.V 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 HNOlt drop still has a negative effect 

at the end of one minute, it is preferably allowed to remain on the 

surface until it begins to dry up, or approximately two 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 scum 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 per cent solution. Mercuric chloride is more troublesome. Davy and Farnham recommend a saturated solution (approximately 7 per cent 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

15122 31 5
54 MICROSCOPIC DETERMINATION OF ORE MINERALS

liable to confuse this mercuric chloride precipitate with an etch 

reaction. For this reason the writer recommends a 5 per cent solution of mercuric chloride (1 gram of HgCl2 with sufficient water 

added to make 20 cubic centimeters of solution). Even with this concentration some mercurous chloride will slowly separate out, and it 

is advisable to filter the solution from time to time. 

The reagents used by the writer are as follows:

HN03 , 1:1(1 part HN08, specific gravity 1.42, to 1 part H2 0). - , HC1, 1:1 (1 part HC1, specific gravity 1.10, to 1 part H2 0). 1

KCN, 20 per cent solution. '"~ '

FeCla, 20 per cent solution.

KOH, 40 per cent solution. |

HgCl2, 5 per cent solution.

Aqua regia (3 parts concentrated HC1 to 1 part concentrated HNOa).

H2 02, commercial reagent.

Aqua regia is used only for those minerals which 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, H2O2 . A drop of 

the commercial reagent placed on the mineral effervesces, but the 

mineral itself is not etched. 89 According to Smitheringale,40 these 

reactions are less positive than 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 author uses the 3 per cent solution manufactured by the Oakland Chemical Co., New York. 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 four 

solutions, on the other hand, 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

80 Thlel, G. A., The manganese minerals; their identification and paragenesls: Econ. 

Geology, vol. 19, p, 107, 1924.

40 Smitheringale, W. V., Etching tests and X-ray examinations of some manganese 

minerals: Econ. Geology, vol. 24, p. 481, 1929.
ETCH REACTIONS 55

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 can not 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 apt to be overlooked. For instance, pyrite will give a slow effervescence, a few bubbles at a time, 

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 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. 13.) 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
56 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 

is applied to the surface until the section is removed for the purpose 

of washing the reagent off. The effect can be obser.ved 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 surf ace 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.41 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 our min41 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 bei preferable.
ETCH REACTIONS 57

eral-grain 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, 

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 can not 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. 12.) 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 HN03j which usually requires 

regrinding the surface on a steel lap.
58 MICROSCOPIC DETERMINATION OF ORE MINERALS

Although the systematic etching scheme just described has many 

disadvantages and can not 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 

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 

can not 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.

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 

71-96 and the hard minerals on pages 97-104.

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 withHN03 ; 

(2) reacts with HN03 without effervescence; (3) does not react with 

HNO3 . 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 65-70. 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 u pigeonholes" are arranged. In the detailed 

descriptions the reactions to KOH and HgCl2 are also given.
DETERMINATIVE TABLE 59

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 pages 76-77. The description 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, HN03 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 91, and the mineral is either stephanite or miargyrite. 

Scratching the mineral with a chisel-pointed needle (see p. 119) 

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 91. Miargyrite is one ' 

of the minerals in that group, thus affording further confirmatory 

evidence.

The formulas given for minerals in the determinative table are 

those of Wherry and Foshag.42

43 Wherry, E. T., The nomenclature and classification of sulfide minerals: Washington 

Acnd. Sci. .Tour., vol. 10, pp. 4S9-49G, 1020. Wherry, E. T., and Foshag, W. P., A new 

classification of the sulfo-salt minerals: Idem, vol. 11, pp. 2-8, 1921.

INDEX

[Minerals marked * are discredited or doubtful]

Page

Aguilarite, Ag2 (Se,S)_______________________________________________ 80, 86

Aikinite, Cu2S.2PbS.Bi2S3 ......__...._........_....._...... 76, 78, 87, 89

Alabandi.te, MnS....___---__________-_------------------- 74

Alaskaite, Ag2S.PbS.2Bi.S3---_ ~ -- ~- --~ ---~-- 81, 87 

Algodonite, CufiAs..-_______-_-___-_______-___--__---_-_-_---_---- 71

Alt'aite, PbTe.__...__ _.__-_-_.__-__. _____ ___-_____._.__ _ 73

Andorite, Ag_S.2PbS.3Sb2S3 ____~_-- ---_-___-_-_-_-----_--- -- 81,94

Antimony, Sb.'__.___________________________________________ 79, 83

Ararnayoite, Ag2S.(Sb,Bi) 2S3 _______---_____--_.__-_---_-__--___--_- 88

Argentite, Ag2S__._._..._..._____.___..._._.___._.______-_._ . 79, 92

Argyrodite, 4Ag2S.GeS.___._________________________________ 84, 85, 92, 93

Arsenic, As_______________________________________________________ 76

Arsenargentite, Ag3As___________________________--_-__----_-_--_- 72

Arsenoferrite, FeAs2 __--__________-_-____--___-----_---..__-_----_- 97

Arsenopyrite, FeAsS..__.___.____________________________ 98, 100

Baumhauerite, 4PbS.3As2S3 .-.________________________________ 104

Benjaminite (Cu,Ag) 2S.2PbS.2Bi2S3 -----__-------------------------- 78

Berthierite, FeS.Sb2S3_ . _ -- - ----- ---- 88, 95

Berthonite, 2(Pb,Cu.)S.Sb2S3---~ - ---- ---- -«----- 89

Bismuth, Bi_____________________________________________________ 73

Bismuthinite, Bi2S3 __'__ _-__--_-__,______---_-_-------_-..---_----_- 74, 78

Bismutoplagionite, 5PbS.4Bi2S3-___-_--_----_---.____.._-_____-.-_.- 74,82

Bornite, Cu.FeS* _____________________________________________ 76

Boulangerite, 5PbS.2Sb2S3 ----- ---~~~~~- ------- 75, 78

Bournonite, Cu2S.2PbS.Sb2S3--.._----_-_---_-_-----------_______ 89, 95

Braunite, 3Mn2 03 .MnSi02 ._____ ____________________ ________ _ 102

Bravoite (Fe,Ni)S2 _ ____________ _______________._____:____._______ 101

Breithauptite, NiSb_._.________________________________________ 100

Brongniardite, Ag2S.PbS.Sb2S3.------________________________ 89,95

Calaverite (Au,Ag)Te______-_________________________________ 76,78

Canfieldite, 4Ag2S.SnS2 (containssome germanium)._________________ 83,84

Carrollite (Co,Cu)jS4--_-__-_ ___ __- _- _!-__ _ _ 87 

Cassiterite, Sn02 _________________________________________________ 103

Cerargyrite, AgCl._..________.____..____.__..__.-..__________-___ 92

Chalcocite, Cu2S_____________________________._______'____________ 72,75

Chalcophanite (Mn,Zn)0.2Mn02 .2H2 0_____ _________________________ 95

Chalcopyrite, Cu2S.Fe.S3 _-_-____ _ ___._-_______ __-____- 87, 89, 94, 95

Chalcostibite, Cu2S.Sb2S3._ _ ____ __ _ ___ ____ .._- 85,90

Chalmersite, CuS.2FeS (identical with cubanite)_____________________ 90, 95

*Chilenite, Ag6Bi..... .____..__._.__-._..._.__....__._._.__....._. 104

*Chiviatite, 2PbS.3BijS3-__ >_ '._ ___ __--__- _ ___ __- 104 

Chloanthite-smaltite (Ni,Co)As2 _._______ __________________________ 97

Chromite, FeO.Cr203 -_-.------_.______.___'._______..__.__________ 102

Cinnabar, HgS_-_______________-___-______-_____-______________ . 95

Clausthalite, PbSe-____---.____________________________ 80

Cobaltite, CoAsS.____-_____--____---_-______________.-- 100,101,102,103

61
62 MICROSCOPIC DETERMINATION OP ORE MINERALS

	Page 

Coloradoite, HgTe_---------.----_------------------------------- 86, 93

*Comuccite _______________________________________________________ 104

Copper, Cu._-__.-._______.___._____ I______ 72, 75

Copper pitch ore (see Tenorite)_--__-_---____--___-__-_-_____-_-__- 82

Corynite, Ni(As,Sb)S-_--._.........._... ........__._...._..... 97,101

Cosalite, 2PbS.Bi2S8 74, 81

CoveUite, CuS 93

Cubanite, CuS.2FeS.______-__________ __________ 90, 95

Cuprite, Cu20_______________.________,________. 73

*Cuprobismutite, 3CusS.4BiaS3. . 104

Cuprodescloizite, Pb2 (Zn,Cu) 2V2 09 .H20__-.______...____________ 82

Cylindrite, 6PbS.Sb2S3 .6SnS2 -. _ 80, 81, 88

Delafossite,CuFe02 .._______._____'________________________ 92

*Dognacskaite, Cu,Bi,S__ _ ___ _ __ 104

Domeykite, Cu3As_ _______________________________________________ 71, 82

Dufrenoysite, 2PbS.As2S3 _ 103

Dyscrasite, Ag3Sb-____-_.___---______--_--________________ 79, 83, 86

Electrum, Ag,Au________________________________________________ 76

Emplectite, Cu2S.Bi2S3 --_____. _________-______-___. 78, 90

Enargite, Cu_S.4CuS.As8S3 --- _ --------- 85, 93

*Epiboulangerite, 3PbS.Sb2S5 ..____...._____________________________ 104

Eucairite, Cu2Se.Ag2Se_________________________________------- 83

Famatinite, Cu2S.4CuS.Sb2 S3 ________________________________ 85

Ferberite, FeW04 .---------------------------------------------- 103

Franckeite, 5PbS.Sb2S3 .2SnS2 .___...___...____________ 82, 89

Franklinite (Fe> Zn,Mn)O.(Fe,Mn).08 . ___ 101, 102

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 -.______ ____ _.___ 86, 87

Freieslebenite, 3Ag2S.4PbS.3Sb2 S3 ...___ _____._________ _________ 92

Galena, PbS__ - 73, 80

Galenobismutite, PbS.Bi2 S3 --._____________._______________ 77

Geocronite, 5PbS.Sb2S8 -__ _ 78, 90

Germanite, CuxGeSy ..____________________________________________ 94

Gersdorffite, NiAsS___ _ 97, 98

Glaucodot(Co,Fe)AsS,.. _ . .._ _ . 100

Goethite, Fe2O3 .H2O.__ __------------_________________ 103

Gold, Au-__. _ _ __ __ 92

Guanajuatite, Bia (Se,S) 8 ____________________________ 78, 90

*Guejarite, CuaS.2SbjS8 __ _ ... 104 

Guitermanite, 3PbS.As2S3 -_________________________ 90

*Hauchecornite (Ni,Co) 7 (S,Sb,Bi) 8 -__ __ 104 

Hauerite, MnS2 -_--.-__. _ _____ _ _ 81, 91

Hausmannite, Mn304 -_-.________i_______________._________ 82, 92, 99, 101

Hematite, Fe2 0S 102, 103

*Hengleinite (Fe,Ni,Co)Sa_. - , 104

Hessite, Ag.Te__ _______ ___ __ _ 79, 81, 86

Hiibnerite, MnWO4 .__________________________._.__________ __ 103

Huntilite, Ag3As (identical with arsenargentite)--___-_____-____-_-___ 72

Ilmenite, FeTiOs ...- ________________ __-____- 103 

Jamesonite, 4PbS.FeS.3Sb.S3 _.. __- - 74, 77

Jordanite, 4PbS.As2S3 -.-___________________:________ 90

*Kalgoorlite, HgAu2Ag6Te8 ._.________________________ 104

Kallilite, Ni(Sb,Bi)S.. 86, 88, 99

Keeleyite, PbS.Sb2S8 ___ ._ 94

Kermesite, Sb2SaO_._ __ __-__ ___-______-___ -________ 84. 95
DETERMINATIVE TABLE 63

Page 

Klaprotholite (also known as klaprothite), SCiisS^BijSs-------------- 76, 85

Klockmannite, CuSe.._.____._-__-_-___-___-___-_____-__-_..___._^ 85

Krennerite, (Ag,Au)Te2 -__- _ _-_-_--- 78, 90

"Lehrbachite (PbSe+HgSe)...___..__..._._......_......_......__.. 104

Lengenbachite, QPbS.(Ag,Cu) 2S.2As2S3 ._ 104

Leucopyrite, Fe3As4 ___________-_ ____ __-_ -_- -_-_ -- 100

*Limonite, Fe2 03 .nH2 0______-_ . __ ___.__._____ 104

Linnaeite, 0384_________--___-----_---_-___-_-___-..-----_--___ 87

Livingstonite, HgS.2Sb8Ss - 84, 93, 95

Lollingite, FeAs2 ___.______ _.. 98, 100

Lorandite, T]2S.As2S3 --__-----_---------------------------__ 90

Luzonite, 3Cu2S.As2Ss (variety of enargite)_____ ____- __-__ _ _____ 85, 93

Magnetite, Fe3 04 - --------- 91, 96, 101, 103

Manganite, Mn_03 -- - ---- -------.---..------- ---_-. -- 92

Marcasite, FeS2 .___-________ _ 98, 100

Matildite, (Ag2 ,Pb)S.Bi3S3 - ----. -- -------------- 73

. Maucherite, Ni 3As2 --------------------------------------------- 97, 08

Melaconite, CuO (variety of tenorite) _____-__-__-_-_-__--__ -______ 82

Melonite, Ni2Te3 _. _. 77

MenegMnite, 4PbS.Sb2S3 _--------------------_ 75, 78

Metacinnabarite, HgS._--_-_--_- __-_-_-_--_---__--__-----_---_--- 96

Miargj'rite, AgjS.Sb2S8 -------------_-----_-------------- 84, 91, 93

Millerifce, NiS 89

Molybdenite, MoS8 _______..-_..-...._-----__..--_- ..-.. 96

Nagyai?ite, Pb,Au,Te,S...__,_.___________-----________ 90, 96

Naunmnnite, (Ag2 ,Pb)Se_-_-_--_-_--_-_--------_.---------_----_-_ 83

Niccolite, NiAs._____________________________________________ 98

*0nofrii;e, Hg(S,Se)____.______.___..___..... .. __.._........__ 105

Orpiment, As2S3 ___--__________.__________________________________ 93

Owyheeite, 8PbS.2Ag2S.5Sb2S3 -- -- - - -- -- -- 82 

Pearcite, 8Ag2S.As2S3 __.__._--..__--_---.--_-..-.--.___-_-_.-.... 83

Pentia-Qdite, (Ni,Fe)S._ -- - - --- -- I- -. . 88

Petzite, (AgjAuJaTe...... -- - _ --- -. _ 80, 86

Plagionite, 5PbS.4Sb.S3 - - - -- -- - 74, 77

Plessite, (Ni,Fe)AsS_---------___. _-__...___..____--__.__...__ 98

Poliamte, Mn02 .______________-__-.-._-_-_.._-._-_._______ 103

*Polyargyrite, 12Ag2S.Sb2 S3 __... ...___-._--.__ ... ..1_...... 105

Polybs,site, 8AgaS.Sb2S3 ._ . --- 83, 84

Polydymite, ^84 -- _ - ._ 77

Proustite, 3Ag_S.As_S3 . _ _ 83, 84

Psilomelane, Mn2 Mn05 ._-__._-_---_--_---___--_-__-_-_--_____-_-_ 99

Pyrargyrite, 3Ag2S.Sb2S3 ___-___-_ ___... 84, 93

Pyrrhotite, FeSn-._- . __>.._-_ _.. ._ .. _ 89 

Pyrite,, FeS8 ___ 98, 99, 100, 101, 102

Pyrolusite, Mn02 _---------------------------------------_----_-- 96

Rammelsbergite, NiAs2 ________--______---..____-_-_.._______-______ 98

Rathite, 3PbS.2As2S3 ._______-__-_--_.___-__---.__-.__.._.___...._ 78,89

Realgar, AsS__-_---_----_-_-____-_____.._____________ ___ _ _____ 96

*Regnolite, 5(CuS,FeS,ZnS).As_S3 _._. . _ -- - _. 105 

Rezbanyite, 2PbS.3Bi2S3 ____.__.___ _.___.__._ _ 78 

Rickardite, Cu4Te8 (?) __-__.._.--____---------________ 72

Rutile, Ti02 _______-----___-___._..____..__.._.. 103

Schapbachite, Ag2S.PbS.Bi2S3 .____..._._....______.. _... 81
64 . MICROSCOPIC DETERMINATION OF ORE MINERALS

Page 

Seligmannite, Cu2S.2PbS.As2S3 ------_-..____._____._.___ 104

Semseyite, 9PbS.4Sb2S3 . 74

Siegenite, (Co,Ni) 3S4 __ _ _ _ _ __ __ __ 87

Silver, Ag____ __ 72, 73, 75

Skutterudite, (Co,Ni)As3____.,-_---__-__,_____.___-._____.-_____ 97, 99

Smaltite-chloanthite (Co,Ni)As2 ----------------------------------- 97

Sperrylite, PtAs2 _------------------ -- - _ 102

Sphalerite, ZnS. 77, 81, 88, 94

Stannite, Cu2S.FeS.SnS2 -..------------------------------------_- 88, 90

Stephanite, fiAg2S.SbaS3 -' 79, 80, 83, 91

Sternbergite, AgFe2S3-4 ------------------------------------------ 82

Stibnite, Sb3S3 73, 80, 85, 89

Stromeyerite, (Cu,Ag) 2S______----.-------_---_--------_--__--____ 79

Sulvanite, Cu3VS4 ------------------------------- 92

Sylvanite (Au f Ag)Te2 - - 77, 78, 87, 91

*Tapalpite,3Ag2 (S,Te).Bi2 (S,Te)3.- - --- 105 

Teallite, SnS.PbS.. 82, 92.

Tellurium, Te.__ 74, 76

Tennantite, 5Cu2S.2(Cu,Fe,Zn)S.2As2S3 ----------------------------- 84, 88

Tenorite, CuO__________.._._-__ _....__-..._._..___ 82

Tetradymite, 'Bi2 (Te,S')3--------- ---------------------------------- 73, 76

Tetrahedrite, 5Cu2S.2(Cu,Fe,Zn)S.2Sb2S3 _--_______-_-__--_--__-_ 84, 88, 94

Tiemannite, HgSe.____.._....-_._-_______..__.___..-_._.-. 93, 94

Troilite, FeS.... . , .. 74

Tungstenite, WS2 _----------------------------------------------- 94

*Turgite, 2Fe2 03 .H2 0.------------------..-------------------------- 105

Ullmanite, NiSbS.. 86, 88, 100

Umangite, Cu3Se2 ..___________-___-_-_-__^--____-__-_--._-____-_._ 79

Uraninite, uranate of U, Pb, etc--..-_------------_----------------__ 102

Violarite, (Ni,Fe) 3S4 ----_-__... ___ ._.____.--____....._ 77

Voltzite, 4ZnS.ZnO.._ 81

Vrbaite, Tl2S.3(As,Sb) 2S3 . 89

Weissite, Cu6Te3 (?)-_-----------___...______._.._____----- 72

Whitneyite, Cu9As.---------------------------------------------- 71

Willyamite, (Co,Ni)SbS_ _-_______.._: _..._ 86,88, 100, 101

Wittichenite,3Cu2S.Bi2S3.---------------___._-------_...._ 86

Wolframite, (Fe,Mn)W04 ----------------------------------------- 103

Wurtzite, ZnS. _____._------__ 88

Zinkenite, PbS.Sb2S3 . ------------------------------------------ 74
DETERMINATIVE TABLE 65

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Page

77

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DETERMINATIVE TABLE 67

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Chloanthite-smaltite.. . . .....

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(Ni, Co) AS2- -- -- _ - 

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N^isAss

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Page

97 

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70 MICROSCOPIC DETERMINATION OF ORE MINERALS

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DETERMINATIVE TABLE 71

SOFT MINERALS 

HNO3 eff., HC1, KCN, FcCl a

ISOTEOPIC

Copper arsenides. Textbooks in mineralogy usually describe three arsenides of 

copper domeykite (Cu3As), algodonite (Cu6As), 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.43

Domeykile (CusAs).- Isometric; color white; hardness C . 

KOH HN03 effervesces, turns dark gray, and brings out a triangular 

HgCIj etch cleavage. HC1 brings out a triangular etch pattern; 

mineral stains slowly. KCN slowly brings out triangular etch 

pattern; surface does not stain. FeCla instantly stains black. KOH stains 

differentially iridescent and brings out triangular etch pattern. HgCk 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 (Cu8As). Isometric; color greenish gray; hardness C . HNOs 

effervesces and blackens; some varieties give parallel (possibly cubic) etch 

cleavage. HC1 slowly stains brown. KCN stains brown; some varieties show 

parallel etch cleavage. FeCls instantly stains black. KOH slowly stains iridescent. HgClz instantly stains black. Algodonite is an iri'tergrowth of two 

constituents, each a definite mineral, termed a and 0 algodonite. a-algodonite 

is greenish gray, /3-algodonite has a suggestion of blue. With HC1 a-algodonite 

etches more rapidly than /3-algodonite. More sectile than domeykite.

Whitneyite (CugAs). Isometric; color pale coppery pink; hardness C . 

HNOa effervesces vigorously and stains iridescent; on some varieties brings out 

grain structure; with others etching brings out a distinct lamellar structure. 

HC1 slowly stains gray. KCN slowly browns; with some varieties reagent 

effervesces slowly. FeCls instantly blackens. KOH slowly stains iridescent. 

HgClj instantly stains brown to black. Four distinct constituents have been 

recognized in whitneyite. These are termed a, /3, 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. 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. 

A good method of distinguishing between copper and whitneyite is to apply a 

drop of 1: 5 HC1. Whitneyite etches readily, whereas copper is negative. The 

best test for whitneyite, however, is a microchemical test for arsenic.

<> Broderick, T. M., in Butler, B. S., and others, The copper deposits of^Michigan: U. S. Geol. Survey 

Prof. Paper 144, p. 56,1929.
72 MICROSCOPIC DETERMINATION OF ORE MINERALS

Copper (Cu). Isometric; color coppery red; hardness B + . HNOs effervesces; 

some varieties tarnish brown; others seem to effervesce without tarnishing. 

HC1 negative; some varieties slowly tarnish brown. KCN slowly browns. 

FeCls quickly darkens. KOH slowly tarnishes brown to iridescent. HgCl2 

quickly blackens. Sectile.

Arsenargenlite (AgsAs). Isometric; color creamy white; hardness B. HNOs 

effervesces and stains black. HC1 tarnishes iridescent. KCN tarnishes brown. 

FeCls instantly tarnishes iridescent. KOH stains brown. HgCla tarnishes 

brown. Almost sectile. Very rare. Identical with huntilite.

Chalcocile (Cu2S). Isometric and orthorhombic; color bluish 

KOH neg. gray; hardness B. Hypogene chalcocite with more than 8 per 

HgCl2 cent of dissolved covellite is isotropic. Very fine grained supergene chalcocite is also apparently isotropic. Most chalcocite, 

both hypogene and supergene, is weakly anisotropic. HNOs effervesces vigorously; mineral stains blue; with some isotropic varieties brings out octahedral 

etch cleavage; very fine grained supergene chalcocite shows no etch cleavage. 

HC1 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. FeCls stains blue; 

some isotropic varieties show octahedral etch cleavage; very fine grained supergene chalcocite, however, does not show etch cleavage. KOH negative. HgCL> 

tarnishes slightly.

Silver (Ag). Isometric; color silvery white; hardness B. HNOs effervesces 

slightly for a moment; stains gray. HC1 fumes tarnish slightly; almost negative. 

KCN tarnishes dark gray to brown. FeCls instantly stains iridescent. KOH 

negative. HgCl2 stains brown to iridescent. Very sectile.

ANISOTROPIC

Rickardite (CuiTe3 ?). Crystal system unknown; color purplish 

KOH red, similar to bornite; hardness B. Strong anisotropism; gives 

HgCIj four extinctions per revolution. Interference color fiery orange, 

very similar to that of umangite. HNOa 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 given in textbooks is 

probably incorrect.

Weissite (Cu5Te3 ?). Crystal system unknown; color light gray, about the 

same as chalcocite; hardness B. HN03 effervesces and stains brown. HC1 

stains light brown. KCN stains light brown. FeCls stains brown. KOH 

slowly stains brown. HgClj stains light brown. Specimen from Vulcan, Colo. 

Always associated with rickardite (C^Tes). Distinguished from rickardite by 

color.

Chalcocite (Cu2S). Isometric and orthorhombic; color bluish 

KOH neg. gray; hardness B. Most chalcocite is weakly anisotropic. HNOs 

HgCl2 effervesces vigorously; mineral stains blue; with most varieties 

brings out parallel etch structure; very fine grained supergene 

chalcocite, howeverj 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. FeCls stains blue; with most specimens brings out parallel etch cleavage. KOH negative. HgCU tarnishes slightly.
DETERMINATIVE TABLE 73

Cuprite (Cu2 0). Isometric; color bluish gray; hardness D .

KOH neg. HNOs effervesces and deposits coating of metallic copper.

HgCI2 neg. HC1 deposits white coating. KCN darkens and brings out

parallel etch structure. FeCls tarnishes iridescent. KOH and

HgClj negative. Ruby-red powder and internal reflection.

HNO3 eff., HCI, KCN, FeCl3 neg.

ANISOTHOPIC

Siibnite (Sb2Ss). Orthorhombic; color gray; hardness B. 

KOH Very strong anisotropism. HNOs stains differentially iridescent; 

HgCh neg. some varieties effervesce slowly. HCI fumes tarnish; some varieties negative. KCN brings out scratches and sometimes stains 

slightly. FeCls negative. KOH instantly tarnishes brown and gives yellow 

coating; this is the most characteristic reaction for stibnite. HgCl2 negative. 

Prismatic. 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

Silver (Ag). Isometric; color silvery white; hardness B. 

KOH neg. HNOs effervesces and stains gray. HCI fumes tarnish; halo 

HgCl2 does not wash off. KCN, some specimens tarnish gray to brown,

others negative. FeCls instantly stains iridescent. KOH negative. HgClj stains brown to iridescent. Very sectile.

Altaite (PbTe). Isometric; color galena-white; hardness B. 

KOH neg. HNOs effervesces and stains through iridescent to dark gray. 

HgCl2 neg. HCI quickly tarnishes iridescent. KCN negative. FeCls

quickly tarnishes iridescent. KOH and HgCl2 negative. Cubic 

cleavage but less marked than galena; shows fewer triangular pits.

Galena (PbS). Isometric; color galena-white; hardness B. HNOs, a small 

percentage of galena specimens effervesce and turn black. HCI tarnishes iridescent. KCN negative. FeCl3 stains iridescent; with galena areas in contact 

with bornitt:, reaction is negative. KOH and HgCl2 negative. Perfect cubic 

cleavage; many'specimens show abundant triangular pits. Galena specimens 

that effervesce with HNOs give off noticeable amounts of H2S.

ANISOTROPIC

Bismuth (Bi). Hexagonal; color pinkish gray; hardness A . 

KOH neg. Strong anisotropism. HNOs effervesces and darkens. HCI 

HgCl2 stains differentially black; sometimes brings out lamellar structure. KCN negative. FeCls instantly stains iridescent. KOH 

negative. HgCl2 stains brown in spots; doubtful.

Telradymtte (Bi2 (Te,S)s). Hexagonal; color silvery white; hardness B. 

HNOs effervesces and stains brown to black; action sometimes slow in starting. 

HCI stains light.brown to iridescent; in some areas action is very slow. -KCN 

negative. FeCls stains iridescent. KOH negative. HgCl 2 , 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 cleava,ge.

Maiildite ((Ag2,Pb)S.Bi2S3). Color galena-white; hardness C.

KOH neg. HNOs effervesces and stains black. HCI quickly tarnishes black.

HgCl2 neg. KCN negative. FeCls stains light brown. KOH and HgClj

negative. Specimen examined is from O'Brien mine, Cobalt,

Ontario. Is intergrown with another mineral containing Pb, Bi, and S without
74 MICROSCOPIC DETERMINATION OP ORE MINERALS

any silver. Microchemical tests on the matildite indicate the presence of a substantial amount of lead.

Tellurium (Te). Hexagonal; color silvery white; hardness B. Strong anisotropism. HN03 stains black with effervescence. HC1 fumes tarnish. KCN 

negative. FeCl3 slowly stains light brown. KOH and HgCl2 negative.

Cosalite (2PbS.Bi2S3). Monoclinic; color galena-white; hardness B. Strong 

anisotropism. HN03 stains black with slow (doubtful) effervescence; fumes 

tarnish. HC1 slowly stains differentially gray to brown. KCN negative. FeCl3 

stains light brown. KOH and HgCl2 negative. Prismatic. Specimen from 

Cobalt, Ontario; described and analyzed by T. L. Walker.

HNO3 eff., HCI, KCN neg., FeCI3 neg.

ISOTROPIC

Aldbandite (MnS). Isometric; color gray, similar to sphalKOH neg. erite; hardness C. HN03 effervesces and gives off H2S; tarHgCl2 neg. nishes brown; sometimes brings out grain structure. HCI

effervesces and gives off H2S; tarnishes black. KCN, FeCI3 ,

KOH, and HgCl2 negative. Powder and internal reflection greenish yellow.

ANISOTEOPIC

Jamesonite (4PbS.FeS.3Sb2S3). Monoclinic; color galenaKOH white; hardness B + . Strong anisotropism. HN03 tarnishes 

HgCl2 neg. iridescent with slow effervescence. HCI fumes tarnish. KCN 

and FeCl3 negative. KOH tarnishes iridescent. HgCl2 negative. Prismatic. Perfect basal cleavage.

Plagionite (5PbS.4Sb2S3). Monoclinic; color white; hardness C. HN03 

effervesces and stains black. HCI fumes tarnish on some areas; other areas 

negative. KCN and FeCl3 negative. KOH stains gray to brown. HgCls negative. Prismatic. Very rare. Distinguished from zinkenite only by crystal 

form.

Semseyile (9PbS.4Sb2Ss). Monoclinic; color gray; hardness C. Strong anisotropism. HN03 effervesces and stains black. HCI fumes tarnish a little; stains 

slightly in places. KCN and FeCls negative. KOH slowly stains iridescent.

HgC) 2 negative. Prismatic.

Zinkenite (PbS.Sb2S3). Orthorhombic; color gray; hardness C. HNO3 effervesces and darkens. HCI fumes tarnish slightly. KCN and FeCls negative. 

KOH stains iridescent. HgClz 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. HN03 effervesces violently and 

gives off H2 S. HCI effervesces and gives off H2S. KCN and FeCl3 negative. 

KOH stains differentially iridescent; slow. HgCl2 negative. Very rare but 

sometimes found in meteorites.

Bismuthinite (Bi2 S3). Orthorhombic; color galena-white;

KOH'neg. hardness B+. Strong anisotropism. HN03 slowly effervesces

HgCl2 and stains black. HCI fumes tarnish. KCN, FeCl3, and

KOH negative. HgCl2 stains light brown; some specimens

negative. Prismatic.

Bismutoplagionile (5PbS.4Bi2 S3). Orthorhombic; color gray; 

KOH neg. hardness B+. HN03 quickly stains iridescent; effervesces 

HgCl2 neg. slightly. HCI 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. 589, 1920).
DETERMINATIVE TABLE 75

Boulangerite (5PbS.2Sb2S3). Orthorhombic; color galena-white; hardness 

B + . Strong anisotropism. HN03 effervesces; surface stains black; action 

sometimes slow in starting. HCl fumes tarnish slightly; doubtful. KCN, 

FeCl3, KOH, and HgClj negative. Prismatic. Distinguished from jamesonite 

by KOH test.

Meneghinite (4PbS.Sb2S3). Orthorhombic; color white; hardness' C . 

Strong anisotropism. HN03 effervesces; surface stains black; usually reaction 

is slow in staxting, but once started the etching rapidly advances as a wave from 

one side of the drop to the other. HC1 fumes tarnish; some areas negative. 

KCN, FeClj,, KOH, and HgCl2 negative. Prismatic. Specimen examined is 

from Bottino, Tuscany, Italy, the type locality. Mineral is intergrown with 

galena, hence formula above given is open to question.

HNO3 eff., HC1 neg., KCN, FeCl3

ISOTROPIC

. Copper (Cu). Isometric; color coppery red; hardness B.

KOH HNO3 effervesces without tarnishing. HCl negative; sometimes 

HgCl2 tarnishes faintly. KCN slowly browns. FeCl3 quickly darkens. 

KOH slowly tarnishes brown to iridescent. HgCl2 quickly 

blackens.

Chalcocite (Cu^S). Isometric and Orthorhombic; color bluish 

KOH neg. gray; hardness B. Hypogene chalcocite with more than 8 per 

HgCl2 cent of dissolved covellite is isotropic. Very fine grained supergene chalcocite is also apparently isotropic. Most chalcocite, 

both hypogene and supergene, is weakly anisotropic. HN03 effervesces vigorously; mineral stains blue; with some isotropic varieties brings out octahedral 

etch cleavage; very fine grained supergene chalcocite, however, will not show etch 

cleavage. 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. FeCl3 stains blue; with 

some isotropic specimens brings out octahedral etch cleavage. KOH negative. 

HgCl2 tarnishes slightly.

Silver (Ag). Isometric; color silvery white; hardness B. HN03 effervesces 

slightly for 8, moment; surface stains gray. HCl fumes tarnish slightly; almost 

negative. E!CN mineral tarnishes light gray to brown. FeCl3 instantly tarnishes iridescent. KOH negative. HgCl2 stains brown to iridescent. Very 

sectile.

Bornite (CusFeS^. Isometric; color pinkish brown; tarnished 

KOH neg. bornite is purple; hardness B. Usually isotropic, but some 

HgCl2 neg. 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 clialcopyrite and other copper minerals.

ANISOTROPIC

Chalcocite (Cu2S). Isometric and Orthorhombic; color bluish 

KOH neg. gray; hardness B. Most chalcocite is weakly anisotropic. 

HgCI2 HN03 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 chalcodte brings out grain boundaries only. HCl negative; some varieties 

tarnish faintly. KCN quickly stains black; first brings out etch cleavage, but if
76 MICROSCOPIC DETERMINATION OF ORE MINERALS

drop is allowed to remain, etch cleavage is destroyed, and a rough, corroded 

surface remains. FeCls stains blue; most specimens exhibit parallel etch cleavage. 

KOH negative. HgCla tarnishes slightly.

Bornite (CusFeSO. Isometric; color pinkish brown; hardness 

KOH neg. B. Most specimens are isotropic, but some exhibit weak anomHgCl2 neg. alous anisotropism. HNOs effervesces; surface stains yellowish 

brown; sometimes develops etch cleavage, which tends to form a 

bricklike pattern. KCN stains brown. FeCl3 stains orange. KOH and HgCl2 

negative. Usually associated with chalcopyrite and other copper minerals.

HNO3 eff., HC1 neg., KCN, FeCl3 neg.

ISOTROPIC

Electrum (alloy of Au and Ag). Isometric; color light yellow; 

KOH neg. hardness B. HNOs fumes tarnish; effervesces slightly. HCl 

HgCI2 negative. KCN quickly stains dark. FeCls and KOH negative. 

HgCU stains iridescent to black. Very sectile.

ANISOTROPIC

Klaprolholite (3Cu2S.2Bi2S3). Probably isometric; color gray 

KOH neg. like tetrahedrite; in oblique light it has a pinkish brown tinge; 

HgCU neg. hardness C. Weak anisotropism, possibly anomalous. HNOs 

slowly stains brown; fumes tarnish; seems to be a slow effervescence. HCl negative. KCN stains differentially iridescent. FeCl3, KOH, and 

HgCl2 negative. Specimen examined came from Butte, Mont. It was determined by F. B. Laney as klaprotholite. It is intergrown with covellite in mutual 

boundary and subgraphic structures.

HNO3 eff., HCl neg., KCN neg., FeCl3

ANISOTROPIC

Arsenic (native) (As). Hexagonal; color gray; hardness C. 

KOH neg. HN03 slowly effervesces; surface stains iridescent. HCl and 

HgCl2 KCN negative. FeCla quickly stains iridescent to black. KOH

negative. HgCl2 slowly stains light brown; some areas almost 

negative. Good basal cleavage. Gives powder when scratched.

Tellurium (Te). Color silvery white; hardness B+. HNOs tarnishes black 

with effervescence. HCl, some areas, fumes tarnish; other areas negative. 

KCN negative. FeCla stains faintly yellow. KOH negative. HgCl2 , some 

areas stain light brown, others negative.

Tetradymite (Bi2 (Te,S) 3). Hexagonal; color galena-white; hardness B+. 

HN03 effervesces; surface stains brown to black; action sometimes slow in 

starting. HCl slowly stains light brown to iridescent; some areas negative. 

KCN negative. FeCls stains iridescent. KOH negative. HgClj, some specimens stain iridescent; others negative. Perfect basal cleavage.

Aikinile (Cu2S.2PbS.Bi2S3). Orthorhombic; color white;

KOH neg. hardness C. Strong anisotropism. HN03 effervesces; surface

HgCl2 neg. tarnishes iridescent to black. HCl and KCN negative. FeCls

; stains light brown and brings out grain structure; action very

slow and effect slight. KOH and HgCl2 negative.

Calaverite ((Au,Ag)Te2). Monoclinic; color light yellow; hardness C. Strong 

anisotropism. HNOs effervesces; surface stains black. HCl and KCN negative.
DETERMINATIVE TABLE 77

FeCla, most specimens negative; some stain slowly light brown. KOH and 

HgCl2 negative. Prismatic.

Galenobismutite (PbS.Bi2Sa). Color galena-white; hardness B. HNOa effervesces and leaves yellow deposit. HC1 and KCN negative. FeCla leaves slight 

yellow stain; almost negative. KOH and HgCl2 negative. Specimen is from 

Fahlun, Sweden. Gives microchemical tests for Pb, Bi, S, and Se. Intergrown 

with galena.

Melonite (Ni2Te3). Color pale yellow; hardness B. HN03 effervesces; surface 

stains black. HC1 and KCN negative. FeCls 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 distinctly pink tinge, 

about that of bismuth.

Sylvanite ((Au,Ag)Te2). Monoclinic; color creamy gray; hardness C. HNOa 

slowly effervesces; surface stains iridescent; effervescence doubtful. HC1 and 

KCN negative. FeCl3 sometimes leaves light-yellow stain; most areas negative. 

KOH and HgCl2 negative. Lighter in color than calaverite.

HNO3 eff., HC1 neg., KCN neg., FeCl3 neg.

ISOTROPIC

Polydymite (Ni3S4). Isometric; color brass-yellow; hardness F. 

KOH neg. HNOa, slow effervescence; fumes tarnish; mineral slowly stains 

HgCl2 neg. iridescent. HC1, drop turns yellow; mineral unaffected. KCN, 

FeCla, 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. HC1, drop sometimes turns 

yellow; mineral unaffected. KCN, FeCla, KOH, and HgCl2 negative. Some 

specimens show resin-colored internal reflection. Powder white to brown, 

depending on proportion of iron. Some specimens exhibit triangular pits from 

dodecahedral cleavage.

Violarite ((Ni,Fe) 3S4 ). Isometric; color violet; hardness C. HN03 slowly 

effervesces; surface stains slowly brown. HC1, KCN, FeCl3 , KOH, and HgCl2 

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.44

ANISOTROPIC

Jamesonile (4PbS.FeS.3Sb2Sa). Monoclinic; color galenaKOH white; hardness B. Strong anisotropism. HN03 tarnishes

HgCl2 neg. iridescent with slow effervescence. HC1 fumes usually tarnish;

sometimes negative. KCN and FeCla negative. KOH stains

differentially iridescent. HgCl2 negative. Prismatic.

Plagionile (5PbS.4.Sb2S3). Monoclinic; color gray; hardness C. HN03 

effervesces; surface stains iridescent. HCl negative; on some areas fumes 

tarnish slightly. KCN and FeCl3 negative. KOH stains gray to brown. 

HgCl2 negative. Distinguished from zinkenite only by its crystal form. Prismatic. Rare.

» See Short, M. N., and Shannon, E. V., Violarite and other rare nickel sulphides: Am. Mineralogist, 

vol. 15. pp. 2-7, 1930.
78 MICROSCOPIC DETERMINATION OF ORE MINERALS

4

Rathite (3PbS.2As2S3). Orthorhombic; color galena-white; hardness C. Weak 

anisotropism. HNOs effervesces slowly and stains black; effervescence not 

always observed. HC1, KCN, and FeCl3 negative. KOH stains brown. HgCl2 

negative. Very rare; reported only from Binnenthal, Switzerland.

Aikinite (CuS.2PbS.Bi2S3). Orthorhombic; color galenaKOH neg. white; hardness C. Strong anisotropism. HNOs slowly efferHgCI2 neg. vesces; surface stains black. HC1, KCN, FeCls, KOH, and 

HgCl2 negative.

Bismuthinite (Bi2S3). Orthorhombic; color galena-white; hardness B. Strong 

anisotropism. HN03 effervesces; surface stains iridescent. HC1, KCN, FeCl3 , 

KOH, and HgCl2 negative. Prismatic.

Boulangerite (5PbS,2Bi2S3). Orthorhombic; color galena-white; hardness B. 

Strong anisotropism. HN03 effervesces; surface stains black; action often slow 

in starting. HC1, KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. Distinguished from jamesonite by KOH test.

Calaverite ((Au,Ag)Te2 ). Monoclinic; color light yellow; hardness C. Strong 

anisotropism. HN03 effervesces; surface stains black. HC1 and KCN negative. FeCls, most specimens negative; some stain slowly light brown. KOH 

and HgCl2 negative. Prismatic.

Emplectite (Cu2S.Bi2S3). Orthorhombic; color creamy; hardness C. Strong 

anisotropism. HNOa slowly effervesces; surface stains light brown. HC1, KCN, 

FeCl3, KOH, and HgCl 2 negative. Rare.

Benjaminite ((Cu,Ag) 2S.2PbS.2Bi2Ss). Orthorhombic; color

white; hardness B. Shows twinning. HNOs quickly turns

KOH neg. black with effervescence. HC1 negative; fumes tarnish slightly.

HgCl2 neg. KCN, FeCl3 , KOH, and HgCl2 negative. Good cleavage in

one direction. Reported only from Outlaw mine, Manhattan,

Nev. Analyzed by E. V. Shannon (U. S. Nat. Mus. Proc., vol. 65, art. 24, p. 1,

1924).

Geocronite (5PbS.Sb2Ss). Orthorhombic; color galena-white; hardness B. 

HNOs stains black and effervesces slowly; effervescence not always apparent. 

HC1, KCN, FeCl3 , KOH, and HgCl2 negative. Prismatic. Rare.

Guanajuatite Bi2 (Se,S) 3 . Orthorhombic; color gray; hardness B . Strong 

anisotropism. HNOa slowly effervesces and stains iridescent; fumes tarnish; 

effervescence not always observed. HC1, KCN, PeCl3 , KOH, and HgCl2 negative.

Prismatic. Rare.

Krennerite ((Ag,Au)Te2). Orthorhombic; color creamy white; hardness C. 

Strong anisotropism. HN03 slowly effervesces; surface stains iridescent. HCl, 

KCN, FeCls, KOH, and HgClz negative. Prismatic. Lighter in color than 

calaverite.

Meneghinite (4PbS.Sb2S3). Orthorhombic; color galena-white; hardness C. 

Strong anisotropism. HNOs 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. HCl, fumes tarnish; some areas negative. KCN, FeCla, 

KOH, and HgCl2 negative. Prismatic.

Rezbanyite (2PbS.3Bi2Ss). Crystal system unknown; color galena-white; 

hardness C. Strong anisotropism. HNOs effervesces strongly; surface turns 

black. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Specimen examined is 

from Rezbanya, Hungary, the type locality. Distinguished from other leadbismuth compounds by the lack of prismatic habit. Rare.

Sylvanite ((Au,Ag)Te2). Monoclinic; color creamy gray; hardness.C. HNOs 

tarnishes iridescent with slow (doubtful) effervescence. HCl, KCN, FeCl 3, KOH, 

and HgCl2 negative. Distinguished from krennerite only by crystal form.
DETERMINATIVE TABLE 79 

HN03 , HC1, KCN, FeCl3

ISOTBOPIC

Antimony (Sb). Hexagonal; color silvery white; hardness B.

KOH neg. Apparently isotropic. HN03 quickly stains black. HC1 fumes

HgCl2 tarnish slightly and surface stains after long etching; practically

negative. KCN stains light brown. FeCl3 stains black. KOH

negative.. HgCl2 quickly stains iridescent.

Argentite (Ag2S). Isometric; color gray; hardness A. Most specimens are 

isotropic,, but some show weak anomalous anistropism. HN03 slowly etches 

differentially. HC1 fumes tarnish; aureole will not wash off; this is the most 

distinctive reaction for argentite. KCN quickly blackens. FeCl3 quickly stains 

black. KOH negative. HgCl2 quickly iridescent. Sectile.

Dyscrasite (Ag3Sb). Isometric; color silvery white; hardness C . HNOs 

quickly sitains black; with some specimens brings out grating structure. HCl, 

some areas seem to stain a little; others doubtful or negative. KCN stains 

differentially and brings out structure. FeCls quickly stains iridescent. KOH

negative. HgCl2 stains brown and brings out structure. Almost sectile but 

yields powder when scratched.

Hessite (Ag2Te). Isometric; color gray; hardness A. Some specimens are isotropic; others show anomalous anisotropism. Some specimens show both isotropic and anisotropic areas on the .same surface. HNOs stains iridescent to 

black. HCl slowly stains black; negative on small areas. KCN slowly stains 

black; woak. FeCla quickly stains iridescent. KOH negative. HgClj tarnishes 

light brown.

ANISOTROPIC

Stephanite (5Ag2S.Sb2S3). Orthorhombic; color gray; hardKOH ness B. HN03 practically negative; after long etching fumes 

HgCl2 tarnish faintly and mineral seems to stain very light brown; 

reaction doubtful. HCl fumes tarnish; halo will not wash off. 

KCN stains dark gray and brings out scratches. FeCl3 tarnishes. KOH 

quickly stains black. c HgCl2 quickly stains iridescent. Metallic powder.

Argentite (Ag2S). Isometric; color gray; hardness A. Some 

KOH neg. specimens are isotropic; others show anomalous anisotropism. 

HgCl2 HN03 fumes tarnish. HCl fumes tarnish; halo does not wash 

off. KCN stains differentially black and brings out structure. 

FeCl3 quickly stains black. KOH negative. HgCl2 quickly stains black. 

Very sectile; does not yield powder.

Hessitc. (Ag2Te). Isometric; color gray; hardness A + . Some specimens are 

isotropic; others show anomalous anisotropism. Some specimens show both 

isotropic and anisotropic areas on the same polished surface. HN03 quickly 

stains black. HCl slowly stains black. KCN stains dark in spots; weak. 

FeCl3 instantly tarnishes iridescent. KOH negative. HgCl2 stains brown. 

Closely resembles argentite in appearance, but hessite will yield powder when 

scratched; argentite yields only metallic shavings.

Slromeyerite ((Cu,Ag) 2S). Orthorhombic; color gray; hardness B. Strong 

anisotropism. HN03 stains through iridescent to black. HCl fumes tarnish; 

halo doeis not wash off; action sometimes almost negative. KCN quickly stains 

black. FeCl 3 quickly stains iridescent. KOH negative. HgCl2 stains iridescent. 

Closely resembles argentite in color and etch tests; stromeyerite has stronger 

ariisotropism and yields a powder when scratched.

Uman\iite (CusSe2). Color purple, similar to slightly tarnished bornite; hardness B. Strongly anisotropic; gives four extinctions; interference color is a fiery
80

orange very similar to that of klockmannite. HN03 fumes tarnish; surface turns 

blue. HC1, surface turns blue. KCN, surface tarnishes differentially blue to 

dark gray, bringing out grain structure. FeCl3 , surface turns bluish. KOH 

negative. HgCl2, surface turns bluish. Associated with clausthalite and klockmannite.

HN03 , HC1, KCN, FeCl3 neg.

ANISOTROPIC

Stephanite (5Ag2S.Sb2S3). Orthorhombic; color gray; hardKOH ness B. HN03 practically .negative; after long etching fumes 

HgCl2 tarnish faintly and mineral stains very light brown; reaction 

doubtful. HC1 fumes tarnish; halo will not wash off. KCN 

quickly stains black. FeCl, some specimens tarnish; others negative. KOH, 

some specimens stain black; others negative. HgCl2 stains brown to iridescent. 

Metallic powder.

Stibnite (Sb2S3). Orthorhombic; color gray; hardness B. 

KOH Very strong anisotropism. HN03 stains differentially iridescent. 

HgCl2 neg. HC1 fumes sometimes tarnish slightly; usually reaction is negative. KCN brings out scratches and sometimes stains slightly. 

FeCl3 negative. KOH instantly tarnishes brown and gives yellow coating; this is 

the most characteristic etch reaction for stibnite. HgCl2 negative. Prismatic. 

Hardness and color vary somewhat with orientation, giving mottled appearance 

that suggests the presence of more than one mineral.

HNO3, HC1, KCN neg., FeCl3

ISOTHOPIC

Aguitante (Ag2 (Se,S)). Isometric; color gray; hardness A. 

KOH neg. HN03 slowly stains light brown. HCl usually negative; fumes 

HgCl2 sometimes tarnish. KCN negative. FeCl3 etches iridescent.

KOH negative. HgCl2 stains iridescent. Sectile. Rare. 

Pelzile ((Au,Ag)2Te). Isometric; color gray; hardnes^ A. HN03 quickly 

stains iridescent. HCl stains iridescent; some specimens negative. KCN

negative. FeCls quickly stains iridescent. KOH negative. HgCl2 slowly

stains brown. Frequently associated with hessite and native gold.

Clausthalite (PbSe). Isometric; color galena-white; hardness 

KOH neg. A . HNOs tarnishes and leaves a coating which is brick-red 

HgCl2 neg. in oblique light; this distinguishes the mineral from galena. 

HCl slowly tarnishes light brown. KCN negative. FeCl3 tarnishes iridescent. KOH and HgCl2 negative. Has cubic cleavage less perfect 

than galena. Usually shows individual triangular pits here and there.

Galena (PbS). Isometric; color galena-white; hardness B. HN03 stains 

black. HCl tarnishes brown to iridescent. KCN negative. FeCl3 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

Cylindrite (6PbS.Sb2S3 .6SnS2). Tetragonal; color gray; hardKOH ness C . Strong anisotropism. HN03 fumes tarnish; slowly 

HgCl2 neg. stains iridescent. HCl fumes tarnish; stains slightly after etching 

two minutes. KCN negative. FeCl3 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.
DETERMINATIVE TABLE 81

Hessite (Ag2Te). Isometric; color gray, resembles argentite; 

KOH neg. hardness A. Most specimens show anomalous anisotropism. 

HgCl2 HN03 stains black. HCl negative; fumes seem to tarnish faintly 

on some areas. KCN negative. FeCls stains iridescent. KOH 

negative. HgCl2 stains brown to iridescent. Almost sectile but yields powder 

when scratched.

Alaskaite (Ag2S.PbS.2Bi2S3). Orthorhombic; color gray; hardKOH neg. ness C. Strong anisotropism. HNOs slowly stains dark. HCl 

HgCU neg. 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 

per cent.

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.

Schapbachile (Ag2S.PbS.Bi2S3). Orthorhombic; color gray; hardness B . 

HNOs slowly stains brown. HCl stains differentially; some areas unaffected. 

KCN negative. FeCls stains iridescent. KOH and HgCl 2 negative. From 

Schapbach, Baden, the only locality reported. Differs from alaskaite in its 

higher silver content (Ag 11.6 per cent).

HNO3 , HCl, KCN neg., FeCl3 neg.

ISOTEOPIC

Hauerite (MnS2). Isometric; color gray; hardness E. HNO 

KOH n<eg. fumes tarnish slowly; some areas practically negative. HCl 

HgCl2 neg. solution turns brown; mineral stains light brown; fumes tarnish; 

some areas do not stain. KCN, FeCls, KOH, and HgCl 2 negative. H2 02 , slow effervescence; surface does not stain. Cubic cleavage; surface 

shows triangular pits. Red powder.

Sphalerite (ZriS). Isometric; color gray; hardness C. HNOs fumes tarnish; 

on areais smaller than the drop, reaction is usually negative. HCl fumes sometimes ta,rnish; usually negative. KCN, FeCl3 , KOH, and HgCl2 negative. 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.

Voltzite (4ZnS.ZnO). Isometric; color gray, like sphalerite; hardness D. 

HNOs fumes tarnish strongly. HCl tarnishes and gives off H2S. KCN, FeCls, 

KOH, and HgCl2 negative. Resin-colored powder and internal reflection. No 

cleavage.

ANISOTROPIC

Andorite (Ag2S.2PbS.3Sb2Ss). Orthorhombic; color gray, simiKOH lar to tetrahedrite; hardness C. Strong anisotropism. HNOs 

HgCl2 neg. fuines tarnish; some areas stain slowly brown, others negative. 

HCl fumes tarnish faintly. KCN and FeCls negative. KOH 

slowly stains brown. HgCl2 negative. Aqua regia effervesces vigorously and 

stains black. Almost invariably associated with stannite. Rare. Prismatic.

Cylindrite (6PbS.Sb2Ss.6SnS2 ). Tetragonal; color gray; hardness C . Strong 

anisotropism. HNOs fumes tarnish; slowly stains iridescent. HCl fumes tarnish; 

stains slightly after etching two minutes. KCN negative. FeCl3 stains slightly 

brown; doubtful. KOH stains light gray, in places iridescent. HgCl negative. 

Basal sections are more or less cylindrical and are made up of concentric shells.
82 MICROSCOPIC DETERMINATION OF ORE MINERALS

Franckeite (5PbS.Sb2S3 .2SnS2). Color gray; hardness B. HN03 stains differentially iridescent; fumes tarnish. HC1 fumes tarnish slightly after long etching; doubtful. KCN and FeCl3 negative. KOH, some areas stain slightly; 

others negative. HgCl2 negative.

Sternbergite (AgFe2S3-4). Orthorhombic; color light brown; hardness B. Very 

strong anisotropism; shows without analyzing nicol. HN03 fumes tarnish. 

HC1 fumes tarnish slowly. KCN and FeCl3 negative. KOH stains differentially iridescent. HgCl2 negative; gives a deceptive "etching," owing to deposition of HgCl2 crystals. Habit prismatic. Usually associated with ruby silver 

minerals.

Bismutoplagionite (5PbS.4Bi2Ss). Orthorhombic; color gray; 

KOH neg. hardness B+. HN03 quickly stains iridescent; effervesces 

HgCl2 neg. 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).

Cuprodescloizile (Pb2 (Zn,Cu) 2 .V2Oo.H2 0). Orthorhombic; color gray, similar 

to sphalerite; hardness D. Weak anisotropism. HNOs etches and deposits 

yellow powder. HCl etches and deposits yellow powder. KCN, FeCl3 , KOH, 

and HgCl2 negative. Resinlike powder. Reddish-brown internal reflection.

Owyheeite (8PbS.2Ag2S.5Sb2S3). Orthorhombic; color gray; hardness B + . 

Strong anisotropism. HNOs stains differentially iridescent. HCl fumes tarnish. 

KCN, FeCla, KOH, and HgCl2 negative. Prismatic structure. Shows triangular 

cleavage pits. Very rare. Specimen is 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 gray; hardness D-. HNO3 

fumes tarnish slowly. HCl stains black and brings out scratches. KCN, FeCl3 , 

KOH, and HgCl2 negative. Has good cleavage in at least two directions and 

shows numerous triangular pits. Specimen contains Pb and Sn, but Sb is absent. 

Does not check in physical characteristics with teallite. May be some other 

compound of lead and tin.

Tenorite or melaconite (CuO). Monoclinic, color gray; hardness E . Moderately strong anisotropism. HN03 fumes tarnish; surface slowly stains differentially brown; some areas unaffected. HCl slowly stains differentially brown;

some areas unaffected; drop colors green. KCN, FeCla, 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 

contains more than about 5 per cent of either of these two components the mineral 

is known as "copper pitch ore."

Hausmannite (Mn3 04). Hexagonal; color white with a faint tinge of blue; 

hardness D . Strongly anisotropic; most specimens show multiple twinning 

resembling that of plagioclase. HN03 fumes tarnish; some specimens negative. 

HCl tarnishes differentially brown in places; some areas negative. KCN, 

FeCl3 , KOH, and HgCl2 negative. H2 02 effervesces slowly without staining 

surface. Good basal cleavage. Powder and internal reflection deep red. Takes 

a high polish.

HNO3 , HCl neg., KCN, FeCl3

ISOTROPIC

Domeykite (Cu3As). Isometric; color creamy gray; hardness 

KOH C. HN03 effervesces and brings out lamellar structure. HCl 

HgCI2 brings out etch cleavage; some areas, action very slow, reaction 

doubtful. KCN brings out etch cleavage. FeCl3 quickly 

stains iridescent. KOH and HgCl2 stain iridescent and bring out lamellar 

structure. Almost sectile but yields powder when scratched.
DETERMINATIVE TABLE S3

Antimony (Sb). Hexagonal; color silvery white; hardness B.

KOH neg. Apparently isotropic. HN03 quickly stains black. HC1 negaHgCl2 tive; fumes sometimes tarnish faintly. KCN stains light

brown. FeCl3 stains black. KOH negative. HgCl2 quickly

stains iridescent. Rhombohedral cleavage. Exhibits triangular pits.

Dyscrasite (Ag3Sb). Isometric; color silvery white; hardness C . HN03 

quickly stains black; with some specimens brings out grating structure. HCl 

negative. KCN slowly stains brown; weak. FeCl3 quickly stains iridescent 

and brings out structure. KOH negative. HgCl2 quickly stains brown. Almost 

sectile but yields powder when scratched.

ANISOTROPIC

Polybasile (8Ag2S.Sb2S3). Monoclinic; color gray; hardness 

KOH C-. HN03 fumes tarnish. HCl negative. KCN quickly 

HgCl2 blackens. FeCl3 tarnishes iridescent. KOH slowly tarnishes 

iridescent; in some specimens negative. HgCl 2 quickly blackens. . Powder dark ruby-red; if particles are coarse powder appears black. 

Internal reflection red; not always seen.

Stephaniie (5Ag2S.Sb2S3). Orthorhombic; color galena-white; hardness B. 

HNOs fumes tarnish. HCl negative. KCN quickly blackens. FeCl3 stains 

slightly light gray, practically negative. KOH tarnishes iridescent to black. 

HgCl2 tarnishes iridescent. Best distinguished from polybasite by weak FeCl3 

reaction, metallic powder, and lack of internal reflection.

Naumannite ((Ag2, Pb)Se). Isometric; color gray; hardness B. 

KOH neg. Weak anomalous anisotropism. HN03 tarnishes iridescent and 

HgCI2 brings out structure. HCl negative. KCN very slowly etches 

brown; brings out cleavage. FeCl3 instantly tarnishes iridescent. KOH negative. HgCl2 tarnishes brown to iridescent and brings out 

structure. Usually shows traces of cubic cleavage. Isomorphous with and 

usually undistinguishable from aguilarite, Ag2Se.

Pearcite (8Ag2S.As2S3). Monoclinic; color white; hardness C. HN03 fumes 

tarnish. . HCl negative. KCN quickly stains dark. FeCl3 stains iridescent. 

KOH negative. HgCl2 stains iridescent. Very thin splinters are dark red at 

the edges.

Proustite (3Ag2S.As2S3). Hexagonal; color bluish gray; hardness B . HNOs 

slowly stainis dark. HCl negative. KCN quickly stains black and brings out 

scratches. FeCl3 slowly stains black. KOH negative. HgCl2 stains brown to 

iridescent. Color in oblique light bright ruby-red, lighter than pyrargyrite. 

Powder and internal reflection ruby-red.

Eucairite (Cu2Se.Ag2Se). Isometric; color gray, similar to 

KOH neg. chalcocite; hardness A . Weakly anisotropic. HN03 fumes 

HgCl2 neg. tarnish strongly; surface slowly turns dark gray. HCl negative. 

KCN instantly stains black. FeCl3 instantly tarnishes differentially iridescent. KOH and HgCl* negative. Associated with clausthalite 

and klockmannite.

HNO3 , HCl neg., KCN, FeCI3 neg.

ISOTROPIC

Canfieldite (4Ag2S.SnS2 , with some germanium). Isometric; 

KOH color gray, with pinkish tinge. Most areas isotropic, but some 

HgCI2 show anomalous anisotropism. HN03 slowly stains brown; 

usually required more than one minute to work. HCl negative. 

KCN stains dark gray and brings out structure. FeCls deposits scum of ferric 

chloride, which tends to cling to surface, but reaction should be termed negative. 

KOH tarnishes through iridescent to dark gray. HgCl2 instantly tarnishes 

iridescent.
84 MICROSCOPIC DETERMINATION OF ORE MINERALS

Argyrodite (4Ag2S.GeS2). Tetragonal, pseudo-isometric; color

KOH neg. gray, similar to enargite; hardness C. Some specimens isotropic;

HgClz others anisotropic. HN03 fumes tarnish faintly; usually negative. HC1 negative. KCN slowly stains light brown. FcCl 3

and KOH negative. HgCl2 quickly stains iridescent.

Tennantite (5Cu2S.2(Cu,Fe,Zn)S.2As2S3). Isometric; color

KOH neg. gray; hardness D. HN03 fumes. tarnish. HC1 negative.

HgCI2 neg. KCN, some specimens stain slowly light, brown, bringing out 

scratches; most specimens negative. FeCl3 , KOH, and HgCl2

negative. Color distinctly greenish when in contact with galena. Powder

metallic in most specimens, reddish brown in some.

Tetrahedrite (5Cu2S.2(Cu,Fe,Zn)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

most specimens, reddish brown in some.

ANISOTROPIC

Canfieldile (4Ag2S.SnS2 , with some germanium). Isometric; 

KOH color gray with pinkish tinge; hardness B. Most areas isotropic, 

HgCl2 but some show anomalous anisotropisin. HN03 slowly stains 

brown; usually requires more than one 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. 

HgCl2 instantly tarnishes iridescent.

Polybasite (8Ag2S.Sb2S3). Monoclinic; color gray; hardness C . HNOs 

fumes tarnish. HC1 negative. KCN stains black. FeCl3 usually tarnishes 

iridescent; some specimens negative. KOH slowly tarnishes iridescent; some 

specimens negative. HgCl2 stains brown to iridescent. Powder dark ruby-red; 

if particles are coarse, poAvder appears black. Internal reflection red; not always 

seen.

Proustite (3Ag2S.As2S3). Hexagonal; color bluish gray; hardness B. HN03 

fumes tarnish. HCl negative. KCN quickly stains black. FeCI3 negative. 

KOH stains black. HgCl2 stains black. Color and internal reflection ruby-red. 

Indistinguishable from pyrargyrite by color and etch tests.

Pyrargyrite (3Ag2S.Sb2S3). Physical properties and etch tests identical with 

those of proustite.

Kermesite (Sb^O). Monoclinic; color gray; hardness B. 

KOH Strong anisotropism. HNOs, some areas tarnish brown; others 

HgCI2 neg. negative. HCl 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).

Livingstonile (HgS.2Sb2S3). Orthorhombic; color lead-gray; hardness C. 

Strong anisotropism. HN03 slowly stains brown to iridescent. HCl negative. 

KCN stains brown and brings out scratches. FeCl3 negative. KOH instantly 

darkens, leaving yellow deposit; action highly characteristic, resembling that of 

stibnite. HgClj negative. Powder is red when finely divided.

Miargyrite (Ag2S.Sb2S3). Monoclinic; color grayish white; hardness B. 

Strong anisotropism. HNOs usually negative; sometimes stains slightly. HCl
DETERMINATIVE TABLE 85

fumes usually tarnish; sometimes negative. KCN stains differentially dark 

gray. FeCl,i sometimes tarnishes faintly; doubtful. KOH tarnishes differentially iridescent. HgC]2 negative. Ruby-red powder and internal reflection. 

For comparison with pyrargyrite see page 91.

Stibnite (SbuSs)'. Orthorhombic; color gray; hardness B . Very strong 

arusotropism. HNOs stains differentially iridescent. HCl fumes sometimes 

tarnish slightly; usually reaction is negative. KCN brings out scratches and 

sometimes stains slightly. FeCls negative. KOH instantly tarnishes and 

deposits a yellow coating; this is the most characteristic etch reaction for 

stibnite. E;gCl2 negative. Prismatic. Hardness and color vary somewhat 

with orientation, giving mottled appearance; this may mislead one into believing that more than one mineral is present.

Klockmannite (CuSe). Hexagonal; color greenish gray; hardness C. Strong 

anisotropism; gives four extinctions per revolution. Interference color fiery 

like covellito but orange rather than red. HNOs fumes tarnish. HCl negative; 

solution turns yellow. KCN quickly stains black. FeCl3 negative. KOH

slowly stains differentially brown. Hgd2 negative. Mottled in appearance.

Close resemblance to covellite indicates that the above formula is correct. 45 

Associated with clausthalite and other selenides.

Argyrodile (4Ag2S.GeS2). Tetragonal, pseudo-isometric; color, 

KOH neg. gray, like enargite; hardness C. Some specimens isotropic, 

HgCl2 others weakly anisotropic. HNOs fumes sometimes tarnish 

faintly; usually negative; HCl negative. KCN stains light 

brown. FeCls and KOH negative. HgCl2 quickly tarnishes iridescent; the 

most characteristic reaction.

Chalcostibite (Cu2S.Sb2S3). Orthorhombic; color galena-white;

KOH neg. hardness C. HNOs fumes tarnish; mineral stains iridescent

HgCl2 neg. after long etching. HCl negative. KCN practically negative;

brings out scratches and stains slightly. FeCls, KOH, and

HgCl2 negative. Lighter in color than tetrahe'drite. Has w^ll-marked cleavage.

Enargiie (Cu2S.4CuS.As2Ss). Orthorhombic; color, two varieties known, one 

gray, the other pinkish; the pinkish variety is known as luzonite; both varieties 

may be seer, in the same specimen; hardness D . Strong anisotropism. HNOs 

fumes tarnish faintly; sometimes negative. HCl'negative. KCN stains black 

and sometimes brings out a parallel structure. FeCls and KOH negative. 

HgCl2 sometimes stains slightly; doubtful. In many places intergrown with 

tennantite.

Famatinite (Cu2S.4CuS.Sb2Ss). Orthorhombic; color pinkish gray, similar 

to pink enargite; hardness D. Strongly anisotropic. HNOs stains differentially 

iridescent; fumes tarnish. HCl negative. KCN stains differentially dark; 

action slower than enargite. FeCls, KOH, and HgCl2 negative. Prismatic 

cleavage. Isomorphous with enargite.

Klaprotholite (4Cu2S.Bi2Ss). Probably isometric; color gray, like tetrahedrite; 

in oblique light it has a pinkish-brown tinge; hardness C. Very weak anisotropism, possibly anomalous: HNOs slowly stains brown; fumes tarnish. 

HCl negative. KCN stains differentially iridescent. FeCls, KOH, and HgCl2 

negative. Specimen from Butte, Mont., determined by F. B. Laney as klaprotholite. It is intergrown with coarse-grained covellite in mutual-boundary 

and subgraphic structures.

49 See also Ramdohr, Paul, Klockinaunite, eiu uouos naturaliches KupfcrsoleniU: Centralbl. Mineralogie, 

1928, Abt. A, No. 7, pp. 225-232. * .

15122 31 7
86 MICROSCOPIC DETERMINATION" OF ORE MINERALS ! . "j 

HNO3 , HC1 neg., KCN neg., FeCl3

ISOTROPIC

Aguilarite (Ag2 (Se,S)). Isometric; color gray; hardness A. 

KOH neg. HN03 stains slowly brown. HCl usually negative; fumes someHgCl2 times tarnish light brown. KCN negative. FeCl3 tarnishes iridescent. KOH negative. HgCl2 stains iridescent. Sectile. Rare.

Dyscrasite (Ag3Sb). Isometric; color silvery white; hardness C. HN03 

stains differentially iridescent. HCl slowly tarnishes differentially; some 

areas negative or doubtful. KCN negative. FeC]3 tarnishes iridescent. KOH 

negative. HgCl2 stains brown. Almost sectile but yields powder when scratched.

Freibergite (5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3). Isometric; color very light gray; 

hardness D. HN03 slowly stains brown. HCl and KCN negative. FeCJ3 

stains iridescent. KOH negative. HgCl2 leaves spotty residue; almost negative. Specimens examined are from Randsburg, Calif., and Mayo district, 

Yukon. Both are relatively high in silver.

Petzite ((Ag,Au) 2Te). Isometric; color galena-white; hardness A. HN03 

stains black. HCl and KCN negative. FeCJ3 stains iridescent. KOH negative. HgCU stains brown to iridescent. 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.

Coloradoiie (HgTe). Isometric; color pinkish gray; hardness 

KOH neg. C. HN03 slowly stains brown; some areas almost negative. 

HgCI2 neg. 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.

Ullmanite (NiSbS). Isometric; color gray; hardness E. HN03 stains brown 

to iridescent. HCl and KCN negative. FeCla stains light brown. KOH and

HgCl2 negative. Has well-defined cubic cleavage and shows triangular pits.

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HN03 

tarnishes iridescent; seems to effervesce slightly. HCl and KCN3 negative. 

FeCls stains slightly; almost negative. KOH and HgCl2 negative. From 

Broken Hill, Australia. Rare.

ANISOTROPIC

Wittichenite (3Cu2S.Bi2S3). Color gray; hardness C . HNOs

KOH quickly stains iridescent. HCl and KCN negative. FeCl3 stains

HgCl2 light brown. KOH stains iridescent. HgCl2 stains slightly;

doubtful. Specimen from Colquijirca, Peru; collected by Dr.

Waldemar Lindgren and analyzed at the Massachusetts Institute of Technology.

Aguilarite (Ag2 (Se,S)). Listed as isometric; color gray; hardKOH neg. ness A. Strong anisotropism. HNOs quickly stains through

HgCl2 iridescent to black; fumes tarnish. HCl and KCN negative.

FeCls quickly stains iridescent. KOH negative. HgCl2

quickly stains iridescent. The strong anisotropism raises the question whether

this mineral is isometric. Resembles argentite but yields powder when scratched.

Hessite (Ag2Te). Isometric; color gray; hardness A+. Most specimens

of Uessite exhibit anomalous anisotropism. HNOs stains black. HCl nega-
DETERMINATIVE TABLE 87

tive; fumes seem to tarnish faintly on some areas. KCN negative. FeCls 

stains iridescent. KOH negative. HgCl2 stains brown to iridescent. Resembles argentite but yields powder when scratched.

Aikinite (CuS.2PbS.Bi2S3). Orthorhombic; color white; hardKOH neg. ness C. Strong anisotropism. HNOs, most areas effervesce

HgCl2 neg. and stain iridescent; some areas stain without effervescence.

HC1 and KCN negative. FeCl3 stains slightly very light brown;

action weak and might be considered negative. KOH and HgCl2 negative.

Alaskaitv (Ag2S.PbS.2Bi2S3). Orthorhombic; color gray; hardness C. Strong 

anisotropism. HN03 slowly stains dark. HCl fumes tarnish a little; doubtful. 

KCN negative. FeCls 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.

Sylvanite ((Au,Ag)Te2). Monoclinic; color creamy white; hardness C. 

Strong anisotropism. HNOs stains iridescent with doubtful effervescence. 

HC1 and KCN negative. FeCls sometimes leaves light-yellow stain; usually 

negative. KOH and HgCl2 negative.

HNO3 , HCl neg., KCN neg., FeCl3 neg.

ISOTROPIC

Carrollite ((Co,Cu)3S4). Isometric; color creamy white, 

KOH neg. similar to linnaeite but somewhat lighter; hardness E. HNOs 

HgCl2 fumes tarnish iridescent. HCl, KCN, FeCl3, and KOH negative. HgCla stains brown to iridescent. Action somewhat 

slow. Micro chemical 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.48

Linnaeite (00384). Isometric; color creamy white; hardness E-K HNOa 

fumes tarnish. HCl, KCN, FeCls, and KOH negative. HgCl2 stains brown 

differentially; some areas almost negative.

Siegenite ((Co,Ni) 3S4 ). Isometric; color creamy white; hardness E + . HNOs 

fumes tarnisih 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 lin- 

'naeite. 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.

Chalcopyrite (CuFeS2). Tetragonal; color brass-yellow; 

KOH neg. hardness C. Most specimens are weakly but discernibly 

HgCl3 neg. anisotropic, but a small proportion are apparently isotropic. 

HNOs 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 arisas 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. HNOs stains iridescent; on some varieties fumes tarnish, but mineral

48 See Shannon, E. V., The identity of carrollite with linnaeite: Am. Jour. Sci., 5th ser., vol. 11, pp. 

489-493, 1926.
88 MICROSCOPIC DETERMINATION OF ORE MINERALS

covered by drop not affected. HC1, KCN, FeCl3, KOH, and HgCl2 negative. 

Silver-bearing tetrahedrite.

Pentlandite ((Ni,Fe)S). Isometric; color brass-yellow; hardness D . 

HN03 fumes tarnish; mineral slowly stains brown. HC1, KCN, FeCls, KOH, 

and HgCl2 negative. Imperfect cubic cleavage. Nonmagnetic. Almost 

invariably associated-with pyrrhotite and chalcopyrite.

Sphalerite (ZnS). Isometric; color gray; hardness C-f HNOs fumes tarnish. HC1, KCN, FeCls, KOH, and HgCl2 negative. Aqua regia effervesces 

and stains dark brown; fumes tarnish. Some specimens show resin-colored 

internal reflection. Some show triangular pits. Powder white to brown. If 

proportion of iron is high powder may be metallic.

Stannite (Cu2S.FeS.SnS2). Isometric; color light brownish gray; hardness D+. 

Most specimens show weak anomalous anistropism; others are isotropic. HN03 

stains iridescent; fumes tarnish. HC1, KCN, FeCl3 , KOH, and HgCl2 negative. 

Usually associated with chalcopyrite.

Tennantite (5Cu2S.2(Cu,Fe,Zn)S.2As2S3). Isometric; color gray; tennantite 

in contact with galena shows distinct greenish tinge; hardness D. HNOs 

fumes tarnish. HC1 negative. KCN usually negative; stains light brown 

and brings out scratches on some specimens. FeCls, KOH, and HgCl 2 negative. Some specimens yield reddish-brown powder, others black metallic powder.

Tetrahedrite (5Cu2S.2(Cu,Fe,Zn)S.2Sb2 S3). Isometric; color gray; hardness D. 

HN03 fumes usually tarnish; some specimens tarnish iridescent, but usually 

requires more than one minute for test to work. HC1, KCN, FeCls, KOH, and 

HgCl2 negative. Some specimens yield reddish-brown powder; others yield 

black metallic powder.

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HNOs tarnishes iridescent; seems to effervesce slightly. HCl and KCN negative. FeCls 

seems to stain slightly; this reaction is easily overlooked. KOH and HgCl2 

negative.

Ullmanite and kallilite. Reactions like willyamite.

Wurtzite .(ZnS). Hexagonal; color dark gray; hardness D . Apparently 

isotropic. HN03 fumes tarnish. HC1, KCN, FeCls, 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 wurzite exhibits 

strong birefrigehce.

ANISOTROPIC

Aramayoite (Ag2S.(Sb,Bi) 2S3). Orthorhombic (?); color gray; 

KOH hardness C. Strong anisotropism. HNOs stains iridescent; 

HgCI2 neg. action slow in starting. HC1, KCN, and FeQ3 negative. KOH 

stains iridescent. HgCl2 negative. Prismatic. Specimen from 

Chocaya, Bolivia, described by L. J. Spencer (Mineralog. Mag., vol. 21, pp. 

156-162, 1927).

Berthierite (FeS.Sb2S3). Orthorhombic; color gray; hardness D . Strong 

anisotropism. HN03 tarnishes differentially brown to iridescent. HCl negative; 

solution sometimes turns yellow. KCN and FeCl3 negative. KOH tarnishes 

iridescent. HgCl2 negative. Prismatic.

Cylindrite (6PbS.Sb2S3 .6SnS2). Tetragonal; color gray; hardness C . Strong 

anisotropism. HNOs slowly stains iridescent. HCl, KCN, and FeCls negative. 

KOH slowly stains brown. HgCl2 negative. Prismatic. Basal sections more 

or less cylindrical and are made up of concentric shells.
DETERMINATIVE TABLE 89

Franckeile (5PbS.Sb2S3 .2SnS2 ). Color gray; hardness B. HN03 stains differentially iridescent; fumes tarnish. HC1 fumes tarnish after long etching; doubtful. KCN and FeCl3 negative. KOH, some areas stain slightly, others negative. 

HgCl2 negative.

Pyrrholile (FeSi+). Hexagonal; color brownish or pinkish cream; hardness D . 

Strong anisotropism. HNOs slowly stains light brown; fumes tarnish. HCl, 

solution sometimes turns yellow, but mineral is unaffected. KCN and FeCls 

negative. KOH tarnishes iridescent. HgCl2 negative. Distinctly magnetic.

Ralhite (3PbS.2As2S3). Orthorhombic; color galena-white; hardness C. 

Weak aniso'tropism. HNOs effervesces slowly and stains black; effervescence 

not always observed. HCl, KCN, and FeCls negative. KOH stains brown. 

HgCl2 negative. Very rare; only locality reported Allchar, Macedonia.

Stibnite (Sb2Ss). Orthorhombic; color gray to galena-white; hardness B + . 

Very strong anisotropism. HNOs stains differentially iridescent. HCl fumes 

usually tarnish slightly on a smooth surface; on most areas negative. KCN on a 

smooth surface brings out scratches and sometimes stains slightly; this reaction 

may not be noticeable on a rough surface. FeCls negative. KOH instantly

tarnishes and deposits a yellow coating; this is the most characteristic etch 

reaction for stibnite. HgCl2 negative. Prismatic. Hardness and color vary 

somewhat with orientation, giving mottled appearance; this may give the impression that more than one mineral is present.

Vrbaite (Tl2S.3(As,Sb) 2S3). Orthorhombic; color gray; hardness C. Weak

anisotropisru. HNOs 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.

Millerite (NiS). Hexagonal; color brass-yellow, between color

KOH neg. of pyrite and that of chalcopyrite; hardness E. Strong anisoHgCl2 tropism. HNOs slowly stains brown; fumes tarnish. HC1,

KCN, FeCls, and KOH negative. HgCl2 slowly stains brown.

Usually but not invariably fibrous in habit.

Aikinite (CuS.2PbS.Bi2S3). Orthorhombic; color galena-white; 

KOH neg. hardness C. Strong anisotropism. HNOs, most areas effervesce 

HgCl2 neg. and stain iridescent to black; a few areas seem to stain without 

effervescence. HCl, KCN, FeCl3, KOH, and HgCl2 negative.

Berlhonii'e (2(PbCu2)S.Sb2S3). Color gray, about like tetrahedrite; hardness C. 

HNOs fumes tarnish; mineral slowly stains brown to iridescent. Other reagents 

negative. Massive; no cleavage. From Tunisia. Described by H. Buttgenbach 

(Soc. ge"ol. Belgique Annales, vol. 46, p. 212, 1923).

Bournonile (Cu2S.2PbS.Sb2S3). Orthorhombic; color gray, about like tetrahedrite; hardness C . HNOs fumes tarnish slowly; some specimens tarnish 

slightly light brown. HCl, KCN, FeCl3) KOH, and HgCl2 negative. Aqua 

regia effervesces and stains iridescent. Distinguished from tetrahedrite by means 

of polarized light.

Brongniardite (Ag2S.PbS.Sb2Ss). Color galena-white; in contact with galena 

it has a suggestion of greenish gray; hardness C. Weak anisotropism. HNOs 

negative; after long etching slowly stains differentially iridescent. HCl, KCN, 

FeCls, KOH, and HgCl2 negative. Aqua regia quickly effervesces and stains 

differentially iridescent. Rare.

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. Chalcopyrite 

associated with pyrrhotite usually shows multiple twinning in polarized light. 

HNOs fumes tarnish; some varieties negative. HCl, KCN, FeCl3, KOH, and
90 MICROSCOPIC DETERMINATION OF ORE MINERALS

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.

Chalcostibile (Cu2S.Sb2S3). Orthorhombic; color white; hardness C, HNOa 

fumes tarnish. HCl, KCN, FeCl3, KOH, and HgCl2 negative. Two good 

cleavages. Distinguished from tetrahedrite by means of polarized light. The 

only undoubted chalcostibite seen by the author is from Raz-el-Azul, Algeria; 

other so-called chalcostibites have proved to be tetrahedrite.

Cubanite (CuS.2FeS). Orthorhombic; color brownish or pinkish cream; hardness C. Strong anisotropism. HNOa, some specimens stain light brown; others 

negative. HCl, KCN, FeCls, KOH, and HgCl2 negative. Invariably intergrown with chalcopyrite and usually associated with pyrrhotite. Color almost 

identical but a little yellower than that of pyrrhotite. 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. 

Strong anisotropism. HNOs, 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, FeCls, KOH, 

and HgCl2 negative. The color is the most characteristic property of this 

mineral.

Geocronite (5PbS.Sb2S3). Orthorhombic; color galena-white. HNOs slowly 

effervesces; mineral turns black; effervescence not always observed. HCl, KCN, 

FeCl3 , KOH, and HgCl2 negative. Prismatic. Rare.

Guanajuatite (Bi2 (Se,S)s). Orthorhombic; color gray; hardness B+- Strong 

anisotropism. HNOs tarnishes differentially iridescent. HCl, KCN, FeCls, 

KOH, and HgCl2 negative. Prismatic.

Guitermanite (3PbS.As2S3). Orthorhombic; color galena-white; hardness C. 

HNOs fumes tarnish; mineral slowly stains iridescent. HCl, KCN, FeCls, KOH, 

and HgCl2 negative. Specimen from Zuni mine, Silverton, Colo., the type 

locality. Rare.

Jordanite (4PbS.As2S3). Monoclinic; color galena-white; hardness C. HNOs 

slowly tarnishes gray to brown. HCl, KCN, FeCls, KOH, and HgCl2 negative. 

Very rare; only localities noted are Binnenthal, Switzerland, and Nagyag, 

Hungary.

Krennerite ((Au,Ag)Te2). Orthorhombic; color creamy white; hardness C. 

Strong anisotropism. HN03 slowly effervesces and stains brown; effervescence 

not always observed. HCl, KCN, FeCl3, KOH, and HgCl2 negative. Prismatic. Distinguished from sylvanite only by external crystal form.

Lorandite (Tl2S.As2Ss). Monoclinic; color gray; hardness B. HNOs fumes 

give permanent tarnish; mineral stains slowly In places. HCl, KCN, FeCl8 , 

KOH, and HgCl2 negative. Very rare; only locality reported Allchar, Macedonia. Red powder and internal reflection.

Nagyagite (Pb,Au,Te,S (?), sulpho-telluride of Pb and Au). Orthorhombic; 

color bluish white; hardness B . HNOs slowly stains iridescent; fumes sometimes tarnish. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Habit platy. 

Plates are flexible and frequently curved like those of molybdenite.

Stannite (Cu2S.FeS.SnS2). Isometric; color light brownish gray; hardness D + . 

Most specimens show weak anomalous anisotropism; others isotropic. HNOs 

stains iridescent; fumes tarnish. HCl, HCN, FeCls, KOH, and HgCl2 negative. 

Usually associated with chalcopyrite.
DETERMINATIVE TABLE 91

Sylvanile ((Au,Ag)Tc2). Monoclinic; color creamy gray; hardness C. HNOs 

stains iridescent with doubtful effervescence. HCl and KCN negative. FeCl 3, 

some areas leave light-yellow stain, others negative. KOH and HgCl2 negative.

HNO3 neg., HCI, KCN, FeCl3

ANISOTROPIC

Miargyrile (Ag2S.Sb2S3). Monoclinic; color gray; hardness B.

KOH Strong anisotropism. HNOs negative. HCl fumes tarnish

HgCl2 slightly; weak. KCN stains black; brings out multitude of

scratches. FeCls tarnishes differentially brown; slow. KOH

quickly stains iridescent. HgCl2 slowly tarnishes; some specimens feeble or

negative. Powder and internal reflection ruby-red. In contact with miargyrite, pyrargyrite is distinctly bluish. Miargyrite is more strongly anisotropic

than pyrargyrite. Miargyrite is almost invariably associated with pyrargyrite.

Stephanile (5Ag2S.Sb2Ss). Orthorhombic; color gray; hardness B. HNOs

practically negative; after long etching fumes tarnisli faintly and mineral seems

to stain very light brown; reaction doubtful. HCl fumes tarnish; halo will 

not wash off. KCN stains dark gray and brings out scratches. FeCl 3 tarnishes. 

KOH quickly stains black. HgCl2 quickly stains iridescent. Metallic powder.

HNO3 neg., HCl, KCN, FeCI3 neg.

ANISOTROPIC

Slephanite (5Ag2S.Sb2Ss) . Orthorhombic; color gray; hardKOH - ness B. HN03 negative. HCl fumes tarnish; action slow in 

HgCl2 starting; halo does not wash off. KCN quickly stains black. 

FeCls, most specimens tarnish iridescent; some practically negative, but reagent leaves a scum on the surface which rubs off easily. KOH, some 

areas stain brown; others unaffected. HgCl2 stains brown to iridescent.

Miargyrile (Ag2S.Sb2Ss). Monoclinic; color gray; hardness B. 

KOH Strong anisotropism. HN03 negative. HCl fumes tarnish; in 

HgCl2 neg. places negative. KCN slowly stains brown. FeCls negative. 

KOH stains iridescent. HgCl2 negative. Powder and internal 

reflection ruby-red. . Usually intergrown with pyrargyrite. Miargyrite has the 

stronger anisotropism. Pyrargyrite has a distinct bluish tinge when in contact 

with miargyrite.

HNO3 neg., HCl, KCN neg., FeCl3 neg.

ISOTROPIC

Knit Hauerite (MnS2). Isometric; color gray; hardness E. HNOs

 _,. " fumes tarnish slowly; some areas negative. HCl solution turns

2 brown; mineral stains light brown. KCN, FeCls, KOH, and

negative. Triangular pits indicate cubic cleavage. Red powder. 

Magnetite (Fe304). Isometric; color gray; hardness F. HN03 negative. 

HCl on smooth surfaces larger than drop, fumes tarnish; solution turns yellow. 

KCN, FeCls, KOH, and HgCl2 negative. Aqua regia solution turns yellow. 

Black powder when scratched. Powder is magnetic. (See description of chromite, p. 102.) 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.
92 MICROSCOPIC DETERMINATION OP ORE MINERALS

ANISOTROPIC

1,___ Delafossite (CuFe02). Color gray; hardness E. Strong aniso- 

 _. tropism; shows radial structure. HNOs negative. HC1 etches 

iigU2 neg. differentially black; drop turns yellow. KCN, FeCl3, KOH, and 

HgCl2 negative. Black powder, showing tiny crystalline plates. Very rare.

Freieslebenite (3Ag2S.4PbS.3Sb2S3). Monoclinic; color gray, like tetrahedrite; 

hardness B. Strong anisotropism. HNOs negative. HC1 fumes tarnish; halo 

does not wash off. KCN, FeCl3, KOH, and HgCl2 negative. Prismatic. Very 

rare. Specimen is from Hiendelencia, Spain.

Hausmannite (MnsO^. Hexagonal; color white with a faint tinge of blue; 

hardness D . Strongly anisotropic; most specimens show multiple twinning 

resembling that of plagioclase. HNOs fumes.tarnish; some specimens negative. 

HC1 tarnishes differentially light brown; some areas negative. KCN, F.eCl3, 

KOH, and HgCl2 negative. H2 02 effervesces slowly without staining surface. 

Good basal cleavage. Powder and internal reflection deep red. Takes a high 

polish.

Manganite (Mn2 03). Orthorhombic; color gray; hardness E+. Strongly 

anisotropic; gives four extinctions per revolution. HN03 negative. HCl fumes 

tarnish; surface stains light brown. KCN, FeCl3, KOH, and HgCl2 negative. 

H2 02 effervesces slowly without staining surface. Prismatic. Gives brown powder.

Teallite (SnS.PbS). Color galena-white; hardness C. HN03 negative. HCl 

quickly tarnishes gray to brown. KCN, FeCl3, KOH, and HgCl2 negative. 

Perfect basal cleavage. Plates are flexible and curved like those of molybdenite.

HNO3 neg., HCl neg., KCN, FeCl3

ISOTEOPIC

Argentite (Ag2S). Isometric; color gray; hardness A. Some 

KOH neg. specimens are isotropic; others show anomalous anisotropism. 

HgCl2 . HNOs and HCl fumes tarnish most specimens, but some are 

negative. KCN stains differentially black and brings out structure. FeCls quickly stains black. KOH negative. HgCl2 quickly stains black. 

Very sectile; does not yield powder.

ANISOTBOPIC 

Argentite. See above.

HNO3 neg., HCl neg., KCN, FeCl3 neg.

ISOTROPIC

Cerargyrite (AgCl). Isometric; color dark gray; hardness A.

KOH HN03 and HCl negative. KCN instantly stains gray brown.

HgCl2 neg. FeCl3 negative. KOH stains brown. HgCla negative. Waxy

yellow in oblique light. Very sectile; the most characteristic

quality of cerargyrite.

Argyrodite (4Ag2S.GeS2). Tetragonal, pseudo-isometric; color

EOH neg. gray; hardness C. Some specimens isotropic, others weakly

HgCl2 anisotropic. HN03 and HCl negative. KCN stains slowly

dark and brings out scratches. FeCls doubtful or negative.

KOH negative. HgCl2 tarnishes iridescent.

Gold(Au). Isometric; hardness B. HNOs and HCl negative. 

KOH neg. KCN etches black, giving roughened surface. FeCls, KOH, and 

HgCl2 neg. HgCl2 negative. Very sectile.

Sulvanite (CusVS^. Isometric; color galena-white; hardness 

C. HN03 and HCl negative. KCN stains slightly and brings out scratches. 

FeCla, KOH, and HgCl2 negative. Cubic cleavage, not prominent. Very rare. 

Specimen from Burra Burra, South Australia.
DETERMINATIVE TABLE 93

Tiemannile (HgSe). Isometric; color gray; hardness B. HN03 and HC1 

negative. KCN, some areas tarnish iridescent; others negative. FeCls, KOH, 

and HgCl2 negative. Aqua regia effervesces and stains iridescent.

ANISOTKOPIC

Pyrargyrite (3Ag2S.Sb2Ss). Hexagonal; color bluish gray; 

KOH hardness C. HNOs fumes tarnish slightly on some areas; others 

HgCl2 negative. HC1 negative. KCN stains differentially gray to 

black. FeCls negative; some areas seem to tarnish slightly. 

KOH instantly tarnishes iridescent. HgCl2 stains brown. Ruby-red powder 

and internal reflection. (See description of miargyrite, p. 91.)

Livingstonite (HgS.2Sb2S3). Orthorhombic; color gray; hardKOH ness C. Strong anisotropism. HN03 , some specimens stain 

HgCl2 neg. iridescent; others negative. HC1 negative. KCN, some specimens stain brown; others negative. FeCls negative. KOH instantly darkens, leaving yellow deposit; reaction highly characteristic and 

resembles that of stibnite. HgCl2 negative. Aqua regia effervesces and stains

iridescent. Powder red when finely divided. Prismatic. Rare.

Miargyrite (Ag2S.Sb2S3). Monoclinic; color gray; hardness B. Strong anisotropism. HN03 and HCl negative. KCN stains differentially black and 

brings out fine scratches. FeCls negative. KOH stains dark. HgCl2 negative. 

Powder and internal reflection ruby-red. Usually intergrown with pyrargyrite. 

Miargyrite has the stronger anisotropism. Pyrargyrite has a distinct bluish 

tinge when in contact with miargyrite.

Orpiment (As2Ss). Monoclinic; color gray in vertically incident light, yellow 

in oblique light; hardness B. Strong anisotropism. HNOs and HCl negative. 

KCN etches and brings out structure. FeCls negative. KOH instantly stains 

dark brown.. HgCl2 deposits sediment but does not etch surface. Yellow powder 

and internal reflection.

Argyrodite (4Ag2S.GeS2). Isometric; color gray, similar to 

KOH neg. enargite; hardness C. Some specimens isotropic; others weakly 

HgCI2 anisotropic. HNOs, sometimes fumes tarnish faintly; usually 

negative. HCl negative. KCN stains light brown. FeCls 

doubtful or negative. KOH negative. HgCl2 quickly stains iridescent; the 

most characteristic etch test.

Covellite (CuS). Hexagonal; color indigo-blue; hardness B+. 

KOH neg. Strong anisotropism; grains range in color from blue to fiery red 

HgCI2 neg. between crossed nicols. HNOs and HCl negative. KCN stains 

black. FeCl3 , KOH, and HgCl2 negative.

Enargite (3Cu2S.4CuS.As2Ss). 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. HNOs 

fumes tarn:ish faintly; sometimes negative. HCl negative. KCN stains black 

and sometimes brings out a parallel structure. FeCls and KOH negative. 

HgCl2 sometimes stains slightly; doubtful. Commonly intergrown with tennantite.

HNOs neg., HCl neg., KCN neg., FeCl3

ISOTROPIC

Coloradoite (HgTe). Isometric; color gray with a distinct

KOH neg. tinge of pink; hardness C . HNO3 , some areas stain slowly

HgCl2 neg. light brown to iridescent; other areas negative. HCl and KCN

negative. FeCls stains differentially iridescent. KOH and

HgCl2 negative.
94 MICROSCOPIC DETERMINATION OP ORE MINERALS

ANISOTUOPIC

Keeleyite (PbS.Sb2S3). Orthorhombic; color gray; hardness 

KOH C. Strong anisotropism. HN03 , HCl, and KCN negative. 

HgCl2 neg. 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.47

Tungstenite (WS2). Hexagonal; color gray; hardness A. 

KOH neg. Strong anisotropism; gives four extinctions per revolution. 

HgCl2 HN03 , HCl, and KCN negative. FeCl3 stains differentially

light brown; some areas affected. KOH negative. HgCl2 

stains iridescent in places; doubtful. 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 R. C. Wells (Washington Acad. Sci. Jour., vol. 7, 

pp. 596-599, 1917).

HNO3 neg., HCl neg., KCN neg., FeCl3 neg., aqua regia

ISOTROPIC

Chalcopyrite (CuFeS2). Tetragonal; color brass-yellow; hardKOH neg. ness C. Most specimens are weakly but discernibly anisotropic; 

HgCl2 neg. a small proportion are apparently isotropic. HNOs fumes 

tarnish; some specimens 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 unaffected. Easily scratched by needle, which distinguishes it from 

pyrite. Gives powder when scratched, in contrast to gold, which is sectile.

Cermanite (CuiGeSy). Isometric; color pinkish gray, about the same as pink 

enargite; hardness C. HN03 , KCl, KCN, FeCl3 , KOH, and HgCl 2 negative. 

Aqua regia slowly stains light brown. From Tsumeb, Southwest Africa; very 

rare.

Sphalerite (ZnS). Isometric; color gray; hardness C+. HNOs fumes usually 

tarnish; mineral stains light brown after long etching; sometimes negative. 

HCl, 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 may be metallic.

Teirahedrite (5Cu2S.2(Cu,Fe,Zn)S.2Sb2 S3). Isometric; color gray; hardness 

D. HN03 , HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia fumes 

tarnish. Some specimens yield reddish-brown powder; others black metallic 

powder.

Tiemannile (HgSe). Isometric; color galena-white; hardness B. HN03 , HC1, 

KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia instantly effervesces and 

stains iridescent.

ANISOTROPIC

Andorite (Ag2S.2PbS.3Sb2 S3). Orthorhombic; color gray,

KOH similar to tetrahedrite; hardness C. Strong anisotropism.

HgCl2 neg. HN03 fumes usually tarnish but sometimes negative. HCl

negative; fumes sometimes tarnish faintly. KCN and FeCl3

negative. KOH slowly stains brown. HgCl2 negative. Aqua regia effervesces

« 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.
DETERMINATIVE TABLE 95

vigorously and stains black. Almost invariably associated with stannite. 

Prismatic.

Berthierite (FeS.Sb2S3). Orthorhombic; color gray; hardness D . HN03 

slowly stains iridescent; usually takes more than a minute before effect begins 

to show. HCl, KCN, and FeCl3 negative. KOH stains iridescent. HgCl2 

negative. Aqua regia effervesces and stains light brown. Prismatic. .

Kermesite (Sb2S2 0). Monoclinic; color bluish gray; hardness B. Strong 

anisotropism. HN03 , HC1, KCN, and FeCla negative. KOH deposits yellow 

coating; this is the most characteristic etch reaction; resembles that of stibnite. 

HgClz negative. Aqua regia stains differentially iridescent. Reddish-brown 

powder. Ruby-red internal reflection with strong polarized light. Habit fibrous.

Living stonUe (HgS.2Sb2S3). Orthorhombic; color gray; hardness C. HN03 , 

some specimens tarnish iridescent; others negative. HC1 negative. KCN, 

some specimens tarnish brown; others negative. FeCl 3 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.

Bournonile (Cu2S.2PbS.Sb2S3). Orthorhombic; color gray,

KOH neg. about the same as tetrahedrite; hardness C . HN03 , usually

HgCl2 neg. fumes tarnish, some specimens negative. HC1, KCN, FeCl3 ,

KOH, and HgCl2 negative. Aqua regia quickly effervesces and

stains differentially iridescent.

Brongniardite (Ag2S.PbS.Sb2S3). Color galena-white; in contact with galena 

has a suggestion of greenish gray; hardness C. Weak anisotropism. HN03 

negative; after long etching slowly stains differentially iridescent. HC1, 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, FeCl3) KOH, and HgCl 2 negative. 

H2 02 effervesces vigorously, but surface does not stain. Aqua regia, drop 

colors brown; surface stains differentially light brown. Prismatic. Occurs at 

Franklin, N. J., associated with franklinite and other zinc minerals.

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. Chalcopyrite 

associated with pyrrhotite usually shows multiple twinning in polarized light. 

HN03 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.

Cubanite (CuS.2FeS). Orthorhombic; color brownish or pinkish cream; 

hardness C. Strong anisotropism. HNOa, some specimens stain light brown; 

others negative. HCl, KCN, FeCl3 , KOH, and HgCl2 negative. Invariably 

intergrown with Chalcopyrite and usually associated with pyrrhotite. Color 

almost identical but a little yellower than that of pyrrhotite. 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.

Cinnabar (HgS). Hexagonal; color bluish gray; hardness B. HNOs, HCl, 

KCN, FeCl3 , KOH, and HgCl2 negative. Aqua regia stains differentially iridescent. Red powder and internal reflection.
96 MICROSCOPIC DETERMINATION OF ORE MINERALS

Metacinnabarite (HgS). Isometric; color gray, black in oblique light; hardness 

B. Usually shows multiple twinning. HN03 , HC1, KCN, FeCl3 , KOH, and 

HgCl2 negative. Aqua regia effervesces and turns black. Almost invariably 

intergrown with cinnabar. In polished sections metacinnabarite has a smooth 

surface and gives a black powder, whereas cinnabar has a rougher surface and 

gives'red powder and internal reflection.

Molybdenite (MoS2). Hexagonal; color galena-white; hardness B±. Strong 

anisotropism. Basal sections are isotropic. Prismatic sections give nearly parallel extinction and four extinctions per revolution. HNOg, KCN, FeCl3, KOH, 

and HgCl2 negative. Aqua regia slowly effervesces and darkens. Some areas 

negative. Perfect basal cleavage; plates tend to be more or less curved.

Nagyagite (Pb,Au,Te,S?). Orthorhombic; color gray; hardness B. HN03 

slowly stains iridescent; practically negative. HC1, KCN, FeCl3, KOH, and 

HgQ2 negative. Aqua regia stains differentially iridescent. Habit platy; plates 

are flexible like those of molybdenite. From Nagyag, Hungary. Formula 

doubtful.

HNO3 neg., HC1 neg., KCN neg., FeCI3 neg., aqua regia neg.

ISOTROPIC

Realgar (AsS). Monoclinic; hardness B. Apparently isoKOH tropic; the mineral is fairly transparent and most of the incident

HgCI2 neg. light is absorbed. HN03, HC1, KCN, and FeCl3 negative. KOH

instantly stains black. HgCl2 and aqua regia negative. Red

internal reflection and powder.

Magnetite (FeaO^. Isometric; color gray; hardness F. HN03 

KOH neg. negative. HC1, on smooth surfaces larger than drop, fumes 

HgCI2 neg. 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. 102.) 

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.

ANISOTROPIC

Molybdenite (MoS2). Hexagonal; color galena-white; hardness B±. Strong 

anisotropism. 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 (Mn02). Orthorhombic; color gray, almost white, similar to hematite; hardness D to E. Strongly anisotropic. Gives four extinctions per revolution. HN03 negative. HC1, drop colors brown, but surface does not stain. 

KCN, FeCl2, KOH, and HgCl2 negative. Aqua regia same as HC1. H2 02 , 

drop effervesces vigorously, but surface does not stain. H^+HjSO^ surface 

stains black. Gives blaci: 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.
DETERMINATIVE TABLE 97

HARD MINERALS 

HNO3 eff., HC1 neg., KCN neg., FeCl3

ISOTROPIC

Chloanlhite-smallite ((Ni,Co)As2). Isometric; colorgray; hardKOH neg. ness-F. HN03 effervesces vigorously and stains dark gray. 

HgCl2 HC1 and KCN negative. FeCl3 stains differentially and brings 

out etch cleavage. KOH negative. HgCla stains differentially 

iridescent. Specimen is from Schneeberg, Germany. Analysis by J. G. Fairchild, 

of the United States 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 a 

formula (Ni,Co)2Ass, which can be expressed for purposes of comparison as 

(Ni,Co) Asj.fi. See description of smaltite-chloanthite. 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 very rare.

Corynile (Ni(As,Sb)S). Isometric; color grayish white; hardness F. HN03

effervesces a.nd brings out etch cleavage. HC1 and KCN negative. FeCls stains 

brown and brings out etch cleavage. KOH negative. HgClz stains brown to 

iridescent. Specimen from Olsa, Carinthia, Austria.

Maucherii'e (Ni3As2). Isometric; color pinkish gray, about same as cobaltite; 

hardness D. Some specimens isotropic, others weakly anisotropic. HN03 

instantly effervesces and stains black. HC1 and KCN negative. FeCls quickly 

tarnishes iridescent. KOH negative. HgCU slowly stains brown.

Smaltite-chloanthite ((Co,Ni)As2). Isometric; color gray; hardness F. HN03 

effervesces vigorously; surface stains differentially, bringing out concentric shell 

structure. HC1 and KCN negative. FeClg stains differentially brown. KOH 

negative. HgCl2 stains brown to iridescent. Specimen is from Schneeberg, 

Germany. Analysis by J. J. Fahey, of the United States 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 

smaltite-chloanthite show considerable variation, ranging from (Co,Ni)Asi. 5 to 

(Co,Ni)As3 . The latter end member is termed skutterudite. Nickel-free varieties are rare. The average corresponds to about (Co,Ni)As2 .

Arsenoferrite (FeA2). Isometric; color light gray, almost 

KOH neg. white; hardness F. HN03 effervesces slowly; stains brown. 

HgCh neg. HC1 and KCN negative. FeCl3 stains brown. KOH and HgCl2 

negative. Specimen from Jachymov (Joachimsthal), Czechoslovakia. Analysis by W. F. Foshag, of the United States National Museum, 

shows Fe 24.88, Cu 1.34, Pb 0.05, CaC03 4.00, MgC03 1.57, As 66.84, S 1.08, 

total 99.76. Very rare. Mineral is the ferriferous end member of the smaltite 

series (Co,Ni,Fe)As2.

Gersdorfllte (NiAsS). Isometric; color grayish white; hardness F . HN03 

effervesces vigorously and turns black. HC1 and KCN negative. FeCl3 stains 

dark gray to brown. KOH and HgCl2 negative.

Skutterudite ((Co,Ni)As3). Isometric; color creamy; hardness F. HN03 

effervesces and turns iridescent to black. HC1 and KCN negative. FeCl3 

stains brown. KOH and HgCl2 negative. Occurs as pyritohedrons with 

concentric shells. Specimen from type locality gave good microchemical tests for 

nickel and cobalt.
98 MICROSCOPIC DETERMINATION OF ORE MINERALS

ANISOTROPIC

Maucherite (NisAs2). Color pinkish gray, about the same as 

KOH neg. cobaltite; hardness D. Some specimens isotropic; others very 

HgCh weakly anisotropic. HNOs instantly effervesces and stains 

black. HCl and KCN negative. FeCl3 quickly tarnishes iridescent. KOH negative. HgCl2 slowly stains brown. Some specimens show a 

radial structure after etching.

Niccolite (NiAs). Hexagonal; color coppery pink; hardness E. Strongly 

anisotropic. HNOs tarnishes differentially with effervescence. HCl and KCN 

negative. FeCls slowly stains differentially brown to iridescent. KOH negative. 

HgCl2 stains brown to iridescent. Appears buff when in contact with breithauptite.

Plessite ((Ni,Fe)AsS). Isometric; color white; hardness F. Weak anomalous 

anisotropism. HN03 effervesces and stains black. HCl and KCN negative. 

FeCls 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. HNOs 

effervesces, stains dark, and brings out lamellar structure. HCl and KCN negative. FeCls stains brown. KOH negative. HgCl2 slowly stains brown.

HNO3 eff., HCl neg., KCN neg., FeCl3 neg.

ISOTROPIC

Gersdorffite (NiAsS). Isometric; color gray, almost white;

KOH neg. hardness F . HNOs effervesces and stains iridescent; in some

HgCU neg. specimens brings out parallel etch structure; usually stains gray

to brown; some specimens negative. HCl, KCN, FeCls, KOH,

and HgCl2 negative.

Pyrite (FeSz). Isometric; color pale brass-yellow; hardness F. HNOs fumes 

tarnish; some specimens slowly effervesce; with small grains or poorly polished 

surfaces reaction may be entirely negative. HCl, KCN, FeCls, KOH, and 

HgCl2 negative. Aqua regia fumes tarnish; some specimens slowly effervesce. 

Not scratched by needle, which is best means of distinguishing from chalcopyrite.

ANISOTROPIC

Arsenopyrite (FeAsS). Orthorhombic; color galena-white; 

KOH neg. hardness F+. Strong anisotropism. HNOs slowly effervesces 

HgCl2 neg. and stains differentially iridescent; effervescence not always 

observed. HCl, KCN, FeCls, KOH, and HgCl2 negative. 

Diamond-shaped cross sections frequently observed. Distinguished from lollingite by whiter color of arsenopyrite.

Lollingite (nickeliferous) ((Fe,Ni)As2 ). Orthorhombic; color galena-white; 

hardness E + . Strongly anisotropic. HNOs slowly effervesces; deposits 

colorless octahedra; mineral tarnishes iridescent. HCl, KCN, FeCls, KOH, 

and HgCU negative. Etch tests practically the same as lollingite. Microchemical tests show abundant nickel as well as iron.

Marcasite (FeS2). Orthorhombic; color pale brass-yellow; hardness E+. 

Strong anisotropism. HN03 stains brown to iridescent with slow effervescence; 

sometimes effervescence not observed. HCl, KCN, FeCls, KOH, and HgCl2 

negative. Prismatic. Marcasite is slightly paler in color than pyrite. The
DETERMINATIVE TABLE 99

only certain, way of distinguishing between pyrite and marcasite is by means of 

polarized light.

Pyrite (FeS2). Isometric; color pale brass-yellow; hardness F. More than 

99 per cent of pyrite specimens are isotropic; very rarely, however, polished 

sections show pyritohedrons with the color of pyrite rather than of marcasite, 

each pyrito hedron being a mosaic of grains of weak anisotropism and diverse 

orientation. HNOs fumes tarnish; some specimens stain slightly; drop effervesces slowly. Other reagents negative. Specimen examined is from Cerro de 

Pasco, Peru. The color and the square rather than prismatic cross sections are 

sufficient criteria to identify mineral as pyrite.

HNO3 , HC1, KCN neg., FeCI3

ISOTBOPIC

Psilomelane (Mn2 MnOs). Amorphous; color gray; hardKOH neg. ness D+. HNOs stains light brown; fumes tarnish. HCl

HgCI2 neg, stains brown to black; action more intense than that of HN03 . KCN negative. FeCl3 stains light brown. KOH and HgC]2 

negative. JI2 02 effervesces vigorously without etching. Massive, botryoidal, 

or reniform structures. Polished surfaces commonly show concentric banding.

HN03 , HCl, KCN neg., FeCI3 neg.

ANISOTROPIC

Hausmannite (MnsO^. Hexagonal; color white with a sugKOH neg. gestion of blue; hardness D . Strongly anisotropic; most 

HgCI2 neg. specimens show multiple twinning resembling that of plagioclase. HN03 fumes tarnish; some specimens negative. HCl 

stains differentially brown; some areas negative. KCN, FeCl3 , KOH, and 

HgCU 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 , HCl neg., KCN neg., FeCI3

ISOTROPIC

Skutterudite (ferriferous) ((Co,Ni,Fe)As3). Isometric; color 

KOH neg. gray; hardness D+. HN03 slowly tarnishes iridescent; someHgCl2 times effervesces slightly. HCl and KCN negative. FeCls 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 United States Geological Survey, shows Co 10.98, Ni 5.14, Fe 5.82, As 75.30, insoluble 1.44, 

total 98.68.

Kallilite (Ni(Sb.Bi)S). Isometric; color gray; hardness D. 

KOH neg. HN03 tarnishes iridescent. HCl and KCN negative. FeCl 3 

HgCl2 neg. stains faintly, almost negative. KOH and HgCl2 negative.

From Schoenstein, Germany. Very rare. According to Thomson,48 kallilite is a mixture. The present writer's specimen is homogeneous and 

contains all three metallic elements.

<s Thomson, Lllis, A inineralographic study of the pyrite group: Toronto Univ. Studies, Geol. ser., No. 

12, p. 36, 1921.
100 MICROSCOPIC DETERMINATION OF ORE MINERALS

Ullmanite (NiSbS). Isometric; color galena-white; hardness E. HNOs stains 

brown to irridescent. HC1 and KCN negative. FeCl3 stains light brown. 

KOH and HgCl2 negative. Distinctly softer than gersdorffite. Has well-defined 

cleavage and shows triangular pits.

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HN03 tarnishes 

iridescent; seems to effervesce slightly. HC1 and KCN negative. FeCl3 stains 

faintly, almost negative. KOH and HgCl2 negative. From Broken Hill, 

Australia. Rare.

HNO3 , HC1 neg., KCN neg., FeC!3

ANISOTROPIC

Breithauptite (NiSb). Hexagonal; color coppery pink; hardKOH neg. ness E. Strong anisotropism. HN03 stains differentially 

.HgCl2 neg. iridescent. HC1 and KCN negative. FeCl3 stains differentially 

iridescent. KOH-and HgCl2 negative.

HNO3 , HC1 neg., KCN neg., FeCI3 neg.

ANISOTROPIC

Arsenopyrite (FeAsS). Orthorhombic; color galena-white;

KOH neg. hardness F+. Strong anisotropism. HN03 stains differentially

HgCI2 neg. iridescent. HC1, KCN, FeCl3 , KOH, HgCl2 negative. Cross

sections frequently observed. Distinguished from lollingite

by whiter color.

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G. Some specimens isotropic; others faintly anisotropic. HN03 fumes tarnish; after long 

leaching stains differentially. HC1, KCN, FeCl3 , KOH, and HgCl2 negative. 

Usually crystallizes in cubes and pyritohedrons.

Glaucodot ((Co,Fe)AsS). Orthorhombic; color white; hardness F. Weak 

anisotropism. HNOs stains iridescent. HC1, KCN, FeCl3 , KOH, and HgCl2 

negative.

Leucopyrite (Fe3As4). Orthorhombic; color white; hardness F. Strong anisotropism. HNOs stains brown; solution deposits tetrahedral and octahedral

crystals. HC1, KCN, FeCl3 , KOH, and HgCVnegative. Distinguished with

difficulty from arsenopyrite and lollingite.

Lollingite (FeAs2). Orthorhombic; color grayish white; hardness F . Strong 

anisotropism. HN03 stains brown to iridescent; fumes tarnish. HCl, KCN, 

FeCl3 , KOH, and HgCl2 negative.

Marcasite (FeS2). Orthorhombic; color pale brass-yellow; hardness E+. 

Strong anisotropism. HN03 stains brown to iridescent; with larger grains 

sometimes gives slow effervescence; with smaller grains effervescence usually 

not observed. HCl, KCN, FeCls, KOH, and HgCl2 negative. Prismatic. 

Color paler than pyrite. The only certain way of distinguishing between marcasite and pyrite is by means of polarized light.

Pyrite (FeS2). Isometric; color pale brass-yellow; hardness F. More than 

99 per cent 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'is 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.
DETERMINATIVE TABLE 101 

HNO3 , HC1 neg., KCN neg., FeCI3 neg.

ISOTROPIC

Bravoite ((Fe,Ni)S2). Isometric; color pale violet; hardness 

KOH neg. E. HNOs fumes tarnish; mineral stains iridescent and brings 

HgCl2 neg. out zonal structure. HC1, 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. 49

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G. Some specimens isotropic; others weakly anisotropic. HNOs, some specimens stain differentially iridescent after long etching and fumes tarnish; other specimens negative. 

HCI, 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, FeCls, KOH, and HgCl2 negative. 

Specimen is 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

Pyrile (FeS2). Isometric; color pale brass-yellow; hardness F. HNOs fumes 

tarnish; some specimens slowly effervesce; with small grains or poorly polished 

surfaces reaction may be entirely negative. HC1, KCN, FeCls, KOH, and 

HgCl2 negative. Aqua regia fumes tarnish; some specimens slowly effervesce. 

Not scratched by needle, which is best means of distinguishing from chalcopyrite.

Willyamite ((Co,Ni)SbS). Isometric; color gray; hardness E. HNOs tarnishes 

iridescent; ueems to effervesce slightly. HC1 and KCN negative. FeCls seems 

to stain faintly; this reaction is easily overlooked. KOH and HgQz negative.

HNO3 neg., HCI, KCN neg., FeCl3 neg.

ISOTROPIC

Franklinitc ((Fe,Zn,Mn)0.(Fe,Mn) 2 03). Isometric; color gray, 

KOH neg. about the same as magnetite; hardness G. HNOs negative. HCI 

HgCl2 neg. usually negative, but fumes sometimes tarnish; solution turns 

yellow. KCN, FeCls, KOH, and. HgCl2 negative. Aqua regia 

solution turns yellow. Dark-brown powder when scratched. Powder is somewhat magnetic. (See description of chromite, p. 102.)

Magnetite (FesO^. Isometric; color gray; hardness F. HNOs negative. 

HCI, on smooth surfaces larger than drop, fumes tarnish; solution turns yellow. 

KCN, FeCls, KOH, and HgCl2 negative. Aqua regia solution turns yellow. 

Black powder when scratched. Powder is magnetic. (See description of chromite, p. 102.)

ANISOTROPIC .

Uausmannitc (MnsO.|). Hexagonal; color white Avitli a sugKOH neg. gestion of blue; hardness D . Strongly anisotropic; most

HgCl2 neg. specimens show multiple twinning resembling that of plagioclase.

HNOs fumes tarnish; some specimens negative. HCI stains

differentially light brown; some areas negative. KCN, FeCls, KOH, and HgClj

48 See Short, M. N., and Shannon, E. V., Violarite and other rare nickel sulphides: Am. Mineralogist, 

vol. 15, pp. Hi-14,1930.

15122 31 8
102 MICROSCOPIC DETERMINATION OF ORE MINERALS

negative. H2 02 effervesces slowly without staining surf ace. 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., HC1 neg., KCN neg., FeCI3

ISOTEOPIC

Uraninite (uranate of U, Pb, etc.). Isometric; color dark gray;

KOH neg. hardness G. HN03 , HC1, and KCN negative. FeCl3 slowly

HgCI2 neg. tarnishes to still darker brownish gray. KOH, HgCl2, and

aqua regia negative. Massive. Powder greenish brown to black.

HNO3 neg., HC1 neg., KCN neg., FeCl3 neg., aqua regia

ISOTROPIC

Pyrile (FeS2). -Isometric; color pale brass-yellow; hardness F. 

KOH neg. HN03 fumes tarnish; some specimens slowly effervesce; with 

HgCI2 neg. small grains or poorly polished surfaces reaction may be entirely 

negative. HC1, 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 , HCl, KCN, 

FeCl3 , KOH, and HgCl2 negative. Aqua regia effervesces slightly and stains 

light brown in places. Occurs as small isometric crystals. Only locality Sudbury, 

Ontario.

HNO3 neg., HCl neg. KCN neg., FeCI3 neg., aqua regia neg.

ISOTEOPIC

Braunite (3Mn2 03 .MnSiO2). Tetragonal; color gray; hardness 

KOH neg. G. Apparently isotropic. HN03 , HCl, KCN, FeCl3 , KOH, 

HgCI2 neg. HgCl2 , and aqua regia negative. H2 02 slowly effervesces without staining surface. Gives dark-brown powder when scratched. 

Frequently associated with hausmannite.

Chromite (FeO.C^Os). Isometric; color gray; hardness G+. Negative to 

all reagents, including H2O2 . Chromite has a fairly light-brown powder; franklinite has a dark-brown and magnetite a black pow'der. 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.

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G. Some specimens isotropic; others show weak anisotropism. HN03 , some specimens, fumes 

tarnish and mineral stains differentially iridescent after long leaching; other 

specimens negative. HCl, KCN, FeCl3 , KOH, HgCl2 , and aqua regia negative. 

Usually crystallizes in cubes and pyritohedrons.

Franklinite ((Fe,Zn,Mn)0.(FeMn) 2 03). Isometric; color gray, about the 

same as magnetite; hardness G. HN03 negative. HCl usually negative, but 

fumes sometimes tarnish; solution turns yellow. KCN, FeCl3 , 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. Negative to all reagents. Fine-grained and earthy varieties of hematite will give a red powder
DETERMINATIVE TABLE 103

when scratched! with a needle. Some specularite specimens, however, yield a 

metallic powder.

Magnetite (Fes04 ). Isometric; color gray; hardness F. HNOs negative. 

HC1 usually ^negative; on smooth surfaces larger than drop, fumes tarnish. 

KCN, FeCls, KOH, and HgCl2 negative. Aqua regia solution turns yellow. 

Black powder when scratched. Powder is magnetic. (See description of 

chromite, p. 10:2.)

ANISOTROPIC

Cassiterite (SnOa). Tetragonal; color gray; hardness G. Negative to all 

reagents. Can not be scratched by needle. Resin-colored internal reflection; 

not always seen. Color of hand specimen white to yellow.

Cobaltite (CoAsS). Isometric; color pinkish gray; hardness G . Usually 

shows weak anomalous anisotropism. Some specimens negative to all reagents.

Ferberite (FeWOi). Monoclinic; color gray, like sphalerite; hardness E. 

Strong anisotropism. Negative to all reagents. Powder dark, almost metallic.

Goethite (PeA-HaO). Orthorhombic; color gray; hardness E. Moderately

strong anisotropism. HNOs negative. HC1 doubtful; fumes tarnish slightly 

after long standing. KCN, FeCls, KOH, and HgClz negative. Aqua regia 

same as HC1. Gives yellow to orange-colored powder.

Hematite (specular) (Fe20s). Hexagonal; color galena-white; hardness G. 

Negative to all reagents. Habit platy.

Hubnerite (MnW04). 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 (FeTiOs). Hexagonal; color brownish gray; some specimens have 

suggestion of violet color; hardness G. Negative to all reagents. Commonly 

intergrown with specularite and magnetite.

Polianite (MnC^). Tetragonal; color gray; hardness G. Strongly anisotropic; 

gives four extinctions. HN03 , HC1, KCN, FeCla, KOH, HgCl2 , and aqua regia 

negative. HzOi 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. Negative to all reagents. Orange to red 

internal reflection and powder. . Almost invariably prismatic.

Wolframite ((Fe,Mn)W02). Monoclinic; color gray, like sphalerite; hardness 

E. Negative to all reagents. Shows cleavage in two directions. Gives darkbrown powder. Distinguished from hubnerite and ferberite only by chemical 

tests.

BINNENTHAL MINERALS

The following data on minerals that have been reported only from 

Binnenthal, Switzerland, are taken from Davy and Farnham's text. 

The present author was unable to obtain specimens of these minerals. 

The effect of polarized light is inferred from the crystal system as 

given in Dana's "System of mineralogy."

Dufrenoysite (2PbS.As2Ss). Orthorhombic; color gray; hardness C. Anisotropic. HNOa, HC1, KCN, and FeCls negative. KOH tarnishes iridescent and 

darkens. Gives! reddish-brown powder.
104 MICROSCOPIC DETERMINATION OF ORE MINERALS

Baumhauerlte (4PbS.3As2Ss).: Monoclinic; color grayish white; hardness C. 

Anisotropic. HN03 negative; sometimes tarnishes along cracks; rubs clean. 

HC1 negative. KCN negative; sometimes tarnishes along cracks. FeCls negative. 

KOH tarnishes iridescent. HgCU slowly tarnishes brown. Brittle. Internal 

reflection red. Gives brown powder.

Lengenbachite (6PbS.(Ag,Cu)2S.2As2S3). Triclinic; color gray; hardness B. 

Anisotropic. HN03 practically' negative, but sometimes tarnishes slightly. 

HC1, KCN, FeCl3 , KOH, and HgCl2 negative.

Seligmannite (Cu2S.2PbS.As2S3). Orthorhombic; color gray; hardness C. 

Anisotropic. HN03) HC1, KCN, FeCl3 , and HgCl2 negative. KOH slowly 

tarnishes iridescent.

DISCREDITED OB DOUBTFUL MINEBALS 60

Chilenite (Ag6Bi). Specimen from Copiapo, Chile, the type locality, is an 

intergrowth of native silver and cuprite (Cu2 0).

Chiviatite (2PbS.3Bi2Ss). 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 37.86, 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).

Cuprdbismutite (3Cu2S.4Bi2S3). 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.Sb2Ss). 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 (Cu2S.Sb2S3). (See 

Dana's "System of mineralogy," Appendix I, p. 16.)

Hauchecorniie ((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 (Talmage). Moderate aniso- 

.tropism. HNOs slowly stains brown to iridescent. HCl, 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. 

HCl, KCN, FeCl3 , KOH, and HgCl2 negative.

Hengleinite ((Fe,Ni,Co)S2). Specimen from Musen, Westphalia, the type 

locality, is a mixture of siegenite ((Ni,00)384) and pyrite.

Kalgoorlite (HgAu2Ag6Tes). Specimen from Kalgoorlie, Western Australia, 

the type locality, proved to be coloradoite (HgTe).

Lehrbachite (PbSe,HgSe) and zorgite. Do not exist. 51

Limonite (mixture of iron oxides). Coarse-grained varieties are anisotropic; 

stalactitic limonite is usually amorphous and isotropic. Limonite is*a mixture

» The formula given after each mineral is the one which is usually given in the literature. 

" Frebold, Georg, tiber einige Selenerze und ihre Paragenesen in Harz: Centralbl. Mineralogie, 1927, 

Abt. A, pp. 16-32.
DETERMINATIVE TABLE 105

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.

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 (Ag2S) and tetrahedrite (5Cu2S.2(Cu,Fe,Zn)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 (Bi2 (Te,S)3) and argentite (Ag2S).

Turgite (2Fe203 .H20). Is a fibrous variety of hematite (Fe203).62

62 See Posnjak, E., and Merwin, H. E., The system ferric oxide-sulphur trioxide-water: Am. Chem. 

Soc. Jour., vol. 44, p. 1971, 1922.

Antimony (Sb) Page

Andorite, AgaS.2PbS.3Sba S8 ___-. __._ -___________-____ ________ 81,94

Antimony (native), Sb __ ---_________-_ -__________________ 79, 83

Aramayoite, Ag2S.(Sb,Bi) 2 S3 . __ ____ _ ______________________________ 88

Berthierite, FeS.Sb2S3 ___.. ... ..... . . . 88, 95

Berthonite, 2(Pb,Cua)S.SbjS3 ---. ... 89

Boulangerite, 5PbS.2Sb2 S3 .. .................. .......... 75, 78

Bournonite, Cu2S.2PbS.Sb2S3 --------__........__._. I............. 89, 95

BreithauptiteJ NiSb.......... ......... ...................... 100

Brongniardite,Ag2S.PbS.Sb2S3 -------------------------------------- 89,95

Chalcostibite, Cu2S.Sb2S3 .----________...___-___-----___ 85, 90

Corynite, Ni(As,Sb)S-.......__...._........_...__.....__........ 97,101

Cylindrite, 6PbS.Sb2S3 .6SnS2 ..-. _.._...____.....__.'....____..._ 80, 81, 88

Dyscrasite, Ag3Sb...__.__..__...___.._________... 79, 83, 86 

.Famatinite, 3Cu2S.Sb2S5 ....______.___._.._...___---------__ 85

Franckeite, 5PbS.Sb2S3 .2SnS2 _--_-___.._...___.._._-------__ 82, 89

Freibergite, S(Cu,Ag) 2S.2(Cuf Fe)S.2SbaS3......... ......--.. . 86, 87

Freieslebenite, 3Ag2S.4PbS.3Sb2 S3 ----------------------------------- 92

Geocronite, 5PbS.Sba S8 --- - ---- -- 78, 90

Jamesonite, .4PbS.FeS.3Sba S3 --. ... . --. 74,77

Kallilite, Ni(Sb,Bi)S._ ..._.... ............_........ .. 86, 88, 99

Keeleyite, PbS.SbaS8 ... . ... -.. - 94

Kermesite, Sb2S2 0.--_____________...___....___._._.__.. 84, 95

Livingstonite, HgS.2SbaS3 -- -- -- - -- - 84, 93, 95 

Meneghinite, 4PbS.SbaS3 ... ------------------------------- 75, 78

Miargyrite, AgaS.SbaS8 .. .. . - . . -. 84, 91, 93 

Owyheeite, 8PbS.2Ag2S.5Sb2 S3 ------------------------------------- 82

Plagionite, SPbS.4Sba S8 .__-._-. -. 74, 77

Polybasite, 8AgaS.SbaS8 .. . ._. ----. _ 83, 84

Pyrargyrite, 3Ag2S.Sb2S3 ....____ __________.___._...__ 84,93

Semseyite, 9PbS.4Sb2S3 ------------------------------------------- 74

Stephanite, 5Ag2S.Sb2 S3 .___..._-____....___-_._.__... 79, 80, 83, 91

Stibnite, Sb2S3 _______......__...__-----------___ 73, 80, 85, 89

Tetrahedrite, 5CuaS.2(Cu,Fe,Zn)S.2SbaS8 . ---- 84, 88, 94

Ullmanite, NiSbS....----------------------------------------- 86, 88,100

Vrbaite, Tl2S.3(As,Sb) 2S3 -------------------------------_----- 89

Willyamite, (Co,Ni)SbS..;.._._..__....._._...._....._.._._ 86, 88,100,101

Zinkenite, PbS.SbaS8 .......---------------------------_--------- 74

Arsenic (As)

Algodonite, CueAs__ _ _ _ ___ -- __ - ---_____ 71

Arsenargentite, AgsAs_______________ _ _____________________________ 72

Arsenic (native), As_ ---- - - ----------------- 76

Arsenoferrite, FeAs2 __________ --_,. ______-_________--___---__--_ 97

Arsenopyrite, FeAsS. ------------------------------- ------- 98,100

107
108 MICROSCOPIC DETERMINATION OP ORE MINERALS

Page

Baumhauerite, 4PbS.3As2S3 ______-_-_-_______-____... 104 

Chloanthite-smaltite, (Ni,Co)As2 __-____--^---_____-___--_-__-_____. 97

Cobaltite, CoAsS............___...__.._....__..._._.____ 100,101, 102,103

Corynite, Ni(As,Sb)S-.-_----...____._._...._.___. ._. ._.___.. 97,101

Domeykite, Cu3As_____.__________________________ ____-_..- 71,82

Dufrenoysite, 2PbS.As2S3 ---_.____ _---_-__-__ __-_ ______- 103

Enargite, 3Cu2S.As2S5 ...______-____-____________...._ 85, 93

Gersdorffite, NiAsS....._ .....__.___________________.______.__..._. 97, 98

Glaucodot, (Co,Fe)AsS-_________.________________________' 100

Guitermanite, 3PbS.As2S3 -_--___________________-----____. 90

Huntilite, AgAs___ _ ____----____-___-----__________----_________.. 72

Jordanite, 4PbS.As2S3 -.__.---________________________________ 90

Legenbachite, 6PbS.(Ag,Cu) 2S.2As2S3 --------___-_---___._.___._.__ 104

Leucopyrite, FesAs4_______--___-_-_-_---_--_----_-___-___-_______- 100

Lollingite, FeAs2 ______________________________________ 98, 100

Lorandite, Tl2S.As2 S3 ---------------------------------------------- 90

Luzonite, 3Cu2S.As2Ss (variety of enargite)----_-_-_____--_---_-_____- 85,93

Maucherite, Ni3As2 ._-_____--_---__-___----_-______-_--___-____.___ 97, 98

Niccolite, NiAs.____________________________________________ ______ 98

Orpiment, As2S3 ._-_-_-___-___-_-_-__--___-________--____________ 93

Pearcite, 8Ag2S.As2S3 _____________ _____________:______ 83

Plessite, (Ni,Fe)AsS_________________________________ 98

Proustite, 3Ag2S.As2S3 _____-.___________________________ 83,84

Rammelsbergite, NiAs2 ____--_-__----_-_-_---___-_________________- 98

Rathite, 3PbS.2As2S3 ___~_____-_--____--------_-_____.____ ______._ 78, 89

Realgar, AsS_________________________________________ 96

Seligmannite, Cu2S.2PbS.As2S3 .----________________________ 104

Skutterudite, (Co,Ni)As3 .____________________________ 97, 99

Smaltite-chloanthite, (Co,Ni)As2 _________________________________ ___ 97

Sperrylite, PtAs2 _________'_______________________________ 102

Tennantite, 5Cu2S.2(Cu) FeJ Zn)S.2As2S3-__--------_ _______ 84, 88

Vrbaite, Tl2S.3(As,Sb) 2S3--------------------------___-----_-_______ 89

Whitneyite, CuflAs.____-----------____-----_-__--_________________ 71

Bismuth (Bi)

Aikinite, SCPb.Ci^S.BijSs------ - -_--- -- -__- 76, 78, 87, 89

Alaskaite, Ag2S.PbS.Bi2S3 -----__.____________.__'_________________ 81, 87

Aramayoite, Ag2S.(Sb,Bi) 2S3 _______________________________^________ 88

Benjaminite, (Cu,Ag) 2S.2PbS.2Bi2S3 _____.-_-_______.________________ 78

Bismuth (native), Bi_____-----_---_____-__________________________ 73

Bismuthinite, Bi2S3 ______________________________________________ 74, 78

Bismutoplagionite, 5PbS.4Bi2 S3 _____________________________________ 74, 82

Cosalite, 2PbS.Bi2S3 _______--_-_-__________________________________ 74, 81

Emplectite, Cu_S.Bi_Ss --- __________________________ 78, 90

Galenobismutite, PbS.Bi2S3 ________ _________________________________ . 77

Guanajuatite, Bi2 (Se,S) 3 ._._-_--_-----___--_________________________ 78, 90

Kallilite, Ni(Sb,Bi)S_...__ _-_.-___-_-__ _______________ ___ 86, 88, 99

Klaprotholite, SCujS.BijSs __-_._ ______ 76, 85

Matildite, (Agj,Pb)S.BiaS8-_ --.--_ __-_ -- _-__.___ _ _ 73 

Rezbanyite, 2PbS.3Bi2S3 _____________________________ 78

Schapbachite, Ag2S.PbS.Bi2S3 .-___________________________________ 81

Tetradymite,Bi2 (Te,S)3----------------------------------------_-_- 73,76

Wittichenite, 3Cu3S.Bi_Si_________________._______..________ 86
DETERMINATIVE TABLE 109

Cobalt (Co) Page

Carrollite, (Co > Cu) 8S4 - -- - - - - .. 87

Cobaltite, CoAsS _....._........._._.__......_..._._.... 100, 101,102,103

Glaucodot, (Co,Fe)AsS---.~ - _.. ... _..... 100

Linnaeite, 0384.--_-__-_-_---_-_---____.__-_-___-____-_____-______ 87

Siegenite, (Co,Ni) aS4 ...-.___________________________ 87

Skutterudite, (Co,Ni)As3 __-_-_-__-_-_______._____________________ 97, 99

Smaltite-chloanthite, (Co,Ni)A$2----------------_--_-_-_---_-_-_-_-_ 97

Willyamite, (Co,Ni)SbS-_-_--__--_-__---_--_--___--_____.-_ 86, 88,100,101

Copper (Cu)

Aikinite, 3(Pb,Cu2)S.Bi2Ss -;.- 76, 78, 87, 89

Algodonite, CueAs---__-__--_-------_-__--__-___-______--_--______- 71

Benjaminite, (Cu,Ag) 2S.2PbS.2Bi2S3 _ .._._.___.__.._. _ __._... 78 

Berthonite, 2(Pb > Cu2)S.Sb2S3 _ ~~ 89

Bornite, Cu5FeS« T - . 76

Bournonite, Cu2S.2PbS.Sb2S3 ~. . .-. 89, 95

Carrollite, (Co,Cu) 8S4 . -- 87

Chalcocite, Cu2S -. . 72, 75

Chalcopyrite, Cu2S.Fe2S3 ____._ _______.._._.___..______._. 87, 89, 94, 95

Chalcostibite, Cu2S.Sb2S3~.._------_.___._..__._.._... ._. 85, 90

Chalmersite. See Cubanite.

Copper (native), Cu--_-_-----------_--_--_----__-____-__-___.____. 72, 75

Covellite, CuS.__.....-. _ _ . _......... 93

Cubanite, CuS.2FeS _ ........................................... 90, 95

Cuprite, Cu2 0_________.'_-____.__________.___... 73

Cuprodescloizite, PbaCZn.CiOaVaOg.HaO 82

Delafossite, CuFe02 __-______--_---------------_.___... 92

Domeykite, Cu3As_____________._____._.__.___ 71, 82

Emplectite, Cu2S.Bi2S3 .__--___-___-------_____'_._.__.. 78, 90

Enargite, Cu2S:4CuS.AsaS3 -.. .-... ... ... 85,93

Eucairite, Cu2Se.Ag2Se_---_---------___--_-_-_--_-___--____--____-_ 83

Famatinite, Cu2S.4CuS.Sb2S3 __._______-------___________ 85

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 . _ . 86, 87 

Germanite, CuTGeSy ______________________________________________ 94

Klaprotholite, 4CuaS.Bi2Ss .__-----___-----______----- 76, 85

Klockmannite, CuSe_--_--_--_--__-___________---____--_-___-_-_-_ 85

Lengenbachite, 6PbS.(Ag,Cu) 2S.2As2S3 ___.______.__._____ 104 

Luzonite, Cu2S.4CuS.As2S3 (variety of enargite) ______________________ 85,93

Melaconite, CuO (variety of tenorite)..._.__________________ .......... 82

Rickardite, Cu4Te3 _ ____________________ - ________- 72 

Seligmannite, Cu2S.2PbS.As2S3 __________.______________ 104 

Stannite, Cu2S.FeS.SnSa . ___ '_......... 88, 90

Stromeyerite, (Cu,Ag) 2S_____________^_____________________________ 79

Sulvanite, Cu3VS4 .___________._______.___.______ 92 

Tennantite, 5Cu2S.2(Cu,Fe,Zn)S.2As2S3 ------------------------------ 84, 88

Tenorite, CuO_____________________..._____...__ 82 

Tetrahedrite, 5Cu2S.2(Cu,Fe,Zn)S.2Sb2S3 _------------------- -. 84, 88, 94

Umangite, Cu3Se2 ___-_-___---------______--___-----___--_--__----- 79

Weissite, Cu6Te3 ---___--------_____._____1. .......^._ 72

Whitneyite, Cu9As__- ..............^.......^...................... 71

Wittichenite, 3Cu2S.Bi2S3 --------_______..._------ 86
110 MICROSCOPIC DETERMINATION OF ORE MINERALS

Germanium (Ge) Page

Argyrodite, 4Ag2S.GeS2 _ _____________________ _ 84,85,92, 93 

Germanite, CuxGeSy ______________________________________________ 94

Gold (Au)

Calaverite, (Au,Ag)Te2 .-._--.--_---.-----.----_--...____--.._-. 76, 78

Electrum, Ag,Au._________________________________________________ 76

Gold (native), Au________________________________ 92

Krennerite,(Ag,Au)Te2 _--_---____-_..____---_-_---_-_-_----___--. 78,90

Nagyagite, Pb,Au,Te,S__-_._______________________ 90, 96 

Petzite, (Ag,Au) 2Te __-__ _-_ _ .-.-_ ._.- _.- __ 80, 86 

Sylvanite, (Ag,Au)Te2 .-___-_---.__-_-_.._.-._--_.._-____ 77, 78, 87, 91

Iron (Fe)

Arsenoferrite, FeAs.--__-__--_-____--__________-___-____---__-___-_ 97

Arsenopyrite, FeAsS ._---------____-----_-----_.______ 98, 100

Berthierite, FeS.Sb2S3 ..-._-._.___.-___.__________________- 88, 95

Bornite, Cu5FeS4 .--_______________________________ 76 

Bravoite, (Fe,Ni)S__ _.__. 1___-_ _-__ _._-_ _ _._ 101 

Chalcopyrite, Cu2S.Fe2S3 ___-_-___---_-_____-__--_-_______ 87, 89, 94, 95

Chalmersite. See Cubanite.

Chromite, FeO.Cr2 03 _________________...___-_____ 102 

Cubanite, CuS.2FeS.__________________________.____ 90, 95 

Delafossite, CuFe2 02 -.-_______________________________ 92

Ferberite, FeW04 ._-_-------__-.______.__.___._____-__.____._._ 103

Franklinite, (Fe,Zn,Mn)0.(Fe,Mn) 203 ______________.____ 101,102 

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 ___.--_--_--_.-------------- 86, 87

Glaucodot, (Co,Fe)AsS------__--------_----_--------------.---_-- 100

Goethite, Fe_Os.H_0 . ....___._______..____._.._.. 103

Hematite, Fe2 03 __.____________________________ 102,103 

Ilmenite, FeTiOa - - - __ _-------------- 103

Jamesonite, 4PbS.FeS.3SbaS3 - .---_ - . 74, 77

Leucopyrite, Fe3 As4 _______________________________________________ 100

Ldfflngite, FeAs2 __----________________________.___ 98,100

Magnetite, Fe3 04 - __________________________ T _ . 91,96,101,103

Marcasite, FeS2 -_ -----_________-______________--_- 98, 100

Pentlandite, (Ni,Fe)S....._.------------______.---__.---.-...-.--. 88

Plessite, (Ni,Fe)AsS...._....__-------_____.__________. 98

Pyrite, FeS2 __.____.........._................. 98, 99,100,101,102

Pyrrhotite, FeSi+______...___....___..._.._.._...____ 89

Stannite, Cu2S.FeS.SnS.. . .....__----------___............. 88, 90

Sternbergite, AgFe2S3 __-____________-____-__--_.._-_-.-_ 82

Tennantite, 5Cu2S.2(Cu,Fe,Zn)S.2As2S3 _________-__________ 84, 88 

Tetrahedrite, 5Cu2S.2(Cu,Fe,Zn)S.2Sb2S3 ------.------_--.-------- 84, 88, 94

Troilite, FeS...........__...._........ ._._. _.__.__. ..__ . 74

Violarite, (Ni,Fe) 3S4 _______....._._.__-_.__.__-.__-_-_.__._.__ 77

Wolframite, (Fe,Mn)W04 __.________--__. _..___--------- 103

Lead (Pb)

Aikinite,3(Pb,Cu2)S.BisS8 . .. .. ..-------- 76,78,87,89

Alaskaite, AgaS.PbS.BiaS3 ..- _ . -.. 81, 87

Altaite, PbTe_________________________. ___. _____ 73 

Andorite, AgS.2PbS.3Sb2S3 ----------------_____.......-......... 81, 94

Baumhauerite, 4PbS.3As2S3 -._.______________.__...._ ___.. 104
DETERMINATIVE TABLE 111

Page

Benjaminite, (Cu,Ag) 2S.2PbS.2Bi2S3-_________-_-_-___... 78 

Berthonite, 2(PbCua)S.SbjS8 ..... 89

Bismutoplagionite, 5PbS.4Bi2S3 -----.------------------------------- 74,82

Boulangerite, 5PbS.2Sb2S3 --------------------------------------- 75, 78

Bournonite, Cu2S.2PbS.Sb2S3 ---_..._._..__.___._...___._-_..--- 89, 95 

Brongniardite, Ag2S.PbS.Sb2S3 .____________________ 1__ 89, 95 

Clausthalite, PbSe....-------------------------------------------- 80

Cosalite, 2PbS.Bi2S3 __-----_____-___-___..____..._._... 74, 81

Cuprodescloizite, Pb,(Zn,Cu) 2V2Ofl .H2 0_--__-____............... 82

Cylindrite, 6PbS.Sb2S3 .6SnS2 . .................................... 80, 81, 88

Dufrenoysite, 2PbS.As2S3 .--___-_____________--_._____ 103 

Franckeite, 5PbS.Sb2S3 .2SnS2 ____--._-_.__..___.-_-__.___.._ 82,89

Freieslebenite, 3AgaS.4PbS.8SbjS3 92

Galena, PbS...--------------------------------------------------- 73,80

Galenobismutite, PbS.Bi2S3 _._-_---__--__-__----_____--_-_______-.- 77

Geocronite, 5PbS.Sb2S3 -------.----------------------------------- 78,90

Guiteraianite, 3PbS.As2 S3 ------------------------------------------ 90

Jamesonite, 4PbS.FeS.3Sb2Sj___ _ 74, 77

Jordanite, 4PbS.As2S3 ------------------------------------------- 90

Keeleyite, PbS.Sb2S3 -__.____...______-----_-_.-.__..__ 94

Legenbachite, 6PbS.(Ag,Cu) 2S.2As8S3 . 104

Matildite, (Aga,Pb)S.Bi 2S3 73

Meneghinite, 4PbS.Sb8S3 75, 78

Nagyagite, Pb,Au,Te,S_-----------__________-_.___------- 90, 96

Naumannite, (Ag2 ,Pb)Se_----_-_---_-------------_------_--_---__-- 83

Owyheeite, 8PbS.2AgjS.5SbaS3 82

Plagionite, 5PbS.4SbjS3 74, 77

Rathite, SPbS^AsaSs... ,.. 78, 89

Rezbanyite, 2PbS.3Bi2S3 _._____...._______.____...___ 78

Schapbachite, Ag2S.PbS.Bi2S3 -----,-------------------------------- 81

Seligmannite, Cu2S.2PbS.As2S3 .__-----------______---------- 104

Semseyite, 9PbS.4Sb2S3 - - , 74

Teallite, SnS.PbS.. _^ 82, 92

Uraninite, uranate of U, Pb, etc-__-___-__-------__----_______-__-__ 102

Zinkenite, PbS.Sb2S3 ..._________..______________._ 74

Manganese (Mn)

Alabandite, MnS____.-.--___-_..________.--_._-_____.___._..._ 74

Braunite, 3Mn2 03 .MnSi02 _.__________________________ 102 

Chalcophanite, (Mn,Zn) 0.2Mn02 .2H2 0__________ _________ 95 

Franklinite, (Fe,Zn,Mn)0.(Fe,Mn) 2 03 _____________________ 101,102

Hauerite, MnS2 _ __ _ _. ___. _______ - ______ 81, 91 

Hausmannite, Mn304 _____ ...______________- 82, 92, 99, 101 

Hubnerite, MnW04 ._.________.____________..______ 103 

Manganite, Mn2 03 ___---_----__---____-___.__________ 92

Polianite, Mn02 ...___-_____-_____________________ 103 

Psilomelane, Mn2MnOs_._._________-_______---.-_..-__.-_ 99

Pyrolusite, Mn02 ---_______-_------__ ____________..._.-_ 96

Wolframite, (Fe,Mn)W04 _____-_.._._-_-----___.___ 103

Mercury (Hg)

Cinnabar, HgS____ ____ _ _ __ _ 95

Coloradoite, HgTe______.____......._ ....................... 86, 93
112 MICROSCOPIC DETERMINATION OF ORE MINERALS

Page

Livingstonite, HgS.2Sb2S3 -_---_.-.-___________._----- 84, 93, 95 

Metacinnabarite, HgS.____________________________________________ 96

Tiemannite, HgSe.__---_---_-------------------_-_.._.__.____.___ 93, 94

Molybdenum (Mo) 

Molybdenite, MoS2 _____________________-____'.______ 96

Nickel (Ni) 

Bravoite, (Fe,Ni)S2 _.... . _.'___ ____.._ .. . ... 101

Breithauptite, NiSb________________________________ 100

Chloanthite-smaltite, (Ni,Co)As2 _--------_________________________ 97

Corynite, Ni(As,Sb)S_--___.-~- --- _ _- 97, 101

Gersdorffite, NiAsS.____________________________________ 97, 98

Kallilite, Ni(Sb,Bi)S_______________.____._._.__-___._.-___------ 86, 88, 99

Maucherite, Ni3As2 __-___-----_-_-___----_----______--__-________ 97, 98

Melonite, NijTes.. - _-_-_ - -_ _-______-__-_ _ _-- 77

Millerite, NiS_________________-________-_--___________________._.__ 89

Niccolite, NiAs________________________________ 98

Pentlandite, (Ni,Fe)S______________________________________________ 88

Plessite, (Ni,Fe)AsS._ _._ _._____.'_____..____________________ 98

Polydymite, Ni3S4 __-_-__-__-___--__-_-_-_---__--__-___-.________ 77

Rammelsbergite, -Ni As2 __________________________________________ 98

 Siegenite, (Co,Ni) 3S4---____________.______________ 87

Skutterudite, (Co,Ni)As3 ____________________________ _ 97, 99

Ullmanite, NiSbS_-_-_____----._-______.-.-__,-______-____-_.- 86, 88,100

Violarite, (Ni,Fe) 8S4 _. ' . _-___ _ __ __ _.____. 77

Wmyamite, (Co,Ni)SbS____--_-.________.__________________ 86, 88, 100,101

Platinum (Pt) 

Sperrylite, PtAs2 _________________________________ 102

Selenium (Se) 

Aguilarite, Aga (Se,S)_....._- __._ __-_-----_---__--_-_-_____._ 80,86

Clausthalite, PbSe________________________________ 80

Eucairite, Cu2Se.Ag2Se____________________________________________ 83

Guanajuatite, Bi2 (Se,S) 3 ________________________--.__._____ 78, 90

Klockmannite, CuSe____--_--_--______-_-__________..____________ 85

Naumannite, (Ag2,Pb)Se______________________________--__. 83

Tiemannite, HgSe._____________-..____.__-.___________ 93, 94

Umangite, Cu3Se2 -___________.______________ 79

Silver (Ag) 

Aguilarite, Ag2 (Se,S)______________________________ 80, 86

Alaskaite, Ag2S.PbS.Bi2S3 ________.______.____________ 81,87

Andorite, Ag2S.2PbS.3Sb2S3 _---------__-----_-________----- 81, 94

Aramayoite, Ag2S.(Sb,Bi) 2S3 _________________________._ 88 

Argentite, Ag2S_ -______________________________ 79, 92

Argyrodite, 4Ag2S.GeS2 __.___-_--_--_.-_--___.-_-______._ 84, 85, 92, 93

Benjaminite, (Cu,Ag) 2S.2PbS.2Bi2S3 ______________________ 78 

Brongniardite,Ag2S.PbS.Sb2S3 _-------_.__-_---_ -_--_-__-__--.__ 89,95

Calaverite, (Au,Ag)Te2 _____-__________________.____ 76, 78

Canfieldite, 4Ag2S.SnS2 with some germanium____ ____________________ 83, 84

Cerargyrite, AgCL____-__ -_-._______________________:.___ 92

Dyscrasite, Ag3Sb_ __^-_______________________ 79, 83, 86

Electrum, Ag,Au__-__-______---.______________________________ 76
DETERMINATIVE TABLE 113

Page 

Eucairite, Cu2Se.Ag2Se.._-__-____-_-__-_____---____--___________--_ 83

Freibergite, 5(Cu,Ag) 2S.2(Cu,Fe)S.2Sb2S3 ---------------------------- 86,87

Freieslebenite, 3Ag2S.4PbS.3Sb2S3 --___...-_-----------____ ._. 92

Hessite, Ag2Te..-_______._______ ____-.__________.___.' 79, 81, 86

Huntilite, Ag3As-_--_-_-______________________________________ 72

Krennerite, (Ag,Au)Te2 _-_____-----_--___---_-___... 78, 90

Lengenbachite, 6PbS.(Ag,Cu) 2S.As2S3 -___--------___.________ 104

Matildite, (Ag2 ,Pb)S.Bi2S3 __.________________.____:_______________ 73

Miargyrite, Ag2S.Sb2S3 ._____-___-_-__________ _____ 84, 91, 93

Naumannite, (Ag2 ,Pb)Se_ ---_-__--_-___-____---__----______-____--_ 83

Owyheeite, 8PbS.2Ag2S.5SbaS3 -_~_ ___'_ - _-_ _ . 82 

Pearcite, 8Ag2S.As2S3 ____________________________________ 83

Petzite, (Ag,Au) 2Te.__.________________________________ 80,86

Polybasite, 8Ag2S.Sb2S3 _----._________________________ 83, 84

Proustite, 3Ag2S.As2S3 _---______________-______________ 83, 84

Pyrargyrite, 3Ag2S.Sb2S3-------------------'---.-------------------- 84, 93

Schapbachite, Ag2S.PbS.Bi2S3 ----___________________________ 81

Silver (native), Ag______--_------_-_--______--___----_-_--_____ 72, 73, 75

Stephanite, 5Ag2S.Sb8S3 ---_-_ _-_ - __.--'-_--. _- _ 79, 80, 83, 91 

Sternbergite, AgFe2S3 __-----__-__-____-______-_____________.._______ 82

Stromeyerite, (Cu,Ag) 2S_---------------------------______________ 79

Sylvanite, (Ag,Au)Te.____ __ _ _._ 77,78,87,91

Tellurium (Te) 

Altaite, PbTe...________________________________________ 73

Calaverite, (Au > Ag)Te2-----------------.------._-------------_------ 76, 78

Coloradoite, HgTe..__________________________________________ 86, 93

Hessite, Ag2Te.--.-__________________________________________ 79, 81, 86

Krennerite, (Ag,Au)Te2 -------------------------------------___---- 78, 90

Melonite, Ni2Te3 -.-____----_-_--__--_--___-_--____-_____-.-_______ 77

Nagyagite, Pb,Au,Te,S__-__ __ _- _---____ 1-___-_________ 90, 96

Petzite, (Ag,Au) 2Te.________________________ ____________________ 80, 86

Eickardite, Cu4Te3 (^ ------------------------------------------- 72

Sylvanite (Ag,Au)Te2 _____________________________________ 77, 78, 87, 91

Tellurium (native), Te___-___-_--------__--__________'_____ 74, 76

Tetradymite, Bi^Te.S). - --- _------------ 73, 76

Weissite, Cu5Te3 (?)____-_-_____---___-________________ 72

Thallium (Tl) 

Lorandite, Tl 2S.As2S3 ______________________________________ 90

Vrbaite, Tl2S.3(Asl Sb)aSJ -.- -_-_ _ _ _ _- __- -__ 89

Tin (Sn)

Canfieldite, 4Ag2S.SnS2 with some germanium._______________________ 83, 84

Cassiterite, SnOz-------------------------------------------------- 103

Cylindrite, 6PbS.Sb2S3 .6SnS2 ____-_____ __________ _ ___________ 80, 81, 88

Franckeite, SPbS.Sb2S3 .2SnS2 _---_____________________________ 82, 89

Stannite, Cu2S.FeS.SnS2 ___________________________________ 88, 90

Teallite, SnS.PbS... ____________________ __________________________ 82, 92

Titanium (Ti) 

Ilmenite, FeTi03 __ __ _ -_-_ -___ _ -_____________-_ 103

Rutile, Ti02 _______________________'_____________ 103
114 MICROSCOPIC DETERMINATION OF ORE MINERALS

Tungsten (W) Page

Ferberite, FeW04 _----------_ 103

Hubnerite, MnWO4 .-_-_____-_________________-.-___ 103

Tungstenite, WS___________ _ _________ 94

Wolframite, (Fe,Mn)W04-----------------------------------------_ 103

Uranium (U) 

Uraninite, uranate of U, Pb, etc... _____-_-__-___-___-__--__-_____ 102

Vanadium (V)

Cuprodescloizite, Pb2 (Zn,Cu)2V209.H2 0-_____________________. 82

Sulvanite, CusVS,. 92

Zinc (Zn)

Chalcophanite, (Mn,Zn)0.2Mn02.2H2 0-------.__-_________________ 95

Cuprodescloizite, Pb2 (Zn,Cu)2V2 09.H2 0_____.-_-__.____..________ 82

Franklinite, (Fe> Zn > Mn)0.(Fel Mn) 2 03 -------------_____-----_ 101, 102

Sphalerite, ZnS______________________________________ 77, 81, 88, 94

Tennantite, 5Cu2S.2(Cu,Fe > Zn)S.2As.S8 .- 84, 88

Tetrahedrite, 5Cu2S.2(Cu,Fe,Zn)S.2Sb2S3-------________________ 84, 88, 94

Voltzite, 4ZnS.ZnO 81

Wurtzite, ZnS__.______..... __ _... ......... 88
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 the thiocyanate test for iron no precipitate is thrown down, but the unknown 

solution turns a deep-red 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 test 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 filtrations. 

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,53 who worked out methods for determining the elements 

in rock-forming minerals.

The methods used by Boricky were improved and expanded by 

Prof. H. Behrens, of the Polytechnic School of Delft, Holland, who 

developed microchemical methods for the identification of practically 

all the then known solid elements.54 His book was later revised

B3 BoriCky, Emmanuel, Elemente eincr neuen chemische-mikroskopisclien Mineral- und 

Gesteins-Analyse: Natnrwiss. Landesdurchf. Bobmen Archly, Band 3, Prague, 1877. 

Translated into English by Winchell, N. H., Minnesota Geol. Survey Ann. Rept., vol. 19, 

pp. 1-80, 1892.

54 Behrens, H., Mikrochemische Methoden, Amsterdam, 1882.

115
116 MICROSCOPIC DETERMINATION OF ORE MINERALS

and translated into English by himself.05 Behrens deserves the 

credit of having done more to further the science of microchemistry 

than anyone else. Several editions of his textbook have appeared, 

and the latest, in 1921, revised by Kley,56 is the foremost work on 

this subject in Europe. .

The best text written in English is that of Chamot,57 who, with 

his coworkers at Cornell University, has carried on instruction in 

microchemistry for nearly 30 years. For many years Cornell was 

the only institution in the United States where microchemistry was 

taught. Recently 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 Prof. 

Waldemar Lindgren, of the Massachusetts Institute of Technology, 

who initiated instruction in this application of the method in 1920. 

The present writer first became acquainted with microchemical 

methods while studying under Professor Lindgren in 1922. The 

possibilities of the method are set forth in a timely editorial.58

Recently McKinstry 59 has contributed a valuable article, references from which will be found in the description of the tests for 

cobalt (p. 137) and elsewhere in this bulletin.

A valuable series of articles on microchemical methods, by Putnam, Roberts, and Selchow,60 has recently appeared. 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 153.

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

55 Behrens, H., A manual of microchemical analysis, London, 1894. Now out of print.

56 Behrens, H., Mikrochemische Analyse, revised by P. D. C. Kley, Leipzig, Leopold Voss, 

1921. No translation into English has yet appeared.

57 Chamot, E. M., Elementary chemical microscopy, New York, 1921.

58 Lindgren, Waldemar, Microchemical methods; Econ. Geology, vol. 19, p. 762, 1924.

68 McKinstry, H. E., Microchemical methods in the identification of opaque minerals; 

Econ. Geology, vol. 22, p. 830, 1927.

60 Putnam, P. C., Roberts, E. J., and Selchow, D. H., Contributions to determinate 

mineralogy; Anr. Jour. Sci., 5th ser., vol. 15, pp. 89-102, 25'3-261, 423-430, 455-460,1928.
PROCEDURE AND APPARATUS 117

means of a sharpened match or splinter of soft wood, the point 

of Avhich 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 from the wood and falls 

to the bottom of the drop. The acid is evaporated off by holding the 

slide over a microflame.61 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 present writer has hitherto rejected tests which have 

called for an area of pure mineral greater than the larger figure. 

The textbooks used in this work are those of Chamot 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. His work 

is open to the objection that many elements, some of great importance in the ore minerals gold, for instance are not even mentioned. Likewise no tests are given for tellurium, selenium, molybdenum, vanadium, tungsten, and the rarer elements. 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.62 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

61 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 economic geology at 

Harvard University.

<° Behrens, H., op. cit. (1921), p. 155.

15122 31 9
118 MICROSCOPIC DETERMINATION OF ORE MINERALS

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.

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 out, especially the Behrens method 

described by Chamot.63 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 end of

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

Chamot. E. M.. op. cit, p. 286.
PEOGEDUEE AND APPAEATUS 119

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 are manufactured by 

K. J. Roberts, Redditch, England, and range from Sharps No. 6 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

11 "1 Lj£V6l6d

contact between needle and _______ . /surface 

wheel should be only mo- h

mentary, a fraction of a sec- ^^ n _EnIarged view of needle for scraplng

ond, as it is not necessary to polished surface

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. 11.)

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 

crater-like cavity is gouged. The depth of this cavity is usually kept 

considerably less than the diameter of its intersection 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 mineral's. 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 rolling 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
120 MICBOSCOPIC DETERMINATION" OF ORE MINERALS

slower will 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 author is 0.35 millimeter, and 

the 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 satisfactory 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. It is then removed, shaken to remove as much of the 

distilled water as possible, and stood up on end to dry.

The upper end of the slide should lean against a clean, smooth,

vertical surface, and the lower end should rest on a paper towel or

filter to remove the water as it collects at the bottom of the slide. 

This procedure obviates the necessity of drying the slide with a 

cloth, which might add lint or a film of grease to the slide. The 

clean slides are kept in a beaker and protected from dust by a watchglass cover.

The tests given by Chamot 64 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. 124.) 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.

64 Chamot, E. M., op. cit., p. 150.
PEOCEDUBE AND APPARATUS 121

I

Jb

v '#?;.' F"\^

**- " * X'^i- * .* egi;;gg 1 1

Microlurner. The ordinary Bunsen flame is not suitable for 

microcheroical work, as it spreads the heat over too great an area. 

The somewhat elaborate auxiliary flame described by Chamot 05 is 

likewise not essential. An improvised alcohol lamp made by the 

writer (fig. 12) 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 

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, can not 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 recommends 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 minute or so.66 

Pyrargyrite, which is almost insoluble when heated on a slide, was 

tested in a crucible and was easily broken up chemically. About 

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 H2 0).

FIGURE 12. Improvised 

alcohol lamp, a, Tinfoil 

cover to keep cork from 

catching fire; 1), brass 

tube; Cj cork; d, ink 

bottle; e, one or two 

strings

05 Chamot, E. M., op. cit., p. 153. 

"Idem, p. 293, fig. 353.
122 MICROSCOPIC DETERMINATION OF ORE MINERALS

Aqua regia (3 volumes of concentrated HC1 plus 1 volume of concentrated EGN"08 ) 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. 118). The capillary tubes used by 

the writer are made from a tube of soft glass originally 3.6 millimeters in outer 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. 13.) During 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

FIGDREI 13. Capillary tube. About natural size

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 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.
MICROSCOPIC DETERMINATION" OF ORE MINERALS 123 

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 concentration is too low, the precipitate will not form at all. 

The happy medium at which the crystals will be of maximum size 

is determined by experiment. For instance, the crystals of silver 

bichromate are of maximum size and most satisfactory in character when a small fragment of ammonium bichromate is placed in 

a 0.5 per cent solution of silver nitrate. Similarly the crystals 

of copper mercuric thiocyanate, which are the most useful for the 

recognition of copper, are most satisfactory in size and shape when 

a solution of potassium mercuric thiocyanate is added to a 0.5 per 

cent solution of copper nitrate. The writer believes that approximately this concentration is most satisfactory 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 be 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. Chamot 6T 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 microchernical 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 most 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 the same size and will lie upon the 

object slide in each experiment in the same positions with respect to faces.

07 Chamot, E. M., op. cit, p. 298.
124

Chamot describes the typical procedures used by him and designates them by roman numerals. His nomenclature is followed by 

the present writer. Some of his 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.68 "A drop of the reagent is placed near a drop of 

the solution to be tested. By means of a platinum wire or drawnout 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 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. 14), 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 III 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 in liquid of a definite 

concentration. Beginners almost invariably use too large a fragment of the reagent, and this causes a large proportion of the 

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 microscopic 

observation, more reagent seems to be needed it can be supplied.

Slide

FIGURE 14. D i a g r a m 

showing Chamot's method I. The reagent drop 

(r) has been made to 

flow into the drop to be 

tested (s) through a 

tiny channel (c). The 

precipitate separates

at p.

08 Chamot, E. M., op. cit, p. 299.
METHODS OF APPLYING TESTS 125

In method III-A a drop of the reagent is added to the test drop 

at its center.09 An example of this method is given on page 157 of 

this paper. A modification not described by Chamot might be 

termed method III-B. The drop containing the unknown elements 

is evaporated over the microburner or, if slow evaporation is desirable, by merely exposing it to the atmosphere. The liquid reagent 

that causes the precipitation is added to the dried residue. An 

example of this method is the dimethyl glyoxime test for nickel 

(p. 134). 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 and a Leitz No. 3 eyepiece. 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 perspective. 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 can not 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 can not 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.

<» Chamot, E. M., op. cit., p. 302.
126 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 

Avith respect to cobalt. These two solutions have an equal number 

of atoms of the metallic element per unit volume.

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

Do.......

Do.......

Do.......

Do.......

Do.......

Do.......

Mercury.- 

Do 

Do ......

DoDo 

Gold... ......

Do 

Do.......

Antimony ... . 

Do 

Bismuth. _ . 

Do 

Tin. ....... ..

Tellurium . 

Do... ....

Selenium.....

Test

Potassium mercuric thiocyanate ... 

.....do......... ...............

.....do-.... 

Potassium mercuric thiocyanate.-.

Cobalt mercuric thiocyanate--- ...

Pyridine and hydrobromic acid. 

Potassium iodide ... ...............

Cesium double iodide - -- . 

Concentration of most dilute solution (grams of salt per 100 cubic 

centimeters of solution)

0.01 Cu(N03)2.3H3 0 . .....

0.02 Ni(N03)j.6HsO ...... .... ..

.. do. ......... ...... ... ....

0.02 coc\j.m.tO. ..................

0.05 Co(N03 )2.6H2 0.. .............

0.02 Fe(NO3 )3.9H2 0. --__.._......

-....do- .

0.03 Fe(N03 )3.9H80.. .--.-..- 

0.005 Pb(C2H3 02).3H2O.. .... ..

0.10 Pb(C2H3 02).3H2 0_ ...........

0.01 Pb(N03)2 - ------ .

.....do.. 

.. do. - ..-- --.-- .

0.02 HAuCl4 - .... - ..

O.lOHAuCU __ ._ - __

0.01 (NHOjAsO.SHzO 

0.02 (NH4 ) 3As0.3H20 ... ... .... 

0.02 SbCl3 -. .

0.01 SbCh _ _ 

0.02 Bi(N03 )3.5H2 0-. --.... 

0.01 Bi(N08 )s.5H2 0- . .

0.01 SnCl . . 

0.01 Te(in HC1) 

0.05 SeOz ........... .. ... 

Concentration 

of element

Gram 

,per 100 

cubic centimeters

0. 00297 

.00263 

. 00406 

. 00406 

.00203 

. 00496 

.1010 

.00277 

.00277 

00415 

.00415 

-.0037 

.00273 

.0546 

. 00625 

.0093 

.0093 

.0093 

.0116 

.058 

. 00304 

. 00608 

. 01056 

.00528 

.0036 

. 00430 

. 00626 

.01 

.01 

.0391

Molality of 

solution

0. 000451 

.000414 

. 000692 

. 000692 

. 000341 

. 000841 

.0171 

. 000496 

. 000496 

. 00742 

. 00742 

. 000185 

. 000132 

.00264 

. 000302 

. 000961 

. 000961 

. 000961 

. 00060 

.00295 

. 000405 

. 000405 

. 000881 

. 000440 

. 000413 

. 000206 

. 000526 

. 000773 

. 000773 

. 00494
MICROSCOPIC DETERMINATION OP ORE MINERALS 127

REAGENTS 

Below is a list of the reagents used.

Adds and ammonia

1:1 HNOs 1 volume of concentrated HN03 (specific gravity 1.42) added to 1

volume of HaO.

1: 7 HNOs 1 volume of 1:1 HNO3 added to 3 volumes of I-LO. 

1 per cent HNOs 1 volume of concentrated HNOs added to 90 volumes of H2 0. 

Aqua regia 1 volume of concentrated HNOs added to 3 volumes of concentrated

HC1 (specific gravity 1.09).

1: 5 HC1 1 volume of 1:1 HC1 added to 2 volumes of H2O. 

5 per cent HCl 5 volumes of 1:1 HCl added to 45 volumes of H2O. 

NH«OH concentrated solution. 

1 per cent NH4OH 1 volume of concentrated NH*OH added to 99 volumes

of H2 0.

1 per cent acetic acid 1 volume of glacial H(C2I-Is02 ) added to 99 volumes

of H2O. 

Pyridine-hydrobromic acid 1 volume of pyridine added to 9 volumes of 40

per cent HBr.

Salt solutions

Dimethyl glyoxirne 1 volume of 2 per cent dimethyl glyoxime in alcohol, 

added to 1 volume of H20.

Potassium mercuric thiocyanate 5 per cent solution in water. (See directions for preparing salt, p. 128.)

a-nitroso )3-naphthol Dissolve 1 gram of nitroso /3-naphthol in 33 cubic centimeters of cold glacial acetic acid, add equal volume of HaO, and filter.

Cupferron 6 per cent solution in H2 0.

Acetate reagent Dissolve 0.5 gram of sodium acetate and 0.5 gram of copper 

acetate in 100 cubic centimeters of 1 per cent acetic acid.

Staunous chloride 2 per cent solution in 1: 5 HCl.

Ammonium molybdate 1.5 per cent solution in 1: 7 HNOs.

Solids ^ ' 

Cesium chloride CsCl. 

Potassium iodide KI. 

Potassium nitrite KN02 . 

Potassium ferrocyanide K4Fe (CN) o. 

Cobalt nitrate Co (NOs) 2 .6H2O. 

Potassium thiocyanate KCNS. 

Ammonium bichromate (NH«) 2Cr2 07. 

Rubidium chloride RbCl. 

Copper acetate Cu (C2Hs02 ) 2 . 

Sodium acetate NaC2H>02 . 

Sodium bismuthate NaBiOs. 

Calcium acetate Ca(C2H302 )2.

Potassium mercuric thiocyanate is not purchasable in the market 

but can be made up in the laboratory as follows:

Prepare about 50 cubic centimeters of a 10 per cent solution of 

potassium thiocyanate (KCNS). Also prepare a solution of mer-
128 MICROSCOPIC DETERMINATION OF ORE MINERALS

curie chloride (HgCl2 ) by dissolving 10 grams of the salt in about 

160 cubic centimeters of water. Slowly titrate the KCNS solution 

into a beaker containing all of the HgCl2 . No precipitate will 

appear until about 13 cubic centimeters of the KCNS solution has 

been added. At that point precipitation of white mercuric thiocyanate proceeds rapidly, according to the reaction 2KCNS + 

HgCl2 =Hg(CNS) 2 + 2KCl. This reaction is reversible, and only 

part of the mercury can be precipitated in this manner. Add 2 cubic 

centimeters more of KCNS solution and let stand 5 minutes. If 

more KCNS is added at this stage the mercuric thiocyanate already 

formed will begin to dissolve, which is to be avoided.

Filter the mercuric thiocyanate precipitate, after washing it in 

order to get rid of adhering traces of KCNS and KC1, and transfer 

it into a 250 cubic centimeter beaker. The mercuric thiocyanate precipitate is soluble in an 

excess of KCNS with the formation of the

,_Ground-glass double salt K2Hg(CNS) 4 . Titrate 10 per cent

surfa.ce

KCNS solution into the beaker containing the 

mercuric thiocyanate precipitate. The solution 

should be added slowly, and the contents of the 

beaker stirred constantly. The first chemical 

change is the formation of KCNS.Hg(CNS) 2 , 

which is almost insoluble in water but is soluble 

in an excess of KCNS with the formation of 

FIGURE is. Glass-stop- 2KCNS.Hg (CNS) 2 . When about 23 cubic cenEt nSnS^il!?08' timeters of KCNS solution has been added, the /i-LMJU L XldtUIcll olZc '

precipitate dissolves suddenly. It is essential to 

be careful at this stage to keep the excess of KCNS at a minimum.

The resulting solution, if of the proper concentration, is ready for

use. It is best, however, to evaporate the solution to dryness, weigh 

the salt, and dissolve it in about twenty times its weight of water. 

About 5.5 grams of solid 2KCNS.Hg(CNS) 2 is formed by the 

procedure above set forth.

The solution should be tested by merging a drop of it into a drop 

of 1 per cent cobalt nitrate solution, weakly acidified with HN03 . A 

blue precipitate should form immediately and should consist of isolated or interlocking needles. (See pi. 4, A.) If precipitation is 

delayed and branching forms appear at the edges of the drop (see 

pi. 5, A), 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 per 

cent cobalt nitrate solution. When the drop approaches dryness tiny
DETAILS OF TESTS 129

blue cobalt mercuric thiocyanate prisms will1 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 10 acids and other liquids 

leaving no residue on evaporation, 7 salt solutions, and 12 solids. 

The liquid reagents are best kept in 15 cubic centimeter glassstoppered dropping bottles. (See fig. 15.) 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. 70 The solids are kept 

in cylindrical phials with cork stoppers.

Potassium mercwrid thiocyanate test. 

Reagent: 5% potassium mercuric thiocyanate solution in H2O.

Product: Zinc mercuric thiocyanate (Zn(CNS) 2 .Hg(CNS) 2 ), white feathery

crosses. 

Limit: 0.01% zinc nitrate solution."

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 tetrahedra form are not always obtainable. The potassium 

mercuric thiocyanate test, on the other hand, is reliable, delicate, 

and easily controlled.

Potassium mercuric thiocyanate (K2Hg(CNS) 4 ; see p. 128) is the 

most useful reagent for microchemistry yet discovered. It yields 

delicate and satisfactory tests with solutions containing zinc, copper, 

cobalt, nickel, iron, silver, and gold, provided that only one metal 

is present in the solutions. .With lead the reaction is less satisfactory. The reactions are given below for comparison:

Zinc nitrate: White feathery crosses.

Copper nitrate: Yellowish-green mosslike clumps and rounded prisms.

Cobalt nitrate: Indigo-blue prisms.

Nickel nitrate: Brown spherulites.

Iron nitrate: No precipitate, but solution is colored red.

Silver nitrate: White curdy amorphous precipitate.

Gold chloride: White amorphous precipitate; more gelatinous than that of

silver. 

Lead nitrate: Colorless prisms; not satisfactory.

70 Glass-stoppered dropping bottles similar to that shown in the sketch may be obtained 

from Arthur H. Thomas Co., Philadelphia, Pa., for approximately 40 cents each.

71 This outline was devised by Putnam, Roberts, and Selchow. See articles already cited.
130 MICROSCOPIC DETERMINATION" OF ORE MINERALS

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 per cent HNO3 . 

The solution was free from cadmium.

1%: The solution gave a strong reaction but was too concentrated.

Amorphous grains and distorted forms predominate.72 

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 2.

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 HNO3 were mixed and 

tested with K2Hg(CNS) 45 with the following results:

0.5% zinc acetate 7S and 0.005% copper acetate. Pure zinc crystals alone form.

No sign of copper. 

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 0.2% 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. 16, a.-) 

0.5% zinc acetate and 0.5% copper acetate. Same as preceding, but color

is a light violet. Simpler crosses predominate over feathered crosses. (See

fig. 16, 6.)

72 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.

78 This solution is actually 1 cubic centimeter of 1 per cent zinc acetate added to 1 

cubic centimeter of 0.01 per cent copper acetate. The percentages as given above are 

percentages of each salt expressed in terms of total solution.
131

0.5% zinc acetate and 0.1% copper acetate. Isolated purple prisms like those 

shown in Figure 16, o. 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. 16, 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. 16, 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.

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 O a

FIGUEE 16. Zinc and copper mercuric thiocyanate 

crystals
132 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 can not be detected 

in a strong cobalt solution. The cobalt should be removed by means 

of potassium nitrite (p. 138) before testing for zinc with potassium 

mercuric thiocyanate.

According to Chamot,74

Inorganic salts of cadmium yield Cd(CNS)2.Hg(CNS) 2 in brilliant colorless, 

probably orthorhombic prisms, usually several times as long as broad, but the 

appearance of these prisms varies with the conditions which obtain at the 

time of their formation, as, for example, the concentration, depth of the test 

drop, amount of reagent added, acidity, etc. These variations are, however, 

not of a kind to render the test doubtful, long prisms either singly or in 

groups being the rule.

Even a small amount of cadmium destroys the feathery and branched character of the skeletons of zinc-mercury thiocyanate, owing to the formation 

of mixed crystals, and there generally result crystallites of the shape of an 

arrowhead. Small amounts of zinc in the presence of much cadmium will 

usually escape detection.

COPPER

Potassium mercuric thiocyanate test. 

Reagent: 5% K2Hg(CNS) 4 in H20.

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. 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 greenish 

yellow, but some of the thicker crystals may be distinctly olive-green. 

(See pi. 3.)

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 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

« Chamot, E. M., op. cit., p. 355.
DETAILS OP TESTS 133

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 " 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 jt. For instance, a 

good copper test can usually be obtained from bismuthinite. The 

student should be on guard for results of this land and ignore faint 

tests.

Potassium iodide. 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. 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 1:7 HN03 and test each one for copper with 

K2Hg(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 1:3 HC1 solution containing copper ions, orange or golden 

colored needles form at the edge of the drop. This result, 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, or Sn.

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.

Triple nitrite reaction. The triple nitrite reaction is described 

on page 145. If the test is being made for copper, lead is supplied, 

preferably in the form of a solution containing 0.5 per cent of lead 

acetate and 0.5 per cent of sodium acetate in 1 per cent acetic acid.

NICKEL

Dimethyl glyoxime test. 

Reagent: 2% dimethyl glyoxime solution in alcohol plus an equal volume of H2O. 

Product: Nickel dimethyl glyoxime (NUCJ^NzOsJa). 

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 

15122 31 10
134 MICROSCOPIC DETERMINATION- OF ORE MINERALS

water but is readily soluble in alcohol and other organic solvents. 

Alcohol spreads over the slide and consequently should be diluted 

with water. Accordingly a 2 per cent solution of the reagent in 

alcohol is diluted by adding an equal volume of water. The resultant 

mixture is a 1 per cent solution of dimethyl glyoxime. 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. 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 filtrate 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 1 per cent 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. Hencp 

it is advisable to eliminate the iron by precipitation with ammonia 

before making the nickel test. The ferric hydroxide formed occludes a considerable percentage of nickel, but most of the nickel 

remains in solution. The ammonia is allowed to evaporate in the 

air. When nearly dry the dimethyl glyoxime solution is added directly to the nearly evaporated ammoniacal solution. The gelatinous 

ferric hydroxide does not interfere with the formation or observation 

of the nickel precipitate. If the concentration of nickel ions is very 

low, the test drop can be evaporated and the reagent, made slightly 

ammoniacal, is added directly to the residue. Dimethyl glyoxime 

gave a good test on a 0.02 per cent solution but a poor test on a 

0.01 per cent solution of nickel nitrate.

Potassium mercuric tMocyanate test. 

Reagent: 5% K»Hg(CNS)« in H2O.

Product: Nickel mercuric thiocyanate (NiHg(CNS)4).

Limit: 0.02% NiCNOa)? solution.
DETAILS OF TESTS 135

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. 4, 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: 5% 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. 4, 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. 4, 6".) 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 

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.

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 par' 

ticularly 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 appear-
136 MICROSCOPIC DETERMINATION OF ORE MINERALS

ance 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 HNO8 ) 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.

The formation of the triple thiocyanate of cobalt, zinc, and mercury is described on page 131. 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 anv 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 )a and 0.10% Fe(NO3 ) 3 . Normal cobalt prisms predominate; also

some formless grains much darker but distinctly blue in strong transverse

illumination. 

0.5% Co(NO3 ) 2 and 0.25% Fe(NO3 ) 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(N03 )a. Same as preceding, but solution turns

very pink, and prisms are envelope-shaped. 

0.25% Co(N03 ) 2 and 0.5% Fe(NO3 ) 3 . Same as preceding. Most of the prisms

are opaque, but a few normal cobalt prisms are also seen. 

0.1% Co(N03 ) 2 and 0.5% Fe(N03 ) 3 . Cobalt prisms black but show blue at thin

edges. 

0.05% Co(NO3 )a and 0.5% Fe(N03 ) 3 Solution strong red. Prisms light

purplish to black. No sign of blue. 

0.02% Co(NO3 ) 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.
U. S. GEOLOGICAL SURVEY BULLETIN 825 PLATE 2

C D

MICROCHEMICAL TESTS FOR ZING

A, B, Zinc mercuric thiocyanate crystals obtained from a 0.5 per cent zinc nitrate solution, X 128; 

6\ D, zinc mercuric thiocyanate crystals obtained from a 0.1 per cent zinc nitrate solution, X 208
U. S. GEOLOGICAL STKVEY

: i

BULLETIN 825 PLATE 3

V

V

MICROCHEMICAL TESTS FOR COPPER

A, Copper mercuric thiocyanate crystals. Obtained from a 0.5% copper nitrate solution. 

X 130. B, Copper mercuric thiocyanate crystals. Obtained from a 0.1% copper nitrate 

solution. X 130. C, D, From same solution as B.
U S. GEOLOGICAL SURVEY * BULLETIN S2-) PLATE 4

MICROCHEMICAL TESTS FOR COBALT AND NICKEL

A, Cobalt mercuric thiocyanate prisms. From a 1% cobalt nitrate solution. X 130. 

B, Cobalt mercuric thiocyanate prisms and spherulites. From a solution containing 

0.5% cobalt nitrate and 0.1% nickel nitrate. X 260. C, Cobalt and nickel mercuric 

thiocyanate spherulites. From a solution containing 0.5% cobalt nitrate and 0.5% nickel 

nitrate. X 210. D, Nickel mercuric thiocyanate spherulites. From a solution containing 0.5% nickel nitrate. X 210.
U. S. GEOLOGICAL SURVKY BULLETIN 825 PLATE 5

C D

MICROCHEMICAL TESTS FOR MERCURY

A, B, Cobalt mercuric thiocyanate. From a 0.5% mercuric chloride solution, x 210. 

C, Mercuric chloride residue. Obtained by evaporating cinnabar (HgS) powder to dryness with aqua regia. X 23. D, Mercuric chloride residue. Obtained by evaporating 

cinnabar powder to dryness on a glass slide with aqua regia. X 130.
DETAILS OF TESTS 137

0.05% Co(N03 ) 3 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 nitroso /3-naphthol or potassium nitrite method.

Nitroso fi-naphthol test. 

Reagent: 1.5% solution of a-nitroso /3-naphthol in 50% acetic acid. 

Product: (CioHo(NO)0) 2Co, a bulky amorphous red precipitate. 

Limit: 0.02% CoCk

NitrOSO j8-naphthol is an organic compound used in quantitative 

analysis for the gravimetric determination of cobalt in the presence 

of nickel. The precipitate is voluminous and comes down quickly. 

For this reason this method has some advantages over the potassium 

nitrite method next described. McKinstry 75 recommends nitroso 

/3-naphthol as a microchemical test for cobalt.

The reagent is made by dissolving 1 gram of nitroso /3-naphthol in 

33 cubic centimeters of cold glacial acetic acid, adding an equal volume of water, and filtering. The addition of water greatly increases 

the surface tension of the liquid and prevents the drop from spreading over the slide.

The cobalt precipitate is almost identical in appearance and character with metallic selenium. Its color ranges from crimson in rela-. 

tively large quantities to brick-red in lesser amounts. The test works 

best in strong HC1 solutions, as the corresponding nickel compound 

is soluble in HC1. However, when method I is used, the precipitate 

does not agglutinate but.spreads over the entire area covered by the 

drop. As the precipitate is not collected into definite crystals, the 

color of the precipitate in very small masses is uncertain, and the 

test thus loses much of its sensitiveness.

A better method is to evaporate the unknown solution quickly over 

the microflame. The material tends to collect as a circular ridge 

around the periphery of the drop. The slide is cooled, and a drop of 

reagent is applied directly to the dried residue. A crimson to brickred ring much bulkier than the original residue forms in its place. 

Nickel does not interfere with this test. The corresponding nickel 

compound is light lemon-yellow and is comparatively soluble in acetic 

acid. As the drop evaporates, the nickel compound begins to come 

down but does not obscure the red cobalt compound already formed.

75 McKinstry, H. E., Microchemical methods In the identification of opaque minerals: 

Econ. Geology, vol. 22, p. 830. 1927.
138 MICROSCOPIC DETERMINATION OP ORE MINERALS

A solution containing 1 per cent of NiCL and 0.02 per cent of 

CoCl2 gave an excellent test for cobalt by this method.

By far the greatest objection to this test is the fact that iron is 

likewise completely precipitated with the cobalt. The iron precipitate is chocolate-brown in color and if present in any quantity completely obscures the cobalt precipitate. Hence iron must be removed 

from solution before testing for cobalt.

Copper and chromium are likewise precipitated from acid solutions, but aluminum, lead, cadmium, manganese, nickel, mercury, and 

zinc are not precipitated by nitroso /?-naphthol.76

Iron can not be separated from cobalt by making the solution 

ammoniacal, as the ferric hydroxide has a strong tendency to occlude 

any cobalt present. 77 This was confirmed by the writer. A drop of 

solution containing 0.5 per cent of CoCl2 and 0.5 per cent of FeCl3 

was evaporated over a microflame. After cooling, concentrated 

NH4OH was added to the residue. The filtrate was transferred to 

another place on the slide and evaporated to dryness. The residue 

was taken into solution .in 1:7 HN03 and tested for cobalt by means 

of potassium mercuric thiocyanate. Only a trace of cobalt was 

found.

A more successful attempt was made by means of cupferron. (See 

p. 141.) A solution containing CoCl2 and FeCl3 was evaporated to 

dryness. The residue was cooled and then leached with a 6 per cent 

solution of cupferron, which was likewise evaporated to dryness. 

The residue was leached with 1:7 HNO3 , and the filtrate was transferred to another place on the slide and evaporated to dryness. The 

residue was cooled and then leached with nitroso /3-naphthol reagent. 

The red cobalt ring formed immediately, and no trace of the brown 

iron compound was observed. Mixtures of various proportions of 

CoCl2 and FeCl3 were tested in this manner. A solution containing 

0.5 per cent of FeCl3 and 0.05 per cent of CoCl2 gave a good test 

for cobalt. One containing 0.5 per cent of FeCl3 and 0.03 per cent 

of CoCl2 gave a distinct cobalt precipitate, but the amount was so 

small that the writer could not be sure of its color, which is the 

only criterion in judging the test.

Potassium nitrite test.-rReagent: Solid potassium nitrite (KN02 ).

Product: Potassium cobalt nitrite (3KNOj.Co(N02 ) 2.l%H20) 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 by Behrens and Kley. The solution containing the

78 Mellor, J. W.. A treatise on chemical analysis, p. 394, London, 1913. 

77 Clennell, J. E., The estimation of cobalt by tbe nitroso 0-naphthol method : Mining 

Mag. (London), vol. 36, p. 272, 1927.
DETAILS OF TESTS 139

cobalt, generally a nitric acid solution, is evaporated to dryness. 

It is then redissolved in a drop of 1 per cent acetic acid, a fragment 

of potassium nitrite is added, and the drop is gently heated but net 

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 arsenomolybdate, described on page 156.

Behrens and Kley 78 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. Chamot 

is also very favorably disposed toward this reaction. He says 79 : 

" This test has its greatest value in a negative way, since failure to 

obtain the very insoluble double nitrite may be considered as indicative of the absence of cobalt."

The test would be very useful indeed if it were as reliable as these 

comments would indicate. The present writer can not 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 per cent 

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 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: 5% K2Hg(CNS>4 in H2 0.

Product: No precipitate forms, but'solution turns pink.

Limit: 0.02% Fe(N03 )s.

Any thiocyanate solution will give a delicate test for iron. The 

iron must be in the lerrjfiLstate. 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. 124). Ferric nitrate solutions of 0.2 per cent

78 Behrens, H., and Kley, P. D. C., op. cit., p. 77. 

" Chamot, E. M., op. cit., p. 413.
140 MICROSCOPIC DETERMINATION OF ORE MINERALS

or greater concentration immediately turn pink near .the junction 

of the two solutions. If the concentration is less than 0.2 per cent 

the color may not appear 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 per cent 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. The writer has obtained distinct tests for iron 

with sphalerite and chalcocite, for instance. 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)e). 

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

get a blue precipitate with neutral ferric nitrate solutions in a concentration of less than 0.1 per cent, but by adding HN03 the solution 

could be diluted to 0.02 per cent and still give a satisfactory test.

The test works equally well in 1: T 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: NEUOH 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
DETAILS OF TESTS 141

all the excess of acid and then add the ammonia directly to the still 

moist residue. (See method III-B, p. 125.)

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.

Cupferron test.

Reagent-: 6% cupferron solution in H2 0.

Product: Amorphous orange-colored to brick-red precipitate.

Limit: 0.03% ferric chloride or nitrate.

Cupferron is a name used for convenience to describe the ammonium salt of nitrosophenyl hydroxylamine (C6HBN.NO.ONH4 ).80 It 

precipitates quantitatively ferric iron, cupric copper, vanadium, zirconium, thorium, cerium, titanium, and tetravalent uranium from 

strongly acid solutions. The reagent is soluble in water and ammonia but insoluble in acids unless the acids are greatly in excess. 

For microchemical work cupferron can be considered practically 

insoluble in 1:7 HNQ3 and 1: 5 HC1.81 In quantitative analysis the 

cupferron solution is slowly added to the beaker containing iron in 

a strong hydrochloric acid solution. In this manner the cupferron 

is kept in solution while the iron is precipitated. But acid should 

not be added to a cupferron solution, because a white precipitate is 

immediately thrown down, and it is difficult to get this precipitate 

into solution again. This property can be utilized in removing iron 

from acid solutions. The unknown solution is evaporated to dryness, and the cupferron solution added to the residue. The iron, 

copper, etc., are precipitated. The drop of cupferron is then likewise 

evaporated. By leaching the precipitate and cupferron residue with 

HNOs or HC1, nickel, cobalt, and other elements not precipitated by 

cupferron are taken into solution, whereas only a small proportion 

of cupferron is dissolved. The nitrate is transferred and tested for 

cobalt and other elements. (See p. 138.)

The iron precipitate is orange-colored to brick-red, the color depending on the quantity present. It greatly resembles ferric hydroxide and likewise tends to cling to the glass slide. The copper 

precipitate is white to gray and crystallizes in orthorhombic needles. 

The vanadium, titanium, and uranium precipitates are light yellow.

Cupferron solutions slowly decompose and give off a peculiar 

sweetish odor of nitrosobenzine. The reagent should be made up 

fresh about once a month.

80 Treadwell, P. P., arid Hall, W. T., Analytical chemistry, p. 838, New York, 1916.

81 Cupferron may be obtained from the Eastman Kodak Co. for 6 cents a gram.
142 MICROSCOPIC DETERMINATION" OF ORE MINERALS

Sumimary. None of the four tests for iron described above precipitate the element 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 test usually made first is 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 HNO3 or HC1 and tested for iron 

with K4Fe(CN) 6 .

The cupferron test is not as good as the others described, but 

cupferron serves as a convenient agent to remove iron from acid 

solutions.

MERCURY

Double tkiocycmate test.-^

Reagents: Solid cobalt nitrate (Co(NOs)2.6H2 0) and solid potassium thiocyanate (KCNS).

Product: Potassium mercuric thiocyanate in blue prisms and branching1 forms. 

Limit: 0.005 Hg012 in 1% HNOs.

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 HNO3 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. 5, C, D.)

The mineral is broken down with aqua regia, the residue leached 

with 1 per cent HN03 , 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. 5, 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 per cent 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.
DETAILS OF TESTS 143

This test is very sensitive, and a 0.005 per cent 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.

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. 6, D.) Mercury compounds leave a similar residue 

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 0.5 per cent acetic acid, and the drop 

is1 transferred and tested with a tiny fragment of KI, using method 

III. A lemon-yellow precipitate of normal lead iodide is thrown 

down.82 If the concentration of lead ions is relatively large (0.1 

per cent Pb(N03) 2 or greater) well-formed hexagonal plates will 

usually appear. (See pi. 6, A, B.) They may, however, be starshaped 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 and a Leitz No. 3 or equivalent 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. 124.)

M 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,
144 MICROSCOPIC DETERMINATION OF ORE MINERALS

As the drop dries, colorless needless of the double iodide 2KLPbI2 

usually appear at the edge of the drop. The double iodide 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 can not 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 132, and the residue afterward tested for lead. In the presence 

of bismuth it is best to prove the lead by the chloride test described 

below. Chamot 83 says 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 the drop 

cools too rapidly when taken from the flame to do much good. The 

same remarks apply to recrystallizing the normal iodide from hot 

water.

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,84 there are also 

seen feathery dendritic X's and long irregular ragged prisms, but 

the prism is the only form that has been observed by the writer.

as Chamot, E. M., op. cit., p. 370, par, .2, 

84 Idem, p. 372,
DETAILS OF TESTS 145

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 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.

Triple nitrite reaction. 

.Reagents: 0.5% of copper acetate and 0.5% of sodium acetate in 1% acetic

acid; solid potassium nitrite. 

Product: Potassium-copper-lead nitrite (2KN02.Cu(N02 )2.PbNO2)2 in brown to

black cubes. 

Limit: 0.01% Pb(N02 )2.

The triple nitrite of potassium, lead, and copper is far more insoluble than the simple nitrites of these elements or the double 

nitrites formed by any two of them. The triple nitrite occurs as 

brown to black cubes, also square and rectangular plates. (See pi. 6, 

C.) The platy crystals are more likely to be brown, and the cubes 

are usually black. In appearance this compound is striking and 

highly characteristic. Either lead or copper can be tested for by 

supplying ions of the other element. However, the potassium mercuric thiocyanate test for copper is so nearly perfect that the triple 

nitrite reaction is not necessary.

As described by Chamot the solution containing the lead which is 

to be tested for is evaporated to dryness over the microflame in 

order to remove strong acids, which otherwise would interfere with 

the test. The residue is taken into solution with very dilute acetic 

acid, and successive fragments of sodium acetate, copper acetate, 

and potassium nitrite are introduced into the solution. If lead is 

present in sufficient concentration to give the test, a cloud of brown 

crystals will form and spread out toward the periphery of the drop. 

When examined in a fairly high magnification (usually a Leitz 

No. 4 or equivalent objective) the crystals are seen to be predominantly cubes.

The sensitivity of the test can be greatly increased by adding a 

fragment of cesium chloride to the drop. The corresponding triple 

nitrite of cesium, lead, and copper is identical in appearance with
146 MICROSCOPIC DETERMINATION OF ORE MINERALS

the potassium salt and more insoluble. On the other' hand, the size 

of the crystals produced by the cesium salt is correspondingly 

diminished.

The objection to adding three and possibly four successive fragments to a solution is that the quantity of material can not be easily 

controlled, and the solution tends to be .clogged up before the triple 

nitrite crystals have a chance to form. The writer modifies this 

procedure by dissolving definite quantities of sodium acetate and 

copper acetate into the dilute acetic acid. Hence these elements are 

all supplied in one operation. By experiment it was found that 0.5, 

per cent of sodium acetate and 0.5 per cent of copper acetate dissolved in 1 per cent acetic acid gave a reagent that worked satisfactorily on test solutions of lead nitrite from 0.01 to 1.0 per cent 

concentration and on all the lead ore minerals tested.

The lead nitrate solution is evaporated to dryness, and a drop of 

the acetate reagent is applied to the residue. Then a fragment of 

potassium nitrite is introduced into the solution. If the concentration of lead is low, the triple nitrite precipitate may not form until 

the drop is nearly dry. If none forms, a drop of 1 per cent acetic 

acid is added, then a fragment of cesium chloride.

If both lead and copper are present in the same solution, both 

can be tested for in one operation. The solution containing the lead 

and copper is evaporated to dryness. The residue is taken up in a 

drop of 1 per cent acetic acid, and successive fragments of sodium 

acetate and potassium nitrite are introduced into the drop. If the 

triple nitrite does not form, more 1 per cent acetic acid is added, 

then a fragment of cesium chloride.

All minerals containing lead tend to form the basic nitrate of lead

when evaporated down to 1:1HN03 . This salt is relatively insoluble in 1 per cent acetic acid and in the acetate reagent above described. 

The procedure recommended by the present writer is as follows: 

Take the mineral in solution by means of 1:1 HNO3 ; evaporate to 

dryness. Leach the residue with a drop of 1:7 HN03 ; transfer, 

drop to another place on the slide and slowly evaporate to dryness. 

Then add acetate reagent or 1 per cent acetic acid and proceed as 

already described. More or less antimony or bismuth if present will 

be carried over with the 1:7 HN03 solution, but these elements tend 

to hydrolyze in the weak acetate reagent and do not materially 

interfere with the test. 

Chamot 85 states:

The addition of an excessive amount of potassium nitrite is objectionable 

because of the fact that the triple nitrite is quite soluble in solutions of this 

reagent. On the other hand it is essential that the amount be very slightly in

86 Chamot, E. M., op. cit., p. 374.
DETAILS OF TESTS 147

excess of that called for by theory. It is therefore necessary to proceed somewhat cautiously. Add a tiny fragment of nitrite; then, after waiting a few 

moments, if no crystals appear, add a little more.

Nickel can 'be substituted for copper in the triple nitrite reaction. 

Before the discovery of the dimethyl glyoxime test for nickel, the 

triple nitrite test was the one relied upon for the identification of 

that element. The nickel .salt 2KN02 .Ni(NO2 ) 2 .Pb(N02 ) 2 is precipitated as colorless, yellow, and light-brown cubes very much 

smaller than those of the corresponding copper salt. The color of 

the nickel salt is also decidedly different from that of the copper 

salt. The nickel salt never forms dark-brown to black cubes, which 

are most characteristic of the copper salt.

In the writer's opinion, the triple nitrite reaction is very valuable 

and should be at the disposal of all who employ microchemical 

methods, The test requires more skill in manipulation than the

iodide test for lead. On the other hand, the triple nitrite is a stable 

compound and once formed persists after the drop has dried up, 

whereas the normal iodide of lead is transitory in character. The 

delicacy of the triple nitrite test is about equal to that of the iodide 

test. The triple nitrite works well in the presence of bismuth, in 

contrast with the iodide test, which is seriously impaired by its 

presence.

In general the writer prefers the iodide test for lead on account 

of its simplicity, but in case there is any doubt about the iodide test, 

the triple nitrite test furnishes a valuable check.

Cesiwn cJdoride. 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 

chloride 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 

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 Kl 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 bichromate 

placed in a 1 per cent HNO3 solution containing lead ions precipi' 

tates a finely divided amorphous yellow powder, which is lead bi15122 31 11
148 MICROSCOPIC DETERMINATION OF ORE MINERALS

chromate. 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 per cent HNO3 will remove only a small 

proportion of the lead. A drop of 1 : 5 HC1 is applied to the original 

residue and then evaporated off over the microflame. A drop of 

water is then applied to the residue and also evaporated off in order 

to remove the last traces of HC1. The residue is cooled and then 

leached with a drop of H2O, and the drop is transferred and tested 

with a fragment of KL

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.

All the silver minerals tested by the writer are broken down by 

1 : 1 HNO3 , 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 can not 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. 

Reagent: Solid ammonium bichromate (

Product : Silver bichromate in ruby-red triclinic crystals ; sometimes also silver

chromate in ruby-red orthorhombic crystals. 

Limit: 0.02% 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 per cent nitric acid. The drop is transferred to a different place on the slide and tested with a fragment 

of ammonium bichromate by method III. The silver separates as 

bichromate and chromate. Both compounds are dark ruby-red in
DETAILS OF TESTS 149

color. 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 7. The color of the crystals 

and their solubility in ammonia are distinctive criteria. With 

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 per cent HN03 in order to 

take 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 per 

cent of Ag. Some f reibergite specimens, however, gave good bichromate tests for silver. No analyses of these specimens were available. 

A solution containing 2.5 per cent of Cu(N03 ) 2 and 0.05 per cent of 

AgN03 gave a good silver test, but a solution containing 2.5 per cent 

of Cu(NO3 ) 2 and 0.025 per cent 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 mask the observations of the silver crystals. A solution containing 0.5 per cent of Pb(N03 ) 2 and 0.05 per cent 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
150 MICROSCOPIC DETERMINATION" OF ORE MINERALS

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 per cent 

HNO.j. 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 per cent 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 per cent of AgNO3 ) the crystals of silver 

bichromate will be very small, needlelike, but usually irregular, and 

will form at the edge of the drop. (See pi. 7, Z>.) 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 0.02 per cent AgNO3 solution in 1 per 

cent HNO3 and prove his skill by making successive tests until consistent positive results are obtained.

Chloride test. 

Reagents: 1: 5 HC1 and concentrated NH4OH.

Product: Silver chloride in minute colorless isometric crystals.

Limit: 0.02% AgNO3 .

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 more simple forms 

predominate. The cube is the most characteristic form, but the 

octahedron is frequently formed.

Some elements have a detrimental 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 HNO3 

until the drop fails to give a copper test with potassium mercuric
DETAILS OF TESTS 151

thiocyanate. Then the silver chloride should be leached with a drop 

of water to remove the nitric acid and 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 

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 per cent 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 per cent each of Cu(N03 ) 2 

and AgN03 in H2 0. 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 HNO3 and one 

of water. When dissolved in NH4OH, the forms reprecipitated were 

small, black, and skeletal. The writer can not 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 per cent of Pb(NO3 ) 2 and 0.05 per 

cent of AgN03 gave well-formed cubes. One containing 0.5 per cent 

of Pb(N03 ) 2 and 0.025 per cent of AgNO3 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. Com-
152 MICROSCOPIC DETERMINATION OF ORE MINERALS

monly 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: 5% K2Hg(CNS)4 in H2 0.

Product: Curdy white amorphous precipitate (KCNS.AgCNS).

Limit: 0.02% AgNOs.

The mineral is broken down with 1:1 HNO3 . The residue is 

leached with a drop of 1:7 HN03 , the drop transferred to another 

place on the slide and joined to a drop of potassium mercuric thioe.yanate (method I). An amorphous cloudy white precipitate forms. 

This reaction is useful in that the precipitate often appears incidentally in 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 precipitate 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. .

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. An etching test with KCN will serve to distinguish gold from chalcopyrite in case the area is too small to 

scratch.

Tellurium is the element that is most frequently found in goldbearing minerals in fact, it is almost the only element which is 

found in nature combined with gold. A selenide of gold is known 

but is exceedingly rare. There are a number of tellurides of gold.
U. S. GEOLOGICAL SURVEY 

" .. *ifA O * ( x

BULLETIN 825 PLATE 6

"I;

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r ;

't,

A

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W

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v- *

MICEOCHEMICAL TESTS FOR LEAD

A, Lead iodide crystals. Obtained from a 0.5% lead acetate solution. X 70. B, From 

same solution as A. C, Potassium-lead-copper triple nitrite crystals. Obtained from a 

0.5% lead acetate solution, x 215. D, Lead nitrate residue. Obtained from evaporating to dryness a drop of 1:1 HNOs containing galena powder. X 70.
U. S. GEOLOGICAL SURVEY BULLETIN 825 PLATE 7

MICROCHEMICAL TESTS FOR SILVER

A, Silver bichromate crystals. Obtained from a 0.5% silver nitrate solution. X 70. 

B. C, From same solution as A. D, Silver bichromate crystals. Obtained from a 0.1% 

silver nitrate solution. X 70.
V. S. GEOLOGICAL SURVEY

\

BULLETIN 825 PLATE 8 

%

I V ,/ V '

I v 1 ^X

IT* A

Halftone by Courtesy of P. C. Putnam, E. J. 

The Colorplato Engraving Co., New York, N. Y., Roberts, and D. Selchow.

CRYSTALS OF GOLD PYRID1NE DOUBLE BROMIDE 

X 250. Agfa color plate
DETAILS OF TESTS 153

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 tellurious acid and metallic gold remain. The gold 

is usually covered and obscured by the tellurious 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 No. 

3 Leitz or other objective of equivalent magnification is most satisfactory for this purpose. This suffices to identify gold jn the 

majority of tests.

Where a confirmatory test is desirable, the gold can be taken into 

solution by means of aqua r'egia. 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. 165.) A drop of aqua 

regia is then applied and the gold dissolved. The residue is gold 

chloride (HAuCl4 ) and is orange-colored and very soluble in water. 

A drop of H2 0 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.

Pyridme double bromdde test. 

Reagent: 1 volume pyridine in 9 volumes of 40% HBr. 

Product: (CUSdNEOAuBn; pleochroic crystals. 

Limit: 0.02% HAuCl4.4H2O 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 Messrs. 

Putnam, Roberts, and Selchow, who discovered and described it. 

(See p. 116.) 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. 8.)

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.
154 MICROSCOPIC DETERMINATION OP ORE MINERALS

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, Eu, Se, Sn (stannic), Te, Th, Ti, U02 , V, W, Zn, and, by 

analogy based on the periodic system, B, Li, Na, K, Eb, 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 before 

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 per cent of gold 

chloride. /

Experiments carried out on lead nitrate solutions of various con- ' 

centrations indicate that the pyridine and HBr solution will cause I 

precipitate from a 0.05 per cent Pb(NO3 ) 2 or more concentrated ! 

solution. The precipitate is not sufficiently characteristic in appear- i 

ance 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: 5% SnCla in 1:5 HC1. ' J

Product: Metallic gold (purple of Cassius). j 

Limit: 0.1% gold chloride. ]

The mineral is dissolved in 1:1 HNO3 . Usually metallic gold j 

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 precipi- ,: 

tate of finely divided metallic gold forms where the solutions come j 

into contact. This spreads out toward the periphery of the test j 

drop in a widening band. The precipitate is amorphous and ranges j 

in color from reddish to bluish tinges. Purple is the most distinc- - 

tive color (purple of Cassius). i

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 uniform-
DETAILS OP TESTS 155

ity 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.

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 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 86 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 is 

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.87 

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 per cent 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 

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 arsenatfe 

test and gives a good reaction with 0.01 per cent ammonium or

80 Chamot, B. M., op. cit, fig. 162, p. 396.

87 Belirens, H., and Kley, P. D. C., op. cit., p. 136. flg. 93.
156 MICROSCOPIC DETERMINATION" OF ORE MINERALS

sodium arsenate; it is effective in strongly acid solutions; the crystals 

obtained are yellow in color 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 HN03 .

Product: Ammonium arseni-molybdate in tiny yellow isometric crystals.

Limit: 0.01% (NH4 ) 3As04 .

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, H3AsO4. 

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: T HNO3 to the residue. Tiny 

yellow octahedra of ammonium arseni-molybdate remain if arsenic 

is present in the unknown solution. (See pi. 9, (7, D.)

These crystals are isometric. Octahedra 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 can not 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 a 

No. 4 Leitz or 8-millimeter Bausch & Lomb 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 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.
DETAILS OP TESTS 157

The writer can not 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. (Method III-A, p. 125.) 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 are 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 

per cent 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% (NEUhAsCX 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. An amorphous yellow to orange-colored precipitate forms 

if arsenic is present. The color of the arsenic precipitate resembles 

that of the copper iodide precipitate but is a darker shade of yellow. 

The color is best observed in obliquely reflected light, with a 16- 

millimeter objective.

With solutions of greater concentration than 0.10 per cent 

(NBL,) 3As04 the precipitation is immediate; with lesser concentration, the precipitate comes down at the edge of the drop a few 

seconds later.
158 MICROSCOPIC DETERMINATION OF ORE MINERALS

Under the same conditions antimony colors the drop yellow but 

gives no precipitate.

The iodide 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% (NHOsAsO, in 1:5 HC1.

The mineral is broken down by 1:1 HN03 or aqua regia. The 

residue is leached with a drop of 1: 5 HC1, 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. 161.) 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 a 

precipitate with arsenic, hence in testing for antimony it woul'd be better to add the cesium chloride before the potassium iodide. The fact 

that potassium iodide alone precipitates arsenic out of an HC1 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 HC1. 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 when KI is added and the solution 

does not turn yellow, antimony can not 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. 160).
DETAILS OF TESTS 159

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 gives no precipitate with arsenic.

CsCl alone gives a well-defined 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 microchemical 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 H2 0, 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 microchemical 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 HNO3 but unfortunately sufficiently soluble to give a 

microchemical test for antimony. Therefore nitric acid can not 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 . Sb2 05 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 

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 origi-
160 MICROSCOPIC DETERMINATION OF ORE MINERALS

nal 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 (b) the formation of the double iodide with 

cesium (Sblg.CsCl or BiIs.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. 

Reagents: Solid cesium chloride (CsCl).

Product: Antimony (or bismuth) cesium chloride (SbCl3 .3CsCl) in colorless

highly refracting plates. 

Limit: 0.02% SbCls solution; 0.02% Bids 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 III).

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. 162.) 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 

ofcalcite. (See pi. 11, G.) In addition to the rhombs, petal-shaped 

and knifeblade-shaped crystals sometimes form, more rarely hexagons 

resembling ttye corresponding antimony compound. In solutions 

containing bismuth, however, hexagons have never been observed 

alone, except with very dilute bismuth chloride solutions (0.02 per 

cent 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 

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.
DETAILS OF TESTS 161

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 present the subordinate ele-' 

ment 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% SbCls and 0.01% Bids. Antimony hexagons and formless grains. No

signs of bismuth. 

0.5% SbCla and 0.02% BiCls. Same as preceding.

0.5% SbCls and 0.03% BiCk Antimony stars, hexagons, and formless grains.

No signs of bismuth. 

0.5% SbCls and 0.06% Bids. Star-shaped crystals, some of considerable size.

These undoubtedly contain both Sb and Bi, as they are not characteristic of

either. 

0.5% SbCla and 0.12% BiCls. Beautiful star-shaped crystals with branching

arms. 

0.5% SbCls and 0.25% BiCl3 . Same as preceding; also some large white

hexagons with ribs.

0.5% SbCls and 0.5% BiCla. Complex star-shaped and treelike crystals. 

0.25% SbCls and 0.5% BiCls. Same as preceding; also characteristic bismuth

rhombs. 

0.12% SbCls and 0.5% BiCls. Rhombs very abundant; stars less so. Many of

the crystals are seedlike, irregular grains. 

0.06% SbCls and 0.5% Bi'Cls 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% SbCls and 0.5% BiCls. Rhombs greatly predominate, but "seeds" also

abundant. 

0.015% SbCls and 0.5% Bids. 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 

himself with the appearance of the mixed crystals.

Double iodide test. 

Reagent: Solid potassium iodide (KI) and solid cesium iodide (CsCl). 

Product: Antimony (or bismuth) cesium double iodide in orange-colored to

red hexagons and stars. 

Limit: 0.01% SbCls solution; 0.005% BiCls solution.

The mineral is broken by placing it in a drop of 1:1 HNO3 

and evaporating to dryness. The residue is leached in a drop of 

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 per cent or
162 MICROSCOPIC DETERMINATION. OF ORE MINERALS

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 per cent 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 oe 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 forms 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 H2SO4 , 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 apt 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: T HN03 . Nearly all the arsenic is removed by 

this treatment, also nearly all the copper and other more basic minerals. Nevertheless it is very difficult to remove the final traces of 

arsenic.

Compounds containing both arsenic and bismuth are rare in nature 

or lacking altogether.
DETAILS OF TESTS 163

The double iodide test was performed on solutions of SbCl 3 in 1 : 5 

HC1 and BiCl3 in 1 : 5 HC1 mixed in various proportions. The folresults were obtained :

0.5% SbCls and 0.01% BiCh. The usual orange-colored hexagons and stars

characteristic of Sb ; no sign of Bi. 

0.5% SbCls and 0.02% BiCla. Orange-colored hexagons predominate, but a few

red hexagons are also present. 

0.5% SbCls and 0.03% BiCla. 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% SbCls and 0.06% Bids. Same as preceding. Orange-colored hexagons

predominate over red. 

0.5% SbCU and 0.12% BiCls. Same as preceding, but red hexagons more

numerous. 

0.5% SbCh and 0.25% Bids. 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 following 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 antimony . 

cesium double iodide. Means for distinguishing between the arsenic 

and antimony compounds are discussed on page 159.

TIN

Cesium chloride test. 

Reagent: Solid cesium chloride.

Product: Cesium chlorostannate (Cs2SnClo), in colorless highly refracting

octahedra. 

Limit: 0.01% SnCh.

The test for tin is similar to the cesium chloride test for antimony. 

The mineral is broken clown by 1 : 1 HN03 . The residue is leached

15122-^31 12
r

~

164 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 

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 III). A 

precipitate of colorless highly refracting octahedra proves tin. (See 

pl.ll,Z>.)

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 octahedra. 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 H2O2 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 octahedra 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.88 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 tliat the individual 

crystal forms can not be recognized. This indicates that the tin was 

not completely oxidized to the stannic condition. This is remedied 

by placing 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

88 The writer differs from Chamot in this regard.
DETAILS OF TESTS . 165

iodide, and tin forms colorless octahedra side by side with the antimony hexagons. (See detailed description of tests for franckeite 

and cylindrite, pp. 187,188.)

The textbooks of Chamot 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 octahedra 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 octahedra are sometimes almost spectacular in appearance. All 

the tin sulpho salts occurring in nature are readily broken down by 

HN03 . Cassiterite (Sn02 ) 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.89 

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 -Na2CO;J 

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 sulpho salts.

TELLURIUM

Cesium chloride test. 

Reagent: Solid cesium chloride (CsCl). 

Product: Cesium chlorotellurite (Cs2TeClo). 

Limit: 0.01% Te in 1.5 HC1.

The mineral is broken down by 1:1 HNO3 . 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 III. 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 octahedra predominate in the precipitate. With lower concentrations pseudohexagonal plates and triangles are more common. These show no double refraction and are

89 Chamot, B. M., op. cit, fig. 154, p. 294.
166 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 tellurious acid (HaTe03 ) forms instead of the normal compound H2Te03 .nH2 0. 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 per cent 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 

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 (seep. 167); a brick-red precipitate proves selenium.

All the amorphous precipitates described here are best observed 

in obliquely reflected light, with a 16-millimeter objective.
DETAILS OF TESTS ' 167

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 173-179.

SELENIUM

Stannous chloride test. 

Reagent: 5% SnClz solution in 1: 5 HCl. 

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 can not be 

taken into solution with nitric acid or aqua regia. With some compounds, such as clausthalite, some metallic selenium is precipitated, 

but most of it goes into solution as selenious acid (H2Se03 ).

The usual procedure is as follows: Break down mineral with 1:1 

HN03 and leach residue with a drop of 1:5 HCl. A red tinge in 

the residue indicates selenium. To confirm it, transfer the filtrate 

and merge with it a drop of 5% 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 (SeL), a reddish-brown powder.

Limit: 0.02% Se02 in ILO.

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 H2Se03 , which remains as a white powder when the solution is evaporated. The residue from evaporation is leached with a drop of 1:5 HCl, 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. 166.) The color is very similar to that of the corresponding Te precipitate but is a lighter shade of brown.
168 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 in solution with 1:5 HC1. 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 HC1. 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.

MANGANESE

Sodiwn ~bisirwthate test. 

Reagent: Solid sodium bismuthate (NaBiOs).

Product: Sodium permanganate (NaMnO*), which colors solution pink to

purple. 

Limit: 0.02% MnCl2 .4H2O 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 .4H2 0, 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 

can not be determined, owing to their small size.

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.
U. S. GEOLOGICAL SURVEY BULLETIN 825 PLATE 9

D

MICROCHEMICAL TESTS FOR ARSENIC AND TELLURIUM 

A, Cesium chloro-tellurite crystals. Obtained from a 1:5 HC1 solution containing 1% 

Te. X 70. B, From same solution as A. C, Ammonium arseno-molybdate crystals. 

Obtained from a 0.5% ammonium arsenate solution in 1:7 HNOa. X 215. D, From 

same solution as C.
U. S. GEOLOGICAL SURVEY BULLETIN 825 PLATE 10

MICROCIIEMICAL TESTS FOR ANTIMONY

v4, B, C, D, Antimony-cesium iodide crystals obtained from a 0.5% antimony chloride 

solution in 1:5 HC1. X 130.
U. S. GEOLOGICAL SURVEY BULLETIN 825 PLATE 11

, '. '

L+

 

O

, o

O

' «> 

(j c- o

D a

D

MICROCHEMICAL TESTS FOR BISMUTH AND TIN

A, B, Bismuth-cesium iodide crystals. X 250. C, Bismuth-cesium chloride crystals. 

X 75. D, Tin-rubidium chloride crystals. X 145.
DETAILS OF TESTS 169

It is advisable to use a fairly large quantity of sodium bismuthate, 

much more than is usual in method III. 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 hiibnerite (MnW04 ) and wolframite 

((Fe,Mn)WO4 ), can not be taken into solution by nitric or hydrochloric acids or by aqua regia. It is obvious that the sodium 

bismuthate test is not applicable to these minerals, and the sodium 

carbonate bead test used in blowpipe analysis is the best one to use.

SULPHUR

Reagent: Solid calcium acetate

Product: Calcium sulphate (gypsum) crystals.

Limit: 0.15% H2SO<.

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 (C6H6 ), 

and a number of other organic solvents and can be recrystallized 

-from these reagents, thus forming 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 benzene 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
170 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 HNO3 , fuming HN03 , and aqua regia. It is 

possible to take this sulphur into solution by treatment in 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.

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 sulpho salts 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 H2SO4 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.
DETAILS OF TESTS 171

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 sulphite 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 octahedra, 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 per cent 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 the center of the prisms. The limit of possible dilution with 

this test is about 0.02 per cent 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 can not 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
172 MICROSCOPIC DETERMINATION OF ORE MINERALS

or weak acid solutions the product is gypsum (CaSO4 .2H2O); from 

strong acid solutions anhydrite (CaSO4 ) 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. 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 per cent or stronger HC1 solution. The best gypsum crystals are obtained with as weak an acid solution as can be 

used. Consequently, the 5 per cent HC1 solution is recommended 

for the sulphate test.

Aqua regia is the most effective reagent for oxidizing sulphur. 

The mineral is broken 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.

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:
A SYSTEMATIC SCHEME OF MICROCHEMICAL ANALYSIS 173

/

Break down the* mineral with two successive drops of aqua regia, 

evaporating each to dryness. Leach the residue with a drop of 5 

per cent 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 per cent 

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 per cent 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 lollingite (FeAs2 ) or between chloanthite 

(NiAs2 ) and gersdorffite (NiAsS). Earthy minerals containing sulphur, such as hauerite (MnS2 ), should be tested for sulphur in this 

manner.

A SYSTEMATIC SCHEME OF MICROCHEMICAL

ANALYSIS 90

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. The opaque minerals are easily identified under the reflecting microscope by their color in vertically reflected light. This is discussed more fully on pages 30 and 34.

As already stated (p. 2), 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

80 This scheme is based on one originated by the writer's former students, Drs. H. J. 

Fraser and H. A. Powers.
174 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 35 

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 cassi'terite, tungstates, and rare-earth 

minerals.

B. Readily soluble in 1:1 HN03 or aqua regia. This group consists mostly of the nickel-cobalt-iron arsenides, antimonides, sulpharsenides, and sulphantimonides; also pyrite and marcasite.

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 mineral, 

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
A SYSTEMATIC SCHEME OP MICEOCHEMICAL ANALYSIS 175

solution with a drop of 1:7 HN03 and tested for iron with a fragmentof K4Fe(CN) 6 .

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 HN03 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, lollingite, gersdorffite, skutterudite, rammelsbergite, 

smaltite, kallilite, willyamite, hauchecornite, safflorite, or ullmanite. 

Arsenopyrite and lollingite can not be told apart by etching, polarization, or microchemical tests. They are slightly different in color, 

however, arsenopyrite being galena-white and lollingite ashy gray. 

Arsenopyrite tends to be associated with pyrrhotite, sphalerite, 

galena, and chalcopyrite; and lollingite 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.

Tests for all the elements of this group can be made from the 

above-mentioned residue. Leach the residue with 1 per cent HN03 .

1. Iron, nickel, cobalt, and arsenic go into the solution. Transfer 

1 per cent HNO3 drop and evaporate almost to dryness. Add drop 

of ammonium molybdate solution and heat very slowly till dry. Add 

drop of 1:7 HNO3 , which dissolves the excess of ammonium molybdate, leaving minute yellow octahedra 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
176 MICROSCOPIC DETERMINATION OF ORE MINERALS

indicates bismuth. Leach original residue with at least three successive drops of 1 per cent. HN03 to remove remaining traces of 

arsenic, then a drop of H2O 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 forms, 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 G. 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 HN03 , 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 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.

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 

orange-colored, 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 per cent 

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;
A SYSTEMATIC SCHEME OF MICBOCHEMICAL ANALYSIS 177

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 H2 0 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 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; tellurium or selenium 

will come down as a fine-grained amorphous chocolate-brown powder ; if antimony or bismuth is present in appreciable amount (0.1 per

cent 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. 167.) 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 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 per cent HN03 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.
178 MICROSCOPIC DETERMINATION OF ORE MINERALS

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 KI. 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 tte 

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; 158. The precipitate is 

best observed in oblique reflected light. Antimony will form orangecolored hexagons; bismuth will form red hexagons; tin will form 

colorless octahedra.

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. Volatilize the HC1 by adding a drop of water 

to the residue and evaporating to dryness. Cool and add a drop of 

water to the residue. Transfer the drop to another place on the slide 

and add a fragment of KI. 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, evaporate to dryness over the microflame. Gold is converted to an orangecolored 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
SPECIFIC TESTS FOB MINERALS 179

present, the HgCl2 residue will usually show a lattice pattern. (See 

pi. 5, 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 

I per cent 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. Mercury 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.

SPECIFIC TESTS FOR MINERALS

The systematic scheme of microchemical analysis described above 

is designed to identify in as few operations as possible all 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 outlined for any particular element is not necessarily the best test for that element. For instance, the tests with 

potassium mercuric thiocyanate for both silver and nickel are given 

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 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 (Se,S)). Decompose mineral with 1:1 HNO8. Usually 

several applications of acid are necessary1. ' The residue is gelatinous and may 

be tinged yellow and reddish. Leach residue with drop of 1 per cent HNQs, 

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 

15122 31 13
180 MICROSCOPIC DETERMINATION OF ORE MINERALS

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. 195.)

Aikinite (Cu2S.2PbS.Bi2S3). Decompose mineral with one or two applications of 1:1 HNO3. Aikinite is very easily attacked by HN08 . The residue is 

white and curdy. Carefully examine residue for lattice structure (lead nitrate). Leach residue with drop of 1 per cent HN03 , transfer drop, and evaporate to dryness. To the residue from the evaporation add a drop of 1 per cent 

acetic acid; add fragment of sodium acetate and potassium nitrite to the drop. 

If black cubes of the triple nitrite do not form at once, add fragment of cesium 

chloride. Black and brown cubes prove lead and copper. Some of the bismuth 

was carried over in the nitric acid solution; most remains in the original 

residue. Leach this with a drop of 1:5 HC1. Transfer drop and add fragment of KI. The solution near the fragment is colored yellow (Sb or Bi). 

Then add fragment of CsCl. Red hexagons prove bismuth.

Alabandite (MnS). Take mineral into solution with 1:1 HN03 . Evaporate, cool, and leach residue with a drop of 1: 7 HN03. Add fragment of sodium 

bismuthate. Solutions turn pink or purple, proving manganese.

AlasJcaite (Ag2 S.PbS.2Bi2S3 ). Decompose mineral with 1:1 HNO3 . Leach 

residue with drop of 1 per cent HNOs. Transfer drop and add fragment of 

ammonium bichromate. A yellow amorphous precipitate indicates lead, and 

red crystals indicate silver. The two precipitates do not mask each other.

The original residue contains bismuth and most of the lead. Add drop of 

1:5 HC1, 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 HC1 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 HC1 and Bi. The second or third drop will yield yellow 

hexagons, which prove lead.

Algodonite (CuoAs). Take mineral into solution with 1:1 HNO.% Residue 

is gelatinous with tinge of green. Leach residue with drop of 1:7 UNO*,

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 till dry. Add drop of 1: 7 HNO.i 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 yellowish-green precipitate proves copper.

Altaite (PbTe). Decompose mineral with 1:1 HNO3 . Examine residue for 

lattice structure. Leach residue with drop of 1:5 HC1. Transfer drop and 

add fragment of CsCl. Honey-yellow isometric crystals prove tellurium. Add 

to original residue a drop of water and evaporate drop over flame to remove 

HC1. Cool and again add drop of water to original residue and transfer drop. 

Add to it a fragment of KI. Yellow hexagons prove lead.

Andorite (Ag2S.2PbS.3Sb2S3 ). 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 the tests. Leach residue with a drop of 1: 5 HC1, transfer 

drop, and test for Sb by mears of KI and CsCl. Orange-colored hexagons 

prove antimony. Leach original residue with two additional drops of HC1 to
 SPECIFIC TESTS FOR MINERALS 181

remove remaining traces of Sb and reject drops. Then leach original residue 

with three successive drops of H2 0 and add to each a small fragment of KI. 

Yellow glistening hexagons prove lead. Finally leach original residue with 

drop of concentrated NHiOH. Observe edge of drop with a 4-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.

Antimony (Sb). Decompose mineral with 1:1 HN03. Kesidue 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. 

Orange-colored hexagons and stars prove antimony.

Aramayoite (Ag2S.(Sb,Bi) 2Sa). Decompose mineral with 1:1 HNOa. Leach 

residue with drop of 1 per cent HNOa, 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 HNOa. Leach residue with 

1 per cent HNOa, transfer drop, and add fragment of ammonium bichromate. 

Red crystals prove silver.

Argyrodite (4Ag2S.GeS2 ). Argyrodite is very resistant to attack by 1:1 HNOa, 

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 

per cent HNOa, transfer drop, and add fragment of ammonium bichromate. 

Red crystals prove silver. No good test for germanium is available.

Arsenargentite (AgaAs). Take mineral into solution with 1:1 HNOa. Leach 

residue with drop of 1 per cent HNOa, transfer drop, and add fragment of 

nmmonium bichromate. Red crystals prove silver. Take more mineral into 

solution with 1:1 HNOs. Add drop of 1:5 HC1 to residue to prec'pitate the 

silver. Evaporate to dryness, cool, and add drop of 1:7 HNOa to residue. 

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 HNOa to residue. Tiny yellow 

isometric crystals prove arsenic.

Arsenic (As). Mineral goes completely into solution in 1:1 HNOa, leaving 

no residue. 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 HNOa. ,Tiny yellow isometric crystals prove 

jirsenic.

Arsenoferrite (FeAsa ). Test identical with that for arsenopyrite.

Arsenopyrite (FeAsS). Decompose mineral with 1:1 HNOa.- The residue 

usually has a tinge of yellow or orange. Leach residue with drop of 1: 7 HNOa, 

transfer drop, and evaporate slowly by pass'ng 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 HNOa 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.

BaumhauerUe (4PbS.3As2 S3 ). Decompose mineral with 1:1 HNOa. Leach 

residue with drop of 1:5 HC1, transfer drop, and evaporate almost to dry-
182 MICROSCOPIC DETERMINATION OF OEE MINERALS

ness. Leach residue from the evaporation with drop of ammonium molybdate 

solution. Evaporate slowly to dryness. Cool and add drop of 1:7 HNOa. 

Tiny yellow isometric crystals prove arsenic. Leach original residue with 

three successive drops of H20, transfer drops, and add to each a fragment of 

KI. Yellow glisten'ng hexagonal plates prove lead.

Benjwvinite ((Cu,Ag) 2S.2PbS.2Bi2S3 ). Decompose mineral with 1:1 HNOs. 

Leach residue with drop of 1 per cent HNOs, 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 HC1, transfer drop, and add 

fragments of KI and CsCl. Red hexagonal plates prove bismuth. Leach 

original residue with two successive drops of 1:5 HC1 to remove most of the 

remaining bismuth; reject both drops, then leach original residue with three 

successive drops of H2O. Add to each a fragment of KI. The first drop may 

not give a satisfactory test, owing to presence of remaining traces of HC1. 

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.

BertMerite (FeS.Sb2S3 ). Decompose mineral with 1:1 HNOs. Leach residue with drop of 1:7 HNOs, transfer drop, and add fragment of K4Fe(CN) e. 

A blue amorphous precipitate proves iron. Leach original residue with drop of 

1: 5 HC1, 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 HNOs. Leach 

residue with drop of 1: 7 HNOs, transfer drop, and evaporate to dryness. To 

residue from this evaporation add drop of 1 per cent acetic acid. Add successive fragments of sodium acetate and potassium nitrite to the acetic acid 

solution. If black and brown cubes do not come down at once add fragment 

of CsCl. The formation of the cubes proves the presence of both lead and 

copper. Leach original residue with two additional drops of HNO3 to remove 

all the copper before testing for Sb. Both drops are rejected. Dry original 

residue and add to it a drop of 1:5 HC1. Transfer drop and add fragment of

KI. Solution is colored yellow near fragment. Then add fragment of CsCl.

Orange-colored hexagons and stars prove antimony.

Bismuth (Bi). Dissolve mineral in 1:1 HNOs. All goes into solution. 

Evaporation leaves white curdy residue. Take this into solution with 1: 5 HC1. 

Add fragment of KI. Solution turns vivid yellow. Then add fragment of 

CsCl. Red hexagons prove bismuth.

Bismuthinite (Bi2S3 ). Decompose mineral with 1:1 HNOs. Add drop of 

1:5 HC1 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.

Bismutoplagionite (5PbS.4Bi2S3 ). Decompose mineral with 1:1 HNO3 . 

Leach residue with" drop of 1:5 HC1, 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 HC1 to remove most of the remaining bismuth; 

reject both drops. Then leach original residue with three successive drops 

of H2O. Add to each a fragment of KI. The first drop may not give a satisfactory lead test, owing to remaining traces of HC1. 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.
SPECIFIC TESTS FOR MINERALS 183

Bornite (CunFeSO. Decompose mineral with 1:1 HNOa. Leach residue 

with 1:7 HNOa. Transfer drop and merge with it a drop of potassium mercuric thiocyanate solution (method I). Part of the drop turns pink, indicating ir.on. A greenish-yellow crystalline precipitate indicates copper. The iron 

test may be faint unless a relatively large amount of mineral is used. A 

microcheinical test for bornite is rarely needed.

Boulangerite (5PbS.2Sb2Ss). 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. Orangecolored hexagons and stars prove antimony. Add drop of water to original 

residue and evaporate over flame to remove remaining traces of HC1. Cool 

arid again add drop of water to original residue. Transfer drop and add 

fragment of KI. Glistening yellow hexagons prove lead.

Bownomte (CuaS.2PbS.Sb2S3 ). Decompose mineral with 1:1 HNOs. Examine residue for lead nitrate lattice. Leach residue with a drop of 1: 7 HNOa. 

Transfer drop and evaporate to dryness. To the residue from tills evaporation 

add drop of 1 per cent acetic acid. Then add successive fragments of sodium 

acetate and potassium nitrite to drop. If brown and black cubes of the triple 

nitrite do not form at once, add a fragment of CsCl. Cubic crystals of the 

triple nitrite prove lead and copper. Leach original residue with two additional drops of HNOs, followed by one of water to remove all the copper before 

testing for antimony. Reject all three drops. Add to original residue a drop 

of 1:5 HC1. Transfer drop and add fragment of KI. Solution is colored 

yellow near the fragment as it dissolves. Add fragment of CsCl. Orangecolored hexagons and stars prove antimony.

Braunite (3Mn2 03.MnSi02 ). 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 HNOj. Leach residue 

with 1: 7 HNOs, transfer drop, and evaporate to dryness. To residue from 

this evaporation add directly a drop of 1 per cent NH4OH. A gelatinous yellow 

or orange-colored precipitate proves iron. Allow drop to evaporate slowly in 

the air. When evaporation is almost complete but the residue is still moist 

add drop of dimethyl glyoxime solution (method III-B, p. 125). . A pink precipitate proves nickel.

Breithduptite (NiSb). Decompose mineral with 1:1 HNOa . Leach residue 

with 1: 7 HNOs, transfer drop, and evaporate to dryness. Add drop of dimethyl 

glyoxime solution directly to the residue from this evaporation. A pink 

precipitate proves nickel. Leach original residue with a drop of 1:5 HC1, 

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.

Brongniardite (AgsS.PbS.StbSs). Decompose mineral with 1:1 HNOs. Leach 

residue with drop of 1 per cent HNOs. Transfer drop and add a fragment of 

ammonium bichromate. Red crystals, usually needlelike, prove silver, and an 

amorphous yellow precipitate proves lead. Leach original residue with a drop 

of 1:5 HC1. 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 HaO to the original residue, transferring drop, and adding 

a fragment of KI. Yellow glistening, hexagonal plates prove lead. . . .
184 MICROSCOPIC DETERMINATION OF ORE MINERALS

Calaverite ((Au,Ag)Te?). 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 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 it. 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, 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 HC1. Then add to original 

residue a drop of concentrated NILOH. Observe with an 8-millimeter objective, 

in a few seconds tiny colorless isometric crystals of AgCl will be seen floating 

in the solution.

CanfielflMe (4Ag2S.SnS2, with some germanium). 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 octahedra prove tin. Germanium is not tested for. Add drop of 

water to original residue and evaporate to dryness to remove last traces of 

HCl. Then add to original residue a drop of concentrated NH<OH. 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)S.CosSa). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1:7 HNOa, transfer drop, and merge with it a drop of 

potassium mercuric thiocyanate solution. Copper precipitates as yellowishgreen prisms and mosslike clumps, and cobalt precipitates separately as indigoblue prisms.

Cassiterite (SnOz). 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 

octahedra prove tin.

Cerargyrite (AgCl). Place fragment of mineral in a drop of concentrated 

NHiOH. 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 HNOa. One application suffices. Leach residue with drop of 1:7 HNOa, 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)O.2MnO2.2HsO). -Take mineral into solution with 

aqua regia. Leach residue with 1:7 HNOs. 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 the greater 

majority of the crystals are arrow-shaped or short prisms with ragged ends. 

Take fresh material into solution with aqua regia, leach residue with drop
SPECIFIC TESTS FOR MINERALS 185

of 1:7 HNOa, transfer drop, and add solid sodium bismuthate fragments. Solution turns purple near fragments, proving manganese.

Chalcopyrite (CuFeS3 ). 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 needlelike prisms prove 

copper. The test drop turns reddish, proving iron.

Chalcostibite (Cu2S.Sb2S3 ). Decompose mineral by means of 1:1 HNO3 . 

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. Leach residue with two successive drops of 1:7 

HNOs to remove copper; reject both drops. Then leach residue with a drop of 

1: 5 HC1, transfer drop, and add fragment of CsCl. The solution near the 

fragment turns yellow as the KI dissolves. Then add' fragment of CsCI. 

Orange-colored hexagons and stars prove antimony.

Chloanthite (NiAs2 ). 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 HN03 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 

dimethyl glyoxime solution directly to the spherulites. 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.

Chromite (FeO.Cr203 ). The powder is very resistant to acids, but enough of 

it can be taken into solution for an iron test by placing powder in aqua regia 

and evaporating to dryness. Usually about 10 successive drops will suffice. 

Leach residue with drop of 1:7 HNO3, transfer drop, and add fragment of 

K4Fe(CN)«. A blue amorphous precipitate proves iron. There is no satisfactory 

test for chromium.

Cinnabar (HgS). Decompose mineral by means of aqua regia. Examine 

residue for mercuric chloride lattice structure. Leach residue with drop of 

1 per cent HN03, transfer drop, and add fragment of Co(NOs) s. 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 HNO3 . 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 stannnous chloride solution. A brick-red amorphous precipitate proves 

selenium. Add a drop of water to original residue and evaporate over microflame to remove traces of HCl. Cool, then add another drop of water to original 

residue. Transfer drop and add to it a fragment of KI. Glistening yellow 

hexagons prove lead.

Cobaltite (CoAsS). Decompose mineral by means of 1:1 HN03 . Leach residue with drop of 1:7 HNOs, 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.
186 MICROSCOPIC DETERMINATION OF ORE MINERALS

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.

Coloradoite (HgTe). Decompose mineral by means of aqua regia. Leach 

residue with drop of 1 per cent HNOs, 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 HNOs. 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 (NiAsS, with Sb; antimonial gersdorffite). Decompose mineral 

with 1:1 HNOs. Leach residue with 1: 7 HNO3 , transfer drop, and evaporate 

slowly. When nearly dry add drop of ammon um 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 HaO 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.Bi2S3 ). 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 HC1, rejecting both. This will remove most of the remaining 

trace of bismuth. Then add three successive drops of water to or'ginal 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 lead 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.

Cubanite (CuS.2FeS). Decompose mineral by several applications of 1:1 

HNO». Residue is orange-colored or yellow. Leach residue with drop of 

1:7 HNO«, 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 (Cu20). Dissolve 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.

Cuprodescloizite (Pb2(Zn,Cu)2V2Oo.H20). Dissolve mineral with 1:1 HNOs. 

Leach residue with drop of 1 per cent HNOs, 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
SPECIFIC TESTS FOR MINERALS 187

should appear. (See description of zinc-copper isomorphous precipitate on 

p. 130.) Two specimens of cuprodescloizite from different localities tested by 

the writer gave negative zinc tests. Leach original residue with drop of 

1:5 HC1, remove drop from residue, and reject it. Add drop of water to 

original residue and evaporate to dryness over microflame to volatilize remaining trace of HC1, cool, leach original residue with drop of water, transfer drop, 

and add fragment of KI. Yellow hexagons prove lead. Vanadium not tested 

for by writer.

Cylindrite (GPbS.SbzSs.GSnSa). Decompose mineral with 1:1 HNOs. 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 octahedra form, proving antimony and tin. Leach original residue with 

drop of water and evaporate over microflame in order to volatilize remainder 

of HC1. Cool, leach original res due with drop of water, transfer drop, and 

add fragment of KI. Yellow hexagons prove lead.

Delafossite (CuFeOz). Dissolve mineral in 1:1 HNOs. Leach residue with 

drop of 1:7 HNOa, 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 (CuaAs). Take mineral into solution with 1:1 HNOs. 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.As2S8 ). Decompose mineral with 1:1 HN03 . Leach 

residue with drop of 1:5 HC1, 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 HZO, transfer drops, and add to each a fragment of KI. 

Yellow glistening hexagonal plates prove lead.

Dyscrasite (Ag3Sb). Decompose mineral with 1:1 HNOs. Leach residue 

with drop of 1 per cent HNOa, 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 HNOa. Leach 

residue with drop of 1 per cent HNOs, 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 dryness. Leach residue with drop of water, transfer drop, and 

merge with it a drop of pyridine-HBr solution. Strongly pleochroic red prisms 

prove gold.

Emplectite (CuaS.BUS»). Decompose mineral with 1:1 HNOS. Leach residue 

with drop of 1 per cent 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 

per cent 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
188 MICROSCOPIC DETERMINATION OF ORE MINERALS

with drop of 1:1 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.As2 S3 ). Decompose mineral with 1:1 HN03 . Leach 

residue with 1:7 HNOs, transfer drop, and evaporate slowly over microflame. 

When nearly dry add drop of ammonium molybclate 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.

Eucairite (Cu2Se.Ag2Se). Decompose mineral with 1:1 HNO3 . Leach residue 

with drop of 1 per cent HNOs, 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 HC1, 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.

Famatimte (Cu2 S.4CuS.As2 S3 ). Decompose mineral with 1:1 HNO3 . Leach 

residue with drop of . 1:7 HNOs, 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 HNO3 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 

arseni-molybdate precipitate. Transfer filtrate and test it for copper with 

potassium mercuric thiocyanate. Leach original residue with two drops of 1 

per cent HNOs and one of water to remove final traces of copper. Then leach 

original residue with drop of 1: 5 HC1, 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 cesiumarsenic iodide precipitate.

Franckeite (5PbS.Sb2 S3 .2SnS2 ). 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 octahedra form, proving antimony and tin. Leach original residue with 

drop of water and evaporate over microflame in order to volatilize remainder of 

HC1. Cool, leach original residue with drop of Avater, transfer drop, and add 

fragment of KI. Yellow hexagons prove lead.

FranJclinite ((Fe,Zn,Mn)0.(Fe,Mn) 2 03 ). Take mineral into solution with 

repeated applications of aqua regia. Leach residue with drop of 1:7 HNOs, 

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 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
SPECIFIC TESTS FOE MINERALS 189

with a drop of 1:7 HN03 , 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 

HNOa. Leach residue with 1 per cent HNOs, 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 two successive drops of 1:7 HNOs and one of 

water to remove final traces of copper. Then leach original residue with drop 

of 1: 5 HC1, 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. Orangecolored hexagons and stars prove antimony.

Freieslebenite (3Ag2S.4PbS.3Sb-jS3 ). Decompose mineral with 1:1 HN03 . 

Leadl residue With drop of 1 per cent HNOs. Transfer drop and add fragment 

of ammonium bichromate, lied crystals, usually needle-shaped, pvove silver, 

and an amorphous yellow precipitate proves lead. Leach original residue with 

a drop of 1:5 HC1. Transfer drop and add fragment of KI. Solution turns 

yellow near fragment. Then add fragment of CsCl to opposite side of drop. 

Orange-colored hexagonal plates prove antimony. If necessary lead can be 

confirmed by adding a drop of ELO to the original residue, transferring drop, 

and adding a fragment of KI. Yellow glistening hexagonal plates prove lead.

Galena (PbS). Decompose mineral with 1:1 HNOa. Examine residue for 

lead nitrate lattice structure. Add drop of 1: 5 HC1 to residue. Remove HC1 

drop. Add drop of water to original residue and evaporate it to dryness in 

order to volatilize remaining trace of HC1. Cool, add drop of water to original 

residue, transfer drop, and add to it a fragment of KI. Yellow hexagons prove 

lead.

OalenoMsmutite (PbS.Bi-Ss). Decompose mineral with 1:1. HNOa. 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 fragmeni 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.

Oeocronite (5PbS.Sb2S3). Decompose mineral with 1:1 HNOa. 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. Add drop of water to original 

residue and evaporate over flame to remove remaining traces of HCl. Cool 

and again add drop of water to original residue. Transfer drop and add fragment of KI. Glistening yellow hexagons prove lead.

Oermanite (Cu*GeSy). 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. There is no satisfactory 

microchemical test for germanium, an element allied chemically to.silicon,
190 MICROSCOPIC DETERMINATION OF ORE MINERALS

Ocrsdorfflte (NiAsS). Decompose mineral with 1:1 HNO3. Leach residue 

with 1:7 HNO3, transfer drop, and slowly evaporate over microflame. When 

nearly dry, add a drop of ammonium molybdate and again slowly evaporate 

over microflame to dryness. 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 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 a drop of the reagent directly to the dried residue from the 

thiocyanate test.

Glaucodot ((Co.Fe)AsS). Decompose mineral with 1:1 HNOa. Leach 

residue with 1 :7 HNOa, 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 HNOs 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. It is best shown in strong oblique illumination.

Goethite (Fe203.H2O). Take mineral into solution in aqua regia. Repeated 

applications are necessary, as the mineral is resistant to chemical attack. Leach 

residue with a drop of 1: 5 HC1 and add K4Fe(CN) e fragments. A blue amorphous powder proves iron. <

Gold (An). The color and sectility of gold are usually sufficient to identify 

it in polished section. If desired, however, a microchemical test can be made1 

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 HNOs. 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 HNOs. The residue is leached with a drop 

of 1:5 HCl, and the drop is transferred and merged with a drop of stannous 

chloride solution (method I). A brick-red powder proves selenium.

Guitermanite (SPbS.AszSs). 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 HNOs to redjssolve excess of ammonium 

molybdate. Tiny yellow isometric crystals prove arsenic. Add drop of water 

to original residue and evaporate to dryness over microflame to volatilize remaining trace of HCl. Cool and add drop of water to original residue, transfer 

drop, and add fragment of KI. Yellow hexagons prove lead. The crystals 

may be somewhat distorted, owing to the presence of arsenic.

Hauerite (MnSa). Take mineral into solution in aqua regia. Take up residue in drop of 1:7 HNOa and add fragments of sodium bismuthate. Solution 

turns pink or purple near fragments, proving manganese.

Hausmannite (Mn304). Take mineral into solution in aqua regia. Leach 

residue with a drop of 1:7 HNO3 , transfer drop, and add fragments of sodium 

bismuthate. Solution turns pink or purple near fragments, proving manganese.
SPECIFIC TESTS FOR MINERALS 191

Hematite (FeaOa). 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 HC1, transfer drop, and 

add fragment of K4Fe(CN)«. A blue amorphous precipitate proves iron.

Hessite (Ag2Te). Decompose mineral with 1:1 HNOa. Leach residue with 

drop of 1 per cent HN03 , transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Leach original residue with drop of 

1:5 HC1. 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.

Jamesonite (4PbS.FeS.3SbjS3 ). Decompose mineral with 1:1 HNOa. Leach 

residue with drop of 1 per cent HNQ3 , 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 drop of water and evaporate to

dryness to volatilize the remaining trace of HC1. Cool again, add a drop of 

water to original residue, transfer drop, and add fragment of KI. Yellow 

hexagons prove lead.

Jordanite (4PbS.AsjS3 ). Decompose mineral with 1:1 HNOs. 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 HNOa to redissolve excess of ammonium 

raolybdate. Tiny yellow isometric crystals prove arsenic. Add drop of water to 

original residue and evaporate to dryness over microflame to volatilize remaining 

trace of HCl. Cool and add drop of water to original residue, transfer drop, and 

add fragment of KI. Yellow hexagons prove, lead. The crystals may be somewhat distorted, owing to the presence of arsenic.

Kallilite (Ni(Sb.Bi)S). Decompose mineral with 1:1 HN03 . Leach residue 

with 1 per cent HN03, transfer drop, evaporate to dryness, and add drop of 

dimethyl glyoxime solution. .A pink precipitate proves nickel. Leach original 

residue with drop of 1:6 HCl, transfer drop, and add fragment of KI. Solution turns yellow. Add fragment of CsCl. Bed hexagons prove bismuth. 

Leach original residue with another drop of 1:5 HCl and again repeat cesium 

double iodide test. Both red and orange-colored hexagons will form, proving 

that antimony is also present.

Keeleyite (PbS.Sb2Ss). Decompose mineral with 1:1 HN03. 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. Add drop of water to original 

residue and evaporate over flame to remove remaining traces of HCl. Cool 

and again add drop of water to original residue. Transfer drop and add 

fragment of KI.. Glistening yellow hexagons prove lead.

Kermesite (Sb2 S2 0). 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 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.
192 MICROSCOPIC DETERMINATION" OF ORE MINERALS

Klaprothite (3Cu2S.2Bi2S3 ). Decompose mineral with 1:1 HN03. Leach 

residue with drop of 1 per cent HNO3 , transfer drop, and merge with it a 

drop of potassium mercuric thiccyanate solution. Greenish-yellow mosslike 

clumps and prisms prove copper. Leach original residue with two additional 

drops of 1 per cent HN03 and one of water in order to remove as much of the 

remaining traces of copper as pos;ible. Ileject 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 be precipitated as a lightyellow 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). T.ike mineral into solution in 1:1 HN03 . Leach 

residue with a drop of 1 per cent HN03, transfer drop, and test for copper 

by merging with a drop of potassium mercuric thiocyanate reagent. Greenishyellow mosslike clumps and prisms prove copper. Leach original residue with 

drop of 1:5 HC1, transfer drop, and merge with drop of stannous chloride 

reagent. An amorphous brick-red precipitate proves selenium.

Kr&nnerite ((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 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, 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 HC1, to remove remaining traces of gold and tellurium, 

then one of water to remove HC1. 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.

LengeribacMte (6PbS. (Ag,Cu) 2S.2AsS3 ). Decompose mineral with 1: 1 HNO^. 

Leach residue with drop of 1 per cent HNOs, transfer drop, and merge 

with drop of potassium mercuric thiocyanate reagent. An amorphous finely 

divided milk-white precipitate proves silver, and greenish-yellow mosslike 

clumps and prisms prove copper. Leach original residue with drop of 1:5 

HC1, take up drop, and reject. Leach original residue with three successive 

drops of H20 and add to each 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 HNO3 . 

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.

Lettcopyrite (FesAs«). Decompose mineral with 1:1 HNO3 . The residue usually has a tinge of yellow or orange. Leach residue with drop of 1:7 HNO3 , 

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 yel-
SPECIFIC TESTS FOR MINERALS 193

low 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 (Fea03 .nH2 0). Take mineral into solution in aqua regia. Repeated applications of acid are necessary, as limonlte is resistant to chemical 

attack. Add drop of 1:5 HC1 to residue and add fragment of KiFe(CN) 0 to 

drop. An amorphous blue precipitate proves iron.

Linnaeite (Co3Si). Decompose mineral with 1:1 HNOa. 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.

Livingstonite (HgS.2Sb2S3 ). Decompose mineral with aqua regia. Examine 

residue for mercuric chloride lattice structure. Leach residue with drop of 1 

per cent 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 HC1. 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.

Lollinffite (FeAs2 ). Dissolve mineral in 1:1 HNOo. The residue usually has 

a tinge of yellow or orange. Leach residue with drop of 1:7 HNO3 , 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 (ThS.AszSs). A fragment of mineral tested in a Bunsen flame 

gave 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 HC1. 

Under the same conditions copper gives a blue flame. Arsenic is tested for 

as follows: Decompose mineral with aqua regia. Leach residue with drop of 

1: 7 HNOa, 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 HNO.i to redissolve excess of 

ammonium molybdate. Tiny yellow isometric crystals prove arsenic.

Magnetite (FeaOO. 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 K4Fe(CN)o. A blue amorphous precipitate proves iron.

Manganite (Mn2Oa). Take mineral into solution in aqua regia. Take up 

residue in drop of 1: 7 HNOs and add fragments of sodium bismuthate. Solution turns pink or purple near fragments, proving manganese.

Marcasite (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 K<Fe(CN)o. A blue amorphous precipitate proves iron.

Matildite ((Ag2,Pb)S.Bi2 S3 ). Decompose mineral with 1:1 HNOa. Leach 

residue with drop of 1 per cent HNOs, transfer drop, and add fragment of 

ammonium bichromate. A yellow amorphous precipitate proves load, and red 

crystals prove silver. Leach original residue with drop of 1: 5 r(Cl. Transfer 

drop and add fragment of KI. Solution stains yellow near fragment as it 

dissolves. Then add fragment of CsCl. Red hexagon's prove bismuth. Leach 

original residue with two successive drops of 1:5 HCl and rnject both. This
194 MICROSCOPIC DETERMINATION OF ORE MINERALS

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.

Maucherite (NiaAs2 ). Take mineral into solution with 1:1 HNO3 . Leach 

residue with drop of 1:7 HN03 . Transfer drop and evaporate slowly over 

tnicroflame. 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 dimethyl glyoxime solution to residue. A pink precipitate 

proves nickel.

Melctoortdte (CuO). Dissolve mineral in 1:1 HNOs. Take up residue with 

drop of 1:7 HNOs, transfer.drop, and merge with drop of potassium mercuric 

thiocyanate reagent. Greenish-yellow mosslike clumps and prisms prove copper.

Melonite (Ni2Te3 ). Dissolve mineral in 1:1 HNOs. 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-yeljow isometric crystals 

prove tellurium. Evaporate other part to dryness and add drop of dimethyl 

glyoxime reagent. A pink precipitate proves nickel.

MenegMnite (4PbS.Sb2S3 ). Decompose mineral with 1:1 HNO3. 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. Add drop of water to original residue 

and evaporate over flame to remove remaining traces of HC1. Cool and again 

add drop of water to original residue. Transfer drop and add fragment of KI. 

Glistening yellow hexagons prove lead.

Metaoinnalxirite (HgS). Decompose mineral by means of aqua regia. Examine residue for mercuric chloride lattice structure. Leach residue with 

drop of 1 per cent HN03 and transfer drop. Add fragment of Co(NOs)j.

When dissolved add fragment of potassium thiocyanate, Indigo-blue prisms

and branching forms of cobalt mercuric thiocyanate prove mercury.

Mfarffyrite (Ag2S.Sb2S3 ). Decompose mineral with 1:1 HNOs. Leach residue with 1 per cent HNOs, transfer drop, and add fragment of ammonium 

bichromate. Red crystals prove silver. Leach original residue with a drop of 

1:5 HC1, 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.

Millerite (NiS). Decompose mineral with 1:1 HNO3 . Leach residue with 

drop of 1:7 HNO3 , transfer drop, and evaporate to dryness. To the residue 

from this evaporation add a drop of dimethyl glyoxime solution. A pink 

precipitate proves nickel.

Nagyagite (Pb,Au,Te,S)?. Decompose mineral with 1:1 HNOs. Leach residue with drop of 1: 5 HC1. Transfer drop and add fragment of CsCl. Honeyyellow isometric crystals prove tellurium. The crystals are smaller with nagyagite than with other tellurides, for some unknown reason. Leach original 

residue with a drop of water and evaporate to dryness over microflame in 

order to volatilize remaining trace of HC1. Cool and add another drop of 

water to original residue. Transfer drop and add fragment of KI. Yellow 

hexagons prove lead. The writer was unable to get a test for gold by the 

pyridine-HBr method.
SPECIFIC TESTS FOE MINEEALS 195

Naumanite (Ag2 ,Pb)Se. Decompose mineral with 1:1 HNOs. Leach residue with 1 per cent HNOs, 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 HC1 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 a drop of water and evaporate over a microflame in order to volatilize the remaining HC1. Cool, again 

leach original residue with a drop of water, transfer drop, and add a fragment of KI. Yellow hexagons prove lead. Naumanite and aguilarite can be 

distinguished only by a microchemical test for lead.

Nicoolite (NiAs). Take mineral into solution with 1:1 HNOs. 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 HNOs 

to redissolve excess of ammonium molybdate. Tiny yellow isometric crystals

prove arsenic. Transfer filtrate and evaporate to dryness. Cool and add a drop

of dimethyl glyoxime solution to residue. A pink precipitate proves nickel.

Orpiment (As2Ss). Decompose mineral with 1:1 HNOs. Leach residue with 

1:7 HNOs, 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.

Oivyheeite (8PbS.2Ag2S.5Sb2S3 ). Decompose mineral with 1:1 HNOs. Leach 

residue with drop of 1 per cent HNOa, 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 HC1. 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. Orange-colored hexagons and stars prove antimony. If necessary the presence of lead can be confirmed by leaching original residue with a 

drop of water, transferring drop, and testing for lead with fragment of KI.

Pearcite (8Ag2S.As2Ss). Decompose mineral with 1:1 HNOs. Leach residue 

with 1 per cent HNOs, 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 

pearcite, 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 per cent HNOs drop. Usually there is enough arsenic 

remaining to give a satisfactory test. Leach original residue with 1:7 HNOs, 

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 HNOa to redissolve 

excess of ammonium molybdate. Tiny yellow isometric crystals prove arsenic.

Pentlandite ( (Ni,Fe)S). Decompose mineral with 1:1 HNOa. Leach residue 

with 1:7 HNOs, transfer drop, and evaporate to dryness. Add drop of 1 per 

cent NH«OH to residue. A yellow gelatinous precipitate of Fe(OH) 8 proves 

iron. Allow the NH4OH to evaporate in air. When nearly dry add directly 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 HNOa. Leach residue 

with 1 per cent HNOs, transfer drop, and add a fragment of ammonium bi15122 31 14
196 MICBOSCOPIC DETERMINATION OF ORE MINERALS

chromate. Red crystals prove silver.. Leach original residue with a drop of 1: 5 

HC1, 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.

Plagionite (5PbS.4Sb2Ss). Decompose mineral with 1:1 HNO3 . 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. Orangecolored hexagons and stars prove antimony. Add drop of water to original 

residue and evaporate over flame to remove remaining traces of HC1. Cool 

and again add drop of water to original residue. Transfer drop and add fragment of KI. Glistening yellow hexagons prove lead.

Plessite ((Ni,Fe)AsS). Decompose mineral with 1:1 HNO3 . 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 HNOs. Tiny 

yellow isometric crystals prove arsenic. Transfer filtrate and merge with drop 

of potassium 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 a drop of dimethyl glyoxime solution. A pink 

precipitate confirms the presence of nickel.

PolyTxisite (8Ag2 S.Sb2S8 ). Decompose mineral with 1:1 HNOs. Leach residue 

with drop of 1:7 HNOs, 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 HC1, 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 (NisSO. 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 dimethyl glyoxime solution. A 

pink precipitate proves nickel.

Proustite (3Ag2S.As2Ss). Decompose mineral with 1:1 HNO3 . Leach residue 

with drop of 1 per cent HNOs, 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 per cent HNOs. Enough is usually left in 

the original residue to give a satisfactory arsenic test. Leach original 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 

evaporate slowly over microflame, this time to dryness. Cool and add drop of 

1:7 HNOs to dissolve excess ammonium molybdate. Tiny yellow isometric 

crystals prove arsenic.

Psilomelane (Mn2MnOo). Take mineral into solution in aqua regia. Take up 

residue in drop of 1:7 HNOs and add fragments of sodium b.ismuthate. Solution turns pink or purple near fragments, proving manganese.

Pyrargyrlte (3Ag2 S.Sb2S3 ). Decompose mineral with 1:1 HNO8. Leach 

residue with 1 per cent HNOa, transfer drop, and add fragment of ammonium
SPECIFIC TESTS FOB MINERALS 197

bichromate. Bed crystals prove silver. Leach original residue with a drop 

of 1:5 HC1, transfer drop, and add fragment of KI. Solution turns yellow 

near the fragment as it dissolves. Then add a fragment of CsCl. Orangecolored hexagons and stars prove antimony.

Pyrolusite (MnOa). Take mineral into solution in aqua regia. Take up 

residue in drop of 1: 7 HNOs and add fragments of sodium bismuthate. Solution 

turns pink or-purple near fragments, proving manganese.

Pyrrhotite (FeSi+). Decompose mineral with 1:1 HNOa. Residue is colored 

yellow or orange. Leach residue with drop of 1: 5 HC1, transfer drop, and add 

fragment of K4Fe(ON)«. A blue amorphous precipitate proves iron.

Pyrite (FeSa). Decompose mineral with 1:1 HNOa. Residue is colored 

yellow or orange. Leach residue with drop of 1: 5 HC1, transfer drop, and add 

fragment of K4Fe(CN)o. A blue amorphous precipitate proves iron.

Rammelsbergite (NiAs2 ). Take mineral into solution with 1:1 HNOs. Leach 

residue with drop of 1:7 HNOa. 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 HNOs to redissolve excess of ammonium.molybdate. Tiny yellow

isometric crystals prove arsenic. Transfer filtrate and evaporate to dryness. 

Cool and add a drop of dimethyl glyoxime solution to residue. A pink precipitate proves nickel.

RatMte (3PbS.2AsaS8 ). Decompose mineral with 1:1 HNOa. 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 HNOs 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, awing to the presence of remaining traces of As, but the second or third 

drop should give yellow hexagons, possibly somewhat distorted, proving lead.

Realgar (AsS). Decompose mineral with 1:1 HNOa. Leach residue with 

1:7 HNOa, 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.

Rezbanyite (2PbS.3Bi2Sa). Decompose mineral with 1:1 HNOa. 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 HCr, 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. The second or third drop should yield yellow 

hexagons, proving lead.

Rickardite (Gu»Tey). Take mineral into solution with 1:1 HNOa. Leach 

residue with 1 per cent HNOa, 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.
198 MICROSCOPIC DETERMINATION OP ORE MINERALS

Schaplachite (Ag2S.PbS.Bi2 S3 ). Decompose mineral with 1:1 HN03. Leacb 

residue with, drop of 1 per cent HNO3 , 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 H l, 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 HC1, rejecting each. 

This removes most of remaining traces of bismuth. Then leach original residue 

with successive drops of H2 0, testing each 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 Pbla 

hexagonal plates.

Semseyite (9PbS.4Sb2S3 ). 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. Orangecolored hexagons and stars prove antimony. Add drop of water to original 

residue and evaporate over flame to remove remaining traces of HC1. Cool 

and again add drop of water to original residue. Transfer drop and add 

fragment of KI. Glistening yellow hexagons prove lead.

Siegenite ((Ni.CohSO. Decompose mineral with 1:1 HNOa. Leach residue 

in drop of 1: 7 HNO3 , 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.

Silver (Ag). Take into solution with 1:1 HN03 . Add drop of 1 per cent 

HNOs to residue. Transfer drop and add fragment of ammonium bichromate. 

Red crystals prove silver.

SkutteruMte ((Co,Ni)As3 ). Take mineral into solution with 1:1 HN03. 

Leach 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 slowly evaporate. When dry, cool and add drop of 1:7 HNOa 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-chloantMte ((Co.NiJAsa). Take mineral into solution in aqua regia. 

Take up residue in a drop of 1:7 HN03 and slowly evaporate drop. When 

nearly dry, add drop of ammonium molybdate reagent and slowly evaporate to 

dryness, cool, add drop of 1:7 HN03 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 HNO3 , transfer drop, and merge with drop of potassium mercuric 

thiocyanate. White feathery crosses prove zinc.

Stannite (Cu2S.FeS.SnS2 ). Decompose mineral with 1:1 HNO3 . Leach residue with drop of 1:7 HNOs, 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 HNO3 and one of H2 0; reject all three drops. Leach residue
SPECIFIC TESTS FOR MINERALS 199

with drop of 1:5 HC1, transfer drop, and add fragment of rubidium chloride. 

Colorless highly refracting octahedra prove tin.

Sternbergite (AgFejSs-*). Decompose mineral with 1:1 HNOs. teach residue with 1 per cent HN03 , transfer drop, and add fragment of ammonium 

bichromate. Red crystals prove silver. Leach original residue with a drop of 

1: 7 HNO3 , transfer drop, and add fragment of KiFeCCNK 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.

 Stephanite (5Ag2S.SbzS3). Decompose mineral with 1:1 HNOs. Leach residue with 1 per cent HNOs, transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Leach original residue with a drop of 1:5 

HC1, 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 HC1, transfer drop, and 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.

Stromeyerite ((Aga ,Cu)S). Decompose mineral with 1:1 HNOS, leach residue with drop of 1 per cent HNOs, transfer drop, and add fragment of ammonium bichromate. Red crystals prove silver. Leach residue with drop of 

1:7 HN03 , transfer drop, and merge with drop of potassium mercuric thiocyanate solution. Greenish-yellow mosslike aggregates and prisms prove copper.

Sulvanite (Cu3VS«). Decompose mineral with 1:1 HNOa. Mineral is very 

resistant to chemical attack, and at least 10 successive applications of acid 

will be necessary. Leach 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. A satisfactory test for vanadium is 

not available.

Sylvanite ((Ag,Au)Tea). 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, transfer drop, and merge with it a drop of pyridine-HBr reagent. 

Strongly pleochroic (colorless to deep red) prisms prove gold. Silver remains, jn 

the original residue as a curdy white chloride. Leach it with two or three 

successive drops of 1:5 HC1 to remove remaining traces of gold and tellurium, 

then one of water to remove HC1. Then add to original residue a drop of concentrated NH«OH. 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 1:1 HNOa. Leach residue 

with drop of 1: 5 HC1, transfer drop, and add fragment of rubidium chloride. 

Colorless, highly refracting octahedra prove tin. Add a drop of water to the 

original residue and evaporate over microflame to volatilize remaining trace
200 MICROSCOPIC DETERMINATION OP ORE MINERALS

of HC1. Cool again, add a drop of water to the original residue, transfer drop, 

and add fragment of KI. Yellow hexagons prove lead.

Tellurium (Te). Dissolve in 1:1 HNO». Leach residue with drop of 1:5 

HC1, transfer drop, and add fragment of CsCl. Honey-yellow isometric crystals 

prove tellurium.

Tennantite (5CuaS.2(Cu,Fe,Zn)S.2As2S3). Decompose mineral with 1:1 

HNOs. Leach residue with 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 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.

Tetradymite (Bia (Te,S)s). Dissolve mineral in 1:1 HNOa. 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.

Tetrahedrite (5Cu2S.2(Cu,Fe,Zn)S.2Sb2S3). Decompose mineral by means 

of 1:1 HNOs. Leach residue with drop of 1: 7 HNOa, transfer drop, and merge 

with it a drop of potassium mercuric thiocyanate solution. Greenish-yellow 

mosslike clumps and prisms prove copper. Leach residue with two successive 

drops of 1: 7 HNOs to remove copper and reject both drops. Then leach residue 

with a drop of 1:5 HC1, transfer drop, and add fragment of CsCl. The solution near the fragment turns yellow as the KI dissolves. Then add fragment of 

CsCl. Orange-colored hexagons and stars prove antimony.

Tiemannite (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 per cent 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 HNOs. Leach residue with drop of 

1:5 HC1, transfer drop, and add fragment of K4Fe(CN) 8. Amorphous blue 

precipitate proves iron.

Ullmanite (NiSbS). Decompose mineral with 1:1 HNO3 . Leach residue with 

drop of 1: 7 HNOa, transfer drop, and evaporate to dryness. To the residue from 

this evaporation add a drop of dimethyl glyoxime solution. A pink precipitate 

proves nickel. Leach original residue with a drop of 1:5 HC1, 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.

Umangite (CusSe2 ). 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 per cent HNO8, 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 brick-red precipitate proves selenium.

Violarite ((Ni,Fe) 8S«), Decompose mineral with 1:1 HNOs. Leach residue 

with 1:7 HNOs, transfer drop, and evaporate to dryness. Add drop of 1 per 

cent NH4OH to residue. A yellow gelatinous precipitate of Fe(OH) a proves 

iron. Allow the NH«OH to evaporate in air. When nearly dry, add directly a 

drop of dimethyl glyoxime solution. A pink precipitate will form on top of the 

gelatinous ferric hydroxide coating, proving nickel.
SPECIFIC TESTS FOR MINERALS 201

Voltzite (4ZnS.ZnO). Decompose mineral with aqua regia. Leach residue 

with drop of 1:7 HNOa, transfer drop, and merge with drop of potassium mercuric thiocyanate. White feathery crosses prove zinc.

Vrbaite (Tl2S.3(As,Sb)jSs). Thallium tested for by flame coloration. A 

small loop at the end of the platinum wire was filled with a drop of 1:1 HC1 

and a small fragment of mineral placed in this loop. This was placed in the 

hottest part of a Bunsen flame. A vivid green color identified thallium. Test 

for arsenic and antimony inicroch em it-ally as follows: Decompose mineral with 

1:1 HNOs. Leach residue with drop of 1:7 HNOa and slowly evaporate over 

microflarne. When nearly dry add drop of ammonium molybdate solution and 

again evaporate slowly over microfluine. When dry add a drop of 1:7 HNO» 

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 (Cu,Te; exact proportions not known). Take mineral into solution 

with 1:1 HNOs. Leach residue with 1 per cent HNOs, 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.

WUlj/amite ((Co.Ni)SbS). Decompose mineral with drop of 1:1 HNO». 

Leach residue with drop of 1:7 I-INOa, transfer drop, and merge with it a drop 

of potassium mercuric tliiocyauate. If both cobalt and nickel are present, both 

blue and brown spherulites will form. If cobalt is present in sufficient concentration, blue prisms will also form. (See description on p. 136.) Leach 

original residue with a drop of 1:5 HCI. 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.Bi 2 S8 ). Decompose mineral with 1:1 HNOa. Leach 

residue with drop of 1:7 HN03, transfer drop, and merge with drop of potassium mercuric thiocyanate solution. Greenish-yellow niosslike clumps and 

prisms prove copper. Leach original residue with drop of 1:5 HCI, 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.

Wurtzite (ZnS). Decompose mineral with aqua regia. Leach residue with 

drop of 1:7 HNOs, transfer drop, and merge with drop of potassium mercuric 

thiocyanate reagent.. White feathery crosses prove zinc.

Zinkenite (PbS.SbaSs). Decompose mineral with 1:1 HNOa. 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. Add drop of water to original residue and evaporate 

over flame to remove remaining traces of HCI. Cool and again add drop of 

water to original residue. Transfer drop and add fragment of KI. Glistening 

yellow hexagons prove lead.

INDEX

Page 

A 

Abrasives, size of_ _........._.... 14

Antimony, microchemical tests for......._. 159

Arsenic, microchemical tests for............. 155

B 

Bakelite..___.__...................... 15

Bateman, A. M............................. 13,42

Behrens, H.... 115,116,117,135,139,155,156,165,166

Binnenthal minerals___________... 103 

Bismuth, microchemical tests for!........... 159

BoriCky, E.................................. 115

Bottle, glass-stoppered dropping.. ...I.... 128

Broderick, T. M............................ 71

Busch microscope lamp...................... 42

Buttgenbach, H...,. . 89

Cabeen, C. K.. _.......................... 49

Cadmium, microchemical test for.... _..... 132

Camera, Bausch & Lomb Le Chatelier model. 23

improvised.__......................... 18

Leitz"Makam"-........ -.....-...-...- 20

Canfleld mineral collection.............. _ vn

Capillary tubes... ......^-................... 122

Chalcography..__ .............. 3

Chamot, E. M..... ........... ........ 116,

117, 118, 120, 121, 123, 124, 125, 132, 135, 139, 

143,144,145,146,155,164,165,169.

Clennell, J. E............................... 138

Cobalt, microchemical tests for.............. 135

Color........................................ 30

Color standards, Ridgway's................. 33

Colored minerals, table. ................. 34

Comparison microscope..____._....... 32

Copper, microchemical tests for.... .. 132

Cupferron_____-___._.......___ 141

D 

Dana, J. D.........-------------------.----- 103

Davy, W. M....... 1,2,3,11,17,36,49, 50,51, 52,103

Determinative table. ....-..-..-.-. 58

Development of photographic plate_....--. 28

Discredited and doubtful minerals........... 104

Etch reactions_. 

Etch reagents..... 

Exposure formula.

Fahey, J. J.................................. 97

Fairbanks, E. E.......-...-.......-..-.--... 3,40

Fairchild, J. G.....--........-. .. 87,97,100,101

Farnham, C. M.... 1,2,3,11,17,36,49,50,51,52,103

Filters, light..... , 23,33

Flotation of polishing powders..__._._ 13

Page 

Foshag, W. F............................ vn,59,97

Fraser, H. J................................ vn, 173

Frebold, Qeorg......_._...................... 104

O

Gold, microchemical tests for...... _____ 152

Qraton, L. C............................... vu,52

H 

Hardness.................................... 35

Harvey, R. D............................... 49

Hind, H. -L.................................. 29

Illuminator, vertical___....... 4

Impregnation of specimen....__....___ 15

Index of minerals, alphabetic..._.____ 61

grouped by elements.---___-____ 107

Internal reflection, table of minerals showing. 34

Iron, microchemical tests for________ 139

K 

Kerr, P. F................................... 49,56

Kley, P. D. C..... 116,117,135,139,155,156,165,166

Koenigsberger, J....._ 39

Kollolith... ......._.... ........ 16

Landes, K. K............................... 117

Laney, F. B................................. 76

Lead, microchemical tests for 143 

Leveling cup.. .._... 7 

Llndgren, Waldemar......_......__... 86,116

Lovering, T. S......................... ... vn

M

Magnesia________ ..... 12

Manganese, microchemical tests for 168 

Mellor, J. W.. 138

Mercury, microchemical tests for 142 

Merwin, H. E.___.__................... 105

Metallographic microscope.. . 3 

Microburner, alcohol._ 121 

Microchemical methods.. ..... 115

Mineragraphy. ............ 3

Mineralography _ . 3 

Mounts, brass....__ -. ........ 8

Murdoch, Joseph...___..... 1,3,30,32,36,50,52

Murdoch ore mineral suite vn, 2,38 

McKinstry, H. E-. .... .. 116,137

N

Needle, chisel-pointed . 

Nickel, microchemical tests for. 

Nitroso/3-naphthol-.-.___..

119

133

137

203
204 INDEX

Page

0

Osborne, F. F.. ... 40,42 

Outline of determinative table . .._.. 65 

Oven, warming____...-- ._..- ._ -. 10

Photomicrography of polished sections....... 17

Plate, Wratten M panchromatic ............ 24

Polarized light..... .... .. .. 39

Polishing process at the U. S. Geological Survey.:..... .. -._ .. 10

at Harvard University-________-. 18 

at University of Minnesota.....___.. 17

Potassium mercuric thiocyanate, preparation

Of..... 127

Posnjak, E _............_ .... 105

Powders, colored, table of minerals with..... 35

Powers, H. A _ ._ vn, 173 

Putnam, P. C . , 116,129,153

R 

Eamdohr, Paul...... ......--. -.-.....-- 85

Randies, W. B.... 29

Reagents, etch......_......-...-.- ..... 54

microchemical.--......-_..---...-.---- 127

Reed, F. S- 13

Roberts, E. J__. ................ 116,129,153

Roebling mineral collection....-..-.....-. vn

Ross, C. S - 14,15

Rouge, black magnetic.....__...._...... 12

Samson cloth.__ -... .- _ -._ 12

Sampson, Edward.....______ vn, 14,40,43,45

Saw, diamond__________. _____ 11 

Schneiderhbhn, Hans.......... 1,12,33,40,43,50,51

Schwartz, O. M .,.. . 17 

Selchow, D.. .. .... .. 116,129,153

Selenium, microchemical tests for_.--.....- 167

Page

Sensitivity of microchemical tests_ .. 126 

Shannon, E. V........ ... 74,77,78,82,87,94,101

Short, M. N.. ...... 10,16,77,94,101

Silver, microchemical tests for________ 148 

Slides, glass---.....___1________ 120

Smitheringale, W. V. 54

Specific tests for minerals_ .. _.- 179 

Spencer, L. J___...________..__. 91 

Steiger, George.._ _ _ vn 

Sulphur, microchemical tests for______ 169 

Systematic scheme of microchemical analysis. 173

Talmage hardness machine.. ___..__ 37 

Talmage, S. B................__._. 31,32,35,37

Tellurium, microchemical tests for_____ 165 

Thiel, G. A............. ... vn,54

Thomson, Ellis.._...._....-..-....--..-. 100

Tin, microchemical tests for. ___._.. 163 

Tubes, capillary____..._________ 122

V

VanderVeen, R. W-...-1 ... 1,5,37,40,51,52 

Vanderwilt, J. W 11,16,38

W 

Walker, T. L. ............ . .. 74,81

Wax, modeling...... .__ _.. ..... 7

sealing.________.________ 9 

Wells, R. C.. . . . 94

Wherry, E. T.. .. ... ....... 59,94

Whitehead, W. L.. .. .. .. .... 3

Winchell, N. H. and A. N................... 39

Wright biquartz wedge_______. 40,44,45,46 

Wright, F. E .- 40

Zinc, microchemical tests for .

O