Geochemistry 

1949

1 GEOLOGICAL SURVEY BULLETIN 992

Contributions to

Geochemistry

1949

By W."_W. BRANNOCK and others

C 0 N_T RIBUTIONS TO GEOCHEMISTRY, 1949

GEOLOGICAL -SURVEY BULLETIN 992

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1953
-3 9

. II

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 65 cents (paper cover)
BY EARL INGERSON

This bulletin is the third of the series "Contributions to Geochemistry" which was begun in 1946 with Bulletin 950, "Contributions to 

Geochemistry, 1942-45." A second member of the series, "Contributions to Geochemistry, 1950" appeared, in 1951 as Bulletin 980. 

References to earlier reports of work done in the laboratories of the 

predecessors of the Geochemistry and Petrology Branch can be found 

in the forewords of the bulletins mentioned above.

A considerable part of the work of the Geochemistry and Petrology 

Branch is the study and analysis of geologic materials for other 

Government organizations, especially other branches of the Geologic 

Division of the U. S. Geological Survey. It is natural, therefore, that 

an important part of the research work of the Geochemistry and 

Petrology Branch is concerned with developing new techniques and 

analytical methods or improving known ones and adapting them to 

special problems.

The object may be to increase the sensitivity or accuracy of a 

determination, to increase speed, or to obtain better and more consistent results in the presence of interfering elements. For example, 

the method for the fluorimetric determination of aluminum in phosphate rock, described by Grimaldi and Levine, was developed in 

response to a request for such a large number of these analyses that 

they could not have been done by the personnel available using 

methods then current. The papers on "The determination of sodium 

and potassium in silicate rocks by the flame photometer" and "A 

photometric method for the estimation of the oil yield of oil shale" 

are also aimed at greater speed without loss of essential accuracy. 

"Determination of phosphorus in rocks containing vanadium" is an 

example of a method for better determination of an element in the 

presence of an interfering one.

Much of the research work of the Geochemistry and Petrology 

Branch is published in specialized journals. Most of the papers that 

are largely mineralogical, for example, are published in the American 

Mineralogist. Papers that deal with geochemical interpretation are 

likely to appear in Geochimica et Cosmochimica Acta and some of the 

work on analytical methods has appeared in Analytical Chemistry.
IV FOREWORD

It is considered worth while, however, to have groups of papers dealing 

with laboratory techniques for work with geologic materials collected 

and published from time to time as bulletins of the U. S. Geological 

Survey. It is hoped that the series will come to the attention not 

only of geochemists and rock analysts, but also of chemists studying 

artificial inorganic materials to which these methods and techniques 

are applicable.
CONTENTS

Page 

Part 1. The determination of sodium and potassium in silicate rocks by

flame photometer, by W. W. Brannock and Sarah M. BerthokL- 1

2. A photometric method for the estimation of the oil yield of oil

shale, by Frank Cuttitta____.______________________________ 15

3. A volumetric method for the estimation of the oil yield of oil shale,

by Frank Cuttitta._________________-_-___________________ 33

4. The fluorimetric determination of aluminum in phosphate rock

with 8-hydroxyquinoline, by F. S. Grimaldi and Harry Levine__ 39

5. The determination of phosphorus in rocks containing vanadium,

by R. B. Randolph'andjF. S. Grimaldi-._____. ._i_._.._..._ 49

6. The probable error of_a chemical'analysis, by William G. Schlecht. 57

7. A confined-spot method for the'determination of traces of silver in

soils and rocks, by Hy Almond, R. E. Stevens, and H. W. Lakin. 71

8. A photographic technique for showing some mineral relations, by

Waldemar T. Schaller.______.__________________________ ... 83

ILLUSTRATIONS

Page

PLATES 1-14. Brightly reflecting surfaces of certain minerals. _ At end of Part 8 

FIGURE 1. Special glass atomizer.__________________________________ 6

2. Correction curve for the effect of potassium on sodium______ 10

3. Relation of optical density to percentage oil yields, by Fischer

assay method________________________________________ 17

4. Relation of optical density to Fischer oil yield, in gallons per

ton_________________________________________________ 18

5. Details of tubular electric furnace. ________________________ 20

0. Effect of 8-hydroxyquinoline concentration on fluorescence

intensity.___________________________________________ 43

7. Effect of pH on fluorescence intensity---__----____--___-_- 44

8. Fluorescence intensity of aluminum quinolate versus extraction time__________________________________________._ 44

9. Effect of different amounts of same buffer on fluorescence

intensity. _ ________________________________________ _ _. 45

10. Working curve_________________________________________ 40

11. Meaning of the terms "accuracy," "precision," and "reliability" of measurements.______________________________ 60

12. Nonnormal distribution of errors in analyses by 10 laboratories. ----__--_-_____-___-_________________-___.____ 61

13. A random sequence and two non-random sequences, all with

the same distribution function._________________________ 62

.1.4. Two complete chromographs in carrying case.______________ 74

15. Extractor for removing acid-soluble constituents from soil and

rock samples.________________________________________ 75

16. Rock sample being treated with boiling nitric acid in outer

chamber.____________________________________________ 77

17. Extract being automatically drawn through filter into inner

chamber on cooling____________________________________ 78

v

Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Part 1. THE DETERMINATION OF SODIUM AND POTASSIUM 

IN SILICATE ROCKS BY FLAME PHOTOMETER

BY W. W. BRANNOCK AND SARAH M. BERTHOLD

CONTENTS

Page 

Abstract__ _-____________________________________________________ 1

Introduction._____________________________________________________ 2

Acknowledgments.________________________________________________ 3

Procedure __-____________-__-____---_-__---_-_--_____-_-_-_______- 3

Outline and discussion.________________________________________ 3

Apparatus and reagents.________-.__-_-___---___-_-___-____--_- 5

Description of special glass atomizer.____________________________ 5

Preliminary steps...____________----_--_---________----_--__-_. 7

Preparation of the sample solution______________.__________.: 7

Starting the flame photometer._____________________________ 7

Determination of the metal-____________________________________ 8

Potassium _________________________________________________ 8

Sodium-______-_-________---_-_-_--_--__-___---_-_-_----_ 10

Results of experiments.____________________________________________ 11

Effect of salts of aluminum, iron, calcium, and magnesium and of

(NH4) 2SO4 on values for sodium__________--______-- _-__-__--_ 11

Effect of magnesium on values for potassium._____________________ 11

Effect of potassium on values for sodium.________________________ 12

Determination of potassium and sodium in solutions containing KC1

and NaCl only._____________________________________________ 12

Retention of sodium or potassium by A12 (OH) 3 Fe2 (OH) 3 , and CaCO3 _ 13 

Determination of sodium and potassium in National Bureau of Standards standard samples--.-------------------.----------------- 13

Literature cited_________________-_-_-____----_-___--_----_----_--- 14

ILLUSTRATIONS

Page 

FIGURE 1. Special glass atomizer.___________________________________ 6

2. Correction curve for the effect of potassium on sodium_______ 10

ABSTRACT

A method is given for the determination of sodium and potassium in silicate 

rocks using the Perkin-Elmer model 52A flame photometer with a special glass 

atomizer, which is described. Samples are decomposed by digesting with HF

1
2 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

and H2SO4. The R2 0s group and calcium are removed by precipitating with an 

excess of NH4OH and (NH4 ) 2CO3 at a definite volume. LiNO3 is added to a portion of filtrate, and sodium and potassium are determined on this solution. Results are as accurate as are normally obtained by the J. Lawrence Smith method, 

and analyses require only about one-fifth the usual time.

INTRODUCTION

In flame photometry, solutions are atomized into the flame of. a 

gas burner, and the concentrations of the relatively easily excited 

metals are determined by measuring the intensity of a part of their 

characteristic spectral emissions and comparing the emissions with 

those of solutions of known concentrations.

Lundegardh (1929; 1934) described a procedure in which the 

intensities of spectral emissions obtained when solutions were atomized 

into a flame were determined by measuring the densities of lines on 

photographic plates as in spectrographic techniques. Flame photometry differs from Lundegardh's procedure in that spectral emissions are measured directly with a photometer.

Several instruments using filters and photocells or phototubes were 

in use in Germany during the 1930's (Schmitt and Breitweiser, pp. 

750-57). In 1945, a flame photometer was described in this country 

by Barnes and others (1945, pp. 605-611). This instrument used 

propane gas for the flame, filters to isolate the desired portions of the 

spectrum, and a phototube and a galvanometer to measure the intensities of the emissions. This was a direct intensity type of instrument, 

employing no internal standard.

In 1946, Berry, Chappell, and Barnes (1946, pp. 19-24) described 

a flame photometer with a dual optical system designed for use with 

lithium as an internal standard, as well as for direct intensity determination. In procedures using an internal standard the intensitv of 

the emission of the unknown element is compared simultaneouslv 

with the emission of the internal standard, which has been added 

to the sample and standard solution in like amounts. This internal 

standard technique reduces the disturbing effects of variation in fla.mo, 

character caused by changes in the gas and air pressures, the presence 

of foreign salts, and differences in viscosity of the solutions.

In 1947 the Perkin-Elmer Corp. brought out their model 52A flame 

photometer, which was similar to the instrument described by Berry, 

Chappell, and Barnes. This is the instrument which has been used 

in the procedure described in this paper.

Sodium and potassium in solution can be rapidly and conveniently 

determined with the model 52A flame photometer by internal-standard 

procedures. A comparatively simple and rapid procedure can be employed for preparation of the sample solution for analysis of silicate 

rocks, as most silicate rocks can be decomposed by digestion with HF
PART 1. DETERMINATION OF NA AND K FLAME PHOTOMETER 3

and H2SO4 . Complete solution of the sample is definitely more 

desirable than an extraction such as is necessary in the commonly 

used J. Lawrence Smith method.

By using the method described in this paper, a person with comparatively little training in chemistry can determine sodium and 

potassium in a group of silicate rock samples in approximately onefifth the time that would be required by a trained analyst using the 

J. Lawrence Smith procedure.

The authors are indebted to Michael Fleischer, Rollin E. Stevens, 

William G. Schlecht, and Leonard Shapiro, colleagues at the U. S. 

Geological Survey, for their helpful suggestions and criticisms.

PROCEDURE

OUTLINE AND DISCUSSION

1. The sample is decomposed by overnight digestion with HF + B^SCX in a 100 ml 

platinum crucible.

2. R2 03 plus CaC03 is precipitated once with NH40H and (NELOaCOa at a fixed 

volume. Only a part of the solution is filtered.

3. The sample solution is prepared for analysis by mixing 50 ml of the filtrate 

from the precipitation of RjOs plus CaC03 with 50 ml of solution containing 

200 ppm lithium as LiNOs for internal standard.

4. Potassium is determined with the flame photometer by comparison with standard solutions of potassium containing 100 ppm lithium.

5. Sodium is determined with the flame photometer by comparison with standard 

solutions of sodium containing 100 ppm lithium.

Most silicate rocks and minerals are completely decomposed by 

digesting a sample overnight with HF and H2S04 in a covered platinum 

crucible. The sample must have.been ground to pass a 100-mesh 

screen.. Boiling the sample with HF and H2S04 for one-half hour to 

an hour in a covered crucible will decompose most silicate rocks and 

minerals also. Samples which are not completely decomposed after 

overnight digestion should, in addition, be boiled for approximately 

one-half hour. Zircon and tourmajine are not completely decomposed 

by either treatment. Zircon does not contain sodium or potassium 

and so decomposition of zircon is not essential. Tourmaline, on the 

other hand, may contain as much as 2 percent Na20, 2 percent Li20, 

and 0.2 percent K20. Most rocks, however, contain little if any 

tourmaline. When tourmaline, or rocks containing appreciable 

tourmaline, are analyzed for Na20 or K2O it is advisable to use the 

J. Lawrence Smith decomposition.

After the sample is decomposed, most of the HF is removed by 

evaporation on the steam bath. A few drops of HN03 are added to 

aid in removal of HF and organic matter, if present; and most of the
4 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

remaining HF is removed by heating until strong fumes of SO3 

evolve. It is advisable not to prolong the fuming of S03 in order not 

to lose appreciable H2S04 and as a consequence reduce the amount of 

(NH4) 2S04 which will be^formed when R2O3 is removed by addition of 

NH4OH. It is desirable to maintain the (NH4) 2SO4 at approximately 

the same concentration in both the sample solutions and the standard 

solutions. The concentration of (NH4) 2SO4 is calculated on the basis 

of the amount of H2SO4 used. Allowing the material to remain at 

the fuming temperature of H2SO4 for an extended time would also 

tend to form basic salts of iron and aluminum which are difficult to 

dissolve.

The crucible and contents are allowed to cool and approximately 75 

ml of distilled water is added. Fifteen to thirty min. digestion on the 

steam bath brings about essentially complete solution of the residue.

In order to avoid contamination by solution of alkalies from the 

volumetric flask, the acid solution, which may contain a small amount 

of HF, should be at room temperature before transfer to the volumetric flask.

Since iron, aluminum, and calcium have an appreciable disadvantageous effect on the determination of sodium, it is advisable to 

remove them from the solution to be analyzed. (See table 1.) Precipitating the R2 03 group and CaC03 at room temperature with 

NH4OH and (NH4 )2C03 , making to volume in a volumetric flask, and 

filtering only a part of the solution gives a satisfactory separation 

which is rapid and simple.

The solution for analysis is prepared to contain 100 ppm lithium 

for internal standard, by mixing 50 ml of the filtrate above with 50 

ml of a solution of LiNO3 in which the concentration of lithium is 

200 ppm.

The use of an internal standard in flame-photometer determinations 

reduces to negligible factors many interferences and variations 

resulting from changes in flame characteristics brought about by 

small changes in gas or air pressures, the presence of foreign ions, and 

differences in viscosities of the solutions being analyzed. The variations mentioned, coupled with a certain "drift," which is considered 

normal to an electronic instrument such as the flame photometer, at 

times reach such levels that calibration curves cannot be relied upon 

to give results as accurate as desired in the analysis of rocks and 

minerals.

These difficulties are satisfactorily overcome by obtaining readings 

for a standard of higher concentration than the unknown, then taking 

the reading for the unknown, and immediately taking a reading for 

a standard of lower concentration than the unknown. Two or three 

such series of readings are taken for each determination. This takes
PART 1. DETERMINATION OF NA AND K FLAME PHOTOMETER 5

only a few minutes and changes or variations in flame or instrument 

that occur affect readings for both standards and sample in a similar 

manner.

Readings are not exactly proportional to concentrations of sodium 

and potassium, but the interval used between the standards is small 

enough so that deviations are not appreciable.

APPARATUS AND REAGENTS

Apparatus: Perkin Elmer Model 52A flame photometer with glass atomizer shown 

in figure 1 and described below, which has been substituted for the 

atomizer furnished by the manufacturer.

Beaker stand.

Constant-voltage transformer.

Propane gas.

Regulators, pressure: 2 for gas, 1 for air.

Air compressor or constant supply of compressed air, 30 to 150 psi.

Air filter for compressed air.

Glass-stoppered bottles (pyrex): 

250 ml.

1 1.

2 1. 

91.

Volumetric flasks: 100, 200, 1000, 2000 ml. 

Pipettes, transfer: 25, 50, 100 ml. 

Tissues: facial type, fairly hard and lint-free. 

Burette: 50 ml. 

Beakers: 30 ml or 50 ml.

Reagents: KC1, NaCl, LiN03 , (NH4) 2S04, H2S04, HN03 , HF, all of ACS grade. 

Stock solutions:

KC1 containing 1,000 ppm potassium: 2 1. 

NaCl containing 1,000 ppm sodium: 2 1. 

LiN03 containing 2,000 ppm lithium: 8 1 or more. 

200 ppm lithium solution for internal standard: 2 1. 

Sample diluting solution containing 100 ppm lithium plus 1

percent (NH4) 2S04 : 2 1.

Standard potassium solutions containing 2.5, 5.0, 10.0, 20.0, 

30.0, 40.0, 50.0 ppm potassium, respectively, plus 100 ppm 

lithium and 1 percent (NH4) 2S04 : 1 1 each.

Standard sodium solutions, 2.5, 5.0, 10.0, 20.0, 30 0, 40.0, 50.0 

ppm sodium, respectively, plus 100 ppm lithium and 1 percent 

(NH4) 2S04 : 1 1 each.

DESCRIPTION OF SPECIAL GLASS ATOMIZER

A special atomizer, shown in figure 1, was designed for use in determining alkalies in solutions prepared from silicate rocks. It has 

certain advantages, when used for this purpose, over the stainlesssteel adjustable funnel-type atomizer which was supplied with the 

instrument, and also over the all-glass funnel-type now available from 

the Perkin-Elmer Corp. It atomizes a solution into the flame at the 

relatively slow fixed rate of approximately 4 ml per minute so that a
CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Tube for liquid 

(capillary tubing)

Avoid enlarging I.D. of 

capillary at this joint

Tube for air 

(capillary or regular tubing)

I.D. 3mm ±.25mm

fire polished

-orifice I.Omm ± 2mm

 orifice .20mm ±0.02mm

O.D. at tip l-2mm

tip bevelled and 

fine ground

3cm

FIGURE 1. Special glass atomizer.

maximum of approximately 10 ml of each sample and standard solution would be consumed for each complete determination. When the 

air pressure is set at 10 psi and propane gas is used as described under 

the section "Starting the flame photometer" it gives a steady flame of 

approximately 15 cm height. This atomizer draws solution up 

through the capillary intake tube from a beaker. When the beaker 

is withdrawn the capillary tube empties at once. The outside of the 

tube can be wiped clean with a hard, lintless tissue; another solution
PART 1. DETERMINATION OF NA AND K FLAME PHOTOMETER 7

can be passed in, and the capillary is flushed clean almost immediately. 

Waiting for solutions to clear is eliminated, and deliberate rinsing and 

flushing is unnecessary with this type of atomizer.

PRELIMINARY STEPS

PREPARATION OF THE SAMPLE SOLUTION

1. Transfer 0.5000 g of sample powder (100-mesh or finer) to a 

100 ml platinum crucible a type commonly used in the determination 

of FeO. If the sample is believed to contain more than 5 percent 

Na2O or 5 percent K2O, a 0.2000 g sample should be taken.

2. Add 6 ml of 1 +1 H2S04 and approximately 25 ml of HF. Cover 

the crucible and digest overnight on a steam bath or gently boil until 

the sample is decomposed.

3. Remove and rinse the cover. Allow the solution to evaporate 

on the steam bath approximately to concentrated H2S04 .

4. Add 5 or 10 drops of HN03 and evaporate on a steam bath until 

acid fumes are no longer given off.

5. Carefully heat the crucible and contents over an electric heater 

or low flame of a gas burner until strong S03 fumes evolve.

6. Allow the crucible and contents to cool. Then nearly fill the 

crucible with distilled water and digest on a steam bath until essentially complete solution of the residue is obtained.

7. Allow the solution to cool to room temperature in the crucible. 

Then transfer the solution to a 200 ml volumetric flask preferably 

one of borosilicate glass or, if the sample is believed to contain not 

more than 1 percent Na2O or 1 percent K2O, use a 100 ml volumetric 

flask. Wash the residual solution from the crucible into the flask 

with distilled water and fill the flask to approximately 20 ml from the 

line.

8. Add NH4OH to the flask until a slight excess is present as indicated by the change to blue of a small piece of red litmus paper.

9. Then add approximately 0.5 ml of freshly prepared concentrated 

ammonium carbonate solution.

10. Make up to 200 ml with distilled water and mix well.

11. Filter 75 to 100 ml of the solution into a pyrex bottle or 

beaker through a dry 12.5 cm Whatman No. 40 paper in a dry funnel.

12. Withdraw 50 ml of the filtrate with a transfer pipette and mix 

with exactly 50 ml of the LiN03 solution containing approximately 

200 ppm lithium. Mix well and set aside for the determination of 

potassium and sodium.

STARTING THE FLAME PHOTOMETER

13. The instrument is set up as described in the instruction manual 

(Perkin-Elmer Corp., 1948), except for the following changes: A
8 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

special glass atomizer as shown in figure 1 and described above is 

substituted, a second pressure gauge is installed in the gas line to give 

a steadier liame, the spring suspension for the burner chimney is 

discarded, and the chimney is allowed to stand upright on the shelf 

through which the burner protrudes. The red-sensitive phototube is 

used for the determination of both potassium and sodium.

14. Adjust the air pressure to 10 psi, open the main valve on the 

propane tank, set the gas regulator which is nearest the main valve to 

10 psi, allow propane to pass to the burner by opening the valve on 

the second regulator. Light the burner and adjust the pressure, using 

the second valve, so that the flame is steady and blue with small 

sharply denned bright-blue inner cones, which do not jump irregularly 

from the burner grid. The burner chimney should be shifted until the 

flame passes up through the center.

15. Turn the electric switch on the panel of the instrument to the 

"on" position and allow the instrument to warm up for approximately 

30 min. before using.

DETERMINATION OF THE METAL 

POTASSIUM

[Steps 16-29, inclusive]

16. If the approximate concentration of potassium in the solution 

can be estimated, omit steps 16 to 22 and proceed with step 23. If 

not, pour 15 to 20 ml of standard potassium solution, in which the 

potassium concentration is 50 ppm, into a 30 ml beaker. Place the 

beaker containing the standard solution in position on the beaker 

stand so that the atomizer inlet tube is immersed in the solution. As 

the solution atomizes into the flame set the element selector to the 

position of maximum response for potassium.

17. While this solution (step 16) is atomizing into the flame, set the 

internal-standard dial to approximately 95 and adjust the coarse- and 

fine-gain controls so that the meter reads 50. Remove the beaker 

containing the standard solution and with no solution atomizing set 

the meter to read 50, using the zero-adjusting knob. Replace the 

beaker with the same standard solution and while the solution atomizes 

adjust the internal-standard dial so that the meter reads exactly 50. 

Remove the beaker and wipe the outside of the liquid inlet tube of the 

atomizer' with a clean lintless facial tissue. Record the setting of the 

internal-standard dial.

18. Put 15 to 20 ml of the sample solution (step 12) in a 30 ml 

beaker and place in position so that it atomizes into the flame. Adjust 

the internal-standard dial so that the meter reads exactly 50. Remove 

the beaker, wipe the atomizer tube with a tissue, and record the setting 

of the internal-standard dial.
PART 1. DETERMINATION OF NA AND K FLAME PHOTOMETER 9

19. Should the meter read more than 50 (step 18) with the internalstandard dial set at 100, while the sample solution is atomizing, a 

concentration of potassium in the sample solution greater than 50 ppm 

is indicated. In this case, in order to get an approximate value for 

the concentration of potassium in the sample solution, set the internalstandard dial to read 100 and adjust the meter to read 50 with the 

coarse- and fine-gam controls while the sample solution is atomizing 

into the flame. Remove the sample solution and with no solution 

passing into the flame set the meter to read 50, using the zero-adjusting 

knob. Again atomize the sample solution into the flame, set the 

internal-standard dial to 100, and adjust the meter to read 50, using 

the coarse- and fine-gain controls. Remove the beaker containing the 

sample solution and wipe the atomizer tube with a tissue.

20. Place the beaker with the standard solution containing 50 ppm 

potassium in position to atomize and adjust the internal-standard dial 

so that the meter again reads 50. Remove the solution, wipe the 

atomizer tube, and record the setting of the internal-standard dial.

21. Assume the readings recorded to be proportional to the concentrations of potassium in the standard and sample solutions and calculate the approximate concentration of potassium in the sample solution, using either the values obtained in steps 17 and 18 or steps 19 

and 20.

22. Should the concentration of potassium in the sample solution be 

greater than 50 ppm a 50 ml aliquot is mixed with 50 or 100 ml of 

diluting solution, containing 100 ppm lithium and 1 percent (NH4 ) 2S04 

to furnish a solution in which the concentration of potassium does not 

exceed 50 ppm.

23. Repeat step 17 using the standard solution with the nearest 

higher concentration to the estimated concentration of potassium in 

the sample solution.

24. Repeat step 18.

25. Repeat step 18 using the standard solution with the nearest 

lower potassium concentration to the estimated concentration of 

potassium in the sample solution instead of the sample solution.

26. With the same solution (step 25) atomizing, shift the position of 

the internal-standard dial and then reset it so that the meter again 

reads 50. Record the setting of the internal-standard dial, remove the 

beaker and solution, and wipe the atomizer tube.

27. Repeat step 18.

28. Repeat step 18 using the higher standard instead of the sample 

solution.
10

29. The equation below is used to calculate the concentration of 

potassium in the sample solution after averaging the readings obtained 

for each standard solution and the sample solution.

*= (y 

where x= concentration of potassium in the sample solution, 

?/=dial reading for the sample solution, 

x\ = concentration of potassium in the lower standard

solution,

2/!=dial reading for the lower standard solution. 

x2 = concentration of potassium in the higher standard

solution, and 

7/2= dial reading for the higher standard solution.

SODIUM

[Repeat steps 16-29, substituting sodium for potassium throughout]

Sodium is determined on a portion of the same solution containing 

the internal standard which was used for the determination of potas50

40

30

o 

o

10

O.I 0.2 0.3 0.4 0.5 0.6 0.7 

Apparent increase in Na concentration (PPM)

FIOURE 2. Correction curve for the effect of potassium on sodium.
PART 1. DETERMINATION OF NA AND K FLAME PHOTOMETER 11

slum. The element selector is set to the position of maximum response 

for sodium and the concentration is determined in the same way as 

that of potassium. Since potassium, when present, causes a small 

increase in the values for sodium, more accurate results are obtained 

when a correction is made for this effect. A correction curve obtained 

for the instrument used in this work from a series of standard sodium 

solutions to which known amounts of potassium were added is shown in 

figure 2. The value for concentration of sodium is calculated the same 

way as for potassium and the correction for potassium is subtracted. 

Then the percent Na20 in the original sample can be computed.

RESULTS OF EXPERIMENTS

EFFECT OF SALTS OF ALUMINUM, IRON, CALCIUM, MAGNESIUM, 

AND OF (NH4 ) 2SO4 ON VALUES FOB SODIUM

Aluminum, iron, calcium, magnesium, and ammonium sulfates will 

be present in appreciable amounts after the breakup of the sample and 

re-solution of the residue. Table 1 shows the results of experiments to 

determine their effect on the determination of sodium. The concentrations include approximately the maximums that would be encountered in the analysis of silicate rocks. These data indicate the advisability of eliminating the effects due to the presence of aluminum, iron, 

calcium, and (NH4 ) 2S04. The effects of magnesium on the sodium 

determination are not serious enough to need consideration.

TABLE 1. Effects of salts of aluminum, iron, calcium, magnesium, and of 

SOi on the determinations of sodium

Na(ppm)

50.0

50.0

,50.0

5. 00

50.0

5.00

50.0

5.00

Added

2.500ppm Al

l,000ppm Fc

500 ppm Ca

500 ppm Ca

500 ppm Mg

.500 ppm Mg

i (NH^SOi

' (NH4 ) 2S04

Apparent 

Na(ppm)

51.7

53.1

50.1

5.84

50. 3

5. 11

50.8

5.01

Change 

(percent)

+3.4

+6.2

+.2

+lfi.

+.«

+2.2

+ 1.6

+.2

Resulting concentration includes 2 percent

EFFECT OF MAGNESIUM ON VALUES FOR POTASSIUM

After the removal of aluminum, iron, and calcium and the elimination of the effects of (NH4 ) 2S04 by maintaining it at the same concentration in samples and standards, there remain to be considered the 

effects of magnesium and sodium on the determination of potassium 

and the effects of potassium on the determination of sodium. Table 2 

shows the effects of magnesium on the potassium determination to be 

negligible.

236156 53 2
12 CONTRIBUTIONS TO GEOCHEMISTRY, 1949 

TABLE 2. Effects of magnesium on the determination of potassium

K (ppin)

50.0 

5.00

Mg 

added 

(ppm)

500 

500

Apparent 

K (ppm)

50. 3 

4.97

Change 

(percent)

+0.6 -.6

EFFECT OF POTASSIUM ON VALUES FOB SODIUM

Figure 2 shows the curve used to correct the values of sodium for 

the effects of potassium. It was prepared from results obtained when 

known amounts of KCI were added to solutions with known sodium 

concentrations. The effects are independent of the sodium concentration.

DETERMINATION OF POTASSIUM AND SODIUM IN SOLUTIONS 

CONTAINING KCI AND NaCl ONLY

A series of solutions containing known amounts of KCI and NaCl 

were prepared and then analyzed by the flame photometer. The 

results are shown in table 3. No corrections were applied to the 

values obtained for KCI.

TABLE 3. Determination of potassium and sodium in solutions containing

KCI and NaCl only

Solution No.

1

1

2

2

3

3

4

4

5

5

fi

6

7

7

8

S

9

9

Operator

8MB

WWB

8MB

WWB

8MB

WWB

8MB

WWB

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KCI

Taken

(mg)

36.1

36.1

36.1

36.1

36.1

36.1

20.1

20.1

20.1

20.1

20.1

20.1

6.11

6.11

6.11

6.11

6.11

6.11

Found

(mg)

36.4

36.0

36.0

35.9

35.7

36.1

20.1

20.1

20.2

20.1

20.2

20.0

6.09

6.04

6.10

6.11

6.16

6.18

Deviation

(percent)

+0.8

-.3

-.3

-.6

-1.1

.0

.0

.0

+.s

.0

+.5 -.5

-.3

-1.1

-.2

.0

+.8

+1.1

NaCl

Taken

(mg)

7.77

.7.77

26.3

26.3

47.0

47.0

7.77

7.77

26.3

26.3

47.0

47.0

7.77

7.77

26.3

26.3

47.0

47.0

Found

(mg)

7.86

7.85

20.3

26.4

46.7

46.4

7.92

7.90

26.6

26.5

47.1

47.1

7.92

7.91

26.7

26.7

47.1

47.3

Deviation

(percent)

+1.2

+1.0

.0

+.4

-.6

-1.3

+1.9

+1.7

+1.1

+.8

+.2

+.2

+1.9

+1.8

+1.5

+1.5

+.2

+.6

As indicated by the data the effects of sodium on the values obtained for potassium is insignificant. The values for NaCl were 

computed after the appropriate corrections, indicated in figure 2, 

were applied. Each solution was analyzed by two operators using
PART 1. DETERMINATION OF NA AND K FLAME PHOTOMETER 13

the same flame photometer. The maximum percent deviation between 

the KC1 taken and the KC1 found is 1.1 percent. The average of the 

deviations for KC1 is 0.4 percent. The maximum percent deviation 

between the NaCl taken and NaCl found is 1.9 percent. The average 

of the deviations for NaCl is 1.0 percent.

RETENTION OF SODIUM OB POTASSIUM BY A1(OH) 3 , Fe(OH) 3 , AND

CaCO3

One may expect that some alkali would be retained by the precipitate of R203 plus CaC03 . Experiments were performed to determine 

if retention of alkalies by the precipitate would be significant when 

the suggested procedure is followed. Table 4 shows the results 

obtained when solutions containing various known amounts of Na2S04 

or K2S04 ; plus A1 2 (S04)3, equivalent to 0.15g A1203 ; Fe2 (S04) 3 , equivalent to 0.05g Fe2O3 ; and CaSO4 , equivalent to 0.05g CaO, were carried 

through the procedure outlined for removal of the R2 03 group and 

calcium. The amounts of aluminum, iron, and calcium present were 

approximately the amounts which would be expected in a typical 

silicate-rock sample. The experiments indicate that sodium and 

potassium are not retained by the precipitate of R203 plus CaC03 

and also that the volumetric error due to the precipitate is insignificant.

TABLE 4. Retention of sodium or potassium by Al(OIT)z-}-Fe(OI-r)z-{-CaCOz

Na2 O

Taken

(mg)

1.3

(i.2

12.5

25.0

Found

(mg)

1.3

6.3

12.6

25.2

K2 0

Taken

(mg)

1.3

6.2

12.5

25.0

Pound

(mf?)

1.3

6.2

12.5

25.0

DETERMINATION OF SODIUM AND POTASSIUM IN NATIONAL 

BUREAU OF STANDARDS STANDARD SAMPLES

Six National Bureau of Standards standard samples were analyzed 

by the suggested procedure. In table 5 the results with the flamephotometer method are compared with values reported by the National 

Bureau of Standards. The results obtained with the flame photometer 

seem to be as good or better than would be expected by gravimetric 

procedures where the Na20 or K2 0 content is above 1 percent; below 

this percentage the flame-photometer method is much to be preferred. 

With the flame photometer measurements can be made in these 

lower ranges of alkali content with essentially the same precision as 

with larger percentages. With gravimetric procedures, on the other 

hand, small percentages are determined with less certainty owing to
14 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

limitations in accuracy of weighings and possible contamination of 

the separated compounds.

TABLE 5. Determinations of sodium and potassium in National Bureau of 

Standards standard samples

N. B. S. sample No. and type

70 (feldspar).--..------.--------------- _ .

97 (clay)... .................................

98 (clay)...........-.....--.....--........

99 (feldspar)....... __ .......... __ ------

Determination

/Na2O__. ..___.___.

\KzO...... ......... 

/NasO.... .........

\K2 0..... .......... 

/Na2 0. ........... .

\K2 0. .......... .... 

Na2O. .-..--.. ....

K2 0. ............. . 

Na2 0. ......... ....

KZ O. ......... ..... 

Na2 0. ............

K2O. ....... .......

N. B. S. 

values

2.38 

12.58 

8.48 

3.25 

.12 

.54 

.28 

3.17 

10.73 

.41 

4. 16 

.16

Flame-photometer method

2.33 

12.73 

8.50 

3.22 

.07 

.54 

.25 

3.24 

10.70 

.41 

4.30 

' .15

2. 3.5 

12.79 

8.51 

3. 25 

.07 

.54 

.25 

3.23 

10.69 

.43 

4.28 

.14

2.32 

12.63

.07 

.55 

.28 

3.23 

10.7-2 

.43 

4.28 

. 15

LITERATURE CITED

Barnes, R. B., Richardson, David, Berry, J. Y., and Hood, R. L., 1945, Flame

photometry: Ind. and Eng. Chem., Anal. Ed., vol. 17, pp. 605-611. 

Berry, J. W., Chappell, D. G., and Barnes, R. B., 1946, Improved method of flame

photometry: Ind. and Eng. .Chem., Anal. Ed., vol. 18, pp. 19-24. 

Lundegardh, H., 1929, Die quantitative Spectroanalyse der Elemente, Ft. ],

Jena, Gustav Fischer. 

 1934, Die quantitative Spectroanalyse der Elemente, Pfc. 2, Jena, Gustav

Fischer. 

Perkin-Elmer Corp., 1948, Instruction manual, model 52A flame photometer,

Glenbrook, Conn. 

Schmitt, L., and Breitweiser, W., 1938, Die Anwendbarkeit verschiedener

Apparaturen sur flammenphotometrischen Bestimmung des Kali urns in

Pflanzenaschen nach Neubauer: Bodenkunde U. Pflanzenernahr; vol. 9-10,

pp. 750-57.
Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Part 2. A PHOTOMETRIC METHOD FOR THE ESTIMATION 

OF THE OIL YIELD OF OIL SHALE

BY FRANK CUTTITTA

CONTENTS

Page

Abstract.._.__-____--...____._____---___-.....__________.______... 16

Introduction _______________________-_____---_-____-_-------.__--__ 16

Acknowledgments. _ _______________________________________________ 19

Procedure for estimating the oil yield______-_--______--_-___-_-___-_. 20

Preparation of the sample______________________________________ 20

Apparatus. ___________________________________________________ 20

Recommended procedure.-------------.------------------------ 21

Calculations _______-_____-______-_--____---______-----_____--_ 22

Experimental data and discussion.-._________________________________ 22

Effect of sample weight on oil yield,_____________________________ 22

Effect of heating temperature on oil yield___ - _____________________ 23

Effect of heating time on yield.__--.___----________._---____--__ 24

Selection of suitable filter.______----____-___----___--_--__-__-__ 25

Effect of solvent on color._________-_____'______________-_______- 25

Color stability of the toluene system...___.--________-________._. 26

Color similarity of various oils__________________________________ 27

Relation of optical density to Fischer oil yield_____________________ 27

Summary.. _ ______________________________________________________ 30

Literature cited.__________________________________________________ 31

ILLUSTRATIONS

Page 

FIGURE 3. Relation of optical density (X10) to percentage oil yield, by

Fischer assay method__________________________________ 17

4. Relation of optical density (X10) to Fischer oil yield, in

gallons per ton________________________________________ 18

5- Details of tubular electric furnace---_-_________--.________ 20

15
16 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

TABLES

Page

TABLE 1. Specific gravities of oil distillates.___________________________ 23

2. Effect of sample weight upon oil yield_______________________ 23

3. Effect of heating temperature on oil yield_____________________ 24

4. Effect of heating time on oil yield__________________________ 25

5: Selection of suitable filter___i______________________________ 25

 6. Effect of solvent on color._________________________________ 26

7. Color stability of the toluene system._______________________ 26

8. Color similarity of various oils _____________________________ 27

9. Relation of optical density to Fischer oil yield_______________ 28

A method is presented for the distillation and photometric estimation of the 

oil yield of oil-bearing shales. The oil shale is distilled in a closed test tube and 

the oil extracted with toluene. The optical density of the toluene extract is used 

in the estimation of oil content and is converted to percentage of oil by reference 

to a standard curve. This curve is obtained by relating the oil yields determined 

by the Fischer assay method to the optical density of the toluene extract of the 

oil evolved by the new procedure. The new method gives in a much shorter time 

results similar to those obtained by the Fischer assay method. The applicability 

of the new method to oil-bearing shale and phosphatic shale has been tested.

INTRODUCTION

As thousands of oil-shale samples have been assayed in the Trace 

Elements laboratory, Geochemistry and Petrology Branch, the purpose of the investigation was to study the applicability of photometric 

procedures to the estimation of oil yield of oil-bearing shales in an 

effort to develop a simple and rapid method for the determination 

of oil content of shales and phosphatic shales.

The results of the determination should have a direct correlation 

with the results obtained by the Fischer assay method so that Fischer 

determinations could still be used.

The analysis of oil shales is usually made by the modified Fischer 

retort method hereinafter referred to as the Fischer method or 

Fischer assay (Stanfield and Frost, 1946, 1949; Karrick, 1926; Guthrie, 

1938). In this method the shale is heated in a special cast-aluminum 

retort, and the oil yield is determined by condensing and measuring 

the volatilized oil. Using one still, an assay by the Fischer method 

requires approximately 2 hours for completion. The Fischer method 

also requires rather elaborate apparatus and is not a convenient 

method for fast routine work where the primary object is to screen 

large numbers of samples.

Closed-tube distillation of oil has been used for qualitative determinations by Trask (1932) and Takahashi (1923). When a rock rich in 

bitumens is subjected to destructive distillation, the oil formed by the
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 17

11

10

6 7

A Mudstones from the Phosphoric formation, 

Montana and Idaho

* Chattanooga shale, Tennessee

1 2 3 4-5 678

PERCENT OIL YIELD ( FISCHER) 

FIGURE 3. Relation of optical density (X10) to percentage oil yield by Fischcr assay method.

distillation accumulates in the horizontal test tube just behind the 

water. Generally, the lightest constituents of the oil are partly 

mixed with the water, and the heavier and darker fractions are found 

toward the hot end of the tube. Looking down the tube during the 

heating, one sees droplets of oil issuing as a fine mist from the sample
18 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

10

3 6 9 12 15 18 21

OIL YIELD IN GALLONS PER TON (FISCHER) 

FIGURE 4. Relation of optical density (X10) to Fischer oil yield, in gallons per ton.

and condensing on the sides. The volume of oil produced by distillation varies greatly and may be so slight that no distillate is visible and 

only the odor of oil can be detected.

In attempting to estimate the oil content of oil-bearing shale, no 

correlation could be established between the Fischer assay and a 

gravimetric determination of the oil evolved in the closed-tube distil-
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 19

lation. However, volumetric determination of the oil evolved in the 

closed-tube distillation was about twice as rapid as the Fischer assay 

method and gave results comparable to those obtained by the Fischer 

method. Details of the volumetric determination are given in figure 3.

The present report shows the feasibility of using the optical density 

of a toluene solution of the oil evolved in a closed-tube distillation as 

a rapid and quantitative method for the estimation of the oil yield. 

The method consists of destructively distilling a 0.5g sample of oil 

shale in a closed test tube, extracting the oil evolved with toluene, and 

measuring the optical density of the solution with a filter photometer. 

The amount of oil is read from a curve (fig. 3) relating the oil yields by 

Fischer assay to the optical density of the toluene extract of the oil 

distilled by the new procedure. To obtain this curve the optical 

densities (X10) of the oil obtained by the new procedure were plotted 

against the percentage of oil as given by the Fischer method. A 

scatter diagram was obtained and a mean straight curve was fitted 

visually through the points. The estimated oil yields as obtained by 

using the standard curve (fig. 1) differed from the Fischer oil yields by 

an average of 0.37 percent absolute or approximately 1 gal per ton. 

The maximum difference between the determined (Fischer assay) 

and the estimated oil yields was 0.94 percent or about 2.5 gal per ton. 

Figure 4 is the same curve expressed in terms of the relation of optical 

density (X10) to Fischer oil yield in gallons per ton of shale.

This linear relationship (fig. 3) was established from a study of 75 

samples of Chat'tanooga shale from Tennessee chosen to represent the 

different grades of shale likely to be encountered in that area. The 

recommended procedure is to establish similar relationships of optical 

density to oil yield for other specific areas, such as Idaho and Montana, 

from representative samples. However, a study of 10 samples from 

the Phosphoria formation in Montana and Idaho showed that figure 

3 established for samples of Chattanooga shale was applicable also 

for assay of these samples. This similarity in relationship of optical 

density to oil yield for the two areas gives some hope that the same 

curve can be applied to the use of this method for oil shales from 

other areas.

The method is simple, rapid, and accurate, and 8 to 10 determinations can be made in the same length of time required for the completion of one Fischer assay.

ACKNOWLEDGMENTS

This work was completed as part of a program undertaken by the 

Geological Survey on behalf of the U. S. Atomic Energy Commission.
20 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

A representative portion of the sample, ground to 80 mesh, is 

dried in an oven at 110 C for 1 hour. Oven drying may be omitted 

for samples to be assayed on the "as received" basis. A 80 mesh 

sample size was selected because this is the size generally used in the 

U. S. Geological Survey for chemical analysis. No attempt was made 

to determine the effect of particle size upon oil yield.

APPARATUS

The test tube used in the closed-tube distillation of the shale is a 

pyrex glass-stoppered tube, 15X50 mm with a no. 16 standard taper 

glass stopper.

The centrifuge tube used to measure the oil yield of the oil shale is 

the Goetz Phosphorus tube, and the centrifuge must be large enough 

to handle the centrifuge tube.

-Chromel-alumel thermocouple 

to indicating pyrometer

Alundum coating 

Asbestos

Nichrome element

No 26 B and S nichrome element 

to powerstat, 8 windings per inch

FlOORE -Details of tubular electric furnace.

The tube furnace used was made by winding a coil of nichrome 

(no. 26 B and S,) the coils spaced about % inch apart, on an iron tube 

that has been covered with a piece of asbestos (or mica) to avoid 

short circuiting the winding. The inner tube and its resistance wire 

are covered with several layers of asbestos and finally coated with 

alundum (alumina with clay binder). The details of this furnace are 

shown in figure 5. The embedding of the nichrome element in asbestos 

and alundum offers protection from physical damage, minimizes 

oxidation, and assures a uniform distribution of heat.
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 21

A chromel-alumel thermocouple is used for the temperature measurement. Such couples, matched to give the temperature to ±5 C, are 

obtainable from various manufacturers. The indicating pyrometer is 

calibrated in both Fahrenheit and centigrade scales in 50-degree 

increments up to 2,000 F and 1,100 C.

Temperatures are regulated by an input control. The variable 

transformer used was. for 115-volt line, frequency 50/60, output 

voltage of 0-135, and a maximum output amperage of 7.5. A fixed 

setting of the variable transformer permitted a reasonably constant 

temperature in spite of the usual line-voltage fluctuations.

The furnace is designed to provide a fast rate of heating with good 

temperature control. It will stand nearly continuous use at temperatures up to 600 C and may be used for short intermittent periods up 

to a maximum of 900 C.

RECOMMENDED PROCEDURE

A O.SOOg sample of the dry shale crushed to pass an 80-mesh sieve 

is weighed into a 25ml pyrex, glass-stoppered test tube. The lower 

one-third of the test tube is heated in a horizontal position in the 

specially constructed small tubular electric furnace at 480 to 500 C 

for 11 to 14 min. It is essential that strict adherence to the predetermined time and temperature conditions be observed. The 

exposed upper two-thirds of the stoppered test tube acts as an air 

condenser. When the distillation period is complete, the closed test 

tube is removed from the tube furnace and is then allowed to air cool 

to room temperature.

Then 10ml of toluene are added, the tube is stoppered, and vigorously shaken for 20 sec. The oil evolved by the destructive distillation 

and extracted by the toluene is filtered through a 9cm, white, no. 589 

S and S paper, and the filter paper and residue are washed with two 

5ml portions of toluene. The combined extract and washings are 

diluted to a final volume of 25ml with toluene. The optical density 

(XIO) of the solution of toluene and oil is measured at 420 mju (blue 

filter) on a filter photometer with pure toluene set at 100-percent 

transmission. The instrument used in this investigation was the 

Lumetron Colorimeter.

The oil yield of the shale is then determined by reference to the 

standard curve established for that particular oil-shale deposit. The 

standard curve was obtained from a study of 80 shale samples assayed 

in the manner described above. The optical densities (XIO) of oil 

obtained by the new procedure were plotted against the percentage of 

oil as given by the modified Fischer retort method (fig. 3.) A scatter 

diagram was obtained and a mean straight line was drawn through 

the points. (See section on relation of optical density (XIO) to
22

Fischer oil yield under "Experimental data and discussion" for further 

details.)

To avoid the effect of regional differences in oil yield the 

recommended procedure is to establish standard curves, as in figure 

3, for specific areas. A study of these particular samples from Tennessee, 

Montana, and Idaho showed the applicability of the relation between 

optical density and oil yield (fig. 3) to the two deposits. It is possible 

that the same standard curve may be applicable to oil shales from 

other areas.

In order to obtain reproducible results, the following precautions 

must be observed:

1. The sample must be mixed thoroughly and be representative 

of the material submitted for assay.

2. The closed-tube system must be gas tight. As a precaution 

against loosening of the stopper, it is advisable to stopper the test 

tube after heating has begun and prior to the evolution of vapors from 

the sample.

3. The proper heating must be maintained within the prescribed 

limits of time and temperature. To insure correct temperature 

measurements, the pyrometer must be checked or standardized 

periodically.

CALCULATIONS

The graph relating the optical density (X10) to the percentage of 

oil yield by the Fischer method is shown in figure 3. The relation of 

optical density (X10) to oil yield in gallons per ton (Fischer) is 

illustrated in figure 4. Figure 4 was derived from figure 3 by taking 

an average specific gravity of oil as 0.903. For oil-shale samples 

from Tennessee, the standard deviation of the specific gravity of the 

oil (table 1) is 0.013. The following formula is used to convert percentage of oil to gallons per ton:

Gallons of oil per ton of shale=2.65Xpercentage of oil yield.

EXPERIMENTAL DATA AND DISCUSSION

The initial work in the development of a new method to determine 

the oil content of oil shales was to ascertain the effects of different 

experimental conditions upon the oil yield.

EFFECT OF SAMPLE WEIGHT ON OIL YIELD

Samples of Chattanooga shale having a Fisclier assay oil yield of 

3 percent ranging from 0.250g to 2.000g, in 0.250g intervals, were 

heated at 480 C to 500 C for 11 to 14 min. The oil evolved was 

extracted with toluene, filtered, and diluted to a final volume of 25 ml.

The results of these tests are shown in table 2. Within limits of 

experimental error, there was no significant difference in the oil yields
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 23

for the different weights of sample. The oil yield of the shale was 

determined by using the standard curve (fig. 3) and interpolated to a 

percent-by-weight yield.

The 0.500g sample was adopted primarily because this size sample 

gives favorable density readings for the range of 0.1 percent to 9.9 

percent oil.

TABLE 1. Specific gravities of oil distillates. (60 F/60 F)

Number of samples, 33 

Specific-gravity, mean, 0.903 

Sd, 0.367

Average deviation, 0.011 absolute 

Standard deviation, 0.013

Sample No.

LC-15-12

.15

31

33

52

LC-IO-32

33

34

35

LC-H-13

15

21

22

31

52

] 711-6- 1

2

3

4

5

6

7

8

9

10

. 11

13

14

15

16

17

18

19

Specific 

gravity

0.924

.920

.902

.910

.895

.915

.926

.915

.926

. 903

.918

920

921

.923

.888

.899

.906

.905

.904

.903

.889

.896

.891

.891

.886

.883

.897

.899

.903

.$,14

.922

.916

.916

Deviation 

(d) from 

mean

+0. 021

+.017

-.001

+.007

-.008

+ 012

+.023

+.012

+.023

.000

+. 015

+ 017

+.018

+.020

-.015

-.004

+.003

+.002

+. 001

.000

-.014

-.007

-.012

-.012

-.017

-.020

-.006

-.004

.000

+.011

+.019

+.013

+ 013

Deviation 

squared 

(d')X109

441

289

t

49

64

144

529

144

529

0

225

289

324

400

225

16

9

4

1

0

196

49

144

144

289

400

36

16

0

121

361

169

169

TABLE 2. Effect of sample weight on oil yield

Sample weight 

(grams)

0.250 

.500 

.750 

1.000

Optical density 

(X 10)

1.55 

3.50 

5.55 

7.70

Oil yield (percent by weight)

3.10 

2.95 

2.86 

2.90

Sample weight 

(grams)

1. 250 

1. 500 

1.750 

2.000

Optical density 

(X10)

10.40 

12.00 

14.50 

17.10

Oil yield (percent by weight)

3.10 

2.92 

3.00 

< 3. 09

EFFECT OF HEATING TEMPERATURE ON OIL YIELD

A study was made of the effect of temperature on the oil yield. A 

heating time of 13 min was chosen arbitrarily. A 0.500g aliquot of a 

Chattanooga shale sample containing 4.5 percent oil determined by
24 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

the modified Fischer assay was heated in the manner prescribed 

under Recommended procedure (p. 21) at the temperatures indicated 

in table 3. The oil evolved by the distillation was extracted with 

toluene, filtered, and diluted to a final volume of 25 ml. Measurements of the optical densities (X 10) were made with a Lumetron 

Colorimeter at 420 mju (blue filter).

The results showed that the oil yield is dependent upon the temperature of heating. Table 3 shows that the best temperature range 

for obtaining reproducible oil yields is from 480 C to 500 C. The 

average optical density (X 10) of the toluene solution of the oil distilled at this temperature range (480 C to 500 C) was 5.95. By consulting the standard curve (fig. 3), it was determined that an optical 

density of 5.95 coincides with a 4.65-percent oil yield, which is very 

near the oil content of the shale by Fischer assay.

TABLE 3. Effect of heating temperature on oil yield

Temperature

(C)

260

335

400

420

440

460

Optical density

(X10)

0.10

.40

2. 95

3.80

4.90

5.60

Temperature

(C)

'480

' i 480

1 480

'500

520

540

Optical density

(X10)

5.90

6.00

5.90

6.00

6.20

6.90

Temperature

(C)

575

600

650

675

705

Optical density

(X 10)

7.30

7.60

8. 10

8.50

9.25

i Average optical density (X10) in the temperature range 480-500 C is 5.95.

EFFECT OF HEATING TIME ON YIELD

The effect of heating time on the oil yield was studied. A 0.500-g 

sample of Chattanooga shale containing 4.5 percent oil was heated at 

480 C to 500 C for the times indicated in table 4. The oil evolved 

by destructive distillation was extracted with toluene, filtered, and 

diluted to a final volume of 25 ml. Measurements of the optical 

densities (X 10) were made with a Lumetron Colorimeter at 420 m/u 

(blue filter).

The results of these tests are shown in table 4. It should be noted 

that two apparently optimum heating times exist, one from 8 to 10 

min and the other from 11 to 14 min. A series of tests with low-grade 

oil shales showed that a greater degree of reproducibility was attained 

when the samples were heated from 11 to 14 min than if heated from 

8 to 10 min. Thus, it was concluded that heating at 480 C to 500 C 

for 11 to 14 min gives the most satisfactory and reproducible oil 

yields. The average optical density (X 10) of the toluene solution of 

the oil distilled at 480 C to 500 C for 11 to 14 min was 6.11. Using 

the standard curve (fig. 3), the oil yield related to an optical density 

of 6.11 is 4.72 percent. It should be noted that these conditions 
PART 2. PHOTOMETRIC ESTIMATION OIL . FROM SHALE 25

heating a 0.500-g sample at 480 C to 500 C for 11 to 14 min give 

results that are in agreement with the Fischer method.

TABLE 4. Effect of heating time on oil yield

Time of heating 

(minutes)

2

4

C

7

8

9

10

Optical density 

(X10)

1.35

4.80

5. 00

5.70

5.85

5.80

5.82

Time of heating 

(minutes)

11

12

13

14

15

1f>

18

Optical density 

(X 10)

5.90

6.15

0.20

0.28

0. 35

(i. 00

0.70

Time of- heating 

(minutes)

20

22

24

:to

35

40

Optical density 

(X 10)

0.90

0.70

0.80

0. 90

7, 00

5. 38

SELECTION OF SUITABLE FILTER

The following tests were made to determine the best filter for 

density measurements. A composite oil was made by combining 1 

ml of each of the oil distillates obtained by the Fischer method from 

19 samples of Chattanooga shale. Series of 0.50-ml aliquots of the 

composite oil were diluted to 50 ml with the solvents indicated in 

table 5 to form stock solutions. A 2-ml aliquot of each of the stock 

solutions was further diluted to 25 ml with the respective solvents. 

The optical densities (X 10) were measured with a Lumetron Colorimeter. A number of filters were available with the filter photometer 

used. The one which gave the greatest optical density with the oil 

solution referred to pure toluene as a blank was the blue filter 

(420 nut). Table 5 shows the variation of the optical density measured with the various filters. The blue filter (420 m/x) was found to 

be best for all solvents tried; it was therefor selected for the final 

procedure.

TABLE 5. Selection of suitable filler

Filter Optical density (X 10)

Color

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

Blue ijre'cn. ..........

Blue... ..............

Violet...............

Wavelength 

(m/j)

050

580

530

490

420

370

Chloroform

0.40

.53

1.05

1.25

3.29

1.14

Carbon 

tetrachloridc

0.70

.95

1.54

1.75

3.57

1.50

Ethyl 

acetate

.20

50

.60

2.59

.72

Carbon 

disulflde

0.40

.56

1.04

1.28

3.62

1.08

Benzene

14

.68

.87

3.02

.90

Diethyl 

ether

.16

K.A

.68

2.58

.70

Toluene

0.25

.34

.70

.85

3.00

.88

EFFECT OF SOLVENTJDN^COLOR

The effect of solvent on color and the stability with time was 

studied. A composite oil was made by combining 1 ml of each of the 

oil distillates obtained by the Fischer method from 19 samples of 

Chattanooga shale. A 0.50-ml aliquot of the composite oil was diluted to 50 ml with the solvents indicated in table 6 to form stock
26 CONTRIBUTIONS TO GEOCHEMISTRY, 1940

solutions. Two milliliters of this stock solution were further diluted 

at once to a final volume of 25 ml with each of the different solvents. 

Optical densities (X 10) were measured at 420 nut (blue filter) with 

a Lumetron Colorimeter after various intervals of time. The results 

are given in table 6.

TABLE 6. Effect of solvent on color through change in optical density

Solvent

Measure of optical density (X 10) 

Immediately

3.29 

3.57 

2.59 

3.62 

3.02 

2.58 

3.00

After 1 

day

3.70 

4.25 

2.63 

2.65 

3.08 

2.59 

3.00

After 2 

days

4.2ft 

4.80 

2.58 

3.72 

 3.02 

2.64 

2.99

After 5 

days

4.75 

5.00 

2. 58 

3.75 

3.04 

2.69 

2.99

After 7 ' 

days

5. 80 

5.90 

2.61 

3.72 

3. 03 

2.68 

2.99

The color of the oil solution was found to be dependent on the solvent used. For the same oil concentration, the colors in chloroform, 

carbon tetrachloride, ethyl acetate, carbon disulfide, benzene, diethyl 

ether, and toluene ranged from pale yellow to reddish amber. Some of 

the solvents yielded unstable color systems; the color intensities increased with time. Benzene, toluene, and ethyl acetate gave stable 

reproducible color systems. The use of ethyl acetate as the solvent 

was precluded by its low boiling point and the ease of its hydrolysis. 

Toluene was chosen as the solvent because it is somewhat less a fire 

hazard and less toxic than benzene.

TABLE 7. Color stability of the toluene system

Aliquot 

(ml)

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

Measure of optical density (X 10) 

Immediately

1.07

1.95

2.71

3.50

4.22

4.87

5.55

6.18

1.11

1.96

2.74

3.51

4.20

4.90

5.55

6.18

After 3

days

1.08

1.92

2.73

3.49

4.21

4.90

5.58

6.25

1.10

1.90

2.77

3.53

.4.20

4.89

5.60

6.20

After 3

months

1.10

1.97

2.75

3.52

4.18

4.75

5.50

6.15

1.10

1.98

2.78

3.52

4.12

4.74

5.55

6.13

COLOR STABILITY OF THE TOLUENE SYSTEM

Table 7 shows^that the'toluene color system is exceptionally stable. 

Approximately the same optical-density readings were obtained
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 27

whether the densities were measured within 5 minutes or after 3 

months. The data in table 7 were obtained on various concentrations 

of the composite oil in toluene.

COLOR SIMILARITY OF VARIOUS OILS

Tests were made to determine if samples from one formation but 

taken at different locations would yield oil having about the same 

color characteristics'" A limited number of tests were also made to 

determine if the relationship of figure 3 would hold for oils from different formations and locations.

Fifteen samples of Chattanooga shale from Tennessee picked at 

random were checked for possible deviations in the color of,the oil. 

Table 8 gives the optical densities (X10) of solutions of the same 

concentrations of the various oils in toluene. The results indicate 

that oils from the same formation but from different locations would 

yield essentially the same densities. The range of the optical densities (X10) is from 9.90 to 10.90.

TABLE 8. Color similarity of various oils

Number of samples, 15. 

Specific gravity, mean, 10.17. 

Average deviation, 0.34 absolute.

Lab. No.

2027

3207... 

3208- 

3216.....--......-.--.....

Optical 

density 

(X10)

9.90 

« i 1Q-.QGL, 

10.00 

10.00 

10.10 

10.30 

10.30 

10.40

Deviation 

from mean 

(d)

-0.27 

Vi_tu _ _17^ 

-.17 

-.17 

-.07 

+.13 

+.13 

+.23

Lab. No.

2053-------------

,3m..,:.,.. . ......

2052-. 

2050.-..-.....---..--.-..

2051....-...-.-.-..-.-...

2054. .....................

2046. ..

Optical 

density 

(X 10)

10.40 

10.60 

10.70 

10.80 

10.80 

10.80 

10.90

"'Deviation 

from mean 

(d)

+0. 23 

+.43 

+.53 

+.63 

+.63 

+.63 

+.73

In addition, it should be noted that the 80 samples studied in the 

determination of the optical density oil yield relationship (fig. 3) 

showed from the linear nature of the scatter diagram that oils from 

different formations and localities were similar.

This relationship was used as the basis for correlating the proposed 

photometric procedure to the modified Fischer retort method.

RELATION OF OPTICAL DENSITY TO FISCHER OIL YIELD

Seventy-five samples of oil shale of the Chattanooga were distilled 

in the manner described under Procedure. The oil evolved was extracted with toluene, filtered, and diluted to a final volume of 25 ml. 

Measurements of the optical densities (X10) were made \vith a Lumetron Colorimeter at 420 M (blue filter). The optical densities (X10) 

were plotted against the percentage of oil yield as given by a Fischer

236156 53 3
28 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

assay. A scatter diagram was obtained, and a straight line was 

drawn which was the mean of the points. The average difference of 

the Fischer assays from the curve is 0.37 percent absolute (or approximately 1 gal. per ton). The maximum difference between the determined (Fischer assay) and the curve is 0.94 percent (or about 2.5 gal. 

per ton). The percentages of oil yield and the optical densities (X10) 

of the toluene-oil solutions are given in table 9.

TABLE 9. Relation of optical density to Fischer oil yield Chattanooga shale

( Tennessee)

Sample Percentage oil 

(Fischer)

Optical 

density 

(X 10)

Difference of 

Fischer assay 

from standard 

curve

Lot 1001

LC-15-11

51

21

53

31

. 12

52

13

15

14

LC-15-33

0.2

.8

1.7

1.8

1.9

2.4

2.4

2.7

2.8

3.4

4.1

0.40

.85

.85

1.35

2.18

1.90

2.25

2.25

2.70

3.20

4.80

-0.35

-.30

+.62

+.38

-.10

+.60

+.36

+.44

+.44

+.70

+.30

Lot 1002

LC-10-14

15

31

LC-11-14

LC-10-13

LC-11-13

LC-10-32

LC-11-15

51

LC-10-34

33 LC-11-52

31

LC-10-35

LO-11-32

LC-11-22

0.8

1.0

1.4

1.8

1.8

2.1

2.7

2.9

3.0

3.0

3.5

4.2

4.3

4.5

4.9

5.0

0.95

1.28

2.00

1.80

2.15

2.15

3.45

2.70

2.25

3.70

4.60

4.90

6.60

6.25

6.70

6.40

-0.36

-.38

-.48

+.06

-.16

+.12

-.18

+.52

+.94

-.06

-.16

+.32

-.74

-.32

-.22

+.08

Lot 1005 (sample LC-105A-X)

LC-105A-43

42

45

41

15

21

12

31

14

13

33-#2

33-#l

34

35-#l

35-#2

32

0.9

1.0

1.0

1.3

2.1

2.4

3.8

4.0

4.1

4.2

4.2

4.2

4.4

4.5

4.5

4.6

0.65

.00

.50

.10

2.90

2.90

5.40

4.40

5.80

4.80

6.30

6.50

6.10

5.60

6.20

6.60

-0.04

+.50

+.18

+.74

-.40

-.10

' -.40

+.46

-.40

+.40

-.64

-.78

-.30

+.12

-.28

-.44
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 29

TABLE 9. Relation of optical density to Fischer oil yield Chattanooga shale

(Tennessee) Continued

Sample Percentage oil 

(Fischer)

Optical 

density 

(X 10)

Difference of 

Fischer assay 

from standard 

curve

Lot 1012 (sample 13M-7-X)

13M-7-22

11

31

52

21

33

16

15

14

32

51

12

13

1.3

2.3

2.5

2.8

3.2

3.3

3.6

3.7

4.4

4.5

4.6

4.9

5.6

1.20

3.10

3. 55

3.45

3.30

5.25

4.60

5.25

6.40

6.70

5.20

5.90

7.10

-0.02

-.34

-.44

-.10

+.42

-.80

-.06

-.42

-.50

-.62

+.52

+.32

+.20

Lot 1052 (Sample 17R-6-X)

17R-6-10

9

8

11

7

15

12

18

6

13

14

Ifi

17

4

. 5

19

3

2

1

2.3

2.5

2.5

2.7

2.8

2.9

2.9

2.9

2.9

3.0

3.2

3.2

4.3

4.3

4.5

4.6

5.2

 6.0

6.7

1.60

2.60

2.20

2.15

3.40

2.50

2.80

3.70

3.80

2.95

3.50

4.20

5.10

6. 50

fi.40

6. 00

6.20

6.60

9.00

+0.70

+.20

+.50

+.70

-.04

+.68 .

+.48

-.14

-.22

+.42

+.28

-.18

+.30

-.70

-.40

-.02

+.40

+.92

-.10

The choice (visual fitting) of the standard curve is based upon the 

assumption that positive and negative deviations of the same magnitude are equally probable. This assumption is verified by the fact 

that a visual inspection shows the pattern of the scatter diagram to be 

essentially symmetrical with respect to the standard curve. Thus, 

the standard curve can be compared to a distribution function of a 

random sequence.

?i=80 

Srf= 29.811|

Average difference of the Fischer assays from the curve

1111 |rfi| + |d2 | + . . + K| 29.81 nof7 . . ., !_L_!Li = - . =0.37 percent absolute. 

n 80

From appearances of the scatter diagram (fig. 3), it is believed that 

the oil content of shales may be determined by the photometric procedure with essentially the same accuracy as by other methods now 

in use.
30 CONTRIBUTIONS TO GEOCHEMISTRY; 1949

To show whether there are variations in the optical properties of 

the oil from widely separated areas (Phosphoria formation and Chattanooga shale) a closed-tube distillation was made of 10 samples of mudstone from the Phosphoria formation in Montana and Idaho (indicated on fig. 3 by the symbol A). The percentage of oil yield and the 

optical densities of the toluene-oil solutions are given in table 9 (f). 

Figure 3 shows that the data for the Phosphoria formation fit the 

curve that was drawn for the Chattanooga shale.

TABLE 10. Relation of optical density to Fischer oil yield Mudstone from the 

Phosphoria formation of Montana and Idaho, Lot

[n, 5 (samples containing no oil were not considered) Srf, 1.80]

Sample

BAG-57-47- .............................................

RAH-250-47 __ ..... .................................

RAH-253-47 i ~ ........ . -

DAB-39... - -. -

ERC-36 . .......... . . .. . ...

LAT-22' -. .-

EEC-26 i. .......... 

LAT-24... 

Percentage 

oil 

(Fischer)

0.0

.0

.0

.0

.0

2.8

3.6

4.5

5.2

5.5

Optical 

density 

(X 10)

0.00

.05

.00

.00

.05

3.75

5.00

G. GO

6.20

Difference of 

Fischer assay 

from standard curve

-0. 30

-.35

1 Oft

+.15

+.70

1 0.250-g samples of ERC-26 and LAT-22 were used.

The recommended procedure in making a standard curve is to base 

it on those samples under study. For other samples, a new curve 

should be made.

SUMMARY

A photometric method for the determination of the oil content of 

oil shale is presented. The proposed method, which utilizes distillation in a closed test tube followed by measurement of the density of a 

toluene solution of the oil, provides a means of determining arbitrarily 

the oil content of oil shale by the destructive distillation of its organic 

matter. This method, like the Fischer method, does not determine 

the total organic matter in shale that may be converted more completely to oil under more favorable conditions. The proposed method 

has several immediate advantages over the modified Fischer retort 

method and is recommended for oil-shale assays. The advantages 

are :

1. Less time is required for the assay. Oil-shale assa}^s by the 

Fischer method require approximately 2 hours for completion, whereas 

by the proposed method 8 to 10 determinations can be completed in 

the same length of time.

2. Less sample is required. A 100-g sample is used in the Fischer 

assay whereas a 0.500-g sample is used in the proposed method. In
PART 2. PHOTOMETRIC ESTIMATION OIL FROM SHALE 31

addition, the same representative sample ( 80 mesh) can be used for 

other analyses.

3. The operation is less difficult and can be controlled more precisely.

The effects are shown of different experimental conditions upon the 

oil yield by the proposed method: effect of heating time on yield, 

effect of heating temperature on yield, effect of solvent on color, 

stability of color in toluene, color similarity of oils, and the relation of 

optical density to the Fischer yield.

The proposed method, like the modified Fischer retort method, is 

an empirical measure, and it has been directly correlated with the 

Fischer method. The estimated oil yields are considered as reliable 

as the values obtained directly by the Fischer assay of the samples. 

The method is fast enough for certain control purposes in mining and 

processing oil shale for which rapid and accurate results are desired, 

such as in the assay of thousands of samples.

LITERATURE CITED

Guthrie, B., 1938, Studies of certain properties of oil shale and shale oil: U. S.

Bur. Mines Bull. 415. 

'Karrick, L. C., 1926, Manual of testing methods for oil shale and shale oil: U. £.

Bur. Mines Bull. 249. 

Stanfield, K. E., and Frost, C. I., 1946, Method of assaying oil shale by a Modified

Fischer Retort: U. S. Bur. Mines Kept. Inv. 3977. 

 1949, Method of assaying oil shale by a Modified Fischer Retort: U. S.

Bur. Mines Rept. Inv. 4477. 

Takahashi, Junichi, 1923, The marine kerogen shales from the oil fields of Japan:

Tohoku Imp. Univ. Sci. Repts., 3d ser., vol. 1, pp. 63-156. 

Trask, Parker D., 1932, Origin and environment of source sediments of petroleum,

Houston, Tex., Am. Petroleum Inst.

Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Part 3. A VOLUMETRIC METHOD FOR THE ESTIMATION 

OF THE OIL YIELD OF OIL SHALE

BY FBANK CUTTITTA

CONTENTS

Abstract__---_-----_--__.________________________._____________ 33

Introduction._____________________________________________________ 33

Procedure for estimating the oil yield- _____-_______--____-_---------__ 34

Preparation of the sample. ______________________________________ 34

Apparatus.____--_____----__.---.-_-_______-_____--__.__.-__-- 34

Recommended procedure ___-_____----____-_--_________-______-_ 35

Calculations. _________________________j_______________________ 35

Discussion._ ______________________________________________________ 36

Reproducibility ____--___--_-_______-___--__-______--__--__---- 36

Advantages of the method-_ _____________________________________ 36

Literature cited___________________________________________________ 37

TABLE

Page 

TABLE 1. Volumetric estimation of oil yield of Chattanooga shale samples. 37

ABSTRACT

A method is presented for the volumetric estimation of the oil yield of oilbearing shale. The shale is distilled in a closed test tube and the oil extracted 

with ethyl acetate. The ethyl acetate is volatilized on a steam bath, and the oil 

residue, while still hot, is centrifuged in a graduated tube at 1,500 revolutions per 

minute (rpm). The volume of the oil yield is read directly off the calibrated 

stem of the centrifuge tube. The method yields much the same results as the 

Fischer assay method in a much shorter time. It is applicable to shale and 

phosphatic shale.

INTRODUCTION

Oil shales have been assayed by the Bureau of Mines retort method 

proposed by Karrick (1926), described by Guthrie (1938) and later 

modified by Stanfield and Frost (1946, 1949). Recently a method 

relating the specific gravity .of the oil shale to its oil yield was developed by the Bureau of Mines (Frost and Stanfield, 1950). In

33
34 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

this procedure the oil yield is determined using a .standard curve 

established for the particular oil-shale deposit. The standard curve 

is obtained by plotting the specific gravities of more than 50 oil-shale 

samples, picked at random, against the percentage of oil given by 

the modified Fischer retort method, hereinafter referred to as the 

Fischer method or Fischer assay.

A rapid method for the photometric estimation of the oil yield of 

oil-bearing shale is described on page 15 of this report. In this 

method the oil shale is destructively distilled in a closed test tube, 

and the oil evolved is extracted with toluene. The optical density of 

the toluene extract is converted to percentage of oil by reference to 

a standard curve. This curve is obtained by relating the oil yields 

by the Fischer assay method (Stanfield and Frost, 1946, 1949) to the 

optical density of the toluene extract.

All these methods are empirical, and the following study is another 

empirical measure that can be directly correlated with the Fischer 

method. Although the proposed method does not supersede the 

photometric method, it can be used to advantage in the estimation of 

the oil yield of comparatively small samples (as small as 1 g) where a 

standard curve is not available for a particular oil-shale deposit. 

In addition the method is rapid, so that four or five determinations 

can be made in the length of time required for the completion of one 

Fischer-method determination.

In this proposed method the shale is distilled in a closed test tube 

and the oil extracted with ethyl acetate. The ethyl acetate is volatilized on a steam bath, and the oil residue, while still hot, is centrifuged in a graduated tube at 1,500 rpm. The volume of the oil 

yield is read directly from the calibrated stem of the centrifuge tube.

This work was completed as part of a program undertaken by the 

Geological Survey on behalf of the Division of Raw Materials of the 

U. S. Atomic Energy Commission.

PROCEDURE FOR ESTIMATING THE OIL YIELD

PREPARATION OF THE SAMPLE

The sample is prepared as in the photometric method (p. 20).

A representati« . portion, ground to 80-mesh, is dried in an oven 

at 110 C for 1 h.:, unless the samples are to be assayed on the "as 

received" basis.

APPARATUS

The test tube used in the closed-tube distillation of the shale is a 

pyrex glass-stoppered tube, 15 by 50 mm with a no. 16 standard-taper 

glass stopper. The-centrifuge tube used to measure the oil yield of 

oil shale is the Goetz phosphorous tube. The tube furnace is the same
PART 3. VOLUMETRIC ESTIMATION OIL FROM SHALE 35

as that used in the photometric method, described and illustrated 

on page 20.

RECOMMENDED PROCEDURE

A 1.000-g sample of the dry shale, ground to pass a 80-mesh 

sieve, is weighed into the test tube. The lower one-third of the glassstoppered test tube is heated in a horizontal position in the specially 

constructed small tube furnace at 480 to 500 C for 11 to 14 min. 

It has been found (see p. 23) that this method of heating gives yields 

of oil that compare with those determined in the Fischer assay method; 

therefore it is essential that strict adherence to the time and temperature conditions be observed. The exposed upper two-thirds of the 

stoppered test tube serves as an air condenser. After the distillation period is complete, the closed test tube is removed from the tube 

furnace and allowed to air cool to room temperature.

Ten ml of ethyl acetate is added, the tube is stoppered and shaken 

vigorously for 20 sec. The oil extracted by the ethyl acetate is 

filtered through a dry, 9-cm, no. 589 (white ribbon) S and S filter 

paper directly into the centrifuge tube. The filter paper and residue 

are washed with, two 3-ml portions of ethyl acetate. The combined 

extract and washings, now in the centrifuge tube, are placed on a 

steam bath to volatilize the ethyl acetate.

To aid in the complete volatilization of the ethyl acetate during 

the final stage of the evaporation, the centrifuge tube is removed 

from the steam bath and tilted to allow the hot oil residue to flow 

from the stem into the pear-shaped body of the centrifuge tube. 

The tube is rotated so that the oil residue coats the lower walls of 

the tube. The centrifuge tube is now returned to the steam bath 

for further evaporation. This removal-and-tilting process is repeated 

until constant volume is obtained. Three to four repetitions generally 

volatilizes the ethyl acetate completely.

The remaining oil residue is then centrifuged at 1,500 rpm for 

3 min. while still hot. The tube is allowed to cool to room temperature, and the volume of oil obtained is read directly from the 

calibrated stem of the centrifuge tube.

CALCULATIONS

The oil yield of the shale is determined by the use of the following 

formula which converts the measured volume, in milliliters of oil, 

to percent oil yield:

Percent oil yield=90.3 X ml oil yield

The following formula is used to convert the measured volume, in 

milliliters of oil, to gallons of oil per ton:

Gallons of oil per ton shale=239.3Xml oil yield
36 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Both of the foregoing formulas were derived by taking an average 

specific gravity of oil as 0.903, which is essentially the density of 

shale oils. (See p. 23.)

DISCUSSION 

REPBODUCIBILITY

In order to obtain reproducible results the following precautions 

must be observed:

1. The closed-tube system must be gas tight. As a precaution 

against loosening of the stopper, it is advisable to stopper the test 

tube after heating has begun and prior to the evolution of vapors 

from the sample.

2. The proper heating must be maintained within the prescribed 

limits of time and temperature. Although no data are shown in this 

report, conditions have been established in the preliminary experimental work done on the photometric method (see p. 23), which 

showed that the closed-tube distillation of the sample at 480 to 500 

C for 11 to 14 min. gave the most reproducible results.

3. If the ethyl acetate is not completely volatilized, positive errors 

will result.

4. The centrifuging of the oil residue after volatilization of the 

ethyl acetate must be done while the oil is still hot. Because the 

viscosity of the oil increases with decreasing temperature, negative errors may result from oil coating the interior walls of the 

centrifuge tube.

This report does not attempt to show the effects upon the oil 

yield of such different experimental conditions as sample size, particle 

size, heating temperature, heating time, and the stability of the ethyl 

acetate system. For details and experimental data on the effects of 

these conditions, the reader is referred to pages 22 to 30.

To test this method 19 samples of Chattanooga shale from Tennessee 

were picked at random. The oil content of the shale samples, having 

a Fischer-assay oil yield ranging from 2.3 to 6.7 percent, was determined by the above method. The results of these tests (table 1) 

show a good correlation with the Fischer-assay results.

ADVANTAGES OF THE METHOD

The proposed method is recommended for oil-shale assays; it has 

two immediate advantages over the modified Fischer retort method, 

as follows:

1. Less time is required for an assay by this method. An oil-shale 

assay by the Fischer method requires approximately 2 hrs. for completion, whereas by the proposed method four or five determinations 

can be completed in the same length of time.
PART 3. VOLUMETRIC ESTIMATION OIL FROM SHALE 37

2. Less sample is required. A 100-g sample is used in the Fischer 

method, whereas a 1.000-g sample is used in the proposed method. 

In addition, the same representative sample ( 80 mesh) can be 

used for other analyses.

TABLE 1. Volumetric estimation of oil yield of Chattanooga shale samples

n, 19

3d, 5.7

Average difference, 0.3 percent absolute

Maximum difference, 0.7 percent

Sample

10

9

8

11

7

6

12

18

13

14

16

15

4

17

5

19

3

2

1

Fischer assay 

(percent)

2.3

2.5

2.5

2.7

2.8

2.9

2.9

2.9

3.0

3.2

3.2

3.9

4.3

4.3

4.5

4.6

5.2

6.0

6.7

Oil yield

Milliliters

0.03

.03

.03

.03

.03

.04

.03

.04

.04

.04

.04

.04

.05

.05

.05

.05

.06

.07

.08

Percent

2.7

2.7

2.7

2.7

2.7

3.6

2.7

3.6

3.6

3.6

3.6

3.6

4.5

4.5

4.5

4.5

5.4

6.3

7.2

Difference from 

Fischer assay

+0.4

+.2

+.2

.0 -.1

+.7

-.2

+.7

+.6

+.4

+.4

-.3

+.2

+.2

.0

-. 1

+.2

+.3

+.5

LITERATURE CITED

Frost, C. I., and Stanfield, K. E., 1950, Estimating oil yield of oil shale from its

specific gravity: Anal. Chemistry, vol. 22, no. 3, pp. 491-492. 

Guthrie, B., 1938: Studies of certain properties of oil shale and shale oil: U. S.

Bur. Mines Bull. 415. 

Karrick, L. C., 1926, Manual of testing methods for oil shale and shale oil: U. S.

Bur. Mines Bull. 249. 

Stanfield, K. E., and Frost, C. I., 1946 and 1949, Method of assaying oil shale by

a Modified Fischer retort: U. S. Bur. Mines Kept. Inv. 3977 and 4477.

Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Part 4. THE FLUORIMETRIC DETERMINATION OF 

ALUMINUM IN PHOSPHATE ROCK WITH 8-HYDROXYQUINOLINE

BY F. S. GlUMALDI AND HAHRY L/EVINE

CONTENTS

Page

Abstract...._--...____._._____.__.__._____....__.____.___..__.___ 40

Introduction. _ __________________________________'__-_-________-____ 40

Apparatus and reagents ____________________________________________ 41.

Experiments and discussion_________________________________________ 41

Effect of other ions________________________________________________ 45

The determination of aluminum in phosphate rock_____________________ 46

Procedure--____-__________-______-__-_______---_-__-----_---_ 40

Results_____________________________________________________ 47

Literature cited _______________________________________:___________ 48

ILLUSTRATIONS

Page

FICIUIVE 6. Effect of 8-hydroxyquinoline concentration on fluorescence intensity. ______________________________________________ 43

7. Effect of pH on fluorescence intensity._____________________ ' 44

8. Fluorescence intensity of aluminum quinolate versus extraction

time.__--_--_____----__-_-_-___-_-____-__----_---_-._ 44

9. Effect of different amounts of same buffer on fluorescence intensity. _-_____-__------__-__-_____-__-__--___________ 45

10. Working curve.__________________________________________ 46

TABLES

Page

TABLE 1. Effect of time and order of addition of reagents on the fluorescence intensity of chloroform solution of'aluminum hydroxyq uinola fce______--__----___-___-_--_--_-------_--------- 42

2. Tests of the effect of various ions_._ _______________________ 46

3. Results of analysis of phosphate rock_______________________ 48

39
40 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

ABSTRACT

A method is described whereby aluminum in phosphate rock is determined by 

the fluorescence of aluminum hydroxyquinolate in chloroform solution. The 

procedure is designed to determine 0.01 to 3 percent A12O3 in a 0.1-mg sample of 

rock. The results are accurate to ±5 percent of the aluminum content. No 

element, in the percentages commonly occurring in phosphate rock, interferes in 

the procedure.

INTRODUCTION

In connection with certain geochemical investigations, the U. S. 

Geological Survey had occasion to analyze many phosphate-rock 

samples for aluminum. For this project a rapid, simple, and reasonably accurate method of analysis was necessary.

The accurate determination of aluminum in phosphate rock by 

gravimetric procedure is difficult, rather tedious, and lengthy. For 

example in the method of Hoffman and Lundell (1938), aluminum is 

separated from most of the calcium, magnesium, and phosphorus by 

several precipitations of the phosphate from acetate-acetic acid solutions, and from iron by several precipitations with sodium hydroxide 

and sodium carbonate. The aluminum is finally determined as aluminum phosphate. It is to be expected that some loss of aluminum will 

occur in the sodium hydroxide precipitation especially if iron is 

high and that some chromium and vanadium and possibly some rare 

earths will contaminate the final aluminum phosphate precipitate. 

Chromium and vanadium are present in western phosphates, and the 

rare earths have been reported in many phosphate rocks.

Colorimetric methods for determining aluminum are based on the 

formation of strongly colored lakes with either aluminon, alizarin S, 

eriochromcyanin, or hematoxylin. These methods usually require 

preliminary separation of aluminum from interfering ions, especially 

iron. Barton (1948) used alizarin S for the determination of aluminum 

in phosphate rock and showed that in measurements at wavelength 

of 370 mu there was practically no interference from iron. He gave 

no data on the effects of other common constituents of phosphate 

rocks.

For the gravimetric determination of aluminum 8-hydroxyquinoline 

(oxine) is commonly used; it is used also for the colorimetric determination of aluminum. Goto (1940) has described the detection of aluminum, zinc, calcium, cadmium, magnesium, beryllium, and zirconium 

by means of the fluorescence of their hydroxyquinolates. Tullo (1949) 

used the fluorescence of aluminum quinolate in chloroform solution 

for the estimation of aluminum in beer. His procedure calls for the 

extraction of the aluminum from slightly alkaline solutions with a 

chloroform solution of 8-hydroxyquinoline. Sandell (1941) used the 

fluorescence of gallium hydroxyquinolate for the detection and determination of gallium, and Merritt (1944) determined zinc fluorimetri-
PART 4. DETERMINATION OF AL IN PHOSPHATE ROCK 41

cally with the same reagent. Other elements mentioned in the literature as giving fluorescent quinolates under certain conditions are 

lithium, scandium, and indium.

The method presented in this paper is based on a study of the 

fluorescence of aluminum hydroxyquinolate in chloroform solution. 

No elements interfere in the concentrations usually occurring in 

phosphate rock. The procedure is designed to determine 0.01 to 3.0 

percent A1203 in 0.1 mg samples of rock. The data given in table 2 

indicate that the method should also be applicable to silicate and 

carbonate rocks.

APPARATUS AND REAGENTS

Fluorimeter: The fluorimeter (Fletcher and Warner, 1951) uses the 

Photovolt electronic photometer model 512 with C search unit. 

The B-H4 high-pressure mercury lamp, operated from a Sola constant-voltage transformer no. 30852 for H4 type lamps, supplies 

the ultra-violet light source. The lamp is cooled by a dark-room 

ventilating fan. Primary filter, Corning no. 5874; secondary filter, 

Corning no. 3385. Diaphragm opening 0.8 in. diamter. Square 

glass cells of 30 ml capacity.

8-hydroxyquinoline (oxine) solution: Dissolve 1.5 g of 8-hydroxyquinoline in 2M acetic acid. 1 ml equals 0.015 g of oxine.

Standard aluminum solution: Prepare a solution of concentration 1 y 

(microgram) A1203 per ml from purified potassium alum.

Buffer: Mix equal volumes of 2M acetic acid (12 g CH3COOH per 

100 ml of solution) and 2M ammonium acetate (15.4 g NH4C2H3O2 

per 100 ml of solution).

Distilled water: If blank is high, redistill from all-glass apparatus.

Chloroform: Technical chloroform, distilled from all-glass apparatus, 

was as good as redistilled C. P. chloroform.

Fluorescein solution A: Dissolve 0.05 g of fluorescein in 200 ml (1 : 1) 

ethyl alcohol and make up to 2 liters.

Fluorescein solution B: Take 3 ml of solution A, add 3 ml of ethyl 

alcohol, and make up to 1 liter.

EXPERIMENTS AND DISCUSSION

Experiments were made to study the following factors that affect the 

fluorescence of the oxine complex of aluminum: (1) Order of addition 

of reagents, (2) pH, (3) concentration of 8-hydroxyquinoline, (4). time, 

and (5) concentration of buffer.

Preliminary work consistently showed that when the aluminumoxine complex is extracted with chloroform from solutions containing 

no phosphate, the intensity of fluorescence of the chloroform solution 

is independent of the order of addition of reagents or time of standing
42 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

of the test solution before extraction. However, when phosphate is 

present, the order of addition of reagents and the time of standing 

before extracting with chloroform affect the intensity of fluorescence. 

The phosphate ion forms a complex with aluminum, but 8-hydroxyquinoline forms h, more stable complex with aluminum. The 

aluminum quinolate is formed immediately if the solution containing 

aluminum and phosphate is added to the 8-hydroxyquinoline. If the 

order of addition is reversed that is, if the 8-hydroxyquinoline is 

added to the aluminum phosphate solution the system reaches the 

same equilibrium in about 20 min. In the first order of addition 

equilibrium is established immediately, because the ratio of concentration of oxine to aluminum phosphate is at each instant relatively 

higher than if the order of addition were reversed. The faster procedure of adding test solution to 8-hydroxyquinoline solution and 

extracting immediately with chloroform was adopted.

By this procedure the intensity of fluorescence of the chloroform 

solution of aluminum quinolate is independent of the time the test 

solution stands before extracting the aluminum quinolate. The 

chloroform solution is stable and the intensity of its fluorescence is 

constant for at least 4 hours.

Table 1 shows the effect of time and order of addition of reagents. 

The aluminum phosphate solution was slightly acid and 2 ml (containing 2 7 A1203 and 50 7 P2 05 was always used. The other reagents 

were water (17 ml), 8-hydroxyquinoline solution (1 ml), 1.M acetic 

acid-lM ammonium acetate buffer (2 ml), and chloroform for extraction (30 ml). Additional data obtained in the absence of phosphate 

are not included in table 1 because the fluorescence intensity obtained 

was independent of time or order of addition of reagents. On the 

basis of these tests, the standard order of addition selected is as 

follows: oxine, aluminum solution, buffer, water, immediate extraction 

with chloroform.

TAB bio 1. Effect of lime and order of addition of reac/enls on the fluorescence intensity 

of chloroform solution of aluminum hydroxy quinolate

[Test solutions contain 27Ah03 and 5()7l:> 205, and order of addition is from loft to right]

Koagents in order of addition

Meter readings

Immediate 

extraction

58 

62 

61 

73

71

72

Extraction 

after 20 min.

73 

f 72 

\ '72 

72 

f 72 

I '73

1 Water added after 20 min., before extraction.
PART 4. DETERMINATION OF AL IN PHOSPHATE R6CK 43

.4 .6 .8 1.0 1.2 1.4 1.6 

Ml OF 8-HYDROXYQUINOLINE

.8 2.0

FIGURE 0. ElToct of 8-hydroxyquinoline concentration on fluorescence intensity.

Figure 6 shows the variation in fluorescence of 2 7 of A1203 with 

different concentrations of oxine. It is seen that 0.5 ml of 8-hydroxyquinoline gives the greatest sensitivity. The fluorescence of aluminum quinolate is particularly sensitive to change of oxine concentration when the amount of quinolate is less than 0.5 ml. Because other 

elements consume oxine, it was preferable to standardize on 1 ml of 

oxine.

Figure 7 illustrates graphically the relationship between the fluorescence intensity of 2 7 A1203 and the pH of the solution before extraction with chloroform. Different pH's were obtained by varying the 

proportions of 1M ammonium acetate and 1M acetic acid in the 2 ml 

of buffer solution. The optimum. pH is clearly 5.4. Because more 

elements react either with phosphate or with 8-hydroxyquinoline at 

higher pH's, standardization was on a pH of 4.6.

The data on the fluorescence of aluminum quinolate obtained with 

different shaking periods are shown in figure 8. The solutions were 

shaken with chloroform for the prescribed period (at the rate of four 

shakes per second), the chloroform layer was then withdrawn, and the 

fluorescence measured. It is seen that, for periods between 5 and 25 

sec. essentially the same fluorescence is obtained. Standardization 

was on a 10-sec. shaking period.

230156 53 4
44 CONTRIBUTIONS TO GEOCHEMISTRY, 1949 

100

S 90

2 

LJ

o 80 

co

UJ

U_

o

60

50

CO

LJ

40

2

- 30

20

I ml 8-HYDROXYQUINOLINE

J I I I I I

4.2 4.4 4.6 4.8 5.0 5.2 5.4 pH

FIGURE 7. Effect of pH on fluorescence intensity.

UJ

o

2 

LJ 

O 

CO 

111

or 

o

z> _J 

u.

u. 

o

en

z

UJ

80

70

60

50

40

30

± 20

2 7 AI2 03

I ml 8-HYDROXYQUINOLINE

pH 4.6

I I I ; I

10 20 30 40 50 60 70 

SHAKING TIME FOR EXTRACTION 

IN SECONDS

FIGURE 8. Effect of extraction time on fluorescence intensity of aluminum quinolatc.
PART 4. DETERMINATION OF AL IN PHOSPHATE ROCK 45

UJ

o

o 

en

80

u. 

O

60 

50 h

UJ f- 

- 40

2 7 AI 2 0 3

I ml 8-HYDROXYQUINOLINE

pH 4.6

I l II I

2345 

Ml OF BUFFER

FIGURE 9. Effect of different amounts of same buffer on fluorescence intensity.

Figure 9 shows the effect of varying amounts of buffer solution on 

the fluorescence intensity of aluminum quinolate.

The working curve of figure 10 is obtained by plotting the fluorescence intensity of aluminum quinolate against micrograms of A1203 . 

The order of addition of reagents is the standard order: 8-hydroxy. 

quinoline (1 ml), AI203 (varying amounts in 2 ml), buffer (2 ml), 

water (17 ml), 10 sec. extraction with 30 ml of chloroform. The 

photometer was set to read 72 for 2 y A12O3 . The same curve was 

obtained in the presence of 50 y P205 .

EFFECT OF OTHER IONS

Tests were made on the elements occurring in phosphate rock to 

determine possible interference. In general the amounts tested were 

considerably in excess of what would normally be present in phosphate 

rock. Fluorine interferes seriously, but it is removed in the preparation of the solution for analysis. Large quantities of zirconium give 

a positive error. In phosphate rocks no interference should be expected from zirconium, because it is present only in small amounts 

and because it precipitates out of solution as phosphate.

Table 2 gives the results obtained in testing the effect of various 

ions. In the analysis of phosphate rock 0.1 mg (1007) of sample is 

always used; therefore the quantity (7) of the element represents the 

percentage.
46 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

ml 8-HYDROXYQUINOLINE 

pH 4.6

.4 .8 1.0 1.2 

0

1.6 1.8 2.0

FIGURE 10. Working curve.

TABLE 2. Tests of the effect of various ions especially those elements present in 

phosphate rocks [Instrument set to read 72 for 2-yAl:>O3; blank reading 10]

Material added to 

test solution as 

soluble ion

2-yMnO--.. --.

1.7Y2 03 --------

lyCetOi

2yThO3 . .........

l7Ti02 __-_. ------

.47Zr02 .. __.-.

5!)7Zr0 2 ---------.

27Cr2 03. . .

27V2 05 -----

Meter readings

No AlzOs 

present

9 

9 

9 

10 

10 

10 

10 

10

9 

10

27Al 2 Os 

present

74 

70 

70 

71 

71 

73 

72 

74 

97 

73 

72

Material added to 

test solution as 

soluble ion

107Fe2 O3 --------.

507P205 . --.-.

367K2 0. .-----.

207Li 2 0-. ...... -

607CaO. .-._..-. -

507MgO ..---..

47ZnO.-_. -_-_--

Meter readings

No AhOa 

present

10 

10 

9

10 

10 

10 

10 

10 

13

27AhOs 

present

74 

71 

72 

54 

74 

72 

72 

73 

73

THE DETERMINATION OF ALUMINUM IN PHOSPHATE

ROCK

PROCEDURE

1. Weigh 0.1000 g of sample, ground to pass 80-mesh, into a 50 ml 

beaker.

2. Add 5 ml HN03 (1:1) and 3 ml of perchloric acid.

3. Evaporate the solution to fumes of perchloric acid and allow to 

fume for several minutes. Cool.
PART 4. DETERMINATION OF AL IN PHOSPHATE ROCK 47

4. Add 5 ml of HN03 (1:1) and 20 ml of water. Digest on steam 

bath for several minutes to dissolve soluble salts.

5. Filter on a small no. 40 Whatman filter paper and wash with 

water. Reserve filtrate in 100 ml beaker. Ignite residue in a platinum crucible.

6. Fuse the residue with a minimum amount of sodium carbonate. 

Cool the melt.

7. Pour a slight excess of HN03 (1:1) into the crucible to dissolve 

the cake. Transfer this solution to the beaker containing the main 

solution.

8. Add 2 ml of perchloric acid and evaporate to fumes of HC104 . 

Allow to fume until about 1 ml of HC104 remains. Cool. 

- 9. Add 30 ml of water and digest the solution on the steam bath to 

dissolve soluble salts.

10. Filter off the silica and wash with water. .Reject residue.

11. Make the solution to 1 liter in a volumetric flask and mix.

12. Take a 1 ml aliquot if a higher dilution is used, aliquots up to 

15 ml containing 0.1 mg of sample may be used and transfer it to a 

60 ml separately funnel containing 1 ml of 8-hydroxyquinoline and 

mix.

13. Add 2 ml of buffer solution, mix, and then add 17 ml of H2 0.

14. Add 30 ml of CHC13 and shake the vessel for 10 sec. (a total of 

about 40 shakes).

15. Remove chloroform layer and measure the fluorescence with 

the photometer.

16. Determine the amount of aluminum from the working curve 

(fig. 10). The photometer may be set to read 72 against a chloroform 

solution of aluminum quinolate (27A12 03) or, alternately, set for a 

standard deflection against fiuorescein solution B, whose fluorescence 

has been compared against 27A1203 . If the fiuorescein solution is 

used, it should be checked daily against the standard chloroform solution of aluminum quinolate.

RESULTS

Two National Bureau of Standards standard samples of phosphate 

rocks were analyzed by the fluorimetric procedure. One of these 

samples was reanalyzed after the addition of known amounts of aluminum. Four western phosphates were also run and again one was reanalyzed after the addition of known amounts of aluminum. The 

western phosphates were previously analyzed by the following unpublished procedure, as developed by the late Norman Davidson, of 

the U. S. Geological Survey. The acid solution of the sample was 

treated with cupferron, and such cupferrates as those of iron and 

titanium were extracted with ethyl acetate. The water layer was
48

treated with 8-hydroxyquinoline and the aluminum precipitated at a 

pH of 5.2. The aluminum quinolate was filtered off, dissolved in acid, 

and reprecipitated with oxine. The precipitate was again filtered 

and ignited to A12 03 under cover of oxalic acid. The results obtained 

by the fluorimetric procedure (table 3) show good agreement with those 

obtained by other methods.

TABLE 3.' Results of analysis of phosphate rock

Sample no. and souicc

National Bureau of Standards, standard sample: 

120... -. . . ..

120. AbOs added to raise percentage in sample to 

1.50.. 

2.00 , 

2.50 

56« .............. ..............................................

Western phosphates: 

1» . - _..-._ . . ... .... ... .

23

3*. ..................................................................

4« ........ ................... ............... ..... 

4. AljOs added to raise percentage in sample to 

2.2...............................................................

2.6 _ _ 

AlsOs (percent) obtained 

Fluorimetrically

f 0.81 

1 .82 

1 .80 

I .81

1.52 

2.07 

2.56 

12.00 

2.02 

2.04 

2.03

6.4 

2.3 

2.6 

1.8

2.2 

2.6

Gravimetrically

"0.80

I ' 2. 02

6.5 

2.2 

2.6 

1.7

1 Average obtained from National Bureau of Standards report.

2 Sample contained 5.8 percent FezOs.

3 Sample contained 2.9 percent Fe20s.

4 Sample contained 3.2 percent FeaOs. 

 Sample contained 3.8 percent FeaOs.

LITERATURE CITED

Barton, C. J., 1948, Photometric analysis of phosphate rock: Anal. Chemistry 20,

pp. 1068-1073. 

Fletcher, Mary H., and Warner, E. Ray, 1951, A Fluorimeter for solutions:

Geological Survey Trace Elements Memorandum Report No. 252, U. S.

Atomic Energy Commission. 

Goto, H., 1940, Fluorescence Analysis Part V; Fluorescence detection: Tohoku

Imp. Univ. Sci. Repts., 1st ser., vol. 29, pp. 204-218. 

Hoffman, J. I., and Lundell, G. E. F., 1938, Analysis of phosphate rock: Nat. Bur.

Standards Research Jour. 20, pp. 607-626.

Merritt, L. L., Jr., 1944, The determination of small mounts of zinc by measurement of fluorescent turbidities: Ind. and Eng. Chemistry, Anal. Ed. 16,

pp. 758-760.

Sandell, E. B., 1941, Detection of gallium by a fluorescence reaction with 8- 

hydroxyquinoline: Ind. and Eng. Chemistry, Anal. Ed. 13, pp. 844-845. ' 

Tullo, J. W., Stringer, W. J., and Harrison, G. A. F., 1949, Estimation of aluminum

in beer: The Analyst, vol. 47, pp. 296-299.
Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY 1949

Part 5. THE DETERMINATION OF PHOSPHORUS IN ROCKS 

CONTAINING VANADIUM

BY R. B. RANDOLPH AND F. S. GRIMA.LDI

CONTENTS

Page

Abstract__. _--_____-________-____-_-_--_--_-__-__--____________ 49

Introduction. _ __-______-__-____-__-_---_--_-------_---____________ 49

Discussion of procedure______-__---_-_--__---__----________.____ 51

Procedure. _______________________________________________________ 52

Preparation of the solution_____________________________________ 52

Determination as ammonium phosphomolybdate__________________ 53

Results of analyses__--____-_______-_---_---___-_----_-____________ 53

Literature cited__---___-__-___--__-----_---_-____--_____-_________ 55

TABLES

Page 

TABLE 1. Results of analyses of pure solutions,_______________________ 53

2. Analyses of standard samples._____________________________ 54

3. Test of procedure--__---------_---_--_-------_---------__ 55

4. Test of procedure--..------------------------------------ 55

ABSTRACT

This paper describes a routine procedure for the determination of phosphorus 

in the presence of vanadium. The vanadium is reduced with formic acid prior 

to precipitation of the phosphorus as ammonium phosphomolybdate, and the 

ammonium phosphomolybdate is weighed as such. Ratios of V2 0s to PzOs as 

high as 100 to 1 the highest tested give no interference. F^Os in amounts from 

0.1 mg to 0.2 g may be determined with no changes in conditions or quantity of 

reagents. The separation of molybdic acid is prevented and the molybdate 

precipitate has a constant composition.

INTRODUCTION

Volumetric and gravimetric methods for the determination of phosphorus usually involve preliminary separation of the phosphate as 

ammonium phosphomolybdate. For volumetric determination the 

phosphomolybdate precipitate is titrated with standard alkali using 

a suitable acid-base indicator. For gravimetric analysis the ammonium phosphomolybdate is sometimes weighed as such but -for 

most accurate work it is dissolved in ammonia and the phosphate 

precipitated as magnesium ammonium phosphate. After two pre49
50 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

cipitations the magnesium ammonium phosphate is ignited bo magnesium pyrophosphate, which is then weighed.

Procedures depending on either the weighing or titrating of the 

ammonium phosphomolybdate presuppose that the ammonium phosphomolybdate precipitate is pure and of definite constant composition. 

It is difficult to get this purity under conditions that usually obtain 

in the determination of phosphorus. The precipitate may be contaminated with arsenic, vanadium, and silica, which form heteropoly 

compounds like phosphorus; with titanium and zirconium phosphates, 

unless some provision is made to prevent their formation; and with 

molybdenum, which may precipitate out as oxide or which may be 

present in the phosphomolybdate in higher proportion than in the 

ideal formula especially if the precipitate is formed at elevated 

temperatures. Silica is easily removed in the preparation of the solution for the analysis. Arsenic interference may be reduced by precipitating at room temperature or eliminated entirely by reducing the 

arsenic to the trivalent state and removing it from the solution by 

volatilizing the chloride.

The proper conditions for the prevention of the separation of 

molybdic acid are not fuPy appreciated in standard procedures. For 

example, the procedure used by Hillebrand and Lundell (1929) calls 

for the addition of a nitric acid solution (20 to 25 percent HN03 by 

volume) of ammonium molybdate to the phosphate solution containing 

5 to 10 percent of nitric acid, by volume. Depending on the phosphate 

concentration, a varying quantity of ammonium molybdate is added. 

The concentration of nitric acid in the final solution could thus vary 

from 8 percent to more than 15 percent, depending on the amount 

of phosphate being precipitated. The final nitric acid concentration 

is critical, and unless it is kept above 12 percent by volume molybdic 

acid is likely to precipitate. Kassner and others (1948) prevent the 

precipitation ol molybdic acid by the addition of citric acid.

Pentavalent vanadium in solution presents a more serious difficulty. 

If the ratio of the concentration of P205 to V205 is high, vanadium is 

largely co-precipitated with phosphorus. If the amount of vanadium 

exceeds that of phosphorus, the precipitation of the phosphomolybdate 

is seriously, retarded and may not be complete even after long standing. 

We have found that no phosphorus is precipitated at all when the 

ratio of the concentrations of V2O5 to P205 is over 5 and the phosphorus 

content of the solution is small.

The interference of vanadium may be eliminated by reduction of 

the vanadium to the quadrivalent state, and precipitation of the phosphomolybdate at room temperature or lower. Under these conditions 

little if any vanadium precipitates, and the retarding effect of vanadium on the precipitation of phosphorus is largely overcome. Hille-
PART 5. DETERMINATION OF P ROCKS CONTAINING V 51

brand and Lundell (1929) advise using FeS04 for the reduction followed by the addition of one gram of iron as ferric nitrate to prevent 

subsequent reduction of the molybdenum.

This report deals with the use of formic acid for the reduction of 

vanadium. Proper conditions vvere developed such that

1. The phosphomolybdate precipitate obtained is constant in 

composition.

2. The separation of molybdic acid is prevented.

3. From 0.1 mg to 0.2 g of P205 may be determined, with no change 

in conditions, or amounts or types of reagents.

The analyst simply prepares a solution of the sample, adds a fixed 

amount of reagents and niters, dries, and weighs the precipitate. No 

guesswork or prior knowledge of the phosphorus and vanadium content is required. The procedure \vill determine phosphorus with good 

accuracy when the V2O5 to P205 ratio is as high as 100 to 1. the highest 

tested. Because the composition of the precipitate is constant, 

titrimetric procedures may also be used.

DISCUSSION OF PROCEDURE

Preliminary investigations determined optimum conditions for the 

precipitation of phosphate with ammonium molybdate. These conditions are always set up for any amount of P2 05 present up to 0.2 g, 

thus:

1. The precipitation is made at room temperature.

2. The total volume of solution is 100 ml after all reagents and water 

are added. It contains 5 ml of formic acid, about 8 g of ammonium 

nitrate, 15 ml of nitric acid (sp gr 1.42), and 45 ml of molybdate 

solution. (See below.)

These conditions are flexible enough to allow of some variation. 

The nitric acid content is a critical factor and should be controlled 

closely. For example, if the concentration of nitric acid in the final 

solution is as low as 9 percent by volume that is, 100 ml of solution 

contains 9 ml of nitric acid, sp gr 1.42 molybdic acid tends to precipitate. A few tests showed that molybdic acid does not separate out 

from solutions containing 12 percent by volume of nitric acid. The 

amount of ammonium molybdate is more than is needed for precipitation of small amounts of P2 05 , but it is advantageous to keep the 

amount added constant; thus, enough is specified to precipitate 

quantitatively as much as 0.2 g P2 05 . Formic acid is necessary to 

eliminate vanadium interference, but it is rapidly destroyed by nitric 

acid if the solution is warmed. For this reason the nitric acid content 

of the solution is kept at a minimum prior to the addition of formic 

acid, and the solution is cooled to room temperature before and after 

the addition of the nitric acid. If the temperature of the room is
52

high enough to cause a slow decomposition of the formic acid, it is 

preferable to cool the solution to a lower temperature. For uniformity of procedure lormic acid is used even when vanadium is 

absent. Under these conditions, the composition of the phosphomolybdate precipitate when dried at 110 C is (NH4) 3P04.12MoO3 , 

and the factor for P2 05 is 0.03783.

PROCEDURE

The special reagents needed are formic acid (reagent grade, sp gr 

1.2, 90 percent) and ammonium molybdate solution. The ammonium 

molybdate solution is prepared by dissolving 16 g of ammonium 

molybdate, (NH4) 6Mo7 02 4.4H2O, in 90 ml of hot water. The solution 

is cooled and then filtered if necessary.

PREPARATION OF THE SOLUTION 

1. PHOSPHATE ROCK

Add 20 ml of HN03 (1:1) to a 0.5-g sample in a glass container, and 

evaporate the solution to dryness. Repeat with 20 ml more of HNO3 

(1:1). Digest the residue with 20 ml HNO3 (1:3) and filter; wash 

with water. Reserve the filtrate. Fuse the residue with a minimum 

of Na2C03 in platinum. Leach the cool melt with water and filter. 

Wash the precipitate with 1 percent Na2C03 solution. Combine the 

filtrate with the filtrate reserved above. Reject the residue. Add 

5 ml of HN03 and evaporate the combined filtrates to dryness. Add 

20 ml of HN03 (1:3), warm to dissolve the salts, and transfer the 

solution to a 100-ml volumetric flask, filtering if necessary. For 

rapid control work, the carbonate fusion need not be made, and the 

residue from the initial attack with nitric acid may be rejected. If 

the sample contains much material that is insoluble in nitric acid, 

prepare solution as in 2.

2. StJLICEOUS ANI> CLAYEY MATERIAL

Treat a 0.5-g sample in a platinum dish with 10 ml of HNO3 (1:1) 

and 10 ml of HF. Cover with a platinum cover and digest on a steam 

bath for 10 min. Remove cover and evaporate to dryness. Fluorides are removed by evaporating twice more after additions of 15 ml 

of HNO3 (1:1). Add 20 ml of HN03 (1:3), digest, and filter, washing 

with water. Reserve the filtrate. Ignite the residue in platinum and 

fuse with a minimum of Na2C03 . Leach the cooled melt with hot 

water. Filter and wash with dilute Na2C03 solution. Reject the 

residue. Combine the filtrate with that reserved previously. Add 5 

ml of HN03 and evaporate the combined filtrates to dryness. Add 20 

ml of HNO3 (1:3), warm to dissolve the salts, and transfer the solution to a 100-ml volumetric flask, filtering if necessary.
PART 5. DETERMINATION OF P ROCKS CONTAINING V 53 

DETERMINATION AS AMMONIUM PHOSPHOMOLYBDATE

1. Transfer an aliquot of the prepared solution, containing 50 mg 

of Pz05 or preferably less, to a 150-ml beaker.

2. Add NH4OH until methyl red indicates neutral.

3. Add 5 ml of formic acid and heat the solution just to boiling.

4. Remove from the source of heat immediately, and cool the 

solution to room temperature in a water bath.

5. Add 8 g of NH4N03 (solid) and stir until it dissolves.

6. Add 15 ml of HN03 . Cool if necessary.

7. Immediately add 45 ml of ammonium molybdate solution slowly 

and stir.

8. Add water to make the total volume 100 ml. Let stand until 

the precipitate settles. The precipate is ready to filter as soon as it 

has settled out. This usually takes about an hour.

9. Filter thru a weighed sintered-glass medium filter or Gooch 

crucible.

10. Wash several times with HN03 (1:99) solution.

11. Dry the precipate at 110 C and weigh.

,. .,. ^ weight of ammonium phosphomolybdateX 3. 783 J he percent P2O5 = - - - : 

weight oi sample

RESULTS OF ANALYSES 

COMPOSITION OF AMMONIUM PHOSPHOMOLYBDATE

A standard phosphate solution was made from Bureau of Standards 

sample No. 186 II (Na2HP04 ) by direct weight and was added to 12 

beakers. The first 6 (series A) contained 0.2, 5, 10, 20, 30, and 50 

mg of PaCV The second 6 (series B) contained duplicate amounts of 

P205 and, in addition, 20 mg of V2O5 added as ammonium metavanadate solution. Phosphorus was determined as described in the procedure. The results in table 1 established the composition of the 

precipitate dried at 110 C as (NH4 ) 3P04.12Mo03.

TABLE 1. Results in milligrams of analyses of pure solutions

j taken

PiOs found, based on

SERIES A

0.2______-_._.

5.0__.____--_.

10.0_________

20.0.-___---_.

30.0___.__-__

50.0.. _... .

.._____._ 0.2

-_._-____ 5.0

_________ 10. 1

_--_-.___ 20.0

_________ 29. 8

_________ 50. 1

PjOs sfound, based on

tOs taken (NHt)tP04.12Mo03

SERIES B 

0.2______.__-_________ 0.2

5.0._. .._____...___... 5.0

10.0-__-_____-__.____- 10.0

20.0_-__-__---_-______ 20. 2

30.0______--___-______ 30. 1

50.0---_------------__ 50. 2
54 CONTRIBUTIONS TO GEOCHEMISTRY, 1949" 

ANALYSES OF PHOSPHATE BOCKS

Nitric acid solutions were prepared from three Bureau of Standards standard samples of phosphate rocks. The certified composition of these standard samples follows:

P2 05 

Sample no. ' Phosphate deposits percent

120____----_---_-___-___ Florida land pebble_-__---___-____-.____ 35. 33

56_ ______________________ Tennessee phosphate.. __ ________________ 31. 33

56a_ _------_-.__________ Tennessee brown. _ _____________________ 33. 01

Duplicate sets of aliquots of each sample were tested, each set 

representing 5.0, 17.0, and 44.0 mg P2 05 . No vanadium was added 

to the samples in series A. Twenty milligrams of V2O5 were added as 

ammonium vanadate solution to each sample in series B. P2O5 was 

then determined according to the procedure described. Table 2 lists 

the results obtained.

TABLE 2. Analyses (in milligramx) of standard samples 

Sample no. and aeries

120:

Series A_

Series B.

56:

Series A.

Series B.

56a:

Series A.

Series B_

PERFORMANCE OF THE PROCEDURE BY OTHER CHEMISTS

Robert Meyrowitz, of the Geological Survey laboratory tested the 

procedure against the Bureau of Standards sample no. 120, employing 

aliquots containing 0.14, 7.1, 17.7, and 35.4 mg of P2O5 to which 20 

mg V2O5 were added.

Taken

( 5.0

17. 0

( 44. 0

C 5. 0

17.0

( 44.0

(5.0

17.0

( 44. 0

( 5.0

17.0

t 44. 0

( 5. 0

17.0

t 44. 0

( 5. 0

17.0

1 44. 0

Found

5. 1

17. 2

43. 8

5. 0

17.0

44. 0

4. 9

16. 8

44. 1

4. 9

17. 1

43. 9

4. 9

17. 1

44. 0

5. 0

17. 2

44. 1
PART 5. DETERMINATION OF P ROCKS CONTAINING V 55

Table 3 shows the results obtained.

TABLE 3. Test of the procedure by Robert Meyrowitz on Bureau of Standards 

sample no. 120 with added vanadium

PtOt found 

(milligrams) (milligrams)

0. 14.-.-. --___.--__--_------------_-.-.._-____. 0. 18

7.1 ----__.---.-.___.-------.-_-.-_-.....--- _ 7.3

17.7 -._-___. __ r ._ _.__.___-___--____-__._...._._ 17.8

35.4 _.._._____ _._.___ _--_.-._.......____.. 35.2

In table 4 Bureau of Standards sample no. 186 II (Na2HP05) was 

used and aliquots containing 100, 150, and 200 mg of P2 05 were 

analyzed by Robert Meyrowitz. No vanadium was added to these 

samples.

TABUS 4. Test of the procedure by Robert Meyrowitz on Bureau of Standards

sample no. 186 II

P^Oi taken PiOt found 

( milligrams) (milligrams)

100 ---------- ------_------------------------- 99. 8

150 _-_ _- _ _----------- ----.-__--_-_ 150. 3

200 - --_-_-._ __ ------ _- --.-.__ 200. 8

The results show that the maximum error is ±0.2 mg of P2 05 when 

the l^Os content is 50 mg or less. For greater amounts of P2 05 for 

example, 0.2 g of P2 05 the maximum error may be ±0.8 mg, which 

may be due to incomplete washing of the precipitate. Although this 

figure seems large, it is actually a small percentage of the total phosphorus present.

LITERATURE CITED

Hillebrand, W. F., and Lundell, G. E. F., 1929, Applied inorganic analysis, p. 562,

New York, John Wiley and Sons. 

Kassner, J. L., Crammer, H. P., and Ozier, Mary A., 1948, Determination of

phosphorous pentoxide in phosphate rock: Anal. Chemistry, vol. 20, no. 11,

p. 1052.

Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Part 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS

By WILLIAM G. SCHLECHT

CONTENTS

Page 

Discussion._ ______________________________________________________ 57

Bibliographic commentary._________________________________________ 64

ILLUSTRATIONS

Page

FIGURE 11. Meaning of the terms "accuracy," "precision," and "reliability" of measurements-.-__________-__-_--_--__-.______ 60

12. Non-normal distribution of errors in analyses by 10 laboratories. ________________________________________________ 61

13. A random sequence and two non-random sequences, all with

the same distribution function__________________________ 62

DISCUSSION

My title may lead the reader to expect me to tell him what the probable error of a chemical analysis is, but my thesis is that there generally is none. Chemists in the Geological Survey have noticed the 

increasing tendency to express estimates of errors of chemical analyses 

in terms of statistical formulas for "probable error," "standard deviation," and the like. Anyone using such formulas ought to consider 

what statistical errors mean, and whether analytical chemical processes are of such a nature that statistical ideas can be useful in studying those processes.

The simple statistical formulas and tables have been widely reprinted in textbooks and handbooks, but without any explanation of 

their basic assumptions. This is unfortunate, because the use of these 

devices without a clear understanding of what they mean can be very 

misleading. Their popularity suggests a spreading uncritical belief 

that they have a magical power to improve observations; statisticians 

intend, on the contrary, that they should be used as skeptical tests of 

hypotheses (1). Statistical calculations are no substitute for practical experience and fundamental understanding of analytical chemical 

processes. If we use algebraic formulas before inquiring into the 

meaning of the numbers we put into them, we may delude ourselves

1 Revision of an informal communication that was presented before the Geological Society of Washington, February 23, 1949.

57
58 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

with what Professor Dingle calls "knowledge without understanding" 

(2). This seems to have happened at times in various fields; a blind 

faith in precision for its own sake has been pushed too far ahead of a 

thoughtful and critical search for sound conceptual schemes (3, 4, 5). 

I think the reason for this premature straining for precision is the 

idea that a thing is not really scientific unless it is quantitative and 

precise (6).

Statistical formulas are designed for treating random errors, but the 

important errors in chemical analysis are not generally random. 

Analytical errors are affected by (a) the nature of the material, (b) 

the way of taking and preparing the sample, (c) the kind of analytical 

procedure used, (d) environmental conditions in the laboratory, and 

(e) the personal traits of the analyst. In practical analytical chemistry we take complex materials, and we subject them to harsh and 

complicated disturbances; the errors we make in instrumental readings 

are likely to be small compared with the uncontrolled variations in the 

chemical processes. The nature of these processes is such that for 

some procedures losses of material are more likely than contamination, 

so that results are likely to be too low (7), whereas for other procedures 

the reverse is true (8). When losses and gains are equal the condition known as "compensating errors" (9) the processes are hardest to 

control.

Many statistical calculations for analytical results have been made 

on the hidden assumption that the errors are all committed in the 

determination or final instrumental reading; but in analyzing complex 

materials the greatest and hardest-to-control errors occur in the 

chemical separations, not in the measuring instrument. The central 

role of chemical separation processes in practical analysis is stressed 

by few writers (10, 11, 12). The chemist generally has satisfactory 

control of the errors in his balance, burets, and other measuring instruments; he shares this problem with all users of instruments. His 

special problem is to control what goes on in his beakers; that is why 

an analyst needs to be a chemist (13). A discussion of errors of analysis 

is likely to be irrelevant if it ignores the chemical behavior of the substances involved. The question is seldom one of how accurately we 

know the weight of a precipitate, but rather how pure and how complete the precipitate is. The kinds of chemical errors likely to occur in 

inorganic analyses are discussed in the better textbooks (13, 14).

It was long ago pointed out that the errors in duplicate determinations run at the same time are not likely to be random (15, 16, 17). 

Thus so simple a calculation as taking the mean of several analyses 

may be misleading. When statistical calculations are valid, the mean
PART 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS 59

is the best estimate of a quantity. Dr. Lundell, in his paper, "The 

chemical analysis of things as they are" (10), discussed some analyses 

whose averages are very unlikely to be correct, and stressed the importance of understanding the chemical processes by which they 

were made. The best way to report such results is to give all the 

values, accompanied by any facts or opinions that bear on their 

worth (18, 19, 20, 21). I wish that consumers of chemical analyses 

would study Dr. Lund-ell's paper.

A particularly misleading practice is to use a formula for "rejection 

of observations." Even statisticians, working with truly random 

phenomena, do not blindly "reject" observations; an unusually large 

discrepancy may call for special attention and should not be ignored, 

in deciding on an action. For one thing, a large discrepancy may be a 

warning that the process is not in statistical control; in that case the 

formulas are not valid. Youclen (22) points out that the user of these 

formulas is almost never warned of the penalty involved namely, 

that any arbitrary criterion for rejection of bad observations is bound 

to throw out at the same time a fixed percentage of the good observations. Eisenhart (23) quotes a joking suggestion that the safest 

criterion is to reject the observations nearest to the mean, because 

they affect the result least.

In judging the results of a process, statistical tests are no substitute 

for experience (23, 24, 25), but statistical theory can be very helpful 

to the practical man. For example, statisticians have found that those 

who judge results by inspection must be conservative about rejection; the fact thaFOTre~ofifr-ffCfrgfcii^nsT??5Tnents differs markedly from 

the rest is not in itself sufficient evidence that it is wrong (23). A 

study of sets of three measurements, made by the Statistical Engineering Group at the National Bureau of Standards (26), shows a surprising frequency of cases in which one of the results differs greatly from 

the other two. Even when the variations are random, the most discrepant result will differ from the middle one by at least 16 times 

as much as the smaller difference, in about 10 percent of the sets.

In figure 11, a simple diagram shows the meaning of the terms used 

in studying errors. Suppose that there is a "true value'' (represented 

by the heavy horizontal line) for a thing being measured. If successive 

observations are plotted in sequence, they will cluster around the 

line marked "average", which in general may be either above or below 

the true value. If the average differs significantly from the true value, 

the difference is called the "bias" (sometimes called the "constant 

error"), and is a measure of the "accuracy." This bias is not always 

constant; it may have a trend, or may even shift or oscillate. If 

limits can be found within which the observations will practically 

always fall, the width of the limits measures the reproducibility of the

236156 53 -5
60

Bias 

(ACCURACY)

Value

TRUE VALUE

'.* *

AVERAGE Variability 

(PRECISION)

RELIABILITY 

(Predictability)

Observations in sequence >

FIGURE 11. Meaning of the terms "accuracy," "precision," and "reliability" of measurements. After

Mitchell (27).

process, and is called the "limits of variation"; it is a measure of the 

"precision." If a process is fairly stable, the average and the precision 

should be fairly reproducible any time that observations are repeated. 

Statistically, a process is said to be "in control" if the precision limits 

do not change much in repeated series of observations. Mitchell (27) 

has proposed that if the process is in control, the condition be called 

"reliability ." If the precision and accuracy of a method are reproducible over a period of time, then the method is reliable.

There are various ways of expressing precision. The simplest of 

these "measures of dispersion" is the range (difference of maximum 

and minimum); some other popular ones are "standard,deviation" 

and "probable error." Professional statisticians have abandoned the 

use of probable-error formulas, even where they are valid, in favor of 

the simpler and more noncommittal expressions involving standard 

deviation (28, 29, 30). "Average deviation" has been a favorite 

among chemists, because it is easy to calculate; its pitfalls have been 

described (31, 32). . Now if the precision is to be calculated by statistical formulas, 

two assumptions are made: first, that the errors are distributed about 

the mean in a certain way approximately in the "normal distribution" and second, that they are random in sequence that no trends
PART 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS 61

0.7

u 0.2 

O

03 O.I

I ' I ' I 

EXPLANATION

 

Ephed ri ne HCI

O 

Benzole acid

O

A rj AVERAGE

 § 

-0.5 -0.1 -0.3 -0.2 -O.I 0 O.I 0.2 0.3 0,4 

EPHEDRINE HCI ANALYSERS IN PERCENT

FIGURE 12. Non-normal distribution of errors in analyses by 10 laboratories. From Youdon (38).

or patterns are shown by successive observations. Incidentally, there 

is nothing inherently "normal" or natural about the sorcalled normal 

distribution; it simply happens to be the simplest and easiest artifice 

for computing a good approximation to most of the random distributions found in practice (33, 34).

To establish the validity of these assumptions, a large sequence of 

observations (35) must be studied according to the modern statistical 

methods worked out by Shewhart (18, 36): they are grouped, plotted 

on control charts, and examined for stability of the mean and control 

limits, and for absence of "runs" or patterns. The subgrouping 

procedure is also essential for detecting and studying systematic 

errors; Deming and Birge (35, p. 160) remind us that "* * * no 

amount of analysis of a single sample, regardless of how large it is, 

can of itself lead one to suspect the presence of constant errors." 

(Note that Deming and Birge use the words "single sample" in the 

statistician's sense, to mean a single group of observations, rather 

than in the chemist's sense of a single lot of material).

The few most thorough of the studies that have been made of 

chemical analyses indicate that analytical errors are not likely to be

236156 53-
62 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

x 80f . .. X 60['- ' '. ' .' «\ .. . . .'. / '. --^- 

40 ^ | 10 20 30 40 50

OBSERVATION NO. 

80^ X60

40

x80

X60 

40

FIGURE 13. A, A random sequence, B, C, Two non-random sequences. All have the same distribution

function. From Schlecht (36).

normally distributed. V. J. Clancey (37) has studied some 250 distributions, involving about 50,000 chemical analyses of industrial 

products, and found 85 to 90 percent of them to be significantly nonnormal. He concludes that "* * * great caution should be 

exercised in applying the ordinary simple statistical methods to the 

data of chemical analyses * * *."

Figure 12 shows a set of especially careful analyses made of 2 pure 

substances by 10 different laboratories. If the errors computed 

from duplicate analyses were normally distributed, about half of the 

results, as plotted (38), would fall inside the dotted box. None 

of them do.

Thus we have some actual examples of chemical analyses that do 

not have a probable error.

Many observations generally at least 200 are required to 

establish adequately that a process is in statistical control. Most 

workers cannot afford to make so many (39); nevertheless, they often 

assume that the process is in control (40, 41, 42). Many workers, 

when they have made enough observations to establish that their 

distribution was normal, have concluded that therefore it was random. This conclusion is not true; Figure 13 shows that a distribution 

may be normal without being random (36). In this figure sequences 

A, B, and C have the same distribution curve, but only A does not 

show patterns in the sequence.

Correspondence with a distribution curve is a necessary but not 

a sufficient condition for statistical control. The reader can see this 

by imagining a sequence of samples scooped from a bucket full of 

gravel with the larger pebbles on the bottom and the smaller ones 

on top. The mean particle sizes of the samples will give a sequence 

something like the plot in figure 13B. If the gravel is well stirred, 

the mixture becomes more random, and the sequence of samples will 

be more like that shown in figure 13^4; but of course stirring does not
PART 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS 63

change the sizes of the particles, and so the distribution curve of 

particle sizes remains the same.

Probable error and similar formulas take into consideration only 

the distribution curve which, as we have seen, may be the same 

whether the sequence is random or not. Such formulas thus suppress 

a critical part of the evidence for statistical control, and before we 

can trust them we must examine the sequence to see whether or not 

it is random (23, lecture 3).

I have said that statistical methods are no substitute for understanding. We should by all means use them when we are ready for 

them. But before we can study a given analytical method statistically, we must have adequate experience to base our study on a 

large number of experiments under various carefully controlled 

conditions, and a sound theoretical understanding of the process. 

These must be laboriously worked out for each kind of determination 

and material.

When we do use them, we must do so responsibly, with a reasonable 

understanding of their meaning, plus the extreme skepticism traditionally expected of scientists. This means checking with professional statisticians (43, 44), and not putting too much trust in books 

written by such statistical laymen as chemists, physicists, engineers, 

economists, and psychologists (45,46). For reliable guidance to the 

amateur, a modern and simply written book by the statistical experts 

of Imperial Chemical Industries, Ltd., is now available. It is "Statistical methods in research and production, with special reference 

to the chemical industry," edited by Owen L. Davies (29). I single 

out this particular book for special notice, because its authors are 

more conscientious than any others that I have read in stressing the 

underlying assumptions and limitations of the various statistical 

tests.

An invaluable service that the statistician can give to the chemist 

has as yet been little appreciated; it relates to the advance planning 

of experiments in such a way that no work is wasted. Wernimont 

(45), for example, has called attention to some published experimental 

data comparing the precision of 10 spectrophotometers. The measurements were numerous, but were so carried out that the results do not 

justify the conclusions drawn. With properly designed procedure, 

the same number of observations would have given significant results.

Most important for the analytical chemist is what Tukey (47) calls 

the "day-to-day part of experimental design," which consists in so 

planning the work that one can make a realistic estimate of its reliability. He points out, for example, that reliable values for the 

parallel analysis of three unknowns, with an honest estimate of the 

errors of analysis, are not most likely to be obtained by the method
64 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

that most chemists now employ unthinkingly by force of habit, 

namely to make three determinations of the first unknown on one day, 

and three of each of the others on the 2 following days. It is better 

to make one determination of each unknown on one day, another of 

each on the second day, and still another of each on the third day.

The procedures referred to as "design of experiments" are very 

pertinent to the investigation of analytical errors, in that they ensure that the evidence collected is complete and significant. Intelligently used, the statistical viewpoint can contribute indispensably 

to solving the problems of analytical chemistry, and to the realistic 

interpretation of chemical analyses by the consumer.

Acknowledgments. I thank Dr. W. Edwards Deming, Executive 

Offices of the President, Bureau of the Budget; Dr. W. J. Youden 

and Mr. Joseph M. Cameron, Statistical Engineering Laboratory, 

and Mr. Shuford Schuhmann, Division of Chemistry, National 

Bureau of Standards; Mrs. Priscilla Schuhmann; and Mr. James A. 

Mitchell, Cellulose Esters Division, Tennessee Eastman Corp., 

for invaluable criticisms and suggestions; and Dr. Youden and Mr. 

Mitchell for permission, to use their illustrations.

BIBLIOGRAPHIC COMMENTARY

1. Anderson, J. A., 1945, The role of statistics in technical papers: Am. Assoc. 

Cereal Chemists Trans., vol. 3, pp. 09-73.

"Statistical methods can be applied to unsatisfactory data with direful 

results. The very elegance of the analysis may then persuade the user that 

the results are true * * *." p. 69.

2. Dingle, Herbert, 1937, Knowledge without understanding: Atlantic Monthly, 

vol. 160, pp. 116-124.

3. Heinlein, C. P., 1937, Some consequences of pseudo-mathematics and quasimeasurement in psychometrics, education and the social sciences: 

Florida Acad. Sci. Proc. 1936, vol. 1, pp. 33-42.

4. Andersen (1).

"* * * statistics tend to mesmerize the beginner into using calculafcions 

as a substitute for common sense. Experience shows that it is all too easy to 

get into a rut in which most investigations are designed mechanically in accordance with standard procedures, and in which the results of statistical analyses 

are blindly accepted as true. But no mechanical process can substitute for 

thought." p. 69.

5. Stevens, N. E., 1941, Botanical research by unfashionable technics: Science, 

vol. 93, pp. 172-176, reprinted in Stevens, N. E., Factors in botanical 

publication and other essays: Chronica Botanica, vol. 11, pp. 166-172.

6. The doctrine that a subject is not scientific unless it is quantitative was firmly 

established through the influence of great men like Lord Kelvin, the 

physicist, and Sir Francis Galton, the statistician, who were widely
PART 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS 65

quoted to the effect that they considered knowledge unsatisfactory if it 

could not be expressed in numbers. John R. van Wazer has pointed out 

that their criterion would ignore such a logically rigorous science as 

classical organic chemistry, which was based almost entirely on qualitative observations (Physics Today, 1951, vol. 4, no. 6, pp. 18-20).

7. Sandell, E. B., 1949, Determination of beryllium in silicate rocks: Analytica 

Chimica Acta, vol. 3, pp. 89-95.

"The results obtained in applying the proposed method to subsilicic rocks as 

represented by two diabases and to a synthetic solution simulating a basic rock, 

containing from 2 to 8 p. p. m. of added beryllium, are uniformly slightly low, 

the worst result corresponding to 80% recovery of beryllium. The average recovery in these samples corresponds to approximately 90%."

8. Hillebrand, W. F., 1919, The analysis of silicate and carbonate rocks: U. S. 

Geol. Survey Bull. 700, p. 153.

"Experience has shown that most analysts are prone to make a very serious 

plus error in determining moderate amounts of magnesia, such as are found in 

limestone and portland cements."

9. Johnston, John, and Adams, L. H., 1911, The phenomenon of occlusion in precipitates of barium sulphate, and its relation to the exact determination 

of sulphate: Am. Chem. Soc. Jour., vol. 33, pp. 829-845.

"* * * there is little chance of discovering any exact method [for separation of sulfate as barium sulfate] which shall at the same time be generally 

applicable and require no corrections, and yet not owe its accuracy to a compensation of errors; for the reason that some of the requisite conditions appear 

to be mutually incompatible."

10. Lundell, G. E. F., 1933, The chemical analysis of things as they are: Ind. Eng. 

Chem., Anal. Ed., vol. 5, pp. 221-225.

11. Kolthoff, I. M., 1950, Analytical chemistry as a technique and as a science: 

.. Chem. and Eng. News, vol. 28, pp. 2882-2886.

"The early realization of the importance of separations in analytical chemistry 

is reflected interestingly by the Dutch word for chemistry, scheikunde, or the art 

of separating. It is a centuries-old word which to my knowledge is not used in 

any other language. * * * The emphasis on separations is entirely justified, 

even nowadays."

12. Mellon, M. G., 1950, Role of separations in analytical chemistry: Anal. Chem., 

vol. 22, pp. 1342-1346, quoted in Chem. and Eng. News, vol. 28, p. 2268 

(1950).

«* * * separations are likely to be the seat of the most numerous and 

most serious errors in many methods of analysis."

13. Hillebrand, W. F., and Lundell, G. E. F., no date [1929?], Applied inorganic 

analysis, N. Y., John Wiley & Sons.

"The mechanical details of quantitative analysis have been quite satisfactorily worked out, the chemical details have not. In other words, balances, 

weights, volumetric apparatus, reagents, glass, quartz, porcelain and platinum 

wear, filtering media, burners, ovens, and even procedures for the determination
66 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

of elements when occurring by themselves need very little improvement for 

ordinary purposes. On the other hand, methods for the quantitative separation 

of the elements from one another are far from perfect."

14. Groves, A. W., 1951, Silicate analysis, 2d ed., London, George Alien & Unwin.

15. "Student", 1909, The distribution of the means of samples which are not 

drawn at random: Biometrika, vol. 7, p. 210, reprinted in "Student's" 

collected papers, pp. 43-48 (1942).

"Thus repetitions of analyses in a technical laboratory should never follow 

one another but an interval of at least a day should occur between them. 

Otherwise a spurious accuracy will be obtained which greatly reduces the value 

of the analyses."

16. "Student", 1927, Errors of routine analysis: Biometrika, vol. 19, p. 151, reprinted in "Student's" collected papers, pp. 135-149 (1942).

"We now come to a phenomenon which will be familiar to those who have had 

astronomical experience, namely that analyses made alongside one another tend 

to have similar errors; not only so but s'uch errors, which I may call semiconstant, tend to persist throughout the day and some of them throughout the 

week or the month."

17. Moran, II. F., 1943, Determination of the precision of analytical control 

methods: Ind. Eng. Chem., Anal. Ed., vol. 15, pp. 361-364.

"Duplicate determinations run at the same time are generally not truly 

random and the averaging of such results does not improve the precision appreciably."

18. Shewhart, W. A., 1939, Statistical method from the viewpoint of quality control, p. 82, Washington, D. C., Graduate School, U. S. Department of 

Agriculture.

"* * * the meaning or interpretation of any summary ins the form 

X ± AX depends upon whether the original data arose under a state of statistical 

control; such a summary of data does not provide an adequate basis for setting an efficient tolerance range."

19. Hillebrand and Lundell (reference 13, p. 5).

20. American Society for Testing Materials, 1951, ASTM manual on quality 

control of materials, p. 33, Philadelphia, American Society for Testing 

Materials.

"Presented data should be accompanied by available relevant information, 

particularly information on precisely the field within which the measurements 

are supposed to hold and the conditions under which they were made, and evidence that the data are good."

21. Beveridge, W. I. B., no date [1950?], The art of scientific investigation, p. 33, 

N. Y., W. W. Norton & Co.

"Occasionally scientific reports are marred by the authors giving their results 

only as averages. Averages often convey little information and may even be 

misleading. The frequency distribution should be given and some figures relating to individuals are often helpful in giving a complete picture."
PART 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS 67

22. Youden, W. J., 1949, Personal communication.

23. Eisenhart, Churchill, 1949, Accuracy and precision of measurements, Lectures, National Bureau of Standards, Junior Professional. Training 

Program. (Processed.)

24. Rider, P. R., 1933, Criteria for the rejection of observations: Washington 

University (St. Louis, Mo.) Studies, new ser., Science and Technology 

no. 8, 23 pp.

"In the final analysis it would seem that the question of the rejection or 

retention of a discordant observation reduces to a question of common sense. 

Certainly the judgment of an experienced observer should be allowed considerable influence in reaching a decision."

25. Barnard, G. A., 1947, 2x2 Tables. A note on E. S. Pearson's paper: 

Biometrika, vol. 34, pp. 168-169.

"* * * while statistics is a most valuable aid to judgement, it cannot 

wholly replace it."

26. Anon., 1949, The fallacy of the best two out of three: Nat. Bur. Standards 

Tech. News Bull., vol. 33, pp. 77-78.

27. Mitchell, J. A., 1947, Control of the accuracy and precision of industrial tests 

and analyses: Anal. Chem., vol. 19, pp. 961-967.

28. Yule, G. U., and Kendall, M. G., 1947, An introduction to the theory of 

statistics, 13th ed., London, Charles Griffin and Co.

"The use of the probable error is declining, and we recommend the student 

to eschew it."

29. Davies, 0. L., Ed., 1949, Statistical methods in research and production 

with special reference to the chemical industry, 2d. ed., London, Oliver 

and Boyd.

"Other measures of dispersion, such as quartile deviation and probable error, 

are mentioned in the literature, but are to be avoided, especially the latter."

30. ASTM manual (reference 20, p. 42).

"Mention should be made of the practice, currently losing favor in scientific

ff work, of recording such limits as: X±0.6745 =: * * *. Investigation of

vn

this problem has given a more satisfactory alternative (Section 4), a procedure 

which provides limits that have a definite operational meaning."

31. Anon., 1950, Misuse of the average deviation: Nat. Bur. Standards Tech. 

News Bull., vol. 34, pp. 9-10.

32. Anon., 1951, Chemists advised to abandon average deviations: Chem. and 

Eng. News, vol. 29, p. 2380.

33. Power, Francis W., 1939, Accuracy and precision of microanalytical determination of carbon and hydrogen: a statistical study: Ind. Eng. Chem., 

Anal. Ed., vol. 11, pp. 660-673.

"For practical purposes of error theory the normal curve is only a mathematical convenience, the use of which is based largely on pragmatic grounds."
68 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

34. Geary, R. C., 1947, Testing for normality: Biometrika, vol. 34, pp. 209-242.

"Normality is a myth; there never was, and never will be, a normal distribution. 

This is an overstatement from the practical point of view, but it represents a 

safer initial mental attitude than any in fashion during the past two decades."

35. Deming, W. E., and Birge, R. T., 1938, On the statistical theory of errors, 

Washington, D. C., Graduate School, U. S. Department of Agriculture, 

reprinted from Rev. Mod. Phys., vol. 6, pp. 119-161, with additional 

notes dated 1937 and 1938.

"Until one has a long series of measurements he is not in a position to judge 

whether his observations are random, and consequently whether he dare apply 

the theory of errors at all."

36. Schlecht, W. G.,'1946, Control chart method applied to errors in radioactive 

counting: MDDC-695 (declassified Feb. 22, 1947).

37. Clancey, V. J., 1947, Statistical methods in chemical analyses: Nature, vol. 

159, pp. 339-340.

38. Youden, W. J., 1948, Multiple factor experiments in analytical chemistry: 

Anal. Chem., vol. 20, pp. 1136-1140.

39. Shewhart (reference 18, p. 111).

"* * * the method of increasing knowledge does not consist in taking more 

and more repetitive measurements under presumably the same conditions as it 

does when one is making drawings from a bowl. In fact, a scientist seldom 

bothers to take more than five or ten observations under what he considers to 

be the same essential conditions (drawings from a bowl), although often he 

experiments with what he thinks may be slightly different conditions."

40. Anderson (reference 1, p. 71).

"It is only when the samples have been selected at random, which does not 

always happen in laboratory investigations, that it is theoretically sound to 

generalize beyond the specific samples and conditions represented in the 

experiment."

41. Shewhart (reference 18, p. 81).

"* * * as emphasized in the previous chapters, measurements of physical 

properties and quality characteristics, including some of the most refined 

physical measurements, are not ordinarily in a state of statistical control."

42. Scheffe, Henry, 1951, Some basic concepts of probability and statistics: Ind. 

Eng. Chem., vol. 43, pp. 1292-1294.

"* * * a state of statistical control practically never exists in manufacturing processes or methods of measurements before the process is stabilized 

by being analyzed and modified by statistical control techniques."

43. Anderson (reference 1, p. 73).

"Routine statistical analysis of data may present many pitfalls, and the advice of a professional statistician is frequently required."
PART 6. THE PROBABLE ERROR OF A CHEMICAL ANALYSIS 69

44. Wiener, Norbert, 1948, Cybernetics, New York, John Wiley and Sons.

"* * * the modern apparatus of the theory of small samples, once it 

goes beyond the determination of its own specifically defined parameters and 

becomes a method for positive statistical inference in new cases, does not inspire 

me with any confidence, unless it is applied by a statistician by whom the main 

elements of the dynamics of the situation are either explicitly known or implicitly 

felt."

45. Wernimont, Grant, 1949, Statistics applied to analysis: Anal. Chem., vol. 21, 

pp. 115-120.

"It is not safe to depend entirely upon any of the books on statistics which 

have been written by chemists or physicists."

46. Clarke, B. L., 1949, Statistical methods in the chemical industry: Chem. 

and Eng. News, vol. 27, pp. 1426-1428.

"Dr. Shewhart * * * first asked to see several outstanding modern textbooks on analytical chemistry. After examining these, particularly the treatment given the subject of error, he told me regretfully that even in the best of 

these books, the discussion of precision and accuracy would have been considered antiquated 40 years ago. Indeed, he pointed out, more in sorrow than in 

wrath, that the handling of the subject of error in one of these books was 

based upon a concept that had been abandoned by statisticians in 1888."

47. Tukey, J. W., 1947, Discussion, Symposium on statistical methods in 

experimental and industrial chemistry: Anal. Chem., vol. 19, pp. 956-957.

Bulletin 992. CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Part 7. A CONFINED-SPOT METHOD FOR THE DETERMINATION OF TRACES OF SILVER IN SOILS AND ROCKS

By HY ALMOND, R. E. STEVENS, and H. W. LAKIN

CONTENTS

Page

Abstract___ _____________________________________________________ 71

Introduction__ ____________________________________________________ 72

Reagents and apparatus____________________________________________ 73

Procedure ________________________________________________________ 75

Preparation of standard spots.__________________________________ 75

Preparation of sample solution_____________ ___________________ 75

Separation and estimation of silver_________ __________________ 75

Discussion._ ______________________________________________________ 76

Results. -...___________________________________________ 79

Usefulness of the method-__________________________________________ 81

Literature cited---_-___--_-_-___-___________-__---__-___-_---___-_ 81

ILLUSTRATIONS

FIGURE 14. Two complete chromographs in carrying case_-_---_---_--_- 74

15. Extractor for removing acid-soluble constituents from soil and

rock samples____-__--_____-_______--___-___-__---__-_ 75

16. Rock sample being treated with boiling nitric acid in outer

chamber.____________________________________________ 77

17. Extract being automatically drawn through filter into inner

chamber on cooling. __________________________________ 78

ABSTRACT

'A procedure for estimating traces of silver in soils and rocks was developed to 

provide a method suitable for geochemical prospecting. Silver can be determined 

in a 0.5 g. sample in concentrations between 0.2 and 20 ppm. A specially designed 

extractor prevents bumping during the acid digestion of a soil and upon cooling 

automatically niters the acid digest. The final estimation is made with a chromograph, a device which automatically controls the rate of flow of test solution 

through a confined spot on filter paper. A preliminary ignition of the sample to 

eliminate organic matter and mercury is followed by a nitric acid digestion. 

Silver is separated from the colored ions in the nitric acid solution by collecting 

the silver salt of p-dimethylaminobenzalrhodanine at the interface between amyl 

alcohol and the aqueous solution and then discarding the aqueous extract. This 

silver precipitate is dispersed by adding ethyl alcohol and is then collected on a 

confined spot by means of the chromograph. The intensity of the red color on

71
72 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

the filter paper, compared with spots prepared from standard solutions, is a measure of the silver content of the original sample. About 20 samples can be analyzed 

per man-day.

INTRODUCTION

The Geological Survey's project of developing trace methods of 

analysis applicable under field conditions to geochemical prospecting 

has resulted in the development of a serviceable method for the determination of traces of silver. The method is an application of the 

confined-spot technique (Stevens and Lakin, 1949; Yagoda, 1937), 

using ^-dimethylaminobenzalrhodanine as the precipitating reagent.

This reagent had the desired sensitivity and was more satisfactory 

than other reagents for reasons discussed below. Dithizone reacts 

with silver to form a silver dithizonate which is soluble in carbon 

tetrachloride, but the yellow color imparted by the silver dithizonate 

to the carbon tetrachloride is indistinguishable from that imparted 

by the oxidation product of dithizone, also soluble in the organic solvent. Furthermore, the silver dithi/.onate-dithizone mixed color is 

not sufficiently sensitive for visual comparisons in the range desired. 

Cupric dithizonate as a reagent for silver is unsatisfactory because 

again silver is determined by the mixed-color method which lacks 

sensitivity. Turbidimetric and confined-spot methods which involve 

the precipitation of the chloride, sulfide, or free silver are also not 

sufficiently sensitive.

To determine the silver content of a soil or rock by the rhodanine 

method described by Sandell (1950), silver must first be isolated from 

colored salts or ions as well as from elements that react with p-dimethylaminobenzalrhodanine (Ettisch and Tamchyna, 1931; Feigl, 1949). 

The acidity of the separated silver solution must be carefully adjusted 

and an exact amount of the reagent added. The color intensity is 

best determined instrumentally (Sandell, 1950; Schoonover, 1935). 

The procedures of Schoonover (1935) and of Sandell (1950) do not 

seem promising for field work because they require precise control of 

acidity and because colored ions have to be removed.

The method described here is based on the fact that the silver salt 

of p-dimethylaminobenzalrhodanine can be separated quantitatively 

from a relatively large volume of aqueous solution by collecting the 

silver precipitate at the interface between aqueous and amyl alcohol 

phases (Strafford, 1933; Monier William, 1950). Most of the 

aqueous phase is discarded and the silver precipitate dispersed into 

the remaining smaller volume by destroying the interface with ethyl 

alcohol. The dispersion is filtered through filter paper in the chromograph, and silver estimations are made by comparing the intensity of 

red color on the paper with spots prepared from standard solutions.
PART 7. CONFINED-SPOT DETERMINATION-TRACES OF AG 73

The nitric acid digestion used in .this procedure dissolves essentially 

all the silver in the sample without introducing the difficulties and the 

time-consuming operations of complete sample solution.

Very exact methods require the solution of the entire sample, but 

the drastic procedures for obtaining complete solution of the sample 

are usually too complex for ease in rapid field methods. The effectiveness of extraction procedures depends in part upon the state of 

subdivision. However, an extraction procedure cannot bring into, 

solution material enclosed in a matrix insoluble in the extracting 

agent.

REAGENTS AND APPARATUS

Ammonium citrate, 25 percent: Dissolve 25 g of ammonium citrate, 

(iNH4 )2HC6H507, in water and dilute to 100 ml. Shake with amyl 

alcohol and allow to stand 4 hours. Discard both the amyl alcohol 

and the solids collecting at the interface.

Ammonium hydroxide, cone, sp gr 0.90.

Ammonium hydroxide, 1 N: Add 10 ml of cone ammonium hydroxide 

to 140 ml of water.

Acetic acid, glacial.

Nitric acid, cone, sp gr 1.41.

Water: Pass tap water through a resin demineralizer to free it of 

heavy metals. Demineralizers are commercially available that do 

not require conductivity meters to indicate the quality of water 

passing through.

p-dimethylaminobenzalrhodanine, 0.05 percent: Dissolve 0.05 g in 

100 ml of glacial acetic acid.

Amyl alcohol, boiling point 136-138 C.

Ethyl alcohol, 95 percent.

Standard silver solution, 0.01 percent: Dissolve 0.158 g of silver 

nitrate, dried at 110 C, in 1 1 of 1 N ammonium hydroxide.

Standard silver solution, 0.001 percent. Transfer 10 ml of 0.01 

percent to a 100 ml volumetric flask containing about 50 ml 1 N 

ammonium hydroxide and dilute to volume with 1 >T ammonium 

hydroxide.

Standard silver solution, 0.0001 percent: Transfer 10 ml of 0.001 

percent standard solution to a 100 ml volumetric flask containing 

about 50 ml 1 N ammonium hydroxide and dilute to volume with 

1 N ammonium hydroxide.

Glass chromographs: These instruments confine areas of definite size 

on a strip of reagent paper and automatically regulate the rate of 

flow of the test solution through the confined spot. The plastic 

parts of the original model (Stevens and Lakin, 1949) are replaced
74 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

FIGURE 14. Two complete chromographs in carrying case.

by glass for the silver test because of the destructive action of the

amyl alcohol on the plastic. Glass chromographs are illustrated in

figure 14. 

Extractor: A cheap and simple extractor is made as follows: Crimp

a piece of tubing 170 mm in length about % inch from one end

with a spot flame. To the other end fuse a ground-glass joint.

Pack the crimped end with glass wool and fit the other end with a

glass stopper. (See fig. 15.)

Glass wool: AAA fineness borosilicate glass fiber, free of heavy metals. 

Separately funnels, 60 ml Squibb.

Culture tubes, borosilicate (16 x 150mm): Calibrate at 2% ml. 

Portable gasoline slove: A pocket stove is satisfactory. 

Filter paper: Hard, double-washed, retentive, close texture. 

Balance, torsion, senstivity, 2 mg. 

Crucibles, porcelain, no. 0. 

1 serological pipet, 1 ml, graduated in 0.01 ml. 

1 serological pipet, 10 ml, graduated in 1.0 ml. 

Litmus paper, blue. 

Borosilicate glass rod, 150 mm long. 

3 graduated cylinders, 5 ml. 

1 graduated cylinder, 20 ml.
PART 7. CONFINED-SPOT DETERMINATION-TRACES OF AG 75

FifiURE 15. Extractor for removing acid-soluble constituents from rock and soil samples.

Fusion and Digestion Rack: A device to support eight test tubes 

over the stove during an acid digestion. It consists of two disks 

of sheet steel welded to a central supporting rod. Each plate 

has eight holes for test tubes; the holes in the bottom plate are 

small so that tubes will not slip through, but large enough to permit 

the ready flow of heat.

Iron mortar and pestle: Any convenient size to crush rocks.

Mullite mortar and pestle; outside diameter of mortar 75 mm.

Sieve, iron, 2 mm mesh.

Sieve: 80-mesh, cloth or stainless steel. Held together by an aluminum holder which fits into an aluminum receiver.

PROCEDURE

Preparation of standard spots. Prepare a series of spots representing values of silver ranging from 0 to 24 ppm as follows:

Label eight porcelain crucibles "a" through "h", and to each add 

0.5 g of finely ground silver-free soil. Ignite each soil sample 10 

min., then cool. To crucible "a" add no silver, but to the other seven 

add 0.1, 0.3, 0.6, 1.0, 3.0, 6.0, and 12.0 micrograms of silver, respectively; the eight samples are considered as standard solutions. Then 

add 2 ml concentrated nitric acid and continue as the procedure 

directs to extract the silver and to prepare confined spots containing 

the red silver dimethylaminobenzalrhodanine compound. Mount 

the series of spots on the edge of a cardboard chart, cover with cellophane tape, and label each spot with the figure of parts per million 

to which it corresponds.

Preparation of sample solution. -Place a 0.5 g soil or rock sample, 

ground to pass through an 80-mesh sieve, in a porcelain crucible. 

Ignite for approximately 10 min., cool, add 2 ml concentrated nitric 

acid and allow the sample to stand in the acid overnight. In the
76

morning transfer sample and nitric acid to a culture tube and wash the 

remaining soil into the test tube with % ml water. Insert a firmly 

stoppered extractor into the culture tube and place in a fusion and 

digestion rack on a gasoline stove. Boil for 10 min. Remove the 

culture tube from the rack and allow to cool. By the time the culture 

tube has cooled, the solution has filtered into the extractor.

Separation and estimation of silver. Transfer the filtrate to a 

separatory funnel containing 10 ml of water. Rinse the extraction 

stick with 10 ml of water and combine with the original portion in the 

separatory funnel. Add 3 ml of ammonium citrate, shake, then add 

ammonium hydroxide in small increments until the solution is strongly 

alkaline to litmus paper. Acidify with 5 ml of glacial acetic acid. 

Add 0.1 ml of ^-dimethylaminobenzalrhodanine solution and shake to 

mix. Allow the solution to cool for about 15 min. Add 3 ml of 

amyl alcohol, shake for 30 sec. and allow to stand for 20 min. Drain 

the aqueous (lower) solution until about % ml of aqueous solution 

remains, add 1 ml of water, and again drain until about % ml of the 

aqueous phase remains. Add 1 ml of ethyl alcohol and shake to 

disperse the silver precipitate and to dissolve any unreacted reagent 

that has precipitated.

Adjust the liquid level in the chromograph to the upper timing 

mark by pressure on the adjuster bulb and turn the stopcock to the 

closed position. Place a strip of filter paper in the head of the chromograph and tighten the coupling to seal the filter paper between the 

two sections of glass pipe. Transfer the contents of the separatory 

funnel to the head of the chromograph, open the fast capillary of the 

chromograph, then turn the stopcock so that the water in the column 

drains through the capillary. After the solution has filtered, drain 

the residual solution from the paper into the deflated adjuster bulb 

by turning the stopcock. Remove the filter paper containing the 

silver spot, dry, and then compare the silver precipitate on the filter 

paper with a standard series of spots. Before using the separatory 

funnel again wash it with nitric acid; rinse four times with tap water 

and then with distilled water to remove traces of ethyl alcohol.

DISCUSSION

Any element or material that may occur in a soil or rock sample, 

as well as introduced reagents, must be considered possible interferences in quantitative methods of analysis, particularly when trace 

constituents are determined. In the colorimetric method described 

herein, copper (I), mercury, palladium, and gold must be eliminated 

because they react with the reagent, ^-dimethylaminobenzalrhodanine, just as silver does. Some means must be taken to destroy 

organic matter, too, because it also may color the confined spot.
PART 7. CONFINED-SPOT DETERMINATION-TRACES OP AG 77

FIGURE 10. Bock sample being treated with boiling nitric acid in outer chamber.

The various steps in the procedure are designed to minimize these 

hazards as much as possible and still retain speed. The sample is 

roasted to expel mercury and to destroy organic matter. Nitric acid 

dissolves essentially all the silver, but it does not appreciably extract 

gold, platinum, and palladium. It furthermore oxidizes copper to

236156 53 7
78 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

FIGURE 17. Extract being automatically drawn through filter into inner chamber on cooling.

copper (II), which does not react with the reagent. The nitric acid 

extract is diluted to prevent precipitation of slightly soluble salts.

The nitric acid digestion of the sample was adopted because other 

methods of attack were less satisfactory. A fusion of the soil with 

potassium bisulfate followed by a nitric acid digestion resulted in a
PART 7. CONFINED-SPOT DETERMINATION-TRACES OF AQ 79

large amount of insoluble residue in the aqueous solution. The 

residue caused the mixture to filter very slowly. To decrease the 

amount of insoluble residue remaining after fusion, the quantity of 

bisulfate flux was lowered. Although the speed of filtration was 

improved, insoluble material now collected at the interface of the 

amyl alcohol-aqueous phases along with the silver and tended to 

carry along reagent. This resulted in high values, especially for 

samples containing little silver.

Treatment of the sample with nitric-hydrofluoric acid proved to 

be unsatisfactory because some of the dissolved platinum finally 

appeared on the paper as a dark-red precipitate.

Nitric acid dissolved most of the silver as indicated by the following 

experiment: Three soil samples were digested with nitric acid. The 

acid-insoluble residue was fused with a sodium carbonate-sodium 

nitrate flux. Both the nitric acid extracts and a nitric acid solution 

of the fused residues were carried through the remainder of the procedure. Considerable silver was found in the nitric acid extracts, 

none in the residues, showing that extraction of the silver from the 

samples by nitric acid was essentially complete.

Extraction with nitric acid and filtration are accomplished automatically by means of an extractor, a description of which has not 

previously been given. A glass-stoppered extractor (fig. 15) is 

placed in a culture tube containing a soil or rock sample and the acid. 

While the culture tube is being heated, air within the extraction stick 

expands, and a continuous stream of air bubbles is forced through the 

glass-wool filter, producing a stirring action which circulates the soil 

through the hot acid (fig. 16) and at the same time prevents bumping. 

When the extraction is complete, the culture tube is allowed to cool. 

A decrease of pressure within the filter stick upon cooling causes the 

acid extract to filter into the extraction stick. (See fig. 17.)

The dispersed silver precipitate is collected in a confined area on 

filter paper by means of a chromograph. (See fig. 14.) Glass pipe 

was substituted for the plastic head in the original chromograph 

described by Stevens and Lakin (1949) because of the solvent action 

of the alcohols on the plastic. Minute amounts of silver in a large 

volume of solution can be precipitated and subsequently filtered, in 

the confined area with surprisingly good reproducibility.

RESULTS

The results of the analysis of standard solutions, given in table 1, 

illustrate the precision of the colorimetric confined-spot method. 

Each standard solution was analyzed six times. Of the six spots 

obtained from each solution, one was judged visually as having the 

average depth of color, and this one was used.as a standard spot.
80 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Each value listed in column 1 was similarly analyzed, and a standard 

series of spots was thus selected. Each of the remaining five spots in 

each group was compared with the standard series, and the results, 

as reported, are arranged in order of increasing values under the 

column "values obtained." Thus for the one microgram of silver the 

five results are 0.7, 0.8, 1.0, 1.0, and 1.0, respectively. These average 

0.9, reported in the last column.

TABLE 1. Precision and accuracy of the estimation as determined by analyses of

standard solutions

Silver

added (micrograms)

0

O.I

.3

.6

1.0

3.0

6.0

10.0

Values obtained >

0

0

.2

.5

.7

3.0

5.0

6.0

0

0.05

.2

.6

.8

3.0

5.0

10.0

0

0.1

.3

.6

1.0

3.0

6.0

10.0

0

0.1

.3

.6

1.0

3.0

6.0

12.0

0

0.2

.4

.8

1.0

4.0

6.0

12.0

Average

0

0.1

.3

.fi

.9

3

G

10

i Compared to average of six spots.

The results of the analysis of seven soils are given in table 2 to 

illustrate the precision of the colorimetric spot method on soils. These 

soils had been assayed and the assay results are given in the first 

column. Results of repeat determinations are given under the heading 

"colorimetric confined-spot values." The colorimetric and assay results 

agree reasonably well for methods of trace analyses that are entirely 

different. Of the 30 colorimetric values by confined spots 27 fall 

within 50 percent of the assay values.

The results of the confined-spot tests listed in table 2 may be considered as representative of the over-all precision of the test, as each 

of the values were obtained on separate soil samples. The precision 

is considerably less than that obtained in the final spot-test estimation 

given in table 1. This is attributable to variations in completeness of 

extraction and to other interferences. The results in table 2 agree 

fairly well with those obtained by fire assay, but this is no real criterion 

of accuracy as both methods are subject to error.

TABLE 2. Comparative results on soils, in parts per million 

[Assays by Charles 0. Parker, Denver, Colo. Analyses by H. Almond]

Assay value

2

2

2.7

5.4

6.8

19

40

Colorimetric confined-spot values

1

2

1.8

3

5

10

20

1.5

3

2

3

5

10

25

2

4

2

3

5

10

25

3

4

4

4

5

15

30

3

5

4

4

6

20

50

Average

2

4

3

4

5

13

29
PART 7. CONFINED-SPOT DETERMINATION-TRACES OF AQ 81 

USEFULNESS OF THE METHOD

The method makes possible the determination of traces of silver 

quickly in a large number of samples and with simple, easily transported equipment. It has been devised particularly for application 

to geochemical prospecting, for outlining the halos of increasing silver 

content in soils surrounding hidden silver ores. Under field conditions 

about 20 determinations can be made per man-day, with accuracy 

and sensitivity sufficient to discover and evaluate abnormal content 

of silver. The normal silver content of soils is given by Rankama 

and Sahama (1950) as 0.1 ppm. The chromographic test for silver, 

which has been described, will give a positive test for silver when 

present in this background range, and, where present in larger percentages, it quickly measures the silver to within 50 percent of the 

actual content, an accuracy sufficient to show significant trends from 

normal values to those approaching ore.

The test may have application to other investigations as well, 

where a quick estimate of traces of silver is wanted.

LITERATURE CITED

Ettisch, G., and Tamchyna, J., 1931, Mikrochemie, vol. 10, p. 92.

Feigl, Fritz, 1949, "Specific, Selective and Sensitive Reactions," p. 660, New

York, Academic Press Inc. 

Rankama, K., and Sahama, Th. G., 1950, "Geochemistry," p. 702, Chicago, The

University of Chicago Press. 

Sandell, E. B., 1950, "Colorimetric Determinations of Traces of Metals," 2d ed.,

pp. 539-544, New York, Interscience Publishers Inc. 

Schoonover, I. C., 1935, J. Research Natl. Bur. Standards, 15, 377. 

Stevens, R. E., and Lakin, H. W., 1949, TJ. S. Geol. Survey Circ. 63. 

Stratford, N., 1933, Institute of Chemistry lecture. 

Monier William, G. W., 1950, "Trace Elements in Food," pp. 423-27, New York,

John Wiley & Sons, Inc. 

Yagoda, Herman, 1937, Ind. Eng. Chem., Anal. Ed., 9, 79-82

Bulletin 992. CONTRIBUTIONS OF GEOCHEMISTRY 1949

Part 8. A PHOTOGRAPHIC TECHNIQUE FOR SHOWING 

SOME MINERAL RELATIONS

By WALDEMAR T. SCHALLER

CONTENTS

Page

Abstract_.__________________________________________ 83

Introduction. _____________________________________________________ 84

Technique used.__________________________________________________ 85

Crystals of quartz.______--___,_______--____-_---__-_--_---_--__--- 88

Microcline________________________________________________________ 89

Albite---_-___---__-___--_---_______________---__------___------- 92

Quartz of graphic granite.--__-______________-_-___--_-__-----_--_-- 92

Halite and sylvite._______-_--______________-__--__---_------------ 93

Crystal faces____________----_________-________---__---_---_-_--_-- 93

ILLUSTRATIONS

[Pis. 1-14. at end of Part 8]

PLATE 1. Brightly reflecting faces of parallel quartz crystals.

2. Brightly reflecting planes of microcline.

3. Additional brightly reflecting planes of microcline.

4. Brightly reflecting cleavage surfaces of microcline.

5. Brightly reflecting plane of microcline.

6. Brightly reflecting planes of microcline.

7. Brightly reflecting cleavage plane of microcline.

8. Brightly reflecting veinlets of variously oriented albite in uniformly oriented compact massive albite.

9. Poor presentation of quartz, not using the brightly reflecting 

method.

10. Brightly reflecting polished surfaces of quart/.

11. Brightly reflecting surfaces of quartz.

12. A, B, Brightly reflecting surfaces of halite. C, Additional 

brightly reflecting surfaces of halite.

13. Brightly reflecting faces of a crystal of bustamit.

14. Bromide enlargement (4X) of plate 13, D.

A photographic technique is described whereby the parallelism of a group 

of crystals, or the presence of a certain mineral in an association of other minerals, 

or a certain structure, can be emphasized and be shown to better advantage 

than by a straight-on photograph. The requirement is that the particular 

mineral, whose presence in a specimen is to be shown photographically, must

83
84 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

have a brightly reflecting surface, either natural or artificially produced. The 

simple technique is so to orient the specimen before the camera that this surface 

reflects a maximum brightness into the camera. In the resulting photograph 

the bright surface will be in strong contrast to the rest of the picture.

Examples are shown of parallel groups of quartz crystals, their parallelism 

accentuated by so turning the specimen before the camera that the same faces 

of the different crystals brightly reflect light into the camera. The presence 

of microcline, interpreted as being residual in albitized pegmatite, can be emphasized, photographically, if a brightly reflecting surface is present. Two 

generations of albite, in albitite, can be demonstrated. The presence of graphic 

granite quartz or of halite in a mixture of halite and sylvite ("potash ore") can 

be well shown in this way.

The brightly reflecting surface may result from cleavage or parting, or it may be 

the polished surface remaining after differential etching.

The technique can be used also for making an accurate crystal drawing, as 

the exact relative lengths of all intersection edges of the various faces present 

can be measured from the photographs when the crystal is so turned before the 

camera that a particular face reflects light brightly.

INTRODUCTION

The object of this paper is to call attention to a photographic 

technique that illustrates certain relations of minerals much more 

strikingly than a straight-on photograph of a mineral specimen. 

The technique, in brief, is so to orient a specimen before the camera 

that one mineral or one face of a crystal will reflect light into the 

camera much more brightly than the other minerals or crystal faces 

present. As a result, the particular mineral or crystal face selected 

stands out strongly from the rest of the specimen. If the mineral 

to be emphasized is to reflect a more intense light into the camera, 

it must possess a brightly reflecting surface or plane or such a, highly 

reflecting surface must be artificially produced. The technique 

therefore is not of universal application to all specimens. The 

fundamental requirement is the presence of a highly reflecting surface 

or plane. The faces of crystals, natural or developed cleavage surfaces, internal reflecting planes, such as a parting that produces 

schiller, and polished surfaces differentially etched and thereby 

partly dulled, all fulfill this requirement.

The resultant photographs are similar, in the contrast produced, 

to those of many associations of metallic minerals where there is a 

marked difference in the reflectivity of several of these minerals, or 

where a metallic mineral, appearing white on a polished surface, 

will reflect light much more brightly than the associated nonmetallic 

gangue minerals. Those metallic minerals with a maximum (almost 

total) reflectivity, such as chalcopyrite, will reflect so much more 

light in comparison with those of greater light absorption and hence 

lesser reflectivity, as sphalerite, that they will photograph almost 

white.
PART 8. PHOTOGRAPHIC TECHNIQUE SOME MINERAL RELATIONS 85

The parallelism of a group of crystals can be well shown if the 

brightly reflecting faces of one form reflect light into the camera by 

properly orienting the specimen. In a study of the genesis of the 

gem-tourmaline pegmatites of southern California, it becomes important to show clearly the presence of microcline or of graphicgranite quartz, in parts of the pegmatite. Such microcline or quartz 

are interpreted as residual remnants indicating their former much 

more widespread abundance. These remnants are abundantly present 

in many parts of what are considered to be albitized parts of the 

original pegmatite. The presence of graphic granite in the albitites 

of the Pennsylvania-Maryland State line pegmatites is well shown 

by this method. The relations and distinction between halite and 

sylvite in the "potash ore" of the Texas-New Mexico potash field, is 

strikingly shown by this method.

The one necessary condition is that one mineral in an aggregate of 

minerals will reflect much more light than the rest of the specimen. 

This may be a natural condition or it may be induced by proper 

treatment, such as etching.

The brightly reflecting technique can also be used if it is desirable 

to prepare an accurate crystal drawing showing the correct relative 

lengths of all intersection edges of a crystal.

The procedure is herein described and illustrated by examples in 

the hope that others may find it useful and be able to extend it to 

show mineral relations and associations other than those here described.

It is a great pleasure to acknowledge the patient and skillful cooperation of Nelson W. Shupe, photographer of the Geologic Division of 

the U. S. Geological Survey, who took the pictures. Several of the 

specimens here illustrated presented considerable difficulty before the 

desired results could be obtained.

TECHNIQUE USED

If a specimen shows a brightly reflecting plane or surface of one 

mineral, or if such a surface can be produced, the specimen should be 

so oriented that this plane or surface reflects a maximum brightness 

into the camera. In order to keep all parts of the picture in focus, 

the surface to be photographed should be fairly flat and should be 

placed nearly normal to the optical axis of the camera. By slightly 

turning the specimen and then changing the position of the lights, 

the reflecting surface can be made to show a maximum illumination. 

This produces a striking effect in the resultant photograph, as shown, 

for example in plate 1.

Much of the microcline in the gem-bearing pegmatites of southern 

California is so oriented in the dikes that some brightly reflecting 

plane is nearly normal to the dip of the dike. As many of these dikes
86 CONTRIBUTIONS TO GEOCHEMISTRY, L949 .

are more horizontal than vertical, many specimens with a broken or 

sawed surface that is nearly vertical will show some brightly reflecting 

plane, of any microcline present, that also is nearly vertical. These 

planes may be either induced cleavage surfaces developed when the 

pegmatite rock is broken or parting planes. (See pis. 2 to 7.)

Where a cleavage plane of the microcline is developed by breaking 

the rock, the rough surface of the specimen is photographed without 

polishing. (See pis. 2, 3, and 4.) For all sawed specimens the reflecting plane photographs better on the sawed but not polished surface 

than it does on the polished surface. This applies to a cleavage plane 

(pi. 7) as well as to an apparent parting plane (pis. 5 and 6). If the 

sawed surface contains too many saw marks it may be lightly ground 

smooth.

Similar photographic results may be obtained by differentially 

etching a polished surface with the proper solution. For quartz and 

the feldspars, albite and microcline, dilute HF (about 1:3) is very 

satisfactory. The etching solution is spread uniformly over the 

polished surface of a hand specimen with an inch-wide strip of white 

blotting paper, is allowed to stand for a few seconds, and the specimen handled with tongs is then well washed, dried, and examined. 

It is better to repeat the operation several tunes with dilute acid 

until the desired effect is obtained. For graphic granite, for example, 

the polish on the albite and microcline can thus be removed, without 

dulling the polish of the quartz. When properly oriented the polished 

quartz will then brightly reflect light into the camera, whereas the 

etched feldspars will reflect comparatively little light. For the results 

obtained, see plates 10 and 11.

A very unexpected result was obtained by so etching a polished 

surface of the albitite from the Maryland-Pennsylvania State line 

pegmatites. (See pi. 8.) The compact matrix albite, uniformly oriented, which constitutes the bulk of the specimen, was readily depolished but the many veinlets of albite, variously oriented, retained 

their polish.

That there is considerable graphic granite in parts of these albitites 

is shown by plates 9 and 10. A straight-on direct photograph yields 

a very poor presentation of the quartz of the graphic granite. (See 

pi. 9A.) By etching the entire surface the presence of quartz can 

be shown distinctly (pi. 91?) but a much more striking effect is 

obtained (pi. 10) when only the feldspar is etched and the highly 

reflecting surfaces of the polished quartz properly oriented before the 

camera.

For the mixture of halite and sylvite in the "potash ore" of the 

Texas-New Mexico potash field, the ground and polished surface is 

immersed in a saturated solution of sodium chloride which attacks
PART 8. PHOTOGRAPHIC TECHNIQUE SOME MINERAL RELATIONS 87

the sylvite and destroys its polish but leaves the polished surface of 

the halite untouched. (See pi. 12.) An immersion of about 10 seconds 

suffices; the specimen is then removed from the solution and the 

surface quickly wiped dry. The remaining polish of the halite can 

readily be accentuated, if necessary, by hand rubbing.

The technique described can be used also if it becomes desirable to 

make an accurate drawing of a crystal, showing the correct lengths of 

all intersection edges. The crystal is mounted in polar position on a 

two-circle goniometer, or set vertical on a one-circle goniometer. 

The crystal holder is then mounted properly before the camera in an 

arrangement which presents what is usually called an orthographic, 

or c-axis-horizontal, projection a projection on the basal plane or 

more generally on the plane normal to the prismatic zone. The 

camera used should give as large an initial magnification as possible, 

without sacrificing any sharpness. The illustrations of a crystal of 

bustamite shown on plate 13 are enlarged 5 times.

A photograph of the top of the crystal of bustamite, corresponding 

in orientation to an orthographic projection, is shown in plate 13/i. 

In order to obtain a much sharper picture of the intersection edges of 

the several faces, the crystal is then revolved on what would correspond 

to its c-axis, and the position of the lights changed, so that one face 

brightly reflects light into the camera.

By properly revolving the crystal and adjusting the lights for each 

face, all the terminal faces can be photographed successively in the 

position wherein they brightly reflect into the camera. (See pi. 135, 

C, and D.) In all these photographs of the terminal faces, the crystal 

is kept in true position for an orthographic view, as it is moved only 

by revolving on its own axis. A separate photograph has to be made 

for each of the larger faces. If the crystal has many terminal faces, 

many photographs may be required, though obviously very small and 

line faces need not be photographed, as the lengths of their intersection edges will be given by pictures of the larger faces. For photographs of the crystal faces with the prism zone vertical, corresponding 

to the clinographic or axiometric projection, the crystal holder is 

remounted. By the use of two arcs, the crystal mounted polar on 

the goniometer can be turned and tilted the proper amounts, either 

the conventional revolution of 18°26' to the left and forward tilt 

of 9°28', or in whatever position is desired. Then, by revolving the 

crystal on its own axis and adjusting the position of the lights so that 

the desired face brightly reflects, the faces of the prism zone can be 

successively photographed. (See pi. 13E" and F.)

The second largest terminal face on the crystal is shown in plate 

13 E, upper right, but the lengths of its intersection edges on the upper
88 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

left do not show clearly enough to be measurable; all intersection 

edges show clearly, however, when the crystal is turned so that the 

face brightly reflects, as in plate 136r. The apparent intersection 

edge cutting diagonally across the face in plate 13£" is due to the 

reflection of an edge of the largest terminal face in the rear of this 

transparent crystal.

The essence of the technique is to have each face brightly reflect 

light into the camera, the crystal being kept in polar position throughout its turning, the successive bright illumination of the different faces 

being accomplished by changing the position of the lights.

If greater magnification than that of the initial photograph is 

desirable, a bromide enlargement is made, as in plate 14, which is 

a four times enlargement of plate 13Z>, and is of a size similar to 

that of a crystal drawing.

This illustration shows eight brightly reflecting faces of the same 

form and the lengths of all intersection edges can easily be measured. 

The lengths of the intersection edge of the two largest faces (shown 

black) can also be measured in this illustration as well as the lengths 

of the three intersection edges of the small triangular face (shown 

gray) in upper center. This same triangular face is shown brightly 

reflecting in plate 13J5 and H.

For smaller crystals and smaller original magnification, the bromide 

enlargements may be increased, possibly to 15 times. Special films, 

prints, and developers can be used to yield minimum grain size of the 

film and of the paper.

A crystal drawing can be made from the goniometric measurements 

in the regular way. If it be made of a size to correspond with the 

bromide enlargements, these will give the exact relative lengths of all 

intersection edges. They also show the abundance and position of 

oscillatory repetitions of the same face (pi. I3E and F) and any 

imperfections on the crystal which it may be desirable to show in the 

crystal drawing.

The bromide enlargements can also be used directly, by inking in 

the intersection edges and any other features to be shown and then 

bleaching the bromide.

CRYSTALS OF QUARTZ

The essential parallelism of a group of crystals of quartz on graphic 

granite can be well shown by turning the specimen so that the same 

faces of the prism simultaneously reflect light brightly into the 

camera, as shown in plate 1A. The specimen was collected at the 

Panama-Pacific Exposition mine in a gem-bearing pegmatite in the 

east side of Chihuahua Valley, San Diego County, California. This
PART 8. PHOTOGRAPHIC TECHNIQUE SOME MINERAL RELATIONS 89

pegmatite yielded a small handful of cassiterite crystals obtained from 

a single pocket and the pegmatite was briefly described by Schaller. 1 

To show the parallelism of a group of euhedral crystals of quartz, 

which are extensions of the quartz rods of the underlying graphic 

granite, the specimen is so turned before the camera that the corresponding rhombohedral faces on the different crystals brightly reflect 

light into the camera, as shown in plate IB. The specimen was collected at the El Molino gem mine in a pegmatite on Hiriart Hill, Pala, 

San Diego County, Calif.

MICROCLINE

The writer has previously postulated 2 that the gem-bearing pegmatites of southern California were originally composed essentially 

of microcline or of microcline graphic granite, both of which were 

later subjected to hydro thermal replacement yielding in large part an 

albitized and banded rock. Part of the evidence consists of demonstrating the presence of residual unreplaced microcline, unreplaced 

microcline graphic granite, and partially replaced microcline graphic 

granite in the albitized part. The presence of residual microcline and 

of residual quartz of graphic granite, partially replaced, can very well 

be illustrated by use of the photographic technique herein described.

Microcline possesses a number of planes which will reflect light 

brightly. There are the several cleavages, easily developed on breaking the host rock, and the parting that yields a schiller. Much of the 

microcline in the pegmatites is so oriented that a broken rock surface 

nearly normal to the dip of the pegmatite dike will show such a brightly 

reflecting plane.

Similar brightly reflecting planes of microcline were observed in 

albitized pegmatite from localities other than those in southern 

California. One such example from the Maryland-Pennsylvania State 

line albitite pegmatites is shown in plate 7.

Plates 2 to 7 show how clearly the presence of this microcline, interpreted as residual, can be illustrated by using the photographic technique described.

A tapering crystal of residual microcline in a sparsely-banded 

albitized pegmatite from near Jacumba, San Diego County, Calif., is 

shown in plate 2A. The broken, not polished, rock surface yielded a 

cleavage face of the microcline which reflects light brightly. The 

corroded sides of the microcline crystal also are emphasized by using 

this technique. In the photograph the wavy bands of garnet show as

i Schaller, W. T., 1916, Cassiterite in San Diego County, California: U. S. Geol. Survey Bull. 620, pp 

351-354.

: Schaller, W. T., 1925,The.genesis of lithium pegmatites: Am. Jour. Sci. ,5th ser. ,vol. 10, pp. 269-279.
90 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

nearly vertical lines. In the dike, however, the orientation of these 

lines is horizontal.

A similar example is shown in plate 2B, where the brightly reflecting 

cleavage planes of two tapering microclines crystal in the banded 

albitized pegmatite stand out strongly. The microcline is in part 

graphic granite. The specimen was collected a short distance north 

of the Panama Pacific Exposition gem mine in the eastern part oJ 

San Diego County, Calif.

In thenonpolished specimen of albitized pegmatite shown in plate 3>A, 

the cleavage face of the microcline crystal interpreted as residualreflects light brightly. The corroded edges of this crystal are accentuated by its bright reflection. Specimen collected in the Himalaya 

mine, Mesa Grande, San Diego County, Calif.

Plate 35 illustrates the largely disconnected residual fragments of 

microcline, whose uniformly brightly reflecting cleavage surfaces 

indicate the former presence of a single larger crystal, now much 

albitized. The broken surface is not polished. The specimen is from 

the Himalaya mine, Mesa Grande, San Diego County, Calif.

Two views of the unpolished surface of the same specimen are 

shown in plate 4. The specimen is from the banded albitized pegmatite from the San Diego mine, San Diego County, Calif., and is similar 

to the specimens shown in plates 5 and 7A. Although the slightly 

darker colored graphic granite, below the upper band of black tourmaline, can be seen in plate 4-4, the presence of the microcline of the 

graphic granite is strikingly shown in plate 4 5, as the specimen was 

turned before the camera so that the microcline cleavage faces reflect 

light brightly. The illustration (pi. 45) does not show the quartz 

of the graphic granite. It could be shown, if desired, by polishing the 

surface and destroying the polish of the feldspars but not that of the 

quartz with HF and then photographing the brightly reflecting 

polished surfaces of the quartz, as shown in plates 10 and 115.

In plate 5, the microcline (interpreted as residual) in the banded 

albitized pegmatite is very well shown. The specimen, 14 inches high, 

represents essentially a vertical section of the lower half of the pegmatite dike of the San Diego mine, Mesa Grande, San Diego County, 

Calif. Part of it is shown, one-half natural size, in plate 5A. The 

specimen was sawed in two vertically, one-half polished and the 

one-half unpolished. In plate 5A a straight-on photograph of part 

of the polished surface the darker area in the lower center suggests a 

large crystal of some mineral, readily identified as microcline. The 

microcline crystal is so oriented in this specimen that a reflecting 

plane probably a schiller parting is nearly parallel to the sawed 

surface. The reflecting plane also can be seen on the polished surface by 

slightly turning the specimen. It can be seen to better advantage on
PART 8. PHOTOGRAPHIC TECHNIQUE SOME MINERAL RELATIONS 91

the unpolished surface of the other half of the sawed specimen. The 

large microcline crystal in this half reflects light brightly and when 

properly oriented before the camera gave the result shown in plate 5B.

This illustration (natural size) also shows very clearly the corroded 

outlines of the microcline crystal produced by its partial albitization. 

The horizontal straight black line above the microcline is the shadow 

of a sawmark. The microcline crystal contains graphic quartz the 

many small dark areas in the white microcline crystal.

An additional example of the advantage gained by photographing 

a brightly reflecting plane is shown in plate 6. In A is shown, one-half 

natural size, part of the polished half of a sawed specimen of pegmatite, 

largely albitized, with a central layer of unreplaced microcline, just 

above the banded albitized lower portion. On the upper right side is 

an area of microcline which is much better shown when the unpolished 

half of the sawed specimen is so turned that the microcline crystal 

reflects light brightly (pi. 6(7, natural size). On the lower left side of 

plate QA, is a darker area of microcline with graphic quartz, hardly 

detectable in the picture. However, by properly turning the specimen again the unpolished half of the sawed specimen the microcline 

crystal brightly reflects, and in this position is clearly shown in the 

photograph (pi. 6B, natural size). Plate 6B also shows the presence 

of graphic quartz in the microcline and well shows the corrosion produced by the albitization.

The corroded outlines of the microcline crystal shown in plate QC 

are emphasized when it is photographed in the brightly reflecting 

position. The large microcline crystal just above the reflecting microcline (C) is albitized and hence does not reflect brightly. The reflecting plane of the microcline is probably a schiller parting. The 

specimen was collected at the San Diego mine, Mesa Grande, Calif.

The conclusion reached by the writer in 1924 that the albite-rich 

portions of the gem-bearing pegmatites of the southern California area 

are hydro thermally replaced microcline pegmatites, led to a study of 

the albitites in serpentine along the Maryland-Pennsylvania State 

line. These, and other similar albitites, have been described as "desilicated pegmatites." Abundant microcline, as well as microcline 

graphic granite, was found in many of them, especially in the area 

around Pilot, Md. Probably these State-line albitites are simply 

albitized microcline pegmatites. A few illustrations of selected specimens from this locality are here presented to show how clearly the 

presence of microcline and of graphic granite in these albitites can be 

demonstrated by using the technique described herein.

A large shining cleavage surface, 2 in. across, on the outside of a 

weathered specimen of albitite pegmatite from a feldspar mine on the
92 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

Blair farm, near Pilot, Md., gave indication of the presence of microcline. The specimen was sawed in two and one surface polished and 

photographed straight on (pi. 7 A). The graphic character of the 

pegmatite can be seen, though poorly, but the illustration gives no 

indication of the presence of a large microcline crystal in the lightercolored area in the center. The unpolished surface of the other half of 

the sawed specimen shows the residuum of a large microcline crystal, 

and as the sawed surface is nearly parallel to a shining cleavage of the 

microcline crystal, by slight turning of the specimen before the camera 

its brightly reflecting surface can be photographed with marked contrast to the rest of the specimens, as shown in plate 7B. A small 

detached area of the microcline crystal, in parallel position and also 

reflecting brightly, is shown at the bottom center, just above the black 

mica. A similar large microcline crystal is at the lower left corner, 

but because of different orientation, it does not reflect in the same position as the first crystal.

In order to demonstrate that the reflecting plane shown in plate 7B, 

is a cleavage, a little of the microcline crystal was broken off at the top 

of the specimen, and the induced cleavage surface reflected light even 

more strongly (pi. 7(7). The microcline crystal contains graphic

quartz.

ALBITE

A specimen of massive albitite from the Pennsylvania-Maryland 

State line, appears very uniform when polished. (See pi. 8A). The 

specimen represents the typical albite mined. When slightly etched, 

however, the apparently homogeneous massive albite is seen to contain many small irregular reflecting veinlets. At first these reflecting 

veinlets were thought to be quartz, but examination of a thin section 

showed that they were albite, variously oriented. There is very little 

quartz in the specimen. No notable difference was seen in the indices 

of refraction of the two albites. The better retention of the polish by 

the veinlet albites may be due to their various orientations. As far as 

a thin section indicates, the massive host albite is uniformly oriented 

over large areas.

QUARTZ OF GRAPHIC GRANITE

The technique will show the presence of graphic-granite quartz 

much better for some specimens than a straight-on photograph. The 

effect obtained in photographing specimens of graphic granite for 

reproduction depends on the color and reflecting properties of both the 

quartz and the feldspars. For the specimens from the Pennsylvania-Maryland State line, the bright-surface technique gives much 

better results. On the other hand, straight-on photographs of 

polished specimens of graphic granite (both microcline graphic
PART 8. PHOTOGRAPHIC TECHNIQUE SOME MINERAL RELATIONS 93

granite and albitized graphic granite) from the pegmatites of southern 

California, give results fully as good as those shown in plate 10, with 

however a reversal of color, the quartz photographing black or very 

dark and the feldspar white or very light gray. The bright-surface 

technique would have no advantage over a straight-on photograph 

for these California specimens. Plate 9 shows two photographs of the 

same surface of a specimen of albitized graphic granite from the 

Pennsylvania-Maryland State line albitites. In plate 9-4, a straighton photograph of the polished surface presents a very poor picture 

of the abundant graphic quartz, hardly discernable in places. By 

etching both the quartz and the feldspar the presence of the abundant 

quartz, here photographing dark, is much better shown. But it is 

shown even better, by the bright-surface technique, if the albite only 

is etched and the polish is left on the quartz, as shown in plate KM, 

in which the brightly reflecting polished surfaces of the quartz stand 

out strongly from the dark microcline. A similar effect is produced 

by partially etching a polished surface of a specimen of microcline 

graphic granite from the same locality, as shown in plate 10J5. The 

graphic quartz in the albitized graphic granite shown in plate 10^4, 

is much more corroded than the graphic quartz in the microcline 

graphic granite shown in plate WB.

The presence of residual quartz rods in the banded albitized lower 

part of a pegmatite from Pala, Calif., is well shown by the reflection 

technique here described. Plate 11 gives two views of the same 

specimen: A, polished but not etched; B, the same after partial 

etching, the polish of the quartz rods being retained and the specimen 

so turned before the camera that the polished surfaces of the quartz

reflect brightlv.

HALITE AND SYLVITE

In a photograph of a mixture of halite and sylvite, the "potash ore" 

of the Texas-New Mexico potash field, the halite can readily be distinguished from sylvite by the procedure already described, as the 

polished surfaces of the sylvite can be etched and dulled; hence, they 

do not reflect brightly whereas the surfaces of the halite retain their 

polish and reflect brightly.

A comparison of the two pictures of the same core specimen before 

and after etching, as given in plate 12^4, B, illustrates the results 

obtained. These results are shown in plate 12(7 for another specimen of potash ore. The contrast between the light, almost white, 

halite and the much darker sylvite, is very striking

CRYSTAL FACES

The results obtained by photographing a crystal so oriented before 

the camera that only one crystal face or the several parallel faces of
94 CONTRIBUTIONS TO GEOCHEMISTRY, 1949

one form reflects brightly have already been described. These 

results are illustrated in plates 13 and 14. The crystal of bustamite 

(U.S.N.M. K-8309) illustrated on these plates is from the Zinc 

Corp. mine at Broken Hill, New South Wales, Australia. It is 

pink and transparent, weighs 0.64 g., and measures 13 mm. by 9 mm. by 

3 mm. It is completely terminated at one end with brilliant and 

singly reflecting faces, giving excellent signals on the goniometer. 

The crystal has two smaller ones attached on one side, in not quite 

parallel position. These are shown in plate 13E on the left side.

Some trials were made years ago to obtain a photographic record of 

the exact relative lengths of the intersection edges of the faces of a 

crystal by covering the faces with an opaque layer of ammonium 

chloride produced as a vapor and blown on the crystal. It was 

hoped that in this way a more even illumination would be obtained 

and that all the intersection edges would photograph sharply and 

evenly on account of the dull nonreflecting coating of ammonium 

chloride. Although the photographs of such coated crystals gave a 

sharper picture than those of uncoated crystals, the sharpness of the 

intersection edges is much less than that obtained by the technique 

here described.

It may be that this technique will find other uses: for etch figures 

or inclusions of parallel oriented crystals of one mineral in another, 

as the hematites in aventurine feldspar.

o
PLATES 1-14
PLATE 1

.' !' : ' .' ";; V/\-'! (T

A. Essentially parallel crystals of quartz on graphic granite. The brightly 

reflecting faces are the prism. Natural size. From the Panama-PacificExposition mine, Chihuahua Valley, San Diego County, Calif.

B. Group of parallel crystals of quartz, which are extensions of the quartz rods 

of graphic granite. The brightly reflecting faces are the rhombohedron. 

From the El Molino mine, Hiriart Hill, Pala, San Diego County, Calif.
GEOLOGICAL SURVEY BULLETIN 992 PLATE 1

BRIGHTLY REFLECTING FACES OF PARALLEL QUARTZ CRYSTALS
GEOLOGICAL SURVEY BULLETIN 992 PLATE 2

BRIGHTLY REFLECTING PLANES OF MICROCLINE
PLATE 2

/I. Reflecting plane of tapering microcline crystal (interpreted as residual) in 

banded albitized pegmatite. The corroded edges of -the microcline are 

well shown. The wavy bands of garnet from upper left to lower right. 

Jacumba, San Diego County, Calif.

 B. Reflecting planes-of two tapering microcline crystals (interpreted as residual) 

in banded albitized pegmatite. Short distance north of Panama-PacificExposition mine, Chihuahua Valley, San Diego County, Calif.
r!"

PLATE 3

A. Reflecting plane of microcline (interpreted as residual) in albitized pegmatite. 

The corroded edges of the microcline are well shown. Himalaya mine, 

Mesa Grande, San Diego County, Calif.

B. Uniformly reflecting cleavage faces of microcline (interpreted as residual). 

The parallel reflecting surfaces of the disconnected areas of microcline 

indicate the former presence of a single larger crystal. In albitized pegmatite. Himalaya mine, Mesa Grande, San Diego County, Calif.
GEOLOGICAL SURVEY BULLETIN 992 PLATE 3

ADDITIONAL HKIUIITLY KKFLKCTING PLANKS OF MICKOCL1NF.
GEOLOGICAL SURVEY BULLETIN 992 PLATE 4

BRIGHTLY REFLECTING CLEAVAGE SURFACES OF MICROCLINE
PLATE 4

A, Graphic granite (interpreted as residual) in banded albitized pegmatite. 

Natural broken surface, not polished. Although the slightly darker colored 

graphic granite, below the upper band of black tourmaline, can be seen, its 

presence is much better shown in B. San Diego Mine, Mesa Grande, San 

Diego County, Calif.

B. The same specimen shown in A, turned slightly so that the cleavage faces of 

the microcline in the graphic granite more strongly reflect the light.
PLATE 5

A. Microcline (interpreted as residual), the darker-colored area in lower center, 

in banded albitized pegmatite. The microcline is full of graphic quartz. 

Specimen polished. One-half natural size. San Diego Mine, Mesa Grande, 

San Diego County, Calif.

B. Reflecting plane of the microcline shown in A. The other half of the sawed 

but not polished specimen. Natural size. The horizontal black line above 

the microcline is the shadow of a saw mark. The corroded character of the 

microcline is well shown. The entire specimen is 14 inches high.
GEOLOGICAL SURVEY 

BRIGHTLY REFLECTING PLANK OF MI
HI<K;IITLY KKl 'l,^,(rl'l^(; PLANK OK MIC
PLATE 6

A. Vertical section of part of complete pegmatite, largely albitized, with a central 

layer of unreplaced microcline, just above the banded albitized lower 

portion. One-half natural size. The reflecting plane of the microcline at 

the right of the central layer is shown, natural size, in C. Microcline graphic 

granite in lower left, just below the band of black tourmaline, is shown, 

natural size, in B. San Diego Mine, Mesa Grande, San Diego County, 

Calif.

B. Reflecting plane of the microcline of graphic granite (interpreted as residual), 

natural size, in the banded albitized pegmatite. Both B and C were photographed from the other half of the specimen shown in A. Surface of 

specimen ground but not polished.

C. Reflecting plane of microcline. The corroded edges of the microcline are 

well shown. The former microcline just above the one shown by the 

stronger reflecting light, is albitized and does not show any strongly 

reflecting plane.
PLATE 7

A. Polished specimen of albitized pegmatite. The specimen is essentially albite 

and the outlines of the microcline (interpreted as residual) in the center cannot be definitely traced. From the Blair farm, near Pilot, Md.

73. The other half of the specimen shown in A. The sawed surface is ground 

smooth but not polished. The reflecting plane of the microcline readily 

shows its outline.

C. Induced basal cleavage of microcline in upper edge of specimen. The freshly 

broken cleavage face reflects light even more strongly.
GEOLOGICAL SURVEY BULLETIN 992 PLATE 7

BRIGHTLY REFLECTING CLEAVAGE PLANK OF MICROCLJNE
GEOLOGICAL SURVEY BULLETIN 992 PLATE 8

A

BRIGHTLY REFLECTING VEINLETS OF VARIOUSLY ORIENTED 

ALB1TE IN UNIFORMLY ORIENTED COMPACT MASSIVE ALBITE
PLATE 8

A. Albitite pegmatite, nearly all albite, polished and not etched. Photographed 

straight on. The lighter-colored areas are due to light reflected from 

incipient cracks. The small black areas represent chloritic material. 

From Squires Quarry, near Sylmar on Pennsylvania-Maryland State line.

B. Same surface etched with dilute HF. The matrix albite is readily etched, 

loses its polish, and photographs dark. The small irregular veinlets and 

small masses of variously oriented albite retain their polish and the brightly 

reflecting surfaces photograph white.
PLATE 9

A. Albitized graphic granite, specimen polished. Photographed straight on 

with no treatment of polished surface. The quartz is barely visible, 

practically invisible in places. Compare with B, and with plate 10 A. 

Plates 9 A and B, and plate 10 A are from the same specimen. From 

albitite pegmatite from Bald Friar no. 12 quarry, near Pilot, Md.

B. Same specimen surface etched with dilute HF. Both the albite and the 

quartz are etched and dulled. The quartz stands out much clearernahotographing dark, than it does in A. But it stands out even better in plate 

10 A.
GEOLOGICAL SURVEY BULLETIN 992 PLATE 9

POOR PRESENTATION OF QUARTZ, NOT USING THE BRIGHTLY 

REFLECTING METHOD
GEOLOGICAL SURVEY BULLETIN 992 PLATE 10

BRIGHTLY REFLECTING POLISHED SURFACES OF QUARTZ
PLATE 10

A. The same polished surface shown in plate 9, partly etched with dilute HF 

just enough to dull the polished surface of the albite but leaving the polish 

on the quartz. Specimen turned before the camera so as to brightly reflect 

the polished surfaces of the quartz.

B. Brightly reflecting polished surfaces of quartz in microcline graphic granite. 

The polished surface of the specimen was treated the same way as in specimen 

shown in A. From same locality. The albitized graphic granite, plate 9 

and plate 10 A, is interpreted as having been derived from microcline graphic 

granite, such as is shown in plate 10 B. Note how much more the quartz 

in A is corroded as compared with the.quartz in B..
PLATE 11

A. Polished surface of bottom-banded albitized pegmatite from the "Crater", 

Hiriart Hill, Pala, San Diego County, Calif. Except for a single long nearly 

vertical quartz rod (right center), there is little evidence shown of the 

presence of many nearly vertical quartz rods (interpreted as residual quartz 

rods of former microcline graphic granite).

B. Same specimen, etched with HF to depolish the albite but not the quartz, 

turned before the camera so that the brightly reflecting surfaces of the 

quartz rods photograph white. (These quartz rods are interpreted as 

residual quartz rods of former microcline graphic granite). The residual 

quartz rods, reflecting brightly, are present also in the upper part of the 

more thickly banded pegmatite, but the specimen would have to be turned 

more to show them.
GEOLOGICAL SURVEY 

BRIGHTLY REFLECTING SURFACES OF 
GEOLOGICAL SURVEY BULLETIN 992 PLATE 12

BRIGHTLY REFLECTING SURFACES OF HALITE
PLATE 12

A. Mixture of halite and sylvite, polished, and photographed straight on. Both 

halite and sylvite are either white or red. Most of the sylvite, being red, 

photographed dark, but it is not possible to distinguish the twp minerals. 

Part of core from the Blanchard no. 2 well, at a depth of 696 feet to 696^ 

feet. Near Carlsbad, New Mexico.

B. Same specimen, etched with a saturated solution of sodium chloride, sufficiently 

to dull the polished surfaces of the sylvite but leaving the surfaces of halite 

polished. Specimen turned before the camera so that the polished surfaces 

of the halite reflect light strongly. Note the brightly reflecting surfaces 

of minute-grains of halite in the darker sylvite and the minute grains of 

darker sylvite in the reflecting halite.

C. Potash ore, mixture of halite and sylvite. From shaft 1 of the U. S. Potash 

Co., at a depth of 980 feet.
PLATE 13

Different orientations of a crystal of bustamite (Mn, Ca) SiO3 , from Broken 

Hill, New South Wales, Australia (U. S. N. M. R-8309). All photographs 

enlarged about 5 times.

A, B, C, and D, Orthographic view: A, No face brightly reflecting; B, small 

triangular terminal faces brightly reflecting; C, large terminal face reflecting; 

D, elongated terminal faces reflecting (shown enlarged in plate 14). E, F, C', 

and H, clinographic view: E, large face, with parallel oscillatory line face, 

reflecting; F, large face on other side of crystal, reflecting; G, terminal face 

reflecting; H, small triangular terminal faces reflecting (shown in orthographic 

view in B).
GEOLOGICAL SURVEY BULLETIN 992 PLATE 13

BRIGHTLY REFLECTING FACES OF A CRYSTAL OF BUSTAMITE
GEOLOGICAL SURVEY 

BROMIDE ENLARGEMENT (4X) OF P
PLATE 14

Shows elongated terminal faces reflecting. Crystal enlarged twenty-one times.