DBPABTMENT OF THE INTE1UOK

BULLETIN

OV THK

UNITED STATES

GEOLOGICAL SURVEY

No. 186

SERIES E, CHEMISTRY Atfl> PHYSICS, 35

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1 0 0 1

~TTJ

UNITED STATES GEOLOGICAL SUIIVEY

CHAKLES .D. WALCOTX, UIRECTOK

ON

PYMTE AND MARCASITE

H . STOKES

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1901

CONTENTS.

Pago.

Letter of transmittal, by Dr. Becker........ ..........-....-..-------..-.. 9

I. Uncertainty of methods of distinguishing pyrite and marcasite.......... 11

Crystalline form............................................... ...... 11

Color.............................---. - --..---.---.---------..- 11

Density....... ........................ ............................... 12

Density according to different authorities................'........... 12

Density not a sure critei'ion of composition .... .... .............. 12

Oxidation ...................... ................................... .. 13

Penfield's method...... ;.......... ......-...--...--.....---..-..--.. 14

Action of copper sulphate on pyrite and marcasite.... ................ 14

II. Behavior of pyrite and marcasite toward ferric solutions............... 14

Action of ferric chloride .....,--........-...--..--..---- ......-...-.- 15

Percentage of sulphur oxidized.:.....--.-..-.- ...................... 15

Ferric sulphate oxidation method..................... .. .... .......... 15

Principle of the method......................................... :., 15

Deduction of an expression for oxidized sulphur....................... 16

. Standard solution and temperature.............................. ...... 17

Oxidation coefficient...... ..................... ............... ...... __ 17

Theory of the reaction.................. .............................. 17

Influence of concentration...... ....................... ....-..-----..-.. 18

Influence of temperature................... -........ ................... 18

Reason for the different behavior of pyrite and marcasite... .......... 19

Further applications of the method............... ................... 20

IIL Details of the method........................ ................. ...... 20

Necessary precautions ................................................ 20

Preparation of the material....--...--....-..-..--..-...-......-.-...-. 20

Oxidation apparatus..................................................... 22

Details of the operation.......-.-....,-..---.---.---...- .-.-...'-..... 23

Accuracy of the method................. ............................. 24

IV. Data for pyrite............ .................. ......................... 25

Table, and description of specimens.... .......... .................... 25

Oxidation coefficient of pyrite. ..............................^ ..-...-.. 25

V. Data for marcasite............. ............................... ........ 25

Table, and. description of specimens..........'.....-....... ..-.....--.. 26

Oxidation coefficient of marcasite.................................. .. 27

VI. Mixtures of pyrite and marcasite...;............... ...... .... ...... 27

Conditions under which the oxidation coefficients of mixtures may liedetermined ..-...--...-.....--.-.-............- .................... 28

VII. Data for artificial mixtures ......---.....-.. .......-.-.. ;............ 29

Discussion of the curve shown in PI. I.................... . . ...... 30

Influence of the minimum: indirect determination of small amounts

of pyrite in marcasite.. - ..--.-.......-.--..-...-...-..-----..- 30

5
6 CONTENTS.

Page.

VIII. Influence of impurities ...................... .___.-._.-....,-...._. 31

Miscellaneous impurities-.--.-.........................--.,-.,.......-. 31

Hematite and magnetite ................. .............................. 32

Galena....---.----._.._..---.-..._......-----......................... 32

Nickel and cobalt.........................._...... ..................... 32

Arsenopyrite ............................................ .............. 32

Chalcocite and bornite................ ............................... 33

Chalcopyrite........ .........__._. .................. . ....-....-.--...- 33

IX. Form of occurrence of copper in copper-bearing pyrites................ 34

Oxidation experiments...---....-....-.... . .. ....... ... .............. 34

Detection of chalcopyrite in pyrite or marcasite and in rocks. .......... 35

X. Miscellaneous specimens and specimens of dou btf ul nature.............. 35

Table, and descriptions of specimens ...................... .............. 36

Conclusions ..'..... ................................................... 38

Dr. Julien's specimens, table, and descriptions ... .............. .,.. 39

Discussion of Julien's hypothesis....... .............-.;......-....,... 39

XI. Paramorphism .... ......................................... . ... . 40

XII. Constitution of pyrite and marcasite; action of cupric salts......... . 41

Experiments with cupric sulphate and pyrite...... .................... 42

Experiments with capric sulphate and marcasite ...................... 43

Experiment with cupric chloride and pyrite..... ................ .... 44

Experiment with ferrous and cupric sulphates ..........----.--......-. 44

Theory of the reaction.............. ............................... 44

General conclusions from the experiments....... .... ............... 45

XIII. Oxidation of pyrite and marcasite by potassium permanganate...... 4*1

XIV. Summary of results................. ............................... 47

Index ..........-..-.-...----.--.-.. -.....--.--- ----.--..-....-...-........ 49
ILLUSTRATIONS.

Page.

PLATE I. Curve showing oxidation coefficients of mixtures of pyrite and

marcasite ..................................................... 30

FIG. 1. Apparatus for washing sulphides in an atmosphere of carbon dioxide. 21 

2. Apparatus for oxidizing sulphides with ferric somtion .-.---....... 22

7

LETTER OE TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY, 

DIVISION OF CHEMICAL AND PHYSICAL RESEARCH,

Washington, D. C., July 5, 1901.

SIR: The subject of the secondary enrichment of ore bodies is one 

of the most important in chemical geology, and probably also one of 

the most complex, because nothing like chemical homogeneity is to be 

anticipated at or beneath the croppings of ore deposits. A long time 

must elapse before the processes involved are fairly clear.

The memoir by Dr. H. N. Stokes which I transmit herewith, for 

publication in the Bulletin series, is an important contribution to this 

subject, describing a method for the quantitative determination of 

pyrite and marcasite when in mixture, and discussing some of the 

relations of these most abundant sulphides to those of copper. This 

memoir represents only a portion of Dr. Stokes's work in this field, 

and it is to be expected that other papers will follow as soon as 

practicable.

Very respectfully, your obedient servant,

GEORGE F. BECKER,

G-eologist in Charge. 

Hon. CHARLES D. WALCOTT,

Director of United States Geological Survey.

ON PTRTTE AND MARGASITE.

By H. N. STOKES.

The object of this paper is to describe a chemical method by which 

the native forms of iron disulphide may be distinguished with certainty, and by which their relative amounts in mixtures of the two 

may be quantitatively determined. The method is applied to various 

supposed pyrites and marcasites, and it is shown that in the absence 

of well-marked crystallographic features it is extremely easy even for 

experts to be deceived as to the nature of a given specimen.

I. UNCERTAINTY OF METHODS OF DISTINGUISHING PYRITE

AND MARCASITE.

Pyrite and marcasite are usually distinguished by their differences 

of crystalline form, color, density, and ease of oxidation.

CRYSTALLINE FORM.

Pyrite usually crystallizes in cubes, often in pentagondodecahedrons 

or octahedrons, and frequently in combinations, of these and more 

complicated forms of the isometric system, while marcasite assumes 

various forms of the orthorhombic system. This criterion is inapplicable when the minerals assume the massive or compact form, or 

when they occur as concretionary nodules in Avhich the crystalline 

form can not be made out. Moreover, pseudornorphs are not uncommon, and paramorphs of pyrite after marcasite and of marcasite after 

pyrite are said to occur, in which events no conclusion can be drawn 

from this source.

COLOR.

The color of pyrite is generally said to be pale brass yellow, while 

that of marcasite is described as tin white, grayish white, greenish, 

brass yellow, bronze yellow, and in other terms. Dana 1 describes

1 System of Mineralogy, Cth ed., p. 94. For a list of terras and references see Julien, Annals 

New York Acad. Sci., Vol. IV, 1887, p. 179.

11 /
ON PYKITE AND MAKCASITE. [BULL. 186.

marcasite as pale bronze yellow. These discrepancies are doubtless 

due to tarnished specimens having been described. Marcasite 

frequently assumes a yellowish tarnish, which may vary to copper 

colored. On fresh surfaces, such as are obtained by breaking or by 

removing the film by warming with dilute hydrochloric acid, the color 

is unquestionably tin white or grayish white, closely matching 

platinum and without a trace of yellow. The white color is not an 

indication of the presence of arsenic, 1 as it is equally shown by specimens which are free from arsenic. To distinguish pyrite and marcasite 

by the color, they must be cleaned with hydrochloric acid and immediately examined under a good white light, in comparison with 

standard specimens also freshly cleaned. The faint yellow of pyrite 

makes it extraordinarily easy to dec'eive oneself if the light be yellowish in tint. The differences of the supposed yellow and light- 

.colored pyrites vanish, except in rare cases, when these precautions 

are observed, and pyrite can usually be distinguished with ease when 

mixed with marcasite. In the case of fine-grained concretions, or 

where the surface is too rough to afford a good -reflection, it is sometimes quite impossible to draw any conclusion from the color.

DENSITY.

Density according to different authorities. The density of both 

minerals is subject to some variation, as shown by the following 

figures from different sources:

Density of marcasite and pyrite.

Authority.

Dana a ................ ...---.........-.--.-:..........

Julien c ........... ..........-.,.--...-.-......--..,.

Marcasite.

4.85-4.90

4.90

4.80

4. 88-4. 90

Pyrite.

4.95-5.10

5.00

5.01

5.02-5.04

a System of Mineralogy, 6th ed., pp. 85,94.

ftZeitschr. Deutsch.geol.Gesell., Vol. XVI, 1864,p.267. Supposed normal density. 

c.Annals New York Acad. Sci. Vol. IV, 1887, pp. 177,210. Supposed normal density. 

d See pp. 25,26.

Density not a sure criterion of composition. My own determinations were made with the pycnometer at 18°-23°, on the purest obtainable material, coarsely powdered. Julien's figure for marcasite is 

certainly too low. The following list, giving the densities of some 

specimens of which I have determined the composition (after deduct1 Op. cit., p. 95.
STOKES.] PHYSICAL DIFFERENCES. 13

ing impurities), shows how illusive may.be conclusions as to the 

relative amounts of pyrite and inarcasite based on density alone:

Densities of some specimens of pyrite and marcasite.

No. of specimen.

5 .............. ..........

3.............. ...........

33............................

7........... ....... ...... . .'.

10............ ............ ...

12

29..... ................. .......

30.... ...... ............ .... ..

81.. ....'........-. ...... .....

17..... . ..--- ..;

Density.

5.041

5.023

 4.987

' 4. 891

4.886

4.880

4.856

4.843

4. 819

4.563

Composition, chemically determined.

100 per cent pyrite.

100 per cent pyrite.

83.5 pet- cent pyrite, 16.5 per cent marcasite. 

100 per cent marcasite.

100 per cent inarcasite.

100 per cent marcasite.

99.5 per cent pyrite.

99.5 per cent pyrite.

100 per cent pyrite.

100 per cent pyrite.

OXIDATION.

While it is unquestionably true that marcasite oxidizes more rapidly than pyrite under the same conditions, a compact, well-crystallized 

inarcasite can be kept almost indefinitely without any change further 

than a superficial tarnish, whereas finely divided or porous pyrite 

oxidizes with great rapidity, and this fact has frequently caused pyrite 

to be mistaken for marcasite.

The amount of oxidation depends upon the time during which the 

specimen has been exposed; upon the peculiar conditions under which 

it has occurred, such as exposure to dry or moist air, or to water holding oxygen or other oxidizing agents in solution; upon whether the 

oxidation products are removed; upon the surface exposed per unit of 

volume; and upon an oxidation factor characteristic of each mineral, 

but probably not more than three times greater for inarcasite than 

for pyrite. It is obvious, therefore, that no trustworthy conclusion 

can be drawn in any case from vitriolization in the absence of definite 

knowledge of these factors; and even when the conditions of time and 

exposure are identical, as when the specimens are kept side by side

in the cabinet, it is necessary to know the - ltl ac^ factor before rapid

mass

vitriollzation justify one in pronouncing a specimen to be marcasite. 

One of the most extreme cases of rapid vitriolization that I have seen 

is No. 21 (pp. 36-3V), which consists of nodules of practically pure 

pyrite. Many of the rapidly vitriolizing specimens consist of mixtures 

of pyrite and marcasite, and apart from the porosity existing in such 

cases it is possible that an electrochemical action between the two
14 ON PYEITE AND MAECASITE. LBULL. 186.

may assist the oxidation. It is hardly necessary to add that the old 

idea that marcasite tends to produce ferrous sulphate,.and pyrite to 

give limonite, is untenable.

PENFIELD'S METHOD.

Penfield 1 has described a method of distinguishing the two minerals, based on treating the finely powdered substance with strong 

nitric acid under identical conditions; pyrite. dissolves completely, 

while marcasite leaves a residue of sulphur. While this procedure 

serves well to distinguish the pure minerals, it is obviously incompetent to detect pyrite in the presence of marcasite.

ACTION OF COPPER SULPHATE ON PYRITE AND MARCASITE.

A. P. Brown a asserts that when marcasite is heated for six hours at 

200° with 10 per cent copper sulphate solution its iron is completely 

dissolved as ferrous sulphate, while under the same conditions pyrite 

yields a mixture of 2 mols. ferric and 1 mol. ferrous sulphate; and on 

this he bases the conclusion that in marcasite the iron is wholly in 

the ferrous condition, Fe"S2 , while in pyrite it is four-fifths ferric, 

Fe"S2 (Fe'"S2 ) 4 . As Brown's work might seem to afford a means of 

distinguishing the two minerals, and as the different constitution, if 

existing, would have an important bearing on the question of their 

formation in nature, I have repeated these experiments, but with 

totally different results. The details of my experiments are given 

below (p. 42), and I here state merely my conclusion that the only 

recognizable difference is the greater ease with which marcasite is 

attacked by the cupric solution, and that no evidence can be obtained 

in this way as to any difference of chemical constitution.

II. BEHAVIOR OF PYRITE AND MARCASITE TOWARD FERRIC

SOLUTIONS.

In the course of an investigation on the action of ferric salts on 

natural sulphides, differences observed in the behavior of pyrite and 

of marcasite were so marked as to make it seem probable that on these 

differences there might be based a method of distinguishing these 

minerals and of determining them quantitatively in mixtures, thus 

establishing a sure means of ascertaining the composition of concretions and other doubtful specimens > and of testing the validity of the 

conclusion of Julien, quoted below (p. 27), that many supposedly pure 

pyrites and marcasites are mixtures of the two. While approximate 

results were soon reached, many fruitless experiments had to be made 

before all important sources of error were eliminated and before the 

process could be brought to a reasonably satisfactory quantitative

1 Brush aud Penfleld, Determinative Mineralogy, 15th ed., p. 352.

"Proc. Am. Philos. Soc., Vol. XXXIII, 1894, p. 225. Chemical News, Vol. LXXI, 1895, p. 179.
STOKES.] ACTION OF FERRIC SOLUTIONS. 15

basis. Only the results obtained by the perfected method are presented in this paper.

ACTION OF FEKRIC CHLORIDE.

It has long been known that ferric salts attack pyrite. L. L. 

de Koninck * found that pyrite is oxidized at 170°-200° C. by ferric chloride or ferric ammonium alum, with formation of a ferrous salt and sulphuric acid. J. H. L. Vogt 2 mentions thatpyriteis very slowly attacked 

in the cold by 30 per cent ferric chloride solution, giving a trace of sulphuric acid. He regards the formation of sulphuric acid as subordinate, and does not ascribe an important role to ferric salts in the 

decomposition of pyrite. Julien B observed a reduction of ferric chloride by marcasite. No quantitative study of these reactions has been 

made, so far as I can ascertain.

PERCENTAGE OF SULPHUR OXIDIZED.

Preliminary experiments showed that very dilute solutions of ferric 

salts attack pyrite rapidly at the boiling temperature, and much more 

slowly, but quite appreciably, in the cold. It is upon the determination 

of the percentage of sulphur oxidized in the total mineral decomposed 

that the new method is based. If ferric chloride be used, the amount 

of mineral decomposed can be found from the increment of iron in the 

solution 'and the SO,, determined as barium sulphate. An experiment 

with a ferric chloride solution containing 1.14 grams Fe'" per liter, 

with a little hydrochloric acid, which was boiled to complete reduction 

with an .excess of powdered pyrite free from sulphate, in a current 

of carbon dioxide, showed that 65 per cent of the sulphur was oxidized, a result agreeing well with the results presented below.

FERRIC SULPHATE OXIDATION METHOD.

Owing to the tediousness of the gravimetric methods and of the SO4 

determination in the presence of iron, I have adopted a much more 

expeditious indirect method, which involves the use of ferric ammonium alum and titration with potassium permanganate.

Principle of the method. The oxidation of FeSa by Fea (SO4 ) 3 may 

be regarded as taking place by two independent reactions:

(1) FeS2 + Fe2 (SO4 ) 3 = 3FeSO4 + 2S.

(2)

If the reaction be carried out in the presence of an excess of FeS2 

to complete reduction of the ferric salt, it is easily seen that equation 

(1) holds even if the theory of Brown be tru<:ythat the iron in pyrite 

is partly ferric, for in this case the ferric salt formed will in turn be

1 Aiinales Soc. geol. Belgique, Vol. X. 1883, p. 101. Zcitschr. anorg. Uhemie. Vol. XXVI, 1!K)1, p. 123. 

3 Traus. Am. Inst. Min. Eng., Richmond meeting, February, 1901. 

3 Annals New York Acad. Sci. , Vol. IV. , 1887, pp. 188-139.
16 ON PYKITE AND MAECASITE. [BUM* 188.

reduced by the excess of pyrite. Upon .the extent to which the second reaction takes place, as compared with the first, depends the percentage of sulphur oxidized in that portion of the mineral which is 

decomposed.

If a ferric solution containing a known amount of iron be boiled 

with an indefinite excess of FeS2 under appropriate precautions, and 

if the increment of iron in the solution be then determined, as well as 

ttie ferrous salt formed, this increment gives the amount of sulphide 

decomposed; three times this increment gives the quantity of ferrous 

salt formed in equation (1); while any excess of ferrous salt is that 

formed by oxidizing sulphur to sulphuric acid as in equation (2). 

From these data the percentage of sulphur oxidized may be computed. "

DEDUCTION OP AN EXPRESSION FOR OXIDIZED SULPHUR.

It is unnecessary to know the absolute quantities of iron and sulphur involved in these reactions, or the strength of the permanganate 

solution. It suffices, in deducing an expression for the oxidized sulphur, to employ symbols expressing the permanganate equivalent of 

the iron, and the symbols used represent simply the volumes of permanganate consumed by a given volume of the solution.

For a given volume let

a = iron in the original solution, 

b = resulting ferrous iron, 

 ~ c = resulting total iron. 

Then.

c a = increment of iron resulting from decomposition of FeS2 , 

and

/ O-J QO \

2 ( - ) (c a)= total sulphur in decomposed sulphide. (A) V55.60/

Also,

3 (c tt) ferrous iron produced according to equation (1) 

and

b 3 (c of) = ferrous iron produced by oxidation ol sulphur.

According to equation (2), 1 atom of sulphur requires for oxidation 

G atoms of ferric iron, producing 6 atoms of ferrous iron; hence,

1 _ 31.83 ^.._ 3 ((. _ aj \ _ sulphur oxidized. (B).

Calling the percentage of sulphur oxidized p, we obtain from (A) 

and (B) ' . 

100 31.83 -

~ 8,333b

It thus appears that three titrations suffice to determine the percentage of sulphur oxidized, and that neither the amount of FeS2 , the
STOKES.] ACTION OF FERRIC SOLUTIONS. - 17

volume of the ferric solution used, nor the absolute titer of the latter 

or of the permanganate need be known. As, however, the proportion 

of sulphur oxidized varies with the strength of the ferric solution, 

the extent of reduction, and the temperature, it is necessary, in. order 

to obtain comparable results, to use a solution of standard composi1km and a standard temperature, and to continue the action to complete reduction.

STANDARD SOLUTION AND. TEMPERATURE.

The standard solution contains 1 gram ferric iron per liter, and is 

prepared by dissolving in water 8.61 grains clear crystals of pure ferric ammonium alum, adding 16 cm. 3 25 per cent sulphuric acid and 

diluting to 1 liter. One hundred cm. 3 of this solution must show a 

decided change of color with one drop of the permanganate solution,

containing about 1.5 grams per liter, and must be free* from nitrate. 

The sulphuric acid is added to prevent precipitation of basic salt. 

The experiment is conducted at the boiling temperature. The 

strength of the ferric solution-may vary 1 per cent without appreciable influence, and at least 99.5 per cent must be reduced.

OXIDATION COEFFICIENT.

The percentage of sulphur oxidized under these conditions and with 

the precautions explained in detail below (p. 20) may be called briefly 

the oxidation coefficient of the pyrite or .marcasite. The oxidation 

coefficient of pyrite varies between 60 and 61, and that of .marcasite 

between 16.5 and 18. The oxidation coefficients of mixtures of the 

two indicate the relative amount of each present, by reference to an 

empirical curve obtained with artificial mixtures of known composition. A theoretical deduction of the relative proportions from the 

oxidation coefficient is impossible in the absence of the necessary 

physical data.

THEORY OF THE REACTION.

The mass law states that the active mass of a solid phase is constant; 

hence, the final composition of the resulting solution must be the 

same, irrespective of the relative amount of reagent and sulphide 

used, provided only that the latter, .the solid phase, be always in 

excess. With more, or more" finely powdered, material 1 that is, 

with a relatively greater exposure of surface, equilibrium is sooner 

reached, but the composition of the resulting solution, and therefore 

the percentage of sulphur oxidized, is the same. It is unnecessary

1 Ostwald (Zeitschr. physik. Chemie, Vol. XXXIV, 1900, p. 405) has shown for certain cases 

that an extremely finely powdered substance is more soluble than the same in coarser particles, 

and that the composition of the resulting solution is therefore subject to slight variations. 

Such effects were looked for in the present instance by using coarse and very fine material 

obtained by sedimentation, but if they exist they are smaller than the errors of the method.

Bull. 186 01 2
18 ON PYRITE AND MARCASITE. [BULL.-186.

either to weigh the material or to measure the liquid accurately, but 

in order to make unavoidable experimental errors as nearly as possible constant, it is desirable always to employ approximately equal 

quantities, and to boil for nearly equal times.

INFLUENCE. OF CONCENTRATION.

Since the equation expressing the oxidation of the sulphur is of a 

higher order than that denoting the oxidation of the iron, it follows 

that the rate of oxidation of the sulphur must fall off more rapidly 

than that of the.iron, with decreasing concentration of the ferric salt; 

hence the value of p must constantly decrease as reduction proceeds, 

and comparable results can be obtained only .by 'carrying the reduction to completion. This is shown in the following cases, where the 

experiment was stopped after partial reduction:

Results of experiments showing constantly decreasing value of p as reduction

proceeds.

Pyrite.

Fe reduced.

Per cent.

67. 8

88.7

90.5

95.2

99.5

100.0

P

Per cent.

71.4

65.2

64.9

63.2

61.5

60,4

Marcasite.

Fe reduced.

Per cent.

69.2

78.5

100.0

p

Per cent.

23.3

22.5

16.2

INFLUENCE OF TEMPERATURE.

The influence of temperature on p is shown by the following experiments at 20° and at boiling temperature, the reduction being complete in each case:

Results of experiments shoiving influence of temperature on p.

\

Pyrite. ......... ..................... ...^.. ................

-Marcasite ..................................................

Temperature.

<> 

j . 20

I 100

j 20

I 100

P

Per cent.

'80;8

60.4

30.6

17.4
STOKES.] ACTION" OF FEEEIC SOLUTIONS. 19

From the variation in the relative oxidation of the sulphur with 

concentration and temperature it appears certain that no single equation expresses the oxidation of pyrite and marcasite by ferric salts to 

ferrous sulphate, sulphur, and sulphuric acid, but that it is the 

result of two, or possibly three, independent reactions of different 

orders, which produce results varying according to circumstances. 

It is therefore extremely probable that other oxidizing agents, as 

atmospheric or dissolved oxygen, behave in a similar manner. The 

oxidation with separation of free sulphur presents no difficulties; as 

is well known, finely powdered pyrite and marcasite on exposure to 

the air for a short time give free sulphur, which can be detected by 

extraction with ether, and the formation of sulphur dioxide has been 

noted by various observers; finely divided moist marcasite evolves it 

strongly when heated on, the water bath. The difficulty is rather to 

account for the complete oxidation of the sulphur usually occurring 

in nature. Whether with stronger solutions and continued reoxidation by air, or the presence of retarding agents, 100 per cent can be 

reached can not be stated at present, and experiments bearing on 

this point are desirable.

REASON FOR THE DIFFERENT BEHAVIOR OF PYRITE AND MARCASITE.

The different proportion of sulphur oxidized under identical conditions in the two cases is doubtless due to the different solubilities of 

pyrite and marcasite, or at least the different rates at which they are 

attacked. That the sulphur is not completely oxidized is due to its 

smaller oxidation rate under the-given conditions. The iron is attacked so fast that the oxidation of the sulphur falls behind and a 

portion is liberated as free sulphur, which, as my experiments show, 

is scarcely oxidized by. the boiling ferric solution when collected, to 

aggregates of sensible size. This phenomenon is a familiar one when 

precipitated sulphides are treated with nitrjc acid; a portion of the 

sulphur is oxidized and the remainder is liberated. When a ferric 

salt is reduced by hydrogen sulphide the sulphur of the latter is partly 

liberated, but a very considerable part is oxidized to sulphuric acid. 1 

Whatever retards th6 solution of the sulphide without affecting the 

concentration of the oxidizer gives more time for the complete oxidation of the sulphur. This may be brought about by certain salts in 

solution or by a difference of physical structure in the crystals. 

Pyrite dissolves more slowly than marcasite, hence its sulphur is more 

completely oxidized. Under conditions where the solution is very, 

slow, it may perhaps approximate completeness in both cases, a subject well worthy of investigation from its bearing on the oxidation of 

pyrites in nature.

1 Gmelin Kraut, Vol. I, pt. 2, p. 219. I have found that when a I per cent boiling solution of 

ferric chloride is reduced by hydrogen sulphide very nearly one-third of the reduction is 

effected by the sulphur.
20 ON PYRITE AND MARCASITE. [BULL. 186. 

FURTHER APPLICATIONS OF THE METHOD.

In general, when a mineral is composed of two or more elements, 

each capable of oxidation, but at different rates, there will be certain 

conditions under which one will be oxidized to a greater extent than 

the other, and the amount of this difference may often be determined 

by analysis of the solution. If the compound is dimorphous, one form 

will be more soluble than the other, and this will show a greater difference in the degree of oxidation of the two constituents than the 

less soluble form. The above method therefore makes it possible to 

distinguish not only between dimorphous sulphides but other dimorphous compounds, such as arsenides, and to determine their relative 

amounts in mixtures. It should also be possible to determine whether 

certain complex minerals are mixtures of two distinct kinds of molecules or whether all the components are combined in one molecule. 

FeAsS, for example, should show a behavior different from that of a 

mixture FeS2 and FeAs2 in equimolecular proportions. When the 

metal is more slowly oxidized than the nonmetallic constituent a separation of a portion of the former may be expected. Gold sulphide, 

for example, might give sulphuric acid and free gold. As pointed 

out below, it is possible to ascertain in certain cases the form of combination of constituents which exist only as impurities; in a 3 per 

cent cupriferous pyrite, chalcopyrite can be distinguished from chalcocite and bornite, a determination which could not be made by 

analysis.

III. DETAILS OF THE METHOD. 

NECESSARY PRECAUTIONS.

In the following the precautions necessary to obtain strictly concordant results are given in detail, as well as a statement of the 

abbreviations which are permissible when only approximate results 

are desired.

To obtain accurate results it is necessary 

1. To employ sulphide absolutely free from oxidation products.

2. To avoid all access of free oxygen during the experiment.

3. To prevent the precipitation of basic ferric sulphate.

4. To prevent change of concentration due to loss by evaporation.

5. To make the titration with the greatest possible accuracy.

PREPARATION OF THE MATERIAL.

The mineral is somewhat finely ground, an operation which is greatly 

facilitated by adding water. After drying, it should be kept in a vacuum or in carbon dioxide until used. As both pyrite and marcasite 

oxidize rapidly when powdered, the oxidation products must be completely removed immediately before the experiment. About one 

gram of the dry powder is washed by decantation two or three times
STOKES.] DETAILS OF THE METHOD.

with ether, two or three times with 20 per cent hydrochloric acid

(1 part concentrated and 1 

part water), and heated with 

the latter for a quarter of 

an hour or more, to remove 

basic salts, oxides of iron, 

and any contaminating sulphides or soluble silicates 

which may thus be extracted. 

It should be remarked that 

under these conditions pure 

pyrite evolves no perceptible hydrogen sulphide 1 if 

tested a few moments with

lead paper, while marcasite

continues to give off minute 

amounts however long it may 

be heated with the acid. The 

evolution of hydrogen sulphide from pyrite indicates 

the presence of other sulphides. The material is then 

collected on a Gooch crucible 

fitted with a disk of Schleicher & Schiill hardened filter 

paper and washed with dilute 

hydrochloric acid, and then 

with Avater which has been 

boiled and cooled in carbon 

dioxide. The wash i ng is conducted in an atmosphere of 

carbon dioxide, and for this 

purpose I use the apparatus 

shown in'fig. J, in which the 

wash water is delivered from 

the dropping funnel, and 

the carbon dioxide, supplied 

from an ordinary automatic 

generator, 2 itself affords the 

pressure necessary to drive 

;the liquid through the filter. 

The crucible is then sucked 

out by attaching a pump to the outlet tube, placed for a moment on

1 One-half grain pure powdered pyrite, boiled in the decomposing apparatus for one hour with 

7 per cent sulphuric acid, in a slow current of carbon dioxide, caused a slight blackening of lead 

paper placed in the top of the condenser.

" Liquefied carbon dioxide contains air and should not be employed.

Fig. 1. Apparatus for washing sulphides in an atmosphere of carbon dioxide.
ON PYRITE AND MAEOASITE. [BULL. 186.

a blotter to remove most of the water and transferred to a vacuum 

desiccator containing sulphuric acid and filled with carbon dioxide,

which is then exhausted. The 

use of carbon dioxide in this connection is essential, as even the 

best vacuum attainable by ordinary means contains enough oxygen to produce sensible oxidation. The drying is complete in 

half an hour.

OXIDATION APPARATUS.

The decomposition apparatus 

is represented in fig. 2. A flask 

holding about 400 cm. 3 is provided with a doubly perforated 

rubber stopper which has been 

well boiled out in caustic soda; 

through this pass the inlet tube 

for carbon dioxide and the condenser. The latter consists of 

an elongated calcium chloride 

tube, into which fits loosely a 

test tube through which cold 

water passes. The condenser has 

a hole at a to allow the passage 

of the steam, so that the return 

water drops quietly from the end 

of the tube. It is absolutely essential to prevent the return 

water from striking the sides- of 

the flask or inlet tube, as at such 

points a film of basic sulphate 

invariably forms.

The flask is placed up to the 

stopper in a cylinder of asbestos 

board provided with windows of 

mica or glass on opposite sides, 

to admit of watching the operation. This rests on a plate 1 of 

asbestos containing a circular 

hole covered with gauze, and is

closed at the top by a similar plate. The object of this arrangement 

is to prevent as far as possible the condensation of water on the sides

Fiy. A,'. Apparatus for oxidizing sulphides with 

ferric solution.

1 In the figure the flask is represented as elevated above the plate. During the boiling it must 

rest on it.
STOKES.] DETAILS OF THE METHOD. 23

of the flask; when this occurs, a ring of: basic sulphate tends to form 

at the surface of the liquid. About 250 cm. 3 of the standard solution 

are used. The small loss by wetting of the condenser is compensated 

by having it wet at the outset.

Blank experiments of this apparatus lasting,, two hours show no 

deposition of basic salt, no loss by volatilization (if the ingoing gas is 

saturated with moisture), and no reduction of the ferric salt.

DETAILS OF THE OPERATION.

As the solution contains enough air seriously to affect the results, 

and as the percentage of sulphur oxidized varies with the temperature, it is necessary to introduce the substance into the boiling liquid, 

and not until all the air has been boiled out. For this purpose it is 

rapidly placed in the small bucket, shown on a large scale in fig. 2, A, 

which consists of a small test tube cut off and constricted at the 

bottom and provided with a loosely fitting platinum disk, made tight 

by pouring in a little iron-free asbestos emulsion, such as is used in 

Gooch crucibles. The bucket is suspended by a platinum wire, and 

is dropped into the liquid after this has boiled ten minutes. It is 

necessary that the powder shall disseminate through the liquid and 

not settle out on the bottom for any considerable time. If this be not 

the case, abnormal results are obtained, for reasons which are not 

clear. The powder, as well as the apparatus, must therefore be 

strictly free from grease, and, to aid the dissemination, the apparatus 

must be so mounted on a stand as to admit of agitating the liquid 

until the powder remains suspended. Pyrite disseminates readily,, 

but marcasite shows a strong tendency to flocculate at first, and the 

two may generally be distinguished by this means.

During the whole operation a very slow current of carbon dioxide 

is passed through. This must, of course, be absolutely free from 

oxygen, and as some air is present even in the most carefully prepared carbon dioxide, I always pass it over red-hot copper. 1 It must 

also be free from hydrogen sulphide, which is not always the case, 

and it can easily be freed from this by passing it through a tube filled 

with beads and containing saturated copper sulphate solution.

The greater part of the ferric sulphate is reduced after a few minutes' boiling, but in order to effect complete reduction the boiling 

should be continued two hours. The liquid is then cooled and filtered 

through dry filters into two accurate 100 cm. 3 flasks, which are filled 

with carbon dioxide and placed in cylinders of the same. In order 

entirely to avoid contact with the air, it is driven over through the 

inlet tube by .means of carbon dioxide introduced through the condenser. The flasks are then brought to the temperature of th. permanganate to be used, and the contents are transferred to flasks

1 When many determinations are to be made, it is convenient to place the copper in an iron 

gas pipe, water jacketed at both ends i'f necessary.
24 ON PTRITE AND MARCASITE. [BULL. 186.

containing.sulphuric acid and filled with carbon dioxide and titrated. 

After' reducing by hydrogen sulphide, filtering off the sulphur, and 

expelling the hydrogen sulphide by boiling in a carbon dioxide current, the total iron is titrated. A portion of the remaining solution 

should always be tested with sulphocyanate to ascertain the extent of 

the reduction. This generally gives a faint reaction for ferric iron. 

If it is decidedly red, either the reduction is incomplete or the mineral 

contains notable amounts of copper. 1

As small errors in titration produce^considerable deviation, an accurately calibrated burette should be used, so narrow that the bottom 

of the meniscus can be distinctly seen against a light. Those burettes 

which have the graduation carried halfway aroimd the tube are particularly good for accurate reading. The permanganate should have 

a strength of about 1.5 grains per liter, and should be run in only to 

the very faintest change of color, as compared with a ferric alum solution of approximately the same strength and acidity.

When less accurate results are required the mineral, after extraction with acid, may be simply collected on a hardened filter, washed 

with acid and water, and dried in the vacuum; special purification 

of the carbon dioxide, filtering in a carbon dioxide atmosphere, filtration of the reduced solution from sulphur, and other exceptional precautions maybe omitted. Results made in duplicate in this way may 

differ 3 or 4 per cent.

ACCURACY OF THE METHOD.

If 100 cm. 3 solution and a permanganate of 1.5 grams per liter be 

used in titrating, an excess of 0.1 cm. 3 permanganate, or an oxidation 

of 0.0003 gram of the material, gives the following errors in the value 

of p:

Pyrite .. ...................'.. ......

a.

+ 1.00

4-0.20

b.

+0.12

' +0. 01

c.

-1.00

-0.20

Oxidation 

of 0.0003 gram 

sulphide.

-1.00

-0.15

Special care is therefore necessary in -standardizing the original 

solution and in the final titration. Practically, duplicate determinations with pyrite tend to agree more closely than those with marcasite, as seen in the table below. This is perhaps due to the greater 

tendency of marcasite to flocculate, as indicated above (p. 23), and to 

its greater tendency to oxidize, although the influence of the latter is 

less than in the case of pyrite. The influence of various impurities 

is considered below (p. 31).

1 For the explanation of this see p. 45.
STO K KS. ] DATA FOR PYRITE AND MARCASITE 

IV. DATA FOR PYRITE.

25

The following table gives duplicate determinations of p, made upon 

well-crystallized samples of pyrite free from visible impurities, but 

otherwise selected at random.

Determinations of p made upon samples of pyrite.

No.

1

2

3

4

5

Locality.

Unknown ...........................

Roxbury, Conn ......................

Leadville, Colo ........ v. ..--.-.......

Old Jordan uiine, Utah _._.-.. .......

Density.

5. 018

5.023

5.041

)' 

1 00. 1

1 59. 9 

| 60.4

1 59. 7 

J 61.2

1 CO. 9 

60.2 

60.2

60.4 

f 60. 5 

{ 61.1

Menu value 

of p.

60.0

I 60.0

I Cl.O

60.3

60.8

Mean value of p, 60.4.

No. 1. A large cube, with slight tarnish and splendent conchoidal fracture. 

Traces of silica, copper, and arsenic.

No. 2. Octahedron, with fracture on peripheral part conchoidal, on the interior 

somewhat uneven or granular. Slight trace of copper.

No. 3. Combination of cube and octahedron, with conchoidal fracture. Traces 

o! copper and arsenic.

No. 4. Cubes. Traces of silica and copper.

No. 5. Cubes, splendent conchoidal fracture. Trace of copper.

OXIDATION COEFFICIENT OF PYRITE.

From the above figures it appears that the oxidation coefficient of 

pyrite is not likely to vary more than 0.6 per cent on duplicate determinations, and that it may vary about 1 per cent in typical specimens, which do not contain more than traces of impurities that can 

influence the result. Whether this difference is due to the chemical 

action of the impurity on the solvent, to its electrochemical action on 

the pyrite, or to slight physical differences in the pyrite itself, can 

not be decided at present. The oxidation coefficient of pure pyrite 

may be assumed to be 60.4. Figures higher than 61 or lower than 60 

may be taken to indicate greater contaminations, which can usually 

be detected by analysis, and the influence of some of which is pointed 

out below.

V. DATA FOR MARCASITE.

The following table gives the values of ,p (mostly in duplicate) -for 

various samples of marcasite, which were selected as being free from
26 ON PYEITE AND MA KG A SITE. [uur.L. 186.

visible impurity and showing characteristic crystallization. Other 

specimens, which were found to contain pyrite, are described in a 

later section.

Values of p for various samples of marcasite.

No.

6

7

8

9

10

11

19

13

14

Locality.

Dover Cliff s. England . . ..................

Galena. Ill .................. .... .... ....

Galena. Ill ................ ..... .........

Linden mine, Wis ...--.... ...............

Galena, 111 ............... ................

Hazel Green, Wis ......'....................

Weardale, England. ......................

Cornwall, England .........................

Webb City, Mo. ............ .... ...... ...

Density.

4. 881

4.891

4. 901

4.886

 4. 896

4.880

4.878

4. 887

p.

16.3

f 16. 9

1 ID. 1 

f 16.5

I 3Q. I

( 16.3

\ 16. 9 

I 17.4

1 17.3 

1 18.1

( 17.5 

f 18.0

1 10. U

f 18.7

1 18.3 

f 18.6

1 18.8

Mean value 

of p.

16.3

} 16. 5

| 16.6

| 16. 6

| 17. 4

\ 17.8

J 18.0

J 18. 5

| 18.7

No. 6. This is the specimen described by Julien' as follows: " No. 14. Marcasite, 

Dover Cliffs, England. A brilliant cluster of spearheaded crystals broad, striated, twinned plates embedded in light gray chalk, grayish white and brilliant 

on fracture." Some of the crystals carry small pyrite crystals along the plane of

twinning, which are visible only .after cleaning with acid. Only those free from 

visible pyrite were used for the determination.

 No. 7. A hollow stalactite from Galena, 111. The inner portion is fibrous, the 

outer columnar, with pyramidal terminations. Tarnish, copper colored. Color 

of fresh fracture or clean surface, tin white. It contains a trace of copper and is 

free from arsenic. Only the outer columnar portion was used.

No. 8. A similar stalactite from Galena, 111., with greenish gray tarnish on 

fracture and bronze colored on surface and with decided vitriolization.

No. 9. Curved wedge-shaped crystals of the Joplin type, from Linden mine, 

Wisconsin. Iridescent blue and yellow tarnish. Fracture, tin-white. The specimen carried a little galena and calcite, from which it was carefully freed by acid 

and by picking out. Analysis showed a trace of copper and a little lead.

No. 10. Galena. 111. A compact mass of large acute pyramidal crystals branching out from a common axis. Tarnish, greenish yellow. Color on fresh fracture 

and on green surface, tin-white. A few crystals of chalcopyrite. Material carefully selected for determination gave traces of silica and copper and no arsenic.

No. 11. Hazel Green, Wis. A thick crust on calcite, with distinct rhombic 

crystallization. Color, tin-white. Contains traces of lead and copper and is free 

from arsenic.

1 Annals New York Acad. Sci., Vol. IV, 1887, p. 172.
STOKES.] MIXTURES OF PYRITE AND MARCASITE. 27

No. 12. Weardale, England. Flat, deeply striated crystals, with slight bluish 

tarnish. Color of clean surface, tin white. No visible impurity was observed, 

and analysis showed the presence of a faint trace of copper and a little silica and 

the absence of arsenic.

No. 18. Cornwall, England. Flat crystals, with slight bluish tarnish. Color 

of cleaned surface, tin-white. A few of the crystals carry minute cubes of pyrite, 

and such were rejected. Analysis showed no arsenic and a trace of copper and 

antimony. The high oxidation coefficient makes it possible that it incloses considerably pyrite, but material was wanting for a special determination of this.

No. 14. Webb City. Mo. Curved wedged-shaped crystals of the Joplin type, on 

sphalerite and calcite and with a little inclosed chalcopyrite. Analysis of a sample used for the determination showed some lead and copper, which probably 

account for the high figure obtained.

OXIDATION COEFFICIENT OF MARCASITE.

From the above figures it appears that specimens of apparently 

pure marcasite differ somewhat more with regard to the oxidation 

coefficient than does pyrite. It will appear below that an admixture 

of small quantities of pyrite lowers the value of p, notwithstanding 

the fact that pyrite has a higher oxidation coefficient. It is probable 

that other sulphides in minute amount exert a similar effect to even 

a greater extent, possibly in part because of the electrochemical 

action which they produce. The marcasite with the lowest oxidation 

number is therefore not the purest. The specimen No. 12 from 

Weardale was the purest examined, and' I incline therefore to adopt 

the figure 18 as being nearest to the oxidation coefficient of pure 

marcasite. Figures higher than this, as well as lower than 16.5, must 

be regarded as indicating contamination, either with pyrite or with 

another oxidizable mineral. It also appears from the examination 

'of certain samples (Nos. 22 and 23) that marcasite may carry considerable amounts of pyrite so intimately interg.rown as not to be 

visible upon examination of small fragments under a lens after 

cleaning with acid.

VI. MIXTURES OF PYRITE AND MARCASITE.

It is desirable to construct a curve which shall give the oxidation 

coefficients of mixtures of pyrite and marcasite. Such a curve would 

enable us to ascertain, in the absence of other indications, whether a 

given specimen contains one or the other or botli of these minerals, 

and in the latter case to determine their relative amounts. It would 

enable us to test the hypothesis of Julien, 1 whose statement I quote:

The forms of iron pyrites occurring in nature are intimate mixtures of these 

three minerals; rarely of pyrrhotite, however, on account of its ready metasornatic 

alteration into one or the other of the triad. These common mixtures of marcasite 

and pyrite may originate by inclosure during crystallization, by alteration, and 

by displacement, and pass progressively into complete paramorphs, well crystalli'/ecl after the form of one or the other mineral.

The latent constitution of these composite minerals is indicated by variation in

'Annals New York Acad. Sci., Vol. IV, 1887, p. 213.
28 ON PYRTTE AND MARC A SITE. [BULX,. 180.

density, exactly proportionate in most cases to the amount of each constituent, 1 

and by a similar variation in other physical properties; e. g., hardness, fracture, 

resistance to decomposition, and even in color, in the case of the paramorphs of 

marcasitic pyrite.

As a parainorph consists simply of minute crystals of the one substance massed together in a form which characterizes the other, it is 

obvious that the oxidation coefficient must correspond to that substance of which the specimen really consists, not to that which it imitates, and such a curve would indicate the proportions in the case of 

a mixed crystal. It would further enable us to follow the possible 

artificial transformation of one into the other, and would serve as an 

aid in synthetic work. In such work it is frequently difficult to obtain 

products sufficiently well characterized to admit of positive identification in the absence of optical properties and well-developed crystallization. Such a curve would render us independent of these.

It is not possible to calculate .the curve for mixtures of pyrite and 

marcasite from the oxidation coefficients of the two minerals. The 

oxidation coefficient is influenced by the concentration of the ferric 

and ferrous salts and the sulphuric acid. With the pure mineral 

these vary in a perfectly constant manner in the case of each sulphide, 

but when each mineral is decomposing in the presence of the products formed by the other, .nothing short of an elaborate investigation 

could give us the data for calculating the effect in any case. It is 

therefore necessary to construct the curve empirically, from .data 

obtained from a sufficient number of artificial mixtures.

CONDITIONS UNDER WHICH THE OXIDATION COEFFICIENTS OF . MIXTURES MAY BE DETERMINED.

We have seen that the oxidation coefficients of pyrite" and marcasite are independent of the relative amounts of solvent and solute, 

and of the degree of comminution, and are constant for a given temperature and concentration provided the reduction be complete. The 

case of a mixture of the two is more complicated. Here each component deprives the other of a portion of- the ferric salt, this effect 

being greater the greater the surface, or, what is the same, the finer 

the powder. An artificial mixture of given composition will therefore show a varying oxidation coefficient depending on the relative 

degree of fineness of the two components. Moreover, since a given 

volume of the standard ferric solution decomposes about 2.26 times 

as much marcasite as pyrite, the ratio between the surfaces is constantly changing and it is no longer a matter of indifference whether 

a large or a small amount of material be acted on by a given volume 

of solution.

If, however, the minerals be ground together and if the grinding be 

tolerably thorough, then, considering their approximately equal 

hardness, it may be assumed that the ratio of the surfaces for a given

1 Compare table oii p. 13.
STOKES.] DATA FOR AKTIFICIAL MIXTURES. 29

mixture will remain practically constant, and if the value of p be 

obtained by using such an excess of the mixture that this ratio is not 

appreciably altered during the experiment, or if the same amount of 

mixture and of solution be used in every "case, it becomes possible to 

obtain concordant values of p and to construct the desired curve. 

The assumption that the ratio of surfaces remains constant during 

grinding is not absolutely true, but practically it is, as the duplicate 

results presented below.were each obtained from different samples, 

so that the slightly varying hardness of different specimens and the 

different degree of fineness to ..which they are ground introduce no 

greater errors than those which are inherent in the method itself. In 

each case the materials were broken up so as to pass a 20-inesh but 

to be retained by a 60-mesh sieve, were carefully weighed, mixed, 

and ground so as to avoid all loss, and 1 to 1.02 grains of the mixture 

and 250 cm. 3 of the solution were used in each experiment.

VII. DATA FOR ARTIFICIAL MIXTURES.

The following table gives the result for different mixtures, the 

numbers in the second column, referring to the above-mentioned 

specimens of pyrite and rnarcasite, respectively:

Values of p for mixtures of pyrite and marcasite.

Pelcent 

pyrite.

0

5

10

' 20

40

6!)

to

90

9

100

Specimens used.

m. 7.... ....... ..................................

m. 10... .......... ..................................

111.12...'............. ...... ........................

p. 5; in. 7..--.-. ..--.........._...-.-...---- .....

p. 5; m. 10...... ...................................

p. 5; m. 12 ................. ..........................

|p. 5; in. 7 ........ ............................ .....

p. 5; in. 10.... ......-..._.--..-..---..--. ...........

lp. 5; in 7 ............................................

lp.5: in. 10 ... ....................................

Jp.o: m. 7 ........ .............. ....... .... .......

Ip.o: in. 10-... ........ . ._.-....--..-...-._.....

 p. 5; m. 10 ......... ........................ .......

ip.4; m. 10. ....... ............................ ...

jp. 5; m. 7 .........................................

|p. 5; ni. 10 ........... . ...........................

|p.5; m. 10 ......................................... 

p. 4; m. 10 .........................................

p. 4; m. 10 .........................................

p.4...... ............... ......................

p.5....... .......................................

p16. 5

17:4

18.0

15.7

16.5

15.9

15.2

15.2

17.3

16. 9

09 9

29 0

29.1

4.0. 5

40.2

48.9 

48.9

52. 9

53.0

60.3

60.8

Mean value 

of p.

17.3(18.0)

16.0

J 15. 2

| 17.1

J 22 3

29.0

I 40.3

\ ' 48. 9

52.9

[ 60.5
30 ON PYEITE AND MABCASITE. [BULL. 186. 

DISCUSSION OF THE CURVE.

The curve in PL I is based on the mean values of p thus obtained, 

with the exception of pure marcasite, where the more probable value 

18 is used (p. 27).

Influence of the minimum: indirect determination of small amounts 

of pyrite in 'marcasite. The most obvious features of this curve 

are the presence of a minimum point when the amount of jjyrite 

reaches about 10 per cent and the increasing steepness as 100 per 

cent is approached. The minimum implies that a mixture of marcasite with a little pyrite has a less percentage of sulphur oxidized 

than has pure marcasite. This unexpected result, which is abundantly confirmed by numerous other preliminary data not published, 

may be partly due to the fact that pyrite uses up more ferric salt than 

does the equivalent amount of marcasite; the latter is therefore acted 

on by a solution weaker in Fe'" than if it alone were present, the 

effect of which is to lower the oxidation coefficient. Possibly there 

may also be an electrochemical action between the two. A minimum 

also occurs in the curve for pyrite-galena, while pyrite-chalcopyrite 

shows a maximum. The existence of these points shows that it is 

impossible to predict, from a knowledge of the values of p for any 

two sulphides, whether the result of the contamination of one with a 

small quantity of the other will be to raise or to lower the oxidation 

coefficient. The variation of p from 16.3 to 18 for nearly pure marcasites is presumably due to small amounts of impurities other than 

pyrite. Practically these have but little effect on the determination 

of pyrite and marcasite, for, as seen in the table, the three samples 

of marcasite, though varying from 16.5 to 18, give essentially the same 

figures when mixed with even as little as 5 per cent of pyrite.

As a result of the minimum, any specimen in which p is about 18

or less may have one of two possible compositions. Which of these' 

is actually present may be tested by adding one-ninth its weight of 

pyrite and determining the value of p for the new mixture. If 

depression is produced, the true percentage of pyrite lies to the left 

of the minimum; if elevation, to the right. In fact, the freedom of a 

marcasite from pyrite can be regarded as absolutely proved only 

when this precaution is taken. The determination of p in the original specimen is not necessary; the marcasite in question may.at once 

be mixed with a known amount of pyrite and the original contents of 

pyrite thus determined. If x represent the percentage of pyrite in 

the original sample, then a mixture of 90 parts of this with 10 parts 

pyrite will give

0.9 x + 10=per cent pyrite in mixture.

The latter is found by one determination and the composition of the 

original sample deduced. An application of this method may be seen 

below (pp. 36-38) in the case of the marcasites from Garfield tunnel
U. S. GEOLOGICAL SURVEY BULLETIN NO. 186 PL. I

60

O 40

« 30

20

10 20 30 40 50 60 70 80

Percentage of pyrite. 

CURVE SHOWING OXIDATION COEFFICIENTS OF MIXTURES OF PVRITE AND MARCASITE.

90 100
STOKKS.] INFLUENCE OF IMPURITIES. 31

(No. 20), Littmitss (No. 22), and Crow Branch mine (No. 23). In the 

latter two cases the selected material showed no pyrite under a' lens, 

although the' specimen as a whole contained it in visible amount.

It further appears from the curve that small amounts of marcasite 

in pyrite can be determined with greater accuracy than small quantities of pyrite in marcasite. In the former case the determination can 

probably be made to 1 per cent with duplicate experiments and in the 

absence of notable amounts of other sulphides; in the latter it can 

hardly be more accurate than 2 or 3 per cent. It must be remembered, however, that there is a slight uncertainty close to both ends 

of the curve, due to the unknown influence of minute Contaminations. 

In other cases, however, greater accuracy than is obtainable could 

hardly be desired.

VIII. INFLUENCE OF IMPURITIES.

When pyrite or marcasite is inclosed in rock, the extraneous material may be removed by digesting with hydrofluoric and hydrochloric 

acids; but small amounts of insoluble siliceous material can usually be

neglected.

MISCELLANEOUS IMPURITIES.

The necessity of extreme care in preparing the material is emphasized above. Since 5>(c a), an inspection of the equation

8.333 b

c a

shows that the presence of iron salts or oxides, or access of air during

the decomposition, affects the ratio in such a manner as to lower c a

the value of p, this effect being greater with pyrite than with marcasite. Free sulphur exerts very little influence, and is moreover 

removed by the preliminary extraction with ether. Without influence are also sulphates, carbonates, or soluble silicates free from iron, 

insoluble silicates, quartz, and in general any subctance neither contributing iron to nor effecting reduction of the ferric solution. Pyrrhotite and liinonite are easily eliminated. Zinc blende, galena, magnetite, and hematite, if present in but small amounts and not inclosed 

in the fine particles of FeS2 , are more slowly removed by prolonged 

digestion with hydrochloric acid. All sulphides are more or less 

readily attacked, and those free from iron, since they do not contribute 

to c a, should, it might be expected, raise the value of p. If present 

in traces, they do not appreciably affect the result; but if existing in 

large amounts, they introduce considerable errors, and abnormal 

results are to be explained by a qualitative analysis. In reality their 

influence can not be always predicted, as pointed out above (p. 30), 

and some experiments bearing on this point -are given below. 

Bull. 186 01 3
32 ON PYEITE AND MAKCASITE. [BULL. 18i5.

The equation

c a

applies, in its original sense of giving the per cent of sulphur oxidized, 

only to FeS2 . We may, however, use it in a broader way as simply 

indicating a certain relation between the permanganate values of the 

ferrous, ferric, and total iron, and in this sense it may be applied to 

any sulphide or other compound capable of reducing ferric salts, and 

is useful in instituting comparisons between these and FeS2 either 

alone or in a mixture.

HEMATITE AND MAGNETITE.

The presence of these is indicated during the extraction of the 

powder with 20 per cent hydrochloric acid if repeated portions of acid 

continue to be colored yellow after heating. Difficultly soluble ferric silicates, of course, show the same result. As these iron oxides 

are scarcely soluble in the weakly acid ferric solution, their presence 

in small amounts is without appreciable effect.

GALENA.

Galena is rather slowly attacked by the .ferric solution, and the 

value of p is infinity. A mixture. of galena and pyrite (p=60.8) containing 3 per cent lead gave

 p=59.5.

From this it appears that small amounts of galena exert a slight 

depressing influence and that the curve for galena-pyrite varies from 

60.4 to infinity, with a minimum near the pyrite end.

NICKEL AND COBALT.

These show a very strongly elevating influence on the oxidation 

coefficient, as shown by specimens Nos. 27 and 32.

ARSENOPYRITE.

The action of the ferric solution on arsenopyrite, or on pyrites or 

marcasites containing arsenic in notable amounts, is very characteristic, in that the value of 5 is always greater than that of c. This is 

due to the fact that a portion of the permanganate represented by 1) 

is consumed in oxidizing arsenious to arsenic acid. It may be further 

observed that in such cases a permanent end-reaction for b can not 

be obtained, as the last portions of arsenious acid are oxidized but 

very slowly in the cold by permanganate. In this event the titration 

must be stopped at the first change of color, and since arsenic acid is 

precipitated very slowly by hydrogen sulphide, it is necessary to conduct the gas through for several hours at a temperature of about 80°
STOKES.] INFLUENCE OF IMPURITIES. 33

in order to render the precipitation complete and to obtain a correct 

value for c. . An experiment made with the usuaf precautions, with carefully 

selected arsenopyrite, gave 

a, 39.50

6, 56,90

c, 45.84

On substituting these permanganate values we get

p = 49.8

The actual sulphur oxidized, as calculated from these figures, assuming that all the arsenic is oxidized to As2 03 , is 10.3 per cent, and in 

fact a notable sublimation of sulphur was observed. . The data do 

not indicate whether any arsenic is liberated, or whether orpiment 

or realgar is formed, but no evidence of these could be obtained. 

More elaborate experiments would be required to decide whether 

under certain conditions they can be formed by the action of ferric 

salts on arsenopyrite.

A carefully prepared mixture of pyrite and arsenopyrite (2.17 per 

cent), containing 1 per cent arsenic, was oxidized with the following

result:

a, 88.33 

&, 43.17 

c, 42.78

The value thus deduced for p, namely, 56.7, corresponds to that of 

a pyrite containing 2 per cent marcasite. As few pyrites contain as 

much as 1 per cent arsenic, it appears that in general the presence of 

arsenic will not interfere greatly with the marcasite determination.

CHALCOCITE AND BOENITE.

Chalcocite by itself gives an infinite value for p. ~ The influence of 

these minerals on pyrite is stated below. Both are very easily attacked 

by cold ferric solution.

CHALCOPYRITE.

As pointed out in the introduction, it is well known that chalcopyrite is more easily attacked by ferric solutions than is pyrite. An 

experiment with crystallized chalcopyrite made with the usual precautions gave

a, 37.44

6, 44.01

c,'44.11

whence

p=30.0

From these figures and the determination of the dissolved copper 

it appears that not more than 2 per cent of the sulphur is oxidized, 

and that more copper than iron is dissolved (0.7 atom Fe to 1 atom
34 ON PYEITE AND MAECASITE. [BUL.U 186.

Cu), from which it may be concluded that pyrite or marcasite is possibly an intermediate product of the oxidation of chalcopyrite by 

ferric solutions. °

IX. FORM OF OCCURRENCE OF COPPER IN COPPER-BEARING

PYRITES.

The problem of determining whether a given specimen of cupriferous pyrite contains its copper in the form of chalcopyrite.or chalcocite 

was suggested by a geologist, and the following shows that such determination is possible when marcasite or other sulphides are not present 

in considerable quantities.

OXIDATION EXPERIMENTS.

Mixtures of pyrite with chalcopyrite, chalcocite, and bornite were 

prepared, each containing 3 per cent copper. - For chalcopyrite and 

chalcocite these mixtures have the following composition:

Cu........ 3.00

....-..-..-. ___-__-.-_--_..- »J..OO I

Chalcopyrite

Pyrite ................. ._.«.__. ..__. . 91.331 ,.

8 67 or ^ ------- 45- 20

S......... 51.80

Pyrite.. ................................. 96.241 JC*--~- 3.00

Chalcocite............................. 3 . 76 or Fe...l... . 44.85

' IS......... 52.15

The bornite mixture has an intermediate composition. It would 

be scarcely possible to distinguish these by ordinary analytical 

methods, especially in the presence of small amounts of impurity. 

The values of p were found to be:

Pyrite-chalcopyrite.... ..........^............................. 62.7

Pyrite-chalcocite .............--..------- ------------------ 75.9

Pyrite-bornite. ........................... T ............. ........ 76.4

It appears that the oxidation method does not admit of distinguishing chalcocite and bornite with certainty in a pyrite with 3 per cent 

copper, but that these may be readily distinguished from chalcopyrite 

even when the copper is considerably less than 3 per cent. It can 

also be determined with some probability whether a mixture of chalcopyrite with the other sulphides of copper is present. In the present 

case the figures given by chalcocite or bornite and chalcopyrite differ 

by more than thirteen times the probable error of a determination.

This method will probably be of value in synthetic studies of copper ores. It is thus made possible, for example, to ascertain whether 

the first action of a cupric salt on pyrite is to produce chalcopyrite 

alone or a mixture of this with the other sulphides, without having 

to wait for the slow process of crystallization to produce the minerals 

in a form in which they qan be recognized under the microscope. By
BTOKKS.]. MISCELLANEOUS SPECIMENS. 35

employ ing fine powders the rapidity of the change will, be vastly 

increased, while the net results, according to the mass law, will be 

the same.

DETECTION OF CHALCOPYRITE IN PYRITE OR MARCASITE AND IN

ROCKS.

Chalcopyrite mixed with pyrite or marcasite may be readily detected, 

if not in too small amount or too finely divided, by exposing the 

sample to bromine vapor, for half a' minute, and then to hydrogen 

sulphide gas; the chalcopyrite is blackened, while the iron sulphides 

remain bright. Particles of chalcopyrite which would otherwise be 

overlooked are thus easily detected and their outlines sharply defined. 

I have used the same procedure to detect and establish the nature of 

minute grains of chalcopyrite inclosed in rocks.

X. MISCELLANEOUS SPECIMENS AND SPECIMENS OF DOUBTFUL

NATURE.

With the aid of the pyrite-marcasite curve we are now in a position 

to determine the quantitative composition of specimens of pyrite and 

marcasite in regard to which there is some uncertainty. The supposed nature of the specimens, according to the geologist, mineralogist, or dealer from whom they were obtained, is stated, as well as 

the composition established by my method. It is to be understood 

that the figures express the relative amount of pyrite and marcasite 

011 a scale of 100, impurities being excluded. In this connection I 

wish to acknowledge the generosity of Dr. A. A. Julien, who has 

placed at my disposal a number of the identical specimens described 

by him in his valuable paper repeatedly referred- to above. As the 

study of these specimens is especially important in confirming or 

refuting his hypothesis (p. 27), I have grouped them together at the 

end of this section, and have in each case quoted his original description-and specific gravity determination, comparing in parallel columns 

the results obtained by the specific gravity and the oxidation methods.
r

36 OK PYRITE AND MAKCASIT.E. [HULL. 186.

Quantitative composition of specimens of doubtful nature.

[Figures in parenthesis were determined by the indirect method.]

No:

15

16

17

18

19

20

21

22

23

24

25

26

27

Original designation, 

etc.

Concretion from

coal.

Pyritized ammonite.

Fibrous marcasite.

Pyrite with marcasite.

Marcasite .... ....

. ...do ............

Marcasite nodules.

Marcasite (selected).

.....do............

Fibrous marcasite.

Marcasite after

pyrite.

Pyrite ore. ... ....

Pyrite octahedra . .

.Locality.

Unknown ..........

Folkestone, England.

Red Cloud mine,

Colorado.

Quartzburg district,

Oregon.

Chautauqua tunnel,

Idaho.

Garfield tunnel,

Idaho.

South Dakota .'......

Littmitz, Bohemia, . .

Crow Branch mine,

Wisconsin.

Sunshine, Colo .-. ...

Folkestone, England

Rio Tinto, Spain. ...

Unknown ...... ....

p30.1

46.9

65.7

38.6

51.3

1C) 1 c/. OQ 

60.3

19. 5

18.6

52.2

55.4

65.7

80.4

Pyrite.

Per cent.

63.0

88.0

100.0

78.0

no R wo. o

30. 5 (34. 4)

100.0

29.5(27.8)

26.0(31.0)

a 94.0

97.0

100.0

100.0

Marcasite.

Pur cent.

37.0

12.0

22.0

6

K 

. <J

69. 5 (65. 6)

70. 5 (72. 2)

74.0 (79?0j

66.0 '3.0

(c)

a Or more. ft Or less. c Much cobalt.

No. 15. Nearly spherical concretion from coal; locality unknown. Weight, 9.2 

grams. Very hard. The structure is coarsely fibrous and radial, with concentric shells of pyrite and marcasite, easy distinguishable by their color.

No. 16. Pyritized ammonite from Folkestone. England. The walls of the cavities are incrusted with pyrite crystals, but the nature of the mass can not be 

determined from the color.

No. 17. Concretion from Red Cloud mine, San Juan County, Colo., supposed 

to be marcasite. It consists of a finely fibrous incrustation on rhyolite, varying 

in thickness from 1 to 10 millimeters, with mammillary surface, and with a banded 

structure and cleavage perpendicular to the fibers. The banded structure is due 

to tha inclosure of quarts and a black substance which, judging from the presence 

of a little copper, lead, and arsenic, may be galena, arsenopyrite, and chalcocite. 

The latter would account for the high oxidation coefficient, which could not be 

caused by the first two (pp. 32-33). Carefully selected material gave a specific 

gravity of 4.563.

No. 18. The occurrence of this material is thus described by Mr. Lindgren, by 

whom it was submitted for a determination of the yellowish-gray material:
STOKES.] MISCELLANEOUS SPECIMENS. 3

"This specimen is derived from the Present Need mine, Quartzburg district, 

Grant County, Oreg. The vein, carrying chiefly gold, partly in free form, partly 

associated with sulphides, is contained in diabase and diabase-porphyry. The ore 

minerals are normal yellow pyrite, a soft, yellowish-gray material supposed to be 

marcasite, zinc blende, and chalcopyrite." The iron sulphides are easily separated 

from the rock and other sulphides by hydrofluoric acid. The yellowish-gray 

material, which is closely veined with pyrite, could not be separated from the 

latter, but the oxidation coefficient of the mixture indicates that it is marcasite. 

The portion examined contains a trace of copper and lead and no zinc.

No. 19. This is thus described by Mr. Lindgren: "This specimen is taken from 

the dump of the Chautauqua tunnel, at De Lamar, Owyhee County, Idaho. The 

specimen consists of quartz, upon which there is a crust of pyrite. The quartz 

itself contains arborescent forms of a mineral supposed to be marcasite and 

described as such in Twentieth Annual Report U. S. Geological Survey, Part III; 

page 130." The mineral in question was easily separated from the quartz and the 

mass of the pyrite by digestion with hydrofluoric acid. It formed dark-colored 

tabular masses, presenting no definite crystalline form, and containing traces of 

copper and lead with a notable amount of arsenic. The oxidation coefficient 

(51.3) shows that it consists of at least 93.5 per cent of pyrite, but the proportion thus determined is somewhat too low, owing to the influence of the 

arsenic.

No. 20. Thus described by Mr. Lindgren: "This specimen is taken from the 

Garfield tunnel, 250 feet from the portal, near De Lamar, Owyhee County, Idaho, 

and is described on pages 131 and 171, Twentieth Annual Report U. S. Geological 

Survey, Part III. The specimen consists of a soft white material, which, according to analysis, is probably kaolinite mixed with sericite. It contains arborescent 

forms of marcasite." The supposed marcasite was isolated by hydrofluoric acid. 

It could not be definitely described as such from the color and crystalline form. 

As the oxidation coefficient (19.8) was too high for pure marcasite, a portion was 

mixed with one-ninth its weight of pyrite, and then gave 

p=22.6=4l per cent pyrite.

We have, then 

0.9 OH-10=41, or o;=34.4.

This corresponds to p=:20.6, a number agreeing within permissible limits with the 

original determination. We have, therefore 

Pei' cent.

Pyrite directly determined................. ............ ..... 30.5.

Pyrite indirectly determined .................................... 34.4

and the original conclusion of Mr. Lindgren is only in part confirmed. A little 

arsenic is also present.

No. 21. Material supposed to be marcasite, from South Dakota. Submitted by 

Dr. G. P. Merrill. The sample consists of spherules not over one-half millimeter 

in diameter and so brittle as easily to be crushed by the finger. They had undergone vitriolization to such an extent that the original substance was entirely concealed in the mass of sulphate. The oxidation coefficient shows that the material 

is pyrite, and the analysis that some copper is present. The normal value of p 

indicates that the copper is probably in the form of chalcopyrite.

No. 22. Marcasite from Littmitz, 1 Bohemia, consisting of flat, deeply striated 

twins of marcasite on a nucleus which on fracture shows both marcasite and

1 Regarding the Littmitz marcasite, see Sadebeck, Monatsber. K. preuss. Akad. Wiss. Berlin, 

Vol. XX, 1878, p. 19, and Hintze, Handbuch der Mineralogie, Vol. I, pp. 732, 825.
38 ON PYKITE AND MARCASITE. [HULL. 180.

pyrite. Only the portions free from visible admixture of pyrite were taken for 

examination, and gave p= 19.5. Admixture with one-ninth pyrite gavep=21.0=:35 

per cent pyrite. We have, then 

Per cent. 

Pyrite directly determined. ................ ............. ...... 21. 5

Pyrite indirectly determined.................................... 27.8

No. 23. A specimen from Crow Branch mine, Wisconsin, consisting of a compact nucleus of tin-white marcasite covered with crystals which are cornplete.y 

inclosed in elongated cubes of pyrite, the line or' demarcation being clearly dis 

cernible on the fracture by the difference of color. Selected portions free from 

visible admixture of pyrite were used, giving a specific gravity of 4.891. To determine the contents of pyrite the indirect method was used.

Per cent. 

Pyrite directly determined ..................... ................. 26.0

Pyrite indirectly determined ................................... 31.0

As only the powdered .material was available for the indirect determination, the 

difference is not surprising (p. 28).

No. 24. Material from Sunshine, Colo. Finely fibrous radial concretions in 

quartz, which on the freshly cleaned fracture show a faint banded structure. Supposed to be marcasite. A little lead and copper and much arsenic and antimony 

are present. As arsenic, and probably antimony, would lower the oxidation 

coefficient, the figure given for pyrite is too low, and the presence of marcasite is 

doubtful.

No. 25. A large flattened concretion from Folkestone, England, described as 

"marcasite after pyrite." The fracture is columnar and the surface is covered 

with rounded projections, apparently tetragonal pyramids, possibly of the type 

described by Penfield. 1 The color on the freshly broken surface exactly matches 

pyrite. 1 have received several other supposed marcasite concretions from Folkestone, all of which are clearly pyrite.

No. 20. Pyrite ore from Rio Tinto, Spain. The fragments are grayish, evolve 

some hydrogen sulphide with acids, and with the bromine hydrogen sulphide test 

(p. 35) show very finely disseminated copper sulphide.

No. 27. Brilliant untarnished octahedra in calcite, containing a large amount of 

cobalt and a little copper, which explain the abnormal oxidation coefficient.

CONCLUSIONS.

Of the above thirteen specimens five have been described as marcasite, though consisting nearly or entirely of pyrite, while three 

other marcasites are shown to contain a large amount of pyrite, which 

could not be detected without the aid of the oxidation method. As 

these samples were selected and examined without preconception'as 

to their nature, it seems probable that a very considerable proportion 

of the concretions which exist in collections or are sold as marcasite 

are iii reality pyrite, while not a few others, even when fairly well 

characterized by rhombic crystallization, may contain inclosures of 

pyrite.

'Am Jour. Sci., 3d series, Vol. XXXVII, 1889, p. 209.
STOKKS.] MISCELLANEOUS SPECIMENS. 39

DR. JULIEN'S SPECIMENS.

Quantitative composition of Julian's specimens. 

[Tho numbers in parentheses tire those given in Julion's paper.]

Number.

20 (pyrite 90). ........

30 (pyrite 08). -.-....

31 (pyrite 101).......

32 (pyrite 106).......

'

33 (marcasite 1).....

Locality.

Franklin. N.J . ......

( Cumberland, Engl land.

Density.

5.015

4.856

4.84*5

4.819

4. 701

} 4.987

?> 

00.3

50.0

50. 6

60.7

67.7

42.9

Composition deduced from p'.

Pyrite .......100.0

Pyrite ....... 00.5

Pyrite ....... 90.5

Pyrite .......100.0

Pyrite...... a 100.0

r Pyrite....... 83.0

iMarcasite .... 17.0

Composition according to Julien, 

deduced from 

density.

rPyrito .......27.51 

IMarcasite.... 72. 40 

rPy rite .......21.18

IMarcasite.... 78. 82 

rPyrite ....... 0.38

IMarcasite.... 90. C>2 

rPyrite ....... 1.00

IMarcasite. ...00. 00

Pyrite .......89.45

Marcasite .... 10. 55

a Contains nickel.

The following descriptions are in part quoted from Ju lien's paper:

No. 28. " Concretionary nodule. Marsden's diggings, Galena, 111. No. 1 (fibrous 

core). Marcusitic pyrite. Very finely fibrous, pale brass-yellow and splendent." 

Analysis (Julien), Si(X, 0.110; Pb, 0.188; As, 0.056. The fracture varies from 

fibrous to prismatic, the prisms being evidently elongated cubes, with terminations consisting of cube modified by pyritohedron and octahedron.

No. 29. "No. 96. Marcasitic pyrite. Franklin, N. J. Sharp, brilliant, brassyellow, striated pyritohedra, yellowish white and splendent on fracture/' No 

admixture of marcasite crystals. The value of p shows that they are practically 

pure pyrite, the low density being probably due to inclosures.

No. 80. " No. 98. Marcasitic pyrite. Somerville, Mass. Very sharply defined, 

glittering yellowish cubes, sometimes distorted or rectangular, rarely striated, 

occasionally with octahedral planes upon their solid angles, yellowish white and 

brilliant on fracture; scattered through a gray argillaceous slate."

No. 81. "No. 101. Pyrite. Monroe, Conn. Deeply striated, distorted, brilliant 

cubes of very pale brass-yellow color, with modifications by the pyritohedron: 

yellowish Avhite, finely granular, and splendent on fracture, and sometimes inclosing grains of white quartz."

No. 32. "No. 106. Marcasitic pyrite. Bastrop, Bastrop County, Tex. Dull 

and pale brass-yellow octahedra, rarely iiright, very pale yellowish white and brilliant on fracture; upon black granular crystalline hematite." The specimen contains a trace of arsenic, a strong trace of copper, and some nickel. It is the latter, 

possibly in conjunction with the copper, which causes the high oxidation coefficient.

No. 33. '-No. I. Marcasite. Cumberland, England. Hollow incrustation pseudomorphs after barite, implanted on a group of barite crystals. The crusts are 

grayish white to tin-white and splendent on fracture, with surfaces drusy with 

pseudo-octahedrons or rhombic pyramids, whose smooth, rectangular terminal faces 

(the basal pinacoid OP) project but slightly; a few minute rhombic prisms also 

occur, and some hexagonal though apparently triangular twins." The supposed 

rhombic pyramids with basis are evidently combinations of cube and octahedron 

belonging to the 83 per cent of pyrite which it contains.

DISCUSSION OF JULIEN'S HYPOTHESIS.

The above list includes some of the most pronounced cases of '' raarcasitic pyrite " described by Julien that is, material with the crystalline
40 OK PYRITE AND MARCASITE. [«DLL. m.

form of pyrite but supposed to consist very largely of marcasite. 

The four specimens, ISTos. 29, 30, 31, and 32, consist of characteristic 

pyrite crystals, and if the oxidation method be admissible they are in 

fact free from marcasite, yet they all show abnormally low densities. 

The only conclusion which can be drawn from this is that the density 

alone does not afford a reliable means of detecting the presence of marcasite in pyrite unless the absence of all other contaminating substances 

be first conclusively proved, and that isometric crystallization in itself 

affords a proof, almost amounting to certainty, that the sulphide is 

actualty pyrite. It is undoubtedly true that perfect pyrite crystals 

may inclose marcasite, a good case of which is the Crow Branch specimen, No. 23, but such cases are comparatively rare, and the inclosure 

can usually be detected by breaking the specimen and carefully examining the color. In case of doubt a'determination of the density is practically of no value, and resort must be had to the oxidation method. I 

have examined some of the marcasite specimens from the same collection, as well as others which are supposed to contain pyrite, and in general the latter can be detected by its color after careful cleaning with 

acid. I have shown that certain specimens of marcasite do contain 

pyrite inclosed in such small masses as not to be capable of detection 

in this way (Nos. 22 and 23). Small, warty excrescences on marcasite 

crystals are frequently pyrite, and when the conditions controlling 

the formation of the two sulphides are practically in equilibrium these 

are likely not only to form but .to be completely overgrown and 

inclosed in the crystal of marcasite. A marcasite which occurs mixed 

with visible masses of pyrite is therefore likely also to contain such 

inclosures which can not be seen and whose presence can be detected 

only by the indirect oxidation method. That both pyrite and marcasite with abnormal density are essentially prone to decomposition

is doubtless true, and while in certain cases this may be traced to an

admixture of the two, in others it is due to inclosures of another 

nature. The lack of homogeneity not only permits the existence of 

fissures admitting air and moisture, but the formation of these is in 

such cases promoted by unequal expansion and contraction.

Fibrous marcasite is so often referred to that it may well exist, but 

so far as my somewhat limited range of examinations goes, a fibrous 

structure affords absolutely no presumption in favor of a specimen 

being marcasite; on the contrary, fibrous pyrite appears to be much 

more common than fibrous marcasite, the latter in concretions 

tending rather to assume the coarsely columnar structure.

XI. PARAMORPHISM.

The literature of the paramorphism of pyrite and marcasite is 

exceedingly scanty, and such as there is lends no support to the 

view that true transformation paramorphs exist. Those described 

are apparently either replacement or incrustation pseudomorphs, the
STOKES.] ACTION OF CUPRIO SALTS. 41

true nature of which is shown by careful inspection. Wohler 1 

attempted, without success, to convert marcasite into pyrite and 

pyrite into marcasite by heating for four hours at about 400° 

experiments which are well worth repeating, with an extension of 

the time and with the aid of the oxidation method. Julien's assumption of extensive paramorphisrn is scarcely longer tenable in view of 

the results described above, although the inclosure of considerable 

portions of pyrite in some specimens of marcasite lends some support 

to it. While theoretically possible, the attractive hypothesis that 

marcasite gradually changes to pyrite without re-solution, and that 

the porosity of certain pyrites can be thus explained, is thus far 

without a sound basis, either of experiment or of observation. It is 

intended to carry out experiments on this point in the immediate 

future.

XII. CONSTITUTION OF PYRITE AND. MARCASITE; ACTION OF

CUPRIC SALTS.

The importance to chemical geology of an experimental study of 

the action of copper salts on pyrite and marcasite is self-evident. The 

following experiments were made, however, with the object of ascertaining whether the method of A. P. Brown 2 namely, heating with 

neutral copper sulphate solution at 200° could be used to distinguish 

these minerals, and incidentally, of testing his hypothesis that the 

iron in marcasite is wholly ferrous, while that of pyrite is four-fifths 

ferric. I have therefore confined myself to the conditions of temperature and concentration observed by him.

Brown, in his published experiments, has limited himself to titrating the ferrous iron in the resulting solution, while no determination 

was made of any ferric iron which might be present either in the 

solution "or in the precipitate: As neutral ferric sulphate is practically completely precipitated as basic salt on heating its solution, and 

as any ferric salt remaining in solution would inevitably be reduced 

by the sulphides present, it seems reasonably clear that a single experiment with pyrite, in which only the ferrous salt is determined, in 

which no account is taken of the ferric salt supposed to be formed, 

and in which no clear evidence of complete decomposition of the mineral is given, can not have great weight in establishing an important 

hypothesis. While not questioning the correctness of Brown's single 

observation so far as it goes, I have been wholly unable to obtain the 

same results.

 In each experiment cited below the material was finely ground, carefully freed from oxidation products in the manner employed in my 

oxidation experiments (p; 20), and about 0.2 gram heated with 50 

cm. 3 of 10 per cent iron-free CuS04 , 5H20 solution which had been

1 Liebig's Anualen, Vol. XC, 25C. 

" Proc. Am. Philoa. Soc., Vol. XXXIII, 180*, p. 235. Chemical News, Vol. LXX1,1895, p. 179.
42 ON PYRTTE AND MARCASITE.' [BUM, 18(5.

freed from air by boiling out and cooling in carbon dioxide. The 

tubes were filled with carbon dioxide, carefully sealed in a current of 

the same gas, and heated for varying times at 200° C. The filtration 

and titration with permanganate of the resulting solution were also 

made in an atmosphere of carbon dioxide, and the end-reaction, 

which is not very sharp, was determined by comparison with a copper 

sulphate solution of about the same strength.

EXPERIMENTS WITH CUPRIC SULPHATE AND PYRITE.

Experiment 1. Pyrite No. 4. Oxidation coefficient, 60.3. Time of 

heating, ten hours. 

Pyrite taken............. ... '..^.......................^..... 0.2024

Ferrous iron in solution........ ................ .... ........... . 0305

Total iron in solution. .....,. .................... ........... .0314

Total iron in pyrite used ........_.._-.........__.. .......... . 0944

The solution therefore contains 33.3 per cent of the iron, practically 

all in the ferrous condition, against 20 per cent as required by Brown's 

hypothesis. The precipitate was red-brown. In a separate experiment the red substance was found to be left behind on oxidizing the 

sulphide with dilute nitric acid, and proved to be ferric oxide or a 

very basic sulphate. It is easily soluble in hot dilute hydrochloric 

acid, and can thus be separated from the sulphide and determined.

" Experiment 2. Pyrite No. 5. Oxidation coefficient, 60.8. Time of 

heating, eleven hours.

Pyrite taken................................................. .2028

Ferrous iron in solution.................... .................. .0510

Total iron in solution ..........:.............-............... Same.

Total iron in pyrite used................. ....... .. ....... .... .0946

The solution contains 53.9 per cent of the iron, all in the ferrous

state. The ferric oxide in the red-brown precipitate was extracted 

by boiling ten minutes with dilute hydrochloric acid in a current of 

carbon dioxide, whereby some copper also dissolved. The ferric iron 

thus determined was 0.0272 gram. The final residue, oxidized with 

aqua regia, gave much iron, which must have come from undecomposed (or imperfectly decomposed) pyrite. From the sum of the two 

irons above determined it appears that 82.7 per cent of the pyrite was 

decomposed, and that this portion gave 

62.5 per cent ferrous iron.

34.8 per cent ferric iron. '

Required by Brown's hypothesis, 20 per cent ferrous iron. 

Experiment 3. Pyrite No. 5. Time of heating, seven hours.

Pyrite used................ ............................ ...... 0.2027

Ferrous iron in solution............ ....-.".........-........-.. . 0527

Total iron in solution .......>-............. ......---......-.. . 0523

Total iron in pyrite used................. ............_....'. . 0945
STOKES.] ACTION OF CUPRIC SALTS. 43

The solution therefore contains 55.3 per cent of the iron entirely as 

ferrous salt. The ferric oxide, extracted as above from the red-brown 

precipitate, was equivalent to 0.0225 gram iron. The final residue 

contained much iron. From these data it follows that 79.1 per cent 

of the pyrite was decomposed, the dissolved portion giving 

69.9 per cent ferrous iron. 

30.1 per cent ferric iron.

Required by Brown's hypothesis, 20 per cent ferrous iron.

EXPERIMENTS WITH CUPRIC SULPHATE AND MARCASITE.

Experiment 4- MarcasiteNo. 10. Oxidation coefficient, 17.4. Time 

of heating, seven hours. . Marcasite used.'......--.-.------.'.....-.---.------..-.... ... 0.2062

Ferrous iron in solution ................................ ..... . 0559

Total iron in solution ... ............................ .._..._- .0562

Total iron in marcasite used .......... ........................ . 0961

After extracting the red-brown precipitate, as above, the residual sulphide was found to be free from iron, indicating complete decomposition of the marcasite. From these data it follows that the marcasite 

yielded 

58.2 per cent ferrous iron. 

41.8 per cent ferric iron.

Required by Brown's hypothesis, 100 per cent ferrous iron. 

Experiment 5. Marcasite No. 10. Time of heating, fourteen hours.

Marcasite used ....... .... .................................. 0.2096 -

Ferrous iron in solution ............. ........................ . 0651

Total iron in solution ... ........ ............................ Same.

Ferric iron from precipitate..... ...--..-.--................- . 0318

Total iron in marcasite used ......................... ........ . 0977

The precipitate further yielded to hydrochloric acid 0.0626 gram copper, which was present either as cuprous oxide or as metallic copper, 

while the final residue was free from iroai, showing that complete 

decomposition had been effected. The above data give for this 

experiment 

66.6 per cent ferrous iron. 

33.4 per cent ferric iron.

Required by Brown's hypothesis, 100 per cent ferrous iron. The copper sulphide remaining after extraction with hydrochloric acid contained 0.2309 gram copper, from which it follows that two atoms of 

copper are precipitated as sulphide for each atom of iron dissolved. 

This necessarily implies a reduction of the copper to the cuprous condition, and a corresponding oxidation of a portion of the ferrous salt 

or of the sulphur, or both. That cupric salts can accomplish both of 

these results is shown in the two following experiments.
44 ON PYRITE AND MARCASITE. [HUM, 180. 

EXPERIMENT WITH CUPRIC CHLORIDE AND PYRITE.

Experiment 6.- 0. 2545 gram pyrite was heated with 50 cm. 3 of a pure 

cupric chloride solution containing as much copper as the sulphate used 

in the foregoing experiments, the tube bein'g filled with the usual 

precautions to prevent oxidation. The tube was heated five hours at 

200° and on cooling was found to contain undecomposed mineral and 

an abundant crystallization of cuprous chloride. The filtered solution gave 

0.0667 gram iron=0.1429 gram pyrite. 

0.5540 gram BaSO4 =0.1427 gram pyrite.

It appeai-s, therefore, that as far as it was decomposed the pyrite was 

completely oxidized to ferrous salt and sulphuric acid, with a corresponding reduction of the copper from the cupric to the cuprous 

state, according to the equation

I have further obtained an abundance of sulphuric acid by oxidizing 

precipitated copper sulphide with cupric chloride solution at 100°, air 

being rigorously excluded. Doubtless in the above experiments with 

pyrite and marcasite and copper sulphate an analogous oxidation 

occurs, being obscured by other reactions. Chalcocite, however, is 

not perceptibly attacked by 10 per cent copper sulphate solution 

after fourteen hours' heating at 200°.

EXPERIMENT WITH FERROUS AND CUPRIC SULPHATES.

Experiment 7. A mixture of equal volumes of 10 per cent solutions 

of crystallized copper sulphate and crystallized ferrous sulphate (carefully freed from ferric salt by reduction with hydrogen sulj>hide) was 

heated, under absolute exclusion of air, for three hours at 200°. A 

considerable red precipitate was obtained, which consisted of a mixture of ferric oxide or basic sulphate with cuprous oxide and apparently metallic copper. The reduction of cupric by ferrous salts in 

neutral or acid solution is frequently referred to in the literature, but, 

so far as I can ascertain, without experimental basis.

Theory of the reaction. The explanation of the result is probably 

the following : In the mixture we have equilibrium between the cupric, 

cuprous, ferrous, and ferric ions and copper, as represented by the 

equations

Cu"+Fe"£Cu'+Fe'" 

Cu"+2Fe"£Cu+2Fe'"

in which but traces of Cu, Cu', and Fe'" exist. 1 On heating the trace 

of ferric salt is precipitated in the basic form, the equilibrium is dis1 The slight solubility of cuprous compounds shows that not more thau traces can be present.
STOKKH.] ACTION OF CUPRIC SALTS. 45

turbed, and the reaction continues to proceed from left to right, 

accompanied by formation of Cu' and Fe'" (and possibly Cu) and their 

precipitation as insoluble products, until, the accumulated acid is in 

equilibrium with both of these and further precipitation can not 

occur. The process is further complicated by a partial precipitation 

of copper as basic salt when the sulphate is used. A mixture of cupric 

and ferrous salts gives a strong reaction for ferric iron with sulphocyanate. This does not imply an appreciable oxidation of the ferrous 

iron in the mixture itself, but a shifting of the equilibrium to the 

right in the above equation through the formation of the slightly 

dissociated ferric sulphocyanate. It is doubtless to the reduction of 

the cupric sulphate by sulphur and by ferrous sulphate that the formation of chalcocite from, pyrite is to be ascribed.

GENERAL CONCLUSIONS FROM THE EXPERIMENTS.

The conclusions which may be drawn from the above experimental 

data are:

.1. At 200° copper sulphate decomposes marcasite more rapidly than 

pyrite.

2. Both ferrous and ferric salts are formed in both cases, the former 

being found wholly in the solution, the latter wholly in the precipitate. There is no marked difference in the relative amounts of these 

in either case. The precipitate also contains, besides cuprous sulphide, cuprous oxide and perhaps metallic copper.

3. The formation of ferric salt is to be ascribed, in part at least, to 

the reaction of the ferrous salt upon the cupric salt (Experiment 7), 

while some oxidation of sulphur to sulphuric acid occurs (Experiment 6). Any ferric salt which may fail to be precipitated will be 

reduced by the sulphides.

4. The relative amounts of ferrous and ferric salts depend simply 

upon the establishment of equilibrium between the solution and the 

products of decomposition of the pyrite or marcasite, and not upon 

any fundamental chemical difference in the two minerals, and consequently the hypothesis of Brown is devoid of valid experimental 

basis.

It would seem that no method can be depended upon to give us an 

insight into the state of combination or valency of either iron or 

copper in their sulphides which does not take due account of the conditions of equilibrium between the reagents and the decomposition 

products. This applies not only to other experiments of Brown quoted 

in his paper, but to the attempt of Starke, Shock, and Smith 1 to prove 

the divalent condition of iron in arsenopyrite and of Morgan and 

Smith 2 to establish the same for chalcopyrite. That the product of 

heating chalcopyrite in dry hydrochloric gas consists of a mixture of 

cupric chloride and ferrous chloride is inconclusive on the question as

1 Jour. Am. Chem. Soc., Vol. XIX, 1897, p. 948.

2 Ibid., Vol. XXIII, 1901, p. 107.
46 ON" PYRITE AND MARCASITE. [HULL. 186.

to whether chalcopyrite has a constitution represented by (Cu"Fe")S2 

or (Cii'Fe'")Sa until the condition of the system CuCH-FeCl82CnC.la + 

FeCl 2 in equilibrium is known, in the absence of water and presence 

of hydrochloric acid gas. That nearly all the iron was obtained in 

the ferrous state in some of the experiments with copper sulphate 

made by Brown and others I can explain only on the assumption that 

enough acid was originally present or eventually formed to enable 

any basic ferric salt to act on and be reduced by the residual sulphides and cuprous oxide. In fact, hot dilute sulphuric acid slowly 

dissolves the mixture of cuprous oxide and basic ferric salt obtained 

in niy experiments, giving a strongly reducing solution.

XIII. OXIDATION OF PYRITE AND MARCASITE BY POTASSIUM

PERMANGANATE. 1

A few experiments were made to determine the percentage of sulphur oxidized in pyrite and marcasite at ordinary temperature by 

acidified permanganate, the mineral being always in excess.

An excess of the powdered mineral, freed from oxidation products, 

was shaken in a stoppered cylinder with a constant amount of dilute 

sulphuric acid and 100 cm. 3 KMn04 , added in quantities of 5 cm. 3 at 

a time, the shaking being continued after each addition until the 

color had disappeared, which required but a few minutes. The filtered solution was then reduced and the iron titrated by the same 

permanganate solution.

The proportions required are 

For complete oxidation to Fe2 (SO4 ) 3 and sulphuric acid ...... 5 FeS2 -t-15 KMnO4

For oxidation of the iron alone to Fe2 (SO4 ) 8 >...__..... ...... 5 FeS2 -i- 3 KMnO4

For oxidation of- the sulphur ..... N ............................. ' 12 KMnO4

Let ' ,

a=volume of permanganate used in oxidation of FeS2 ,

and

Z>=volume of permanganate required to reoxidize FeO to Fe._,O3 ;

then

3&=volume of permanganate used in oxidizing Fe in FeS2 to Fe.O3 ,

and

a-'3b volume of permanganate used in oxidizing S in FeS2 to SO3 ;

also

12&;=volume of permanganate required to oxidize all the sulphur in the

mineral decomposed; 

whence

T> , ,11. -vi 100(0-36) 8.338a Percentage ot sulphur oxidized =p= ^ - , 

Where 190 cm. 3 permanganate are used,

833.3 _

1 For previous experiments on the action of permanganate on pyrite and marcasite, see A. P. 

Brown, Proc. Am. Philos. Soc., Vol. XXXIII, 1894, and Chemical News, Vol. LXXI, Ib95, pp. 131,155, 

171. The experiments of Brown show that with an excess of pei-niaugauate the total sulphur 

oxidized in a given time is greater in the case of murcasite than of pyrite, but they do not establish any relation between the amount of sulphur oxidized and the quantity of mineral decomposed.
STOKES.] SUMMARY OF RESULTS. 47

The values ofc' p obtained were:

i 3 

Pyrite................................................... 94.0 90.7

Marcasite ............................................... 84.0 t'G. 7

The duplicate results are therefore not sufficiently concordant, nor 

is the difference between pyrite and marcasite great enough, to give 

the method more than a confirmatory value. The difference here, as 

iu the case of oxidation by the ferric-sulphate method, is due to the 

greater ease with which marcasite is attacked, but the relative oxidation rate of the sulphur is clearly much greater. The difference might 

possibly be greater at 100°, but here the experiments failed because 

of the strong tendency of the permanganate to deposit peroxide, even 

in the presence of much sulphuric acid.

XIV. SUMMARY OF RESULTS.

The chief results of this investigation may be thus summarized:

1. When pyrite or marcasite is boiled with an excess of a solution 

of ferric salt to complete reduction of the latter the ratio of sulphur 

oxidized to mineral decomposed is perfectly definite and characteristic 

of each mineral, provided certain standard and easily controllable 

conditions are observed. Under these conditions the percentage of 

sulphur oxidized in pyrite is about 60.4 per cent and in marcasite 

about 18 per cent of the total sulphur. These figures are the characteristic oxidation coefficients.

2. The oxidation of pyrite or marcasite to ferrous salt, sulphuric 

acid, and free sulphur can not be expressed by any single equation, 

but takes place according to two or more;, the relation between which 

varies with the special conditions.

3. An empiric.il curve for the oxidation coefficients of mixtures of 

pyrite and marcasite is constructed by aid of which the composition 

of naturally occurring mixtures may be quantitatively determined.

4. The influence of various impurities on the results is described.

5 Various concretions and other specimens are examined, arid it is 

shown that in many cases much uncertainty exists in distinguishing 

pyrite and marcasite by the usual .methods.

G. There is no well-established evidence of the existence of true 

paramorphs of pyrite after marcasite or of marcasite after pyrite.

7. The hypothesis that most natural specimens, even when well crystallized, are intimate mixtures of the two is without foundation.

8. Specimens crystallizing in the regular system are true pyrite, 

while those forming rhombic crystals are true marcasite. When the 

two are mingled or intergrown it is generally possible to distinguish 

each by the color after cleaning with acid, a rule to which there are 

a few exceptions.

9. The density does not afford a trustworthy means of determining 

one mineral in the presence of the other. 

Bull. 186 01 4
48 ON PYRITE AND MAKCASITE. . [BULL 186.

10. Chalcopyrite maybe sharply distinguished'.from chalcocite or 

boruite in py rite-carry ing 3 per cent copper or less. A method is given 

for detecting small amounts of chalcopyrite in pyrite and marcasite, 

or the reverse, or in rocks.

11. There is no evidence as t;o the state of combination or valency 

of iron in pyrite and marcasite, or that these differ in the two minerals. 

Experiments which have been regarded as proving the existence of 

such differences are inconclusive.

12. The principle of the oxidation method, with appropriate modifications, is probably capable of wider application in distinguishing 

dimorphous minerals, in determining whether a given complex mineral 

is a mixture or compound of its simpler constituents, and in determining the nature of small amounts of impurities.

13. The action of acid permanganate on pyrite and marcasite'is analogous to that of ferric salts, the percentage of sulphur oxidized in 

pyrite being higher than in marcasite.

My thanks are due to Dr. A. A. Julien, whose valuable assistance 

has been noted above,' and to Professors Van Hise, Emerson, Merrill, 

and Penfield, and Messrs. Tassin, Eminons, Ransome, Weed, and 

Lindgren, who have generously supplied most of the material upon 

which the forea'oiuy: investigation is based.
INDEX.

Page. 

Apparatus for oxidizing sulphides with

ferric solution, figure showing... 22 

Apparatus for washing sulphides in an 

atmosphere of carbon dioxide, figure showing ..I.................... 21

Arsenides, application of method to...... 20

Arsenopyrite,influence of presence cf... 82-33 

Bastrop, Tex., composition of specimen

from ............................... 89

Becker, George F., letter of transmittalby !) 

Bohemia, composition of specimen f rom.. 30,37 

Bornite, influence of presence of ......... 33

Brown, A. P., cited on action of cupric

salts on pyrite and inarcasite..... 14,41

cited on action of potassium permanganate ou pyrito and rnarcasite... 46 

Chalcopyrite, influence of presence of. 33-34,35 

Chalcocite,influence of presence of ...... 33,34

Cobalt,influence of presence of........... 32

Color of pyrite and inarcasite ............ 11-12

Colorado, composition of specimens from. 25,

3(5,38 

Concentration, influence of ............... 18

Connecticut, composition of specimens

from ............................... 25,39

Copper-bearing pyrites, form of copper

in................................... 34-35

Cornwall, England, determination of

marcasite from.................... 20,27

Crystalline form of pyrite and niarcasite. 11 

Cumberland, England, composition of

specimen from .................... 39

Cupric chloride, experiments with....... 44

Cupric salts, action of..................... 41-40

Cupric sulphate, experiments with.... 42-43,44

Dana, J.D., cited on color and density of

pyrite and marcasite.............. 11,12

Density of pyrite and marcasite.... ..j... 12-13

Dimorphous compounds, application of

method to....._................... 20

Dover Cliffs, England, determination of

marcasite from. . ................ 20

Emerson,B.K.,acknowledgments to..... 48

Euimous, S. F., acknowledgments to...... 48

England, composition of specimens from. 20,39 

Experimental details...................... 20-24

Ferric solutions,action of................. 14-20

Ferric sulphate oxidation method, description of ...._.........-..-.--..:. 15-10

Ferrous sulphate, experiments with ..... 44

Folkstoue, England, composition of specimens from ...... ................... 30

Franklin, N. J., composition of specimen

from................................ .39

Page. 

Galena, influence of presence of .......... 32

Galena, HI., composition of specimens

from................................ 30,39

Gmelin-Kraut, cited on action of hydrogen sulphide on ferric salts ....... 19

Hazel Green, Wis., determination of marcasite from......................... 26

Hematite, indication of presence of ...... 32

Idaho, composition of specimens troin. '.. . l!(i, 37 

Illinois, composition of specimen from.... 39

Impurities, influence of.................... 81

Julien, A. A., acknowledgments to........ i{5,48-

cited on action of ferric chloride on

marcasito .......................... 15

cited on density of pyrite and marcasite ................................. 12

cited ou terms and references pertaining to pyrite and marcasite ....... 11

composition of specimens loaned by .. 39 

hypothesis of .......................... 39-40

quoted on composition of certain specimens of pyrite-niarcasite......... 39

quoted 011 features of inarcasite from

Dover Cliffs, England.............. 20

quoted on origin and character of

mixtures of pyrite and marcasite- 27-28 

Koninck, L. L. de, cited on action of ferric

chloride on pyrite............ ^...... 15

Leadville, Colo., determination of pyrite

from................................ 25

Lindgren, Waldemar, quoted on features

of specimens of pyrite-rnarcasito . 30,37 

Magnetite,indications of presence of.-... 32 

Marcasite,action of copper sulphate on.. 14 

amount of, oxidized by potassium permanganate....... ................... 40

attempted conversion of pyrito to ... 41 

color of ................................ 11-12

crystalline form of ........._........ 11

data obtained for...................... 25-27

density of.............................. 12-13

oxidation of...................._..... 13-14

oxidation coefficient of ............... 27

Marcasite and pyrite,reasons for different

behavior of, in experiments made. 19 

uncertainty of methods of distinguishing .... ............_..... 1.1-14

Ma'ssa -htisetts, composition of specimen

from ...........'.................... 39

Merrill,G.P.,acknowledgments to...._ 48

determination of specimen submitted

by.................................. 37

Monroe, Conn., composition of specimen

from ............................... 39

49
50 INDEX.

Page.

Morgan and Smith, cited on divalent condition of iron in chalcopyrite..... 45

Missouri, determination of marcasite

from ............. ........ .......... 26

Mixtures of pyrite and murcasite, curve

showing oxidation coefficients of. 30 

data obtained for...................... 27-31

New Jersey, composition of specimen

from ........1.......- ..- - 39

Nickel, influence of presence of .......... 32

Oregon, composition of specimens from . 36 

Ostwald, W., cited on increased solubility

due to pulverizing ................ 17

Oxidation of pyrite and marcasite, conditions of .............. ...... 13-14

by ferric sulphate, method of .... 15-l(i

Oxidation coefficients, marcasite ......... 27

pyrite......--...............----......- 25

pyrite and marcasite, statement of .. 17 

mixtures of pyrite and marcasite,

curve showing................. 30

determinations of................. 28-29

Paramorphism, observations on...... .... 40-41

Penfleld, S.L., acknowledgments to...... 48

cited on method of distinguishing

pyrite and marcasite--............ 14

cited on type of pyrite-marcasite

from Folkestone, England........ 38

Potassium permanganate, oxidation of

pyrite and marcasite by ... .... 46-47

Pyrite, action of copper sulphate on..... 14

action of ferric solutions on ... .... 14-20

color of................-....-.....--. 11-12

crystalline form of... .............. 11

data obtained for...................... 25

density of.............................. 12-13

determination of small amounts of... 30 

oxidation of......................... 13-14,40

oxidation coefficient of................ 25

impurities in, influence of ............ 31-34

Pyrite and marcasite, data obtained for

mixtures of........................ 27-31

oxidation coefficient of................ 17

preparation of, for tests.............. 20-23

reasons for different behavior of,

toward ferric solutions ........... 19

Page. 

Pyrite and marcasite, uncertainty of

methods of distinguishing ........ 11-14

Raininelsberg, (J. F., cited on density of

pyrite and marcasite...... ...... 12

Rausome, F. L., acknowledgments to.... 48

Rio Tinto, Spain, composition of specimen from...--.-.....--,........... 36,38

Roxbury, Conn., determination of pyrite

from --.........._-..........__..__. "25

Sadebeck, A., cited on marcasite from

Littmitz, Bohemia ................ 37

Somerville, Mass., composition of specimen from.......................... 39

South Dakota, composition of specimen

from ............................... 36,37

Spain, composition of specimen from.... 36,38

Starke, Shock,and Smith cited on divalent

condition of iron in arsenopyrite. 45 

Stokes, H. N., cited on density of pyrite

and marcasite ...................... 12

Sulphides, apparatus for oxidizing, figure

showing............................ 22

apparatus for, washing, figure showing ................................. 21

Sulphur oxidized in ferric solutions acting upon pyrite, amount of..._. 15 

deduction of expression for .......... 16-17

Tassin, Wirt, acknowledgments to....... 48

Temperature, influence of................ 18-19

Temperature and strength of standard

solution ._...;.-.......... . t ........ 17

Texas, composition of specimen from.... 39

Utah, determination of pyrite from ..... 25

Van Hise, C. R., acknowledgments to.... 48

Vogt, J. H. L., cited on action of ferric

- chloride on pyrite................. 15

Weardale, England, determination of

marcasite from.................... 26,27

Webb City, Mo., determination of marcasite from........................ 26,27

Weed, W. H., acknowledgments to ...... 48

Wisconsin, composition of specimen from. 36,38 

Wisconsin, determinations of marcasite

from ............................... 26

Wo'hler, F., cited on attempted conversion of marcasite into pyrite ..... 41

o