DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

BULLETIN 548

ELECTRIC ACTIVITY IN 

ORE DEPOSITS

BY

ROGER C. WELLS

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1914

CONTENTS.

Preface, by George Otis Smith............................................. 5

Introduction.............................................................. ' 7

General discussion of the phenomena...........:................-............ 9

Effect of various solutions on the potentials shown by minerals.......... 9

Availability of the combinations to furnish current...................... 12

Electric currents in the earth................'........................... 19

Effect of a current from solution to mineral............................. 21

Method of experiment..........................................'....' 21

Effects at the cathode with moderate currents........................ 21

Effects at the cathode with feeble currents............,......;....... 22

Effect of a current from mineral to solution............................. 23

Electric conductivity of minerals...................................... 24

Detailed study of various potentials......................................... 26

General results of the measurements..................................... 26

Measurements of potential assumed by certain minerals.................. 30

. - Pyrolusite......................................................... 30

Pyrite............................................................ 32

Marcasite.......................................................... 39

Pyrrhotite. ...................................:.................: 41

Magnetite........................................................ 41'

Galena........................................................... 42

Silver sulphide................................................... 44

Bismuth sulphide................................................. 44

Mercuric sulphide................................................. 45

Nickel and other sulphides........................................ 45

Electromotive behavior of soluble sulphides............................ 45

Reducing power of soluble sulphides................................ 45

Relation between potential and concentration....................... 47

Preparation of sodium hydrosulphide............................... . 48

Chemistry of the sulphide electrode................................. 49

Behavior of polysulphides........................ . ................ 51

Effect of dilution.................................................. 51

Conclusions.............-..............'........................... 54

Correlation of the measurements of potential............................. 54

Application to ore deposition............................................... 62

General conditions.........................;.......................... 62

Deposition of ores..................................................... 64

Solution of ores............................................. I......... 69

Summary................................................................ 72

Index................................................................... 75
ILLUSTRATIONS.

Page.

FIGURE 1. Apparatus for studying currents produced by combinations of solutions and minerals.............................................. 13

2. Method of supplying fresh solution to mineral electrodes without

affording access of air........................................... 27

3. Half cell of mineral suspended in a solution for observation over a

long period............ . ....................................... 29

4. Apparatus for boiling out au\under reduced pressure before measuring electromotive force....................................... 35

5. Curves showing rate of polarization............................... 37

6. Mounting of p.yrite to obtain a saturated solution over a long period

without access of air........................................... 38

7. Single potentials of oxygen and hydrogen electrodes in water...... 58

4 . '
PREFACE.

By GEORGE OTIS SMITH.

Until the last few years the contributions to the study of ore deposits by members of the United States Geological Survey have been 

primarily geologic. Close observation of the facts of geologic occurrence and mineral association has been the rule, less attention having 

been given to the chemical and physical questions involved in problems of ore deposition. Notable exceptions have been studies by 

G.. F. Becker and Carl Barus 1 of electric activity in the Comstock 

lode and the lodes of Eureka, Nev., the chemical investigation by 

Becker and Melville of solution and precipitation of metallic sulphides,2 

studies by C. R. Van Hise and C. K. Leith 3 of the chemistry of ironore deposition, and the work of Waldemar Lindgren 4 on the relations 

between ore deposition and physical conditions. Recently, however, unusual interest has been awakened in the geochemical and 

geophysical phases of the problem. This interest has manifested 

itself in such work as that of Victor Lehner,5 .of the University of 

Wisconsin, and A. D. Brokaw, 6 of the University of Chicago, on the 

solution and deposition of gold, and in studies begun by L. C. Graton, 

of Harvard University, and his associates, of the enrichment of 

copper ores.

Among the contributions already made by the United States Geological Survey are the published results of studies by W. H. Emmons7 

of the general subject of enrichment, and of investigations now being 

carried on by E. S. Bastin 8 and Chase Palmer of the enrichment of 

silver ores as exemplified in our western mining camps. The work 

reported in the present paper on electric activity in ore deposits elaborates and extends the work of Gottschalk and Buehler."

1 Geology of the Comstock lode and the Washoe district: U. S. Geol. Survey Mon. 3, pp. 309-367,1882. 

.» Geology of the quicksilver deposits of the Pacific slope: U. S. Geol. Survey Mon. 13, chap. 15,1888. 

»Summarized in U. S. Geol. Survey Mon. 52, pp. 518-545,1911.

* Ore deposition and deep mining: Econ. Geology, vol. 1, pp. 34-46,1905; The relation of ore deposition 

to physical conditions: Idem, vol. 2, pp. 105-127,1907.

* The transportation and deposition of gold in nature: Econ. Geology, vol. 7, pp. 744-750,1912. 

«The secondary precipitation of gold in ore bodies: Jour. Geology, vol. 21, pp. 251-267,1913.

* The enrichment of sulphide ores: U. S. Geol. Survey Bull. 529,1913.

8 Metasomatism in downward sulphide enrichment: Econ. Geology, vol. 8, pp. 51-63, 1913; Metallic 

minerals as precipitants of silver and gold: Idem, pp. 140-170,1913.

* Econ. Geology, vol. 5, pp. 28-35,1910; vol. 7, pp. 15-34,1912.

5
6 ELECTRIC ACTIVITY IN ORE DEPOSITS.

It is interesting to note that this bulletin by Mr. Wells relates to 

a subject mentioned by Becker 1 in his first administrative report to 

Director King as deserving study, a suggestion that was followed by 

the investigation by Barus. The electromotive forces detected in the 

present detailed quantitative laboratory studies of the activity of 

various metalliferous minerals in various solutions show few intensities 

as great as one volt, though some of them are many times greater 

than the largest electromotive forces actually found in ore deposits 

by the earlier observers. Moreover, the effects of difference in 

solutions are found to be greater than those due to mineralogic 

differences.

It should be emphasized that the results thus far obtained afford 

no adequate basis for any method of electric prospecting nor any 

promise of the development of such a method by connecting the 

presence of ore deposits with readily or definitely measurable electric 

activity. Nevertheless, the data here presented are believed to 

possess value in the broader investigation of ores, for even feeble 

currents might exert a directional influence on ore deposition, and 

chemical conditions, even at a distance, might be a factor in determining mineral association.

i U. S. Geol. Survey First Ann. Kept., p. 46,1880.
ELECTRIC ACTIVITY IN ORE DEPOSITS.

By ROGER C. WELLS.

INTRODUCTION.

As long ago as 1830 R. W. Fox called attention to electric activity 

in ore deposits.1 Such currents, as he was able to detect seemed to 

have no relation to the points of the compass, but appeared to be 

due to connections existing between different bodies of ore, or between different parts of the same body. In one mine the OP6 appeared 

to be increasingly negative with depth, a fact which he suggested 

might be dependent on temperature. His original paper contained 

a table showing the order of the electric conductivity of about 

twenty minerals and mineral combinations. In a later paper 2 he 

took pains to show that certain ores may act like metals in galvanic 

combinations, and his principal conclusions are summed up in the 

statement that the electric phenomena in veins "bear a striking 

resemblance to galvanic combinations." From 1830 to 1844 discussion of these points was carried on by Henwood 3 and Fox 4 in 

England and by Von Strombeck 5 and Reich 8 in Germany without 

important advances.

The subject was considered from a somewhat different point of 

view in 1870 by W. Skey.7 Whereas the preceding investigators had 

sought for currents over large areas, Skey confined his observations 

to laboratory experiments on single minerals. He enlarged the 

known list of conducting minerals and determined the direction of

i Fox, R. W., On the electromagnetic properties of metalliferous veins in the mines of Cornwall: Philos. 

Trans.;i830,.pt. 2, p. 399.

a Fox, R. W., Note on the electric relations of certain metals and metalliferous minerals: Philos. Trans., 

1835, pt. 1, p. 39.

s Henwood, W. J., Sur les courants dlectriques observes dans les filons de Cornouailles: Annales des mines, 

3d ser., vol. 11, p. 585,1837.

< Fox, R. W., Account of some experiments on the electricity of the Huel Jewel mine: British -Assoc. 

Adv. Sci. Rept., vol. 3, p. 572,1834; Report on some experiments on the electricity of metallic veins, etc.; 

Idem, vol. 6, p. 133,1837; Some experiments on subterranean electricity, made at Pennance mine, near 

Falmouth: Philos, Mag., 3d ser., vol. 23, pp. 457,491,1843.

6 Strombeck, A. von, Ueber die von Herrn Fox angestellten Untersuchungen in Bezug auf die elektromagnetischen Aeusserungen der Metallgilnge: Karsten's Archiv, vol. 6, p. 431,1833.

6 Reich, F., Notiz iiber elektrische Stro'me auf Erzgangen: Poggendorff's Annalen, vol. 48, p. 287,1839; 

Versuche iiber die Aufsuchung von Erzen mittelst des Schweiger'schen Multiplicators: Berg- und hiittenmann'sche Zeitung, vol 3, pp. 342,386,1844.

' Skey, W., On the electromotive power of metallic sulphides: New Zealand Inst. Trans. and Proc., vol. 

3, pp. 232-236.1871.
8 ELECTRIC ACTIVITY IN ORE DEPOSITS.

the current when conducting minerals in contact with solutions are 

connected by a wire: Besides pointing out anew that conducting 

minerals are capable of forming the electrodes of galvanic batteries 

he called attention to the accelerating or retarding action of one 

mineral on another in chemical changes action due to electric 

activity.

In 1880, at the instance of G. F. Becker, Carl Barus 1 investigated 

the electric activity of the Comstock lode and of the ore deposits at 

Eureka, Nev. Although he followed the experimental methods of 

Fox, Barus appears to have purposely avoided contact with metalliferous minerals. He concluded, from the measurements made at the 

Comstock lode 2 "that the electromotive forces due purely to chemical 

difference and polarization of the terminals are of the same order as 

the data expressing the electric activity of the lode." At Eureka 

the potentials of 21 points were measured against a single point of 

reference with terminals particularly designed to make a good contact 

with the solutions in the rocks. The maximum potential above the 

point of reference was 0.018 volt and the maximum below 0.093 volt. 

"In the words of Barus: 3 "On reviewing the results described it is 

strikingly evident that the electromotive forces met with are invariably small, very frequently, indeed, quite at the limit of the accurately measurable." Electric prospecting, therefore, appeared to 

Barus impracticable, but he adds: 4

It will be desirable to carry out Fox's original idea, namely, of investigating the 

electrical properties of ores and minerals of the heavy metals * * *. The knowledge we possess of the conductivity and the position of ores in the electrical scale is 

largely the result of experiments made a long time ago. Recent observers have made 

but few quantitative additions, and even these probably from improperly chosen 

methods are frequently discordant.

In 1891 Braun 5 clearly proved that certain phenomena attending 

the formation of sulphides and the deposition of copper in capillary 

spaces, previously noted by A. C. Becquerel, are of an electrochemical 

nature, as Becquerel had in fact suggested.6 These phenomena are 

partly dependent, according to Ostwald,7 on the semipermeability of 

precipitated membranes. It is not possible in this paper, however, 

to discuss the complications that would be introduced into the question by so extending it as to consider capillary spaces and semipermeable membranes.

1 Becker, G. F., Geology of the Comstock lode and the Washoe district: TJ. S. Geol. Survey Mon. 3, pp. 

309-367 (chap. 10, On the electrical activity of ore bodies, by Carl Barus), 1882.

a Idem, p. 322.

»Idem, p. 365.

«Idem, p. 366.

6 Braun, F., Electrocapillare Reactionen: Annalen der Physik und Chemie, vol. 44, p. 507,1891.

6 Becquerel, A. C., Sur des nouveaux eflets chimiques produits dans les actionscapillaires: Compt. rend., 

vol. 64, pp. 919-924, 1867; vol. 65, pp. 51-60, 720-729, 1867; vol. 66, pp. 77-81, 245-247, 766-924, 1066-1072, 

1868; particularly vol. 65, p. 51, 1868.

i Ostwald, W., Elektrische Eigenschaften halbdurchlassiger Scheidewande: Zeitschr.physikal. Chemie, 

vol. 6, p. 75,1890. ' .
GENERAL DISCUSSION OF PHENOMENA. 9

Experiments along the line indicated by Bams were made in 1897 

by Bernfeld, 1 who studied the electric behavior of galena particularly, and more recently by Gottschalk and Buehler, who had previously shown that the oxidation and solution of certain natural 

sulphides are accelerated under certain conditions by the presence 

of pyrite or marcasite.2 In explanation of this action E. T. Alien 3 

and I 4 separately ventured to express the opinion that it might be 

due to* the production of sulphuric acid by the pyrite and marcasite. 

Soon afterward, in another paper, Gottschalk and Buehler pointed 

out once more that there may be electric action between different 

sulphides in contact; further, that marcasite and pyrite, which are 

electrically positive to stibnite and sphalerite when in moist contact 

with them, are in fact themselves somewhat protected from oxidation 

by the complementary action of the more oxidizable sulphides. They 

accordingly ascribed the chemical effects observed by them partly to 

electric action, and presented a list of conducting minerals and a 

table giving the electromotive forces shown by several minerals with 

respect to copper, water serving as electrolyte.

This explanation of the alteration.of ores by electrolysis is similar 

to the electrolytic theory of the corrosion of iron and steel and of the 

zinc of zinc plate. In view of-the importance of the subject it has 

seemed desirable to extend the data, not only to correlate field results 

with laboratory experiments, but also to elucidate the effect of various 

solutions on the potentials and to harmonize the whole subject with 

modern theories, such as those of electricity and solution. The data 

presented in this bulletin greatly enlarge the scope of possible investigation. It is well realized that more detailed experimental work 

is both desirable and necessary, but it has seemed best to set forth at 

this time what has already been done.

GENERAL DISCUSSION OF THE PHENOMENA.

EFFECT OF VARIOUS SOLUTIONS ON THE POTENTIALS SHOWN BY

MINERALS.

The potentials of different minerals as presented by Gottschalk and 

Buehler were determined by using water as the electrolyte and metallic copper as the second electrode. 5 Their opinion was that the 

electrolytic action of the sulphides "would be analogous in every respect to the action of metals." But although it is well known that the 

potential shown by a metal as electrode depends on the concentration 

of the metallic ion in the solution in contact with it, Gottschalk and 

Buehler presented no data on the effect of variation in the solution.

1 Bernfeld, I., Studien iiber Schwefelmetallelektroden: Zeitschr. physikal. Chemie, vol. 25, p. 40, 1898.

2 Buehler, H. A., and Gottschalk, V. H., Oxidation of sulphides: Econ. Geology, vol. 5, p. 28,1910.

» Econ. Geology, vol. 5, p. 387,1910.

< Idem, p. 480.

* Gottschalk, V. H., and Buehler, H. A., Econ. Geology, vol. 7, p. 31,3912.
10 ELECTRIC ACTIVITY IN OEE DEPOSITS.

This was therefore the first subject to be investigated. A few measurements soon showed that not only do different minerals employed 

as electrodes exhibit different potentials in a given solution but also 

that the potentials shown by most minerals, certainly the initial values, depend to a marked degree on the nature of the solutions in contact with the minerals. The variation shown by a mineral in passing 

from an acid to an alkaline solution is in fact generally greater than 

the differences shown by diverse minerals in the same solution'. The 

potentials also depend on the oxidizing or reducing nature of the 

solutions. In general, acid and oxidizing solutions give the highest 

potentials, alkaline and reducing solutions the lowest.

A most significant fact for the elucidation of these phenomena is 

" that any unattackable electrode, such as a piece of smooth platinum, 

shows somewhat similar behavior in the various solutions. The 

table below gives a few single potential measurements which illustrate 

this point. The solutions were approximately normal and the sign 

is that assumed by the electrode with reference to the normal calomel 

electrode as +0.560 volt, the measurements being made soon after 

the specimens were placed in the solutions. Of course neither equilibrium nor constancy was wholly obtained under these circumstances 

in the time allowed.

Effect of various solutions on the potential, in volts.

Pyrite.

+1.10

+ .86

+ .72

+ .38

- .17

Galena.

+0.80

o+ .49

+ .49

<H- .16

- .22

Magnetite.

+0.91

+ .88

+ .68

j- 11

- .14

Pyrrhotite.

+0.97

+ .89

+ .56

a+ .13

a- .14

Platinum.

+1.11

4- 97

+ .76

+ .38

- .26

a Mineral appreciably attacked, yielding an indefinite value.

On considering these results a number of important questions at 

once arise. Is it possible to frame a consistent explanation of all the 

values ? How constant and reproducible are they ? Which of them, 

if any, are capable of furnishing noteworthy currents for electrolytic 

action ? Is such action possible in ore deposits ? If so, what are the 

effects of electrolytic action on various minerals ? I shall attempt to 

give answers to these questions by discussing known facts as well 

as by presenting new experimental evidence on the subject.

It may be said at once that most of these potentials are reproducible to tenths of a volt and some to hundredths. Our knowledge of 

the behavior of various electrodes would lead us to expect, however, 

that the products formed by solution of the minerals would have an 

effect on the potentials. To yield significant potentials the solutions
GENEBAL DISCUSSION OP PHENOMENA. 11

should contain definite concentrations of the possible reacting substances, but as the concentrations could not be regulated by the 

method of experimentation used above the values are simply.illustrative and have no exact quantitative significance.

The measurements are suggestive, however, because they represent temporary stages in slow chemical adjustments. Some of these 

adjustments occur very slowly, so slowly that fairly constant potentials are soon obtained; others occur more rapidly and the reaction 

products cause a changing potential. Measurements of electromotive force may be used to indicate the direction and intensity of a 

given chemical action and generally furnish.such indications with 

as great accuracy as chemical experimentation. Moreover, such 

measurements may be made quickly.

The potentials shown by the minerals and the platinum as indicated in the preceding table evidently have something in common

and are affected in a similar way by the nature of the solutions. The 

variations shown seem to be characteristic of the potentials ordinarily termed "oxidation and reduction" potentials. When the 

electrode appears positive the usual assumption is that positive 

electricity has passed'from some ion in the solution to the conductor (platinum ordinarily), or, what amounts to the same thing, that 

.negative electricity has passed from the electrode to the solution. 

The ferric ions present in a ferric salt solution, for example, are capable of acting as oxidizers that is, of parting with a portion of their 

electrification and thereby becoming converted into ferrous ions. 

The electric potential measures the tendency of this chemical process 

to occur. Evidently this general effect is shown even with the mineral electrodes in the above measurements, but with the minerals 

there is the added possibility that their constituents may ionize and 

carry electric charges with them into the solution as they dissolve. 

Considering the similar effects shown by the minerals and by platinum, however, it must be said that the potentials indicate in a general way the order of the oxidizing power of the solutions.

For further measurements of such potentials one may refer to the 

experiments made by Bancroft on a large number of oxidizing and 

reducing solutions.1 In 1898 R. Peters showed that the value of the 

ferric-ferrous potential is dependent on the concentrations of both 

ferric and ferrous salt, and succeeding investigators have found that 

such potentials are much more definite when the salts of both valencies are present in about equal amounts. The present custom is to 

take as the normal oxidation and reduction potential the value shown 

when the two ions concerned, if these can be determined, are present

i Bancroft, W. D., Ueber Oxydationsketten: Zeitschr. physikal. Chemie, vol. 10, p. 387,1892.
12 ELECTRIC ACTIVITY IN ORE DEPOSITS.

in equal concentration. The single ferric-ferrous potential, for 

example, can be represented by the following equation : x

["FV+++1 

E= 1.016 + 0.059 lor

in which E is the potential in volts and the bracketed symbols stand 

for concentrations of the respective ions. It can be seen that at 

equal concentrations of ferric and ferrous ions the second term vanishes. The value 1.016, then, represents the single potential of the 

ferric-ferrous electrode provided the normal calomel electrode has a 

potential of 0.560 volt. The electromotive behavior of substances 

possessing several stages of oxidation has been very fully studied and 

discussed by R. Luther.2

There can be no question, then, that the potentials shown by 

unattackable electrodes are related to the oxidizing or reducing 

nature of the solutions in contact with them, and further, that a number of the common metalliferous minerals exhibit initial potential 

values which may obviously be referred to the same cause.

These facts may possibly help us to explain in part the electromotive forces noted in the earth by Fox. The solutions in the upper 

levels of ore deposits are likely to be oxidizing and acid, but with 

increase in depth they become more reducing and less acid. For 

these reasons isolated portions of ore in the upper levels may possess 

a higher electric potential than portions in the lower levels, and such 

detectable electric currents as might be caused by this difference 

would be more likely to take a vertical than a horizontal direction. 

Of course variations in the nature of the solutions might also give 

rise to differences in potential in horizontal directions. However, 

the presence of dissimilar solutions in contact with ores suggests a

very probable cause of differences of potential in the earth. 

AVAILABILITY OF THE COMBINATIONS TO FURNISH CURRENT.

It appeared to be a matter of practical importance to determine 

whether the above potentials could yield currents available for producing appreciable chemical action or whether such electrodes are 

very easily polarizable. This question was accordingly put to an 

experimental test.

There seems to be no question about the competency of an oxidizing solution like acidified ferric sulphate to furnish a noteworthy 

current when a platinum electrode is employed as a cathode and 

conjoined with any unpolarizable anode. In order to compare this 

action with that occurring when electrodes of pyrite are involved

i See Noyes, A. A., and Brann, B. F., The equilibrium of the reaction between metallic silver and ferric 

nitrate: Am. Chem. Soc. Jour., vol. 34, p. 1025, 1912. 

a Luther, R., Zeitschr. physikal. Chemie, vol. 34, p. 488, 1900; vol. 36, p. 385, 1901.
GENERAL DISCUSSION OF PHENOMENA. 13

the following experiments were performed: In one beaker was placed 

an acidified solution of ferric sulphate, in another beaker a solution 

of sodium sulphide (see fig. 1), both solutions being approximately 

normal. The platinum electrodes measured 2 by 2 centimeters. 

The beakers were connected by a wick saturated with normal sodium sulphate. (See 

fig. 1.) The potential of this combination on 

open circuit, calculated by the data in the 

table on page 10, is 0.93 volt. The external 

circuit was completed by a voltmeter and 

sufficient additional resistance was introduced 

to bring the total resistance of the circuit, 

including the resistance of the liquid (all resistances being actually determined), up to FIGURE i.-Apparatus for study. 

3,000 ohms, which is comparable to the resistance of some geologic strata. On closing the 

circuit the electromotive forces and currents 

tabulated below were noted, the electromotive 

force stated being equivalent to the fall in potential over the whole

circuit.

. Oxidation and reduction current with platinum electrodes.

ing currents produced by combinations of solutions and minerals. A, B, beakers containing 

solutions; E, resistance; V, 

voltmeter; W, wick or tube.

Time.

0.1 minute. ......

Effective 

electromotive force.

Volt. 

0.96 

.94

.94

.88

.76

Current.

Milliampere. 

0.31 

.30

.30

.29

.25

The current was found to vary, of course, with the resistance of the 

circuit. As it continued to flow reduction ensued at the cathode and 

oxidation at the anode, thereby equalizing the differences and slowly 

lowering the potential and current. It was not thought necessary 

to extend the readings further, as the availability of the combination 

to furnish a noteworthy current was shown. Electrodes of pyrite 

were then substituted for the platinum. The external resistances 

were lowered to bring down the total again to 3,000 ohms, when the 

following results were noted:

Oxidation and reduction current with electrodes of pyrite.

Time.

Minutes.

0.1

1.0

5.0

10.0

Effective

electromotive force.

Volts.

1.04

1.02

1.00

1.00

Current.

Milliampere.

0.34

.33

- .33

.33
14 ELECTRIC ACTIVITY IN ORE DEPOSITS.

It is evident from these results that chemical differences between 

solutions are capable of producing appreciable currents for some time 

whether the electrodes are of platinum or of a conducting mineral. 

The polarization appeared to be a little greater, in fact, with the 

platinum than with the pyrite. The important point for geologic 

application is that the chemical differences may be equalized at considerable distances as well.as locally by electrolytic action when the 

proper circuits are present. For the elucidation of other similar 

combinations of solutions and minerals the following additional 

experiments were performed, all the solutions being approximately 

normal:

Currents produced in a circuit of 3,000 ohms resistance by different solutions.

Combination. Minutes.

0.1

1.0 

5.0 

10.0

.1

1.0 

5.0 

10.0

.1

1.0 

5.0 

10.0

.1

1.0 

5.0 

10.0

.1

1.0 

5.0 

10.0

. .1

1.0 

5.0 

10.0

.1

- 1.0 

.1

1.0 

5.0 

10.0

1.0

10.0

Volt.

0.51

.39 

.31 

.25

.71

.61 

.47 

.41

.22

.21 

 .21 

.21

.17

.15 

.14 

.14

.24

.21 

.19 

.17

45

.38 

.28 

.25

.02

(a) 

.40

.08 

.06 

.04

.06

.05

Milliampere.

0.17

.13

.10 .08-

.23

.19 

.15 

.13

.08

.07 

.07 

.07

.06

.05 

.05 

.05

.10

.08 

.07 

.06

.16

.13 

.10

. .08

.007

(a) 

.13

.03 

.02 

.01

.019

.016

a Negligible.

As the same conductors Were used on each side of the above combinations the currents developed must be ascribed to the effect of the 

solutions. It will be seen, however, that only those solutions which 

are ordinarily thought to be capable of fairly easy oxidation or reduction furnish noteworthy currents with platinum electrodes. Most 

other solutions quickly polarize the platinum, leaving only very small
GENERAL DISCUSSION OF PHENOMENA. 15

'' residual currents'' flowing. Such'' residual currents'' are ordinarily 

considered to be due to the fact that the polarizing substances slowly 

diffuse away from the electrodes while the supply is renewed by the 

current. With the mineral electrodes the polarization is a little less 

evident, so that appreciable currents are produced for some time, 

and with solutions that are also capable of oxidation and reduction 

the currents may equal or even exceed those produced with platinum 

electrodes.

In contrast to the preceding results obtained by using different 

solutions it was found that the currents yielded by different minerals 

in the same solution are much smaller, being more of the order of 

"residual currents." In the following experiments the circuit was 

similar to that above, except that the wick was not used and that the 

two minerals were dipped into the same solution:

Currents produced by different minerals in the same solution.

Combination. Minutes.

0.1

1.0 

5.0 

10.0

.1

1.0 

5.0

1.0

5.0 

1.0

1.0

1.0

1.0

Volt.

0.20

.10 

.08 

.06

.18

.09 

.03

.04

<.02 

<.02

<.02

<.02

<.02

MilHampere.

 0.07

.03 

.02 

.02

. .08

.04 

.013

. .013

<.01 

<.01

<.01

<.01

<.oi

Evidently polarization here put a stop to the action in a moment 

or so, although the potentials on open circuit, as shown on page 10, 

might have led one to expect appreciable currents.

In order to proceed further in the elucidation of these phenomena 

it appeared desirable to reduce the number of variables by considering 

each electrode separately. It follows from the first results that if a 

solution is easily oxidizable or reducible no electrode will be quickly 

polarized. It is also well known that the base metals dissolve as 

anodes with very little polarization. It therefore seemed the simplest 

way to set up cells with the various minerals as cathodes and a piece of 

zinc or copper as anode and record the polarization at the cathode 

with a given solution. In this way every difference but that of the 

mineral alone was eliminated. The results in.potassium chloride 

follow.
16 ELECTRIC ACTIVITY IN ORE DEPOSITS. 

Polarization at the cathode.

Combination. Minutes.

0.1

1.0 

. 10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1.

1.0 

 10.0

Volt.

1.31

1.29 

1.17

.82

.80 

.82

.83

.73 

.69

.78

.68 

.60

.76

.64 

.62

.61

.55

.48

.43

.43 

.43

:45

.35 

.35

Milliampere.

0.45

.44 

.40

37

.37 

.37

97

.24 

.23

.26

.23 

. .20

.25

.21 

.21

.21

.19 

.16

.14

.14 

.14

.15

.12 

.12

The minerals have been arranged in the order of increasing-polarizability as cathodes. This polarization doubtless depends partly on the 

solubility of the mineral and possibly also on the rate of solution or 

diffusion, but whatever may be its cause the experiments indicate an 

effective difference in action between different minerals. The same 

intensity was striving to evolve potassium (or hydrogen) in every 

experiment, but the minerals show different abilities to combine with 

or remove the hydrogen. The order is roughly that of the effective 

oxidizing power of the minerals. If the currents had been smaller, 

the polarization would have been less until, with the limiting condition of no current, the potential would represent exactly the oxidizing 

intensity of the cathode system. The limiting values would have 

theoretical rather than practical value, however, because they would 

contain no expression of the efficiency or the rate of the action, which 

is significant for geologic application. Moreover, there are various 

difficulties in determining the limiting values, among which are the 

facts that exceedingly small concentrations of certain ions determine 

the results obtained, that these small concentrations are not reproducible without rigorous exclusion of air, and finally that the minerals 

tnemselves often contain impurities. These points will be more fully 

considered under the headings covering the separate minerals.

The same, minerals were polarized by a smaller current that 

obtained with copper. The results follow.
GENERAL DISCUSSION OF PHENOMENA. 

Polarization at the cathode with a copper anode.

17

Combination.

Chalcocite / potassium chloride / copper. .................................

Pyrite/ potassium chloride /copper.... .................................

Minutes.

0.1

1.0 

10.0

.1

1.0 

. 10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

10.0

1.0

1.0

5.0

Volt.

0 9Q

1 .29 

.25

91

.17 

.08

90

.08 -.06

12

.08 

.06

.24

.08 

.02

.04

(a)

.04

(a)

Milliampere.

0.10

.10 

.08

.07

.05 

.03

.10

.03 

.02

.04

.03 

.02

.08

.03 

<.01

.01

(a)

.01

(a)

a Negligible.

The order of polarizability here shown is about the same as before. 

Pyrolusite is evidently the least polarizable cathode in potassium 

chloride solution.

Now using pyrolusite as a cathode, other minerals were studied 

as anodes, with the following results:

Polarization at the anode in potassium chloride solution.

Combination. Minutes.

0.1

1.0 

5.0

.1

1,0 

5.0

.1

1.0 

.1

1.0 

.1

1.0

Volt.

0.14

.06 

.02

.12

.02 

.02

.06

.02 

.16

.04 

.06

.02

Milliampere.

0.03

.02 

.01

04

.01 

.01

!02

.01 

.05

.013 

.015

.01

Most of these combinations did not yield sufficient electromotive 

force to show differences in polarization at the anode. The experiments were therefore repeated with a cathode of slightly higher 

electromotive force, namely, pyrite / acid ferric sulphate. This was 

connected to the beaker of potassium chloride by means of a wick,

33477° Bull. 548 14 2
18 ELECTRIC ACTIVITY IN ORE DEPOSITS.

the total resistance being 3,000 ohms, as in all these experiments. 

Each cell was of the form 

Pyrite / acid ferric sulphate / potassium chloride / mineral.

The results are tabulated below in the order of increasing polarizability, only the anodes being stated:

Polarization at the anode.

Anode.

Potassium chloride / pyrite ................

Potassium chloride / magnetite ...........................................

Minutes.

0.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

 10.0

..1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

.1

1.0 

10.0

Volt.

0.92

.92 

.92

60

.60 

.60

OQ

.39 

.39

.39

.35 

.35

.39

.33 

.25

qq

.25 

.16

OC

.24 

.22

oq

.08 

.03

.21

.19 

.16

1Q

.14 -.06

19

.08 

.03

(a)

Milliampere.

n 31

.31 

.31

.20

.20 

.20

10

.13 

..13

.13

.12 

.12

.13

.11

.08

.08 

.05

.08

.08 

,07

.08

.03 

<.01

.07

.06 

.05

.06

.04 

.02

OA

.03 

<.01

(a)

a Negligible.

Here, one may say, the same electromotive force was striving to 

deposit oxygen on the anode. The order in which the minerals are 

arranged in the table is therefore the order of their effective reducing 

power (or of their oxidizability) under the conditions of the experiment. The results show a wide range of polarization. That currents 

smaller than 0.01 milliampere may be generated for long periods by 

the less favorable combinations has been proved by several tests. As 

a current of 0.01 milliampere could deposit 1 milligram of silver a day, 

it can be seen that even currents so small have geologic significance 

and that larger currents may produce noteworthy chemical effects,
GENERAL DISCUSSION OF PHENOMENA. ' 19

But the chief object of the above experiments was to establish the 

magnitude of the currents actually produced by various combinations 

as compared with that of currents generated by ordinary galvanic 

cells. On viewing the results as a whole it is evident that different 

minerals in the same solution are much less efficient than combinations containing oxidizing. and reducing solutions. This difference 

may probably be ascribed to the fact that the differences in concentration produced by the solution of different minerals are necessarily small, whereas with more soluble salts the concentrations of the 

effective ions are enormously greater.

ELECTRIC CURRENTS IN THE EARTH.

In the earth the two electrodes and wire of figure 1 might have 

their counterpart in a single portion of ore or several ores in contact. 

The liquid connection might consist of moist rocks or vein solutions. 

Many possible combinations in the earth's crust might produce electric action. By a judicious use of the imagination.it is possible to 

perceive that this action might affect either fairly large zones or, on 

the other hand, might contribute to the development and alteration 

of the most minute particles in tiny veinlets.

In the preliminary measurements of the potential shown by various minerals in various solutions, tabulated and discussed at the 

beginning of this paper, it was recorded that some of the minerals 

were appreciably attacked. It may now be stated that there are 

good reasons to 'believe that all minerals are attacked by all solutions, but in widely varying degrees. For example, even pyrite, one 

of the least attackable sulphides, is affected slowly by acidified ferric 

sulphate. The action results in the production of ferrous salt in 

the solution, which may proceed both from the reduction of the ferric 

salt and from the solution of a part of the iron of the pyrite. Experiments have shown that a dilute acid mixture of ferric sulphate and 

potassium ferricyanide causes the development of an adherent blue 

precipitate on pyrite, as well as on marcasite, chalcopyrite, and 

pyrrhotite. That this precipitate adheres in a very thin film seems 

to be evidence that the minerals function in the reaction.

As pyrite, therefore, is capable of slowly reducing ferric sulphate, 

it is obvious that any electric action which could arise from the oxidizing power of ferric sulphate would occur chiefly on account of the 

fact that pyrite enters into direct action very sluggishly that is, the 

electric reduction of the ferric solution may occur far more readily 

under some circumstances than direct reduction by the pyrite. With 

the more attackable minerals the possibility of electric action would 

be less than with pyrite, but sufficient experimental evidence has 

been presented above to show that appreciable currents may be 

developed by various combinations of solutions and minerals. There
20 ELECTRIC ACTIVITY IN ORE DEPOSITS.

is always the possibility that electric action'may be a more ready 

way of equalizing chemical differences than direct action. This .possibility is in accord with the statement of Becker that it appears to 

be a law of nature for available energy to be expended as rapidly as 

possible. Accordingly, chemical energy should be converted into 

electric energy whenever the attainment of equilibrium would be 

thereby hastened. Becker has called this a principle of maximum 

dissipativity.1

In addition to the ever-present chemical sources of electric currents in the earth, mention may be made of the fact that sulphides 

are capable of developing thermoelectromotive forces, a subject which 

has been investigated by A. Abt, 2 who gives the following thermoelectric series:

Abt's thermoelectric series.

1. Chalcopyrite.

2. Pyrolusite.

3. Bismuth.

4. Zinc.

5. Nickel.

6. Copper..

7. Cadmium.

8. Nickel ore.

9. Arc-light carbon.

10. Iron.

11. Pyrrhotite.

12. Antimony.

13. Pyrite.

Diffusion is also capable of developing electric currents and is the 

cause of potentials between different solutions. These facts are sufficient to justify a study of the behavior of minerals under electric 

influences.

The fact that appreciable earth currents have seldom been found 

at any given point does not exclude the possibility that local electric 

action may be a potent agency in hastening chemical adjustments, 

or that very small currents acting for long periods would be capable 

of accomplishing great results. Of course present conditions in the 

earth are the result of adjustments and readjustments which have 

been going on for ages. The same forces are available now as always, 

but ore deposits may represent the result of very long accumulation. 

It would be incorrect to say that the electric batteries have run down, 

but, on the other hand, we can hardly expect to find batteries in the 

field comparable in intensity with those which we can construct in 

the laboratory. The laboratory results enable us to detect the tendencies at work. Where the action is distributed over a vast distance 

it appears that even the best experimentation might be unable to 

detect the action going on.

It will be desirable to discuss separately the several parts of such 

circuits as those suggested, consisting of various solutions and min1 Becker, G. F., A new law of thermochemistry: Am. Jour. Sci., 3d set., vol. 31, p. 120,1886.

2 Abt, A., Thermoelectromotive force of some metal oxides and metal sulphides in combination with 

one another and with simple metals with 100° difference of temperature of the contact points: Annalen 

der Physik, 4th ser., vol. 2, p. 266,1900.
GENERAL DISCUSSION OF PHENOMENA. 21

erals.. The part requiring the least attention for the present purpose 

is the liquid connection, for it is well known that solutions of inorganic substances, although varying in conductivity with their nature, 

concentration, and temperature, are on the whole good conductors 

of electricity.1 One important point, however, must be emphasized. 

A current in a solution is due to the actual movement of ions through 

the solution, a subject thoroughly studied by Hittorf as long ago as 

1850. The positively charged cations move in one direction, the 

direction usually called the " current," the negatively charged anions 

in the opposite direction. It follows from this that wherever electric currents flow in liquid circuits the cations migrate in one direction, the anions in another. For example, an electric current passing 

up a vein solution would actually consist in the transport of cations 

upward and of anions downward in the vein solution. Either migration or interchange of ions would also necessarily occur in "local 

action."

EFFECT OF A CURRENT FROM SOLUTION TO MINERAL. 

METHOD OF EXPERIMENT.

A current flowing from a solution to an electrode makes a cathode 

combination, and according to the well-known principles of electrochemistry, must be accompanied by " reduction." The chief question that arises is whether the reduction affects the constituents of 

the solution or the mineral forming the electrode. Experiments to 

determine this point were made in two ways: First, fairly large currents several hundredths of an ampere were applied from an outside source; second, more feeble currents were employed a few milliamperes, such currents as might actually be generated by the 

combinations already described.

EFFECTS AT THE CATHODE WITH MODERATE CURRENTS.

Ferric sulphate solution in contact with pyrite. was reduced. In 

dilute sulphuric acid hydrogen and a trace of hydrogen sulphide were 

evolved. The smooth crystal faces of. the pyrite appeared to be 

irregularly corroded, minute cavities being distributed over their 

surfaces. Iron was electroplated upon pyrite from a solution of ferrous sulphate, copper from cupric sulphate, silver from silver sulphate, 

gold from a solution of chlorauric acid, and platinum from a solution 

of chlorplatinic acid. A piece of pyrite weighing 35 grams was made 

cathode for five hours in a weak solution of sodium carbonate with a 

current of 0.04 ampere. The specimen remained bright and lost 

only 0.0025 gram. Hydrogen was evolved. From caustic soda or 

sodium sulphide solutions hydrogen was also evolved, but very little

1 On the conductivity of mine walls and veins see Barus, Carl, U. S. Geol. Survey Mon. 3, p. 345,1882. 

The resistances noted were of the order of thousands of ohms.
22 ELECTRIC ACTIVITY Iff ORE DEPOSITS.

or none was evolved from a solution of sodium polysulphide, which, 

suggests that the polysulphide was reduced by the current. With 

neutral salt solutions, such as those of sodium chloride or sodium 

sulphate, the solution on electrolysis became alkaline around the 

cathode.

Marcasite and pyrrhotite behaved much like pyrite as cathodes 

but were not studied in detail. Galena in sodium sulphate solution 

suffered some mechanical disintegration along cleavage planes, a fact 

which suggests that the electric polarization may extend into the 

minutest capillary spaces. As a result of the action sodium sulphide 

was formed in the solution. The electrolysis of cathodes of galena 

in normal sodium hydroxide was stated by Bernfeld to yield one 

equivalent of metallic lead for each equivalent of sulphur set free or 

passing into solution.1

Magnetite was reduced very slowly if at all in the solutions tried.

Pyrolusite is known to be readily reduced when acting as cathode, 

its use as depolarizer in the Leclanche cell depending on this fact.

EFFECTS AT THE CATHODE WITH FEEBLE CURRENTS.

Ferric sulphate in contact with pyrite was reduced. Gold, platinum, silver, and mercury were precipitated from their soluble salts 

in metallic form on pyrite. The products resulting from sulphuric 

acid with pyrite, as far as they could be identified, were hydrogen, 

some hydrogen sulphide, and ferrous sulphate. The pyrite appeared 

to remain bright and untarnished, however. With sodium chloride 

and pyrite hydrogen was evolved and the solution became alkaline 

and showed a trace of soluble sulphide. When cupric sulphate was 

electrolyzed, if the solution was neutral, some copper was deposited; 

but under the microscope the copper was seen to shade off into a 

dark-colored deposit of microscopic cubic crystals which were too 

small to be identified with certainty. A white precipitate, presumably basic sulphate, was also formed in the solution. No evolution of 

gas occurred. In a solution that was weakly acid and free from 

chlorides copper was the first visible product observable on the pyrite. 

In the presence of chlorides, however, a film consisting of microscopic 

tetrahedra appeared on the surface of the pyrite, and this film proved 

to be cuprous chloride.

The time at my disposal has not been sufficient to enable me to 

make some other experiments of this kind, especially with feeble 

currents, and the effects obtained might in some experiments differ 

from the effects produced by larger currents on account of the slow 

rate at which the very insoluble minerals react with cold solutions.

i Bernfeld, I., Studien uber Schwefelmetallelektroden: Zeitschr. physikal. Chemie, vol. 25, p. 53,1898.
GENERAL DISCUSSION OP PHENOMENA. 23

The experiments made indicate that the changes consist principally 

in a reduction of the constituents of solutions in contact with pyrite. 

Other possible effects are the deposition of free metals, the formation 

of hydrogen sulphide, the development of alkalinity, or the evolution 

of hydrogen or hydrogen sulphide. The sulphides at least are much 

more resistant as cathodes than- as anodes, as will be next shown.

EFFECT OF A CURRENT FROM MINERAL.TO SOLUTION.

When sulphides function as anodes they are attacked much more 

vigorously than when they function as cathodes. Thus pyrite lost, 

fifty times as much in weight with the same current when anode as 

when cathode in dilute sodium carbonate solution, and seven times 

as much in dilute nitric-acid solution. Pyrrhotite lost seven times 

as much when anode in sodium sulphide solution as when cathode.

The loss of weight of a pyrite anode in a solution of sodium carbonate, in spite of the fact that it.became coated with ferric hydroxide, must have been due to oxidation and solution of the sulphur, 

which was converted, in part at least, into sulphate, thus showing 

that both constituents of the pyrite were oxidized. Anodes of pyrite 

in a solution of sodium sulphide were immediately blackened by 

ferrous sulphide, which was formed as soon as current was applied. 

A pyrite anode in a solution of copper sulphate became coated with 

black iridescent copper sulphide. Under similar conditions with a 

solution of ferrous sulphate, the anode suffered very little change in 

weight and extremely slight discoloration, the chief effect of the current consisting in the oxidation of the ferrous sulphate. .As compared 

with its behavior in a solution of sodium sulphide or any other metallic 

.salt the behavior of pyrite in a solution of ferrous sulphate suggests 

a sort of simultaneous decomposition and regeneration of the pyrite, 

the net result being hardly noticeable.

In acid solutions no oxygen was evolved on anodes of pyrite with 

currents of slight intensity. Instead, iron passed into solution as 

ferric salt. Part of the sulphur was oxidized to sulphate; the rest 

remained on the anode in the free state. The electrode was noticeably tarnished by films of various shades gray, purple, and black 

the color changing in the order indicated as the experiment was 

continued.

In some of these experiments higher potentials were applied than 

those produced by natural combinations, and the results must be 

judged accordingly and taken as suggestions of the tendencies at 

cathode and anode respectively. Further study along this line should 

be made by those who are interested in special problems likely to be 

related to the phenomena.
ELECTRIC ACTIVITY IN ORE DEPOSITS.

ELECTRIC CONDUCTIVITY OF MINERALS.

Another part of the suggested circuit is formed by the mineral 

conductors. Fox, whose early study of the electric activity of mineral 

veins has already been cited, determined that pyrite, arsenical pyrite, 

galena, pyrolusite, and tetrahedrite are conductors of electricity; 

that molybdenite is a.very imperfect conductor; and that argentite, 

cinnabar, stibnite, bismuthinite, realgar, and blende are nonconductors. Later observers added other minerals to these categories from 

time to time.

A very complete list of conducting minerals has recently been 

compiled by H. A. Wentworth* from a study of their behavior during 

electrostatic concentration. The theory of electrostatic separation 

is that fine particles of most substances are attracted toward an 

electrically charged body, but that only those that are conductive 

will acquire the same charge and be repelled. If this theory is correct it should be possible to set down a list of the conductive minerals 

by observing their electrostatic behavior. Wentworth's list is as 

follows: , ;

Conducting and nonconducting minerals.

Good conductors.

Argentite

Arsenic, native

Arsenopyrite

Bismuth, native

Bismuthinite

Bornite

Brookite

Calaverite

Carborundum

Chalcopyrite

Chalcocite

Cobaltite

Copper, native

Covellite

Enargite

Ferrosilicon

Franklinite

Zinc blende 

Quartz 

Feldspar 

Epidote

Galena

Graphite

Hausmannite

Hematite

Ilmenite

Jamesonite

Leucopyrite

Linnaeite

Magnetite

Manganite

Marcasite

Mercury, native

Millerite

Molybdenite

Niccolite

Pentlandite

Proustite

Poor conductors.

Garnet 

Apatite 

Rutile

Psilomelane

Pyrargyrite

Pyrite

Pyrolusite

Pyrrhotite

Redruthite

Silicon

Smaltite

Sperrylite

Stannite

Stephanite

Sylvanite

Tellurium

Tetrahedrite

Wad

Wolframite

Zincite

Nearly all the silicates, carbonates, and sulphates. 

Most of the siliceous rocks.

i Wentworth, H. A., Electrostatic concentration or separation of ores: "'Am. Inst. Min. Eng. Bull. 66, 

p. 642,1912.
GENERAL DISCUSSION OF PHENOMENA. 25

The tables of Landolt-Bornstein-Meyerhoffer (1905) give the following values for the conductivities of a few minerals and conducting 

solids in reciprocal ohms.

Conductivity of minerals in reciprocal ohms.

Substance.

PbS............................................................................

Pyrrhotite ......................................................................

FeS.. ....................... ...................................................

Hematite. .................. ..................................................

Mn02 .. ...................................................................

Temperature.

0 C. 

25 '

20.7

15

20

20

0

0

19

0

20

0

18

Conductlvity.

10,386

3,470

820

313

119

91

8.98

1.68

/ 1.24

\ 2.41 

.98

.16

.013

The data in the table are obviously scanty and in part indefinite. 

Large variations are to be expected on account of inclusions, and 

small quantities of a second constituent may be sufficient to transform a nonconducting mineral into a fair conductor; sphalerite, for 

example, seems to possess conductivity by virtue of its content of 

ferrous sulphide.

It was pointed out by Braun * that the conductivity of some 

minerals is a function of the direction and time of passage of the 

current. Dufet 2 took exception to these conclusions, which were 

supported later, however, by the work of Bernfeld 3 on galena.

The conductivity of some sulphides increases with a rise of temperature, as in electrolytic conductors, the behavior of silver sulphide in 

this respect having been noted by Faraday.4 The suggestion has 

been made that the passage of a current in these sulphides causes 

them to decompose into their elements, but no such decomposition 

could be observed by Bernfeld with galena. In some experiments 

heretofore made the contacts used in mounting sulphides for tests 

have been the chief source of irregularities in their conductivity. 5 At 

high temperatures even silicates possess appreciable electric conductivity. It thus appears that a very large number of minerals are 

susceptible to electric influences.

1 Braun, F.,Ueber die Stromleitung durcli Schwefelmetalle: PoggendorfE's Annalen, vol. 153, p. 556,1874. 

3 Dufet, H., Sur la conductibilite" e"lectrique de la pyrite: Compt. Rend., vol. 81, p. 628,1875.

3 Bernfeld, I., Studien iiber Schwefelmetallelektroden: Zeitschr. physikal. Chemie, vol. 25, p. 50,1898.

4 Poggendorfi's Annalen, vol. 31, p. 242,1834.

6 Hayes, H. V., Note on the electrical conductivity of argentic sulphide: Am. Acad. Arts and Sci. Proc., 

vol. 46, p. 613,1910.
26 ELECTRIC ACTIVITY IN ORE DEPOSITS.

DETAILED STUDY OF VARIOUS POTENTIALS. 

GENERAL RESULTS OF THE MEASUREMENTS.

The object so far has been to establish and exemplify the proposition that electric currents may be generated in the earth and bring 

about chemical effects. We must now, somewhat more theoretically, 

consider the relations between electromotive forces and minerals, 

and must attempt to explain them consistently and point out the 

conditions under which they are constant and reproducible. In this 

connection it may be mentioned that measurements of electromotive 

force are quantitative expressions of physical or chemical differences 

between different systems differences made evident by a proper 

separation of the systems. When the systems are properly separated 

the tendency toward electric equalization between them may easily 

be measured. By union or mixture of the systems the differences 

would generally be equalized without visible electric action.

The most general statement that can be made about the potentials 

shown by minerals in different solutions is that the potentials are 

chiefly determined by the solutions. Without denying that there 

may be specific effects due to the minerals themselves it may be positively stated that the effect produced on the potentials by changing 

the solutions is enormously greater than the effect of changing the 

minerals. This is of course due partly to the fact that wide variations in the concentrations of the effective constituents are possible in 

different solutions.

The very great similarity of all the potentials noted, with mineral 

electrodes to "oxidation and reduction" potentials has already been 

pointed out.. When substances capable of giving such potentials to 

inert electrodes are present in relatively large amount in a solution in 

contact with minerals, very constant potentials may generally be 

developed, but the action of the mineral is then scarcely more than 

that of an electric conductor, as the amount of oxidizable or reducible 

material in the solution is so large. Such combinations are not 

adapted to show differences between minerals, although the data obtained by the experiments may aid in elucidating the chemistry of ores. 

The following values were obtained with various iron sulphides in a 

mixture of ferric and ferrous sulphates. The measurements were all 

made by the usual compensation method of Poggendorff, and the 

values of the single potentials are referred to the normal calomel electrode without correction for liquid potentials. The minerals, supported by suitable clamps, were partly immersed in the solutions and 

connection was made to the calomel electrode by a U tube filled with 

normal potassium chloride.
STUDY OF VARIOUS POTENTIALS. 

Potentials shown'by iron sulphides in ferric-ferrous sulphate solutions.

Substance.

{

FeSO< (0.1 normal) ..................... . ...........................

Fe2(SO4 )3 (0.1 normal) ................................................

H2S04 (0.18 normal) ... ; ..............................................

Marcasite collected by Howell. ..................................................

Nodule from Florida .............................................................

Pyrite in above solution diluted 10 times. .......................................

Marcasite in above solution diluted 10 times. ....................................

Pyrrhotite in above solution diluted 10 times. ...................................

f FeS04 (0.95 normal) .................. '. ...............................

|.H2S04 (0.162 normal) .................................................

Time.

Minutes. 

( 1

12

29

I 120

I '

I 78

f 1

1 32

1 45

I «24

Volt.

0.939

.935

.939

.926 

.933 09*;

.929 

.939

.934

.932

.932

.939

.946

.940

.908

.850

.852

.852

.866

Hours.

These results show that under the conditions of the experiments 

the ferric-ferrous potential practically overcame any differences in

o=S=o

JDL JDL

FIGURE 2. Method of supplying fresh solution to mineral electrodes without affording access of air.

the iron sulphides. In one experiment pyrrhotite gave a lower value, 

presumably because of the reduction of ferric salt by the hydrogen 

sulphide formed. That a similar effect occurred to a lesser degree 

with the other sulphides is at least suggested by the fact that the 

values are slightly lower than those computed by the equation on 

page 12, although it is difficult to make a good evaluation of the 

ionic concentrations in the rather concentrated sulphate solutions. 

The suggestion that the sulphides slowly reduce ferric sulphate is correcty however, if less concentrated solutions are employed, and, as 

will be shown later, ferric sulphate is incompatible with most sulphides.
ACTIVITY QBE

An effort was made to determine some potentials in a neutral solution of ferrous sulphate wholly free from ferric salt. For this purpose 

a concentrated solution of ferrous sulphate was prepared and preserved in contact with iron wire in a closed flask fitted with a siphon. 

An atmosphere of carbon dioxide was maintained in the flask (fig. 2). 

This solution could be delivered to tubes in which the electrodes were 

placed. For the sake of comparison a smooth platinum electrode was 

introduced into a third tube. From time to time fresh solution was 

allowed to flow over the minerals. At first bubbles of gas appeared 

on the sulphides, but in the course of tune these dissolved. The 

results are stated in columns headed "A," under the headings "Pyrite'' and ' 'Marcasite.'' To test whether the platinum wires obscured 

any effect of the sulphides, specimens of the sulphides were attached 

to platinum wire and the exposed platinum was then wholly covered 

with paraffin. These specimens gave the results shown in the columns headed "B," under "Pyrite" and "Marcasite," which are practically identical with those shown in the columns headed "A."

Potentials in 3.7 normal ferrous sulphate.

Time.

2 hours .......................

48 hours. ......................

72 hours. . ....................

120 hours.......... ...........

Pyrite.

A

Volt. 0.52" 

.48 

.61 

.57 

.58 

.59 

.62 

.63 

.64 

.61 

.49 

.43

B

Volt.

0.67

.64

.62 

.44 

.43

Marcasite.

A

Volt. 

0.32 

.38 

.43 

.46 

.52 

.60 

.61 

.58 

.61 

.62 

.48 

.47

B

Volt.

0.73

.62

.59 

.44 

.44

Platinum.

Volt. 

0.10 

.08 

.13 

.21 

.34 

.55 

.59 

.55 

.62 

.60 

.48 

.47

Galena.

Volt.

0.56

.52 

.51 

.53

Although no satisfactory explanation of these results can be given, 

it appears that pyrite, marcasite, and platinum present no significant 

differences. Evidently such changes as were going on in the solutions 

were influenced in only a secondary way by the minerals. Galena, 

however, appeared to maintain a more constant value. The.values 

may possibly be interpreted as ferric-ferrous potentials, in which the 

concentration of ferric salt was reduced to a constant point.

The following potentials were obtained in normal sodium hydrosulphide saturated with hydrogen sulphide at atmospheric pressure. 

The specimens were supported in small bottles) as shown in figure 3, 

the platinum wire being well paraffined and the bottles closed except 

when measurements were made.
DETAILED STUDY OF VARIOUS POTENTIALS. 

Potentials in normal sodium hydrosulphide.

29

Time.

0.1 hour.......................

11 days.......................

Pyrite.

Volt. 

 0.02

 .01

 .02

 .03

Marcasite.

Volt. 

 0.03

 .03

 .03

 .03

Pyrrhotite.

Volt. 

-0.02

 .03

 .03

 .04

Chalcopyrite.

Volt. 

. -0.02

 .02

 .03

Galena. :

Volt.

_ n ru

 .04

 .03

 .03

Platinum.

 Volt.

 0.01

 .04

- .02

To assume that the small differences here shown should be ascribed 

to differences in the respective minerals would be unwarranted. 

Rather it appears that the minerals, as well as platinum, merely served 

as conductors for the attainment of a fairly constant potential of the 

sodium hydrosulphide. Under these conditions, although the minerals 

are in a polarized state, there is little, if any, indication that one 

mineral is more stable than another. Electric activity under these circumstances would 

affect chiefly the solution rather than the 

minerals.

. Many attempts were made to obtain characteristic potentials of the different minerals 

in the absence of such pronounced oxidizing 

and reducing solutions as those above mentioned.

The only positive results obtained at first 

were that one group of. sulphides, including 

pyrite, marcasite, and chalcopyrite, gave potentials higher by one or two tenths of a volt 

than another group which included pyrrhotite 

and galena. None of the specimens of sphalerite examined had enough conductivity to yield a result. In attempting to differentiate pyrite, marcasite, and chalcopyrite it was found that 

the differences between the minerals were of the same order as the differences between different specimens of the same mineral, and that 

none of their potentials was much removed from that shown by smooth 

platinum similarly treated. Of course all minerals have some solubility, so that no matter what other substances there may be in a 

solution the products resulting from the minerals must also be present. 

As these products are in general oxidizable or reducible ions, the conclusion is inevitable that the potential of most minerals will be of the 

nature of an oxidation and reduction potential. We thus return to 

the thought that even if the minerals have specific potential differences the'effect of their solution products on the potential can not be 

neglected, and this effect may in fact be all there is to the "potential 

of a mineral" in water. If pure water could be saturated with only 

the solution products of a pure mineral the problem would be easy,

FIGURE 3. Hall cell of mineral' 

suspended in a solution for observation over a long period.
30 ELECTRIC ACTIVITY IN ORE* DEPOSITS.

but in practice the complete removal of foreign substances from the 

water and the mineral is extremely difficult if not impossible.

The potential shown by some specimens of a given mineral depends 

considerably on the previous treatment of the specimens, the explanation of the differences found being that if a mineral is long immersed 

in a solution some of the solution penetrates its invisible pores-and 

subsequently diffuses out very slowly when the mineral is placed in 

another solution. When once the pores are filled with water or an 

indifferent electrolyte the mineral is almost as responsive as a platinum electrode to changes in acidity or concentration of the solution; 

that is, to such changes as determine the equilibria of the effective 

ions in the solution.

To what extent observed potentials may be ascribed to minerals, 

as distinguished from the solutions in contact with them, will best 

appear, however, from the detailed measurements that follow.

MEASUREMENTS OF POTENTIAL ASSUMED BY CERTAIN

MINERALS.

PYEOLUSITE.

Electrodes of manganese dioxide were studied by Tower, 1 who prepared them by precipitating manganese oxide upon platinum anodes 

by electrolysis. In order to develop, a theory of the potential of the 

electrode, the solution bathing the electrode was assumed to contain 

manganous ions. The possible presence of manganese ions of higher 

valence was apparently not considered by Tower. He proved that 

the potential of such electrodes depends on the concentration of acid 

and manganese salt in the solution, the effect of acid being fourfold 

that of the manganese salt. An increase of acid raises the potential; 

an increase of manganese salt lowers it. Some of his values are given 

in the following table:

Potential of manganese dioxide in various solutions at 14° C. as determined by Tower.

Volts.

0.1 normal NaOH. .... ..;.........:....................;.... 0. 70

0.0125 normal NaOH.......................................... .75

0.00156 normal NaOH......................................... .80

0.000391 normal NaOH........................................ .84

Neutral 0.01 normal Mn(N03 ) 2 ................................. .87

0.00195 normal H2S04 , 0.2 normal Mn(N03 )2 .................... 1. 44

Normal H?S04, 0.1 normal Mn(N03)2 .........:.................. 1. 72

Normal H2S04, 0.000391 normal MnS04 ......................... 1. 80

0.000977 normal H2S04 , normal MnS04 ......................... 1.44

0.99 normal H2S04, 0.000782 normal MnS04 ...................... 1. 79

A number of measurements were made by the writer on natural 

specimens of various oxides of manganese, some of which were said 

to be pyrolusite and others known to be manganite, but the potentials observed agreed only in part with those recorded by Tower.

i Tower, O. P., StudientiberSuperoxyd-Elektroden: Zeitschr. physikal. Chemie, vol. 18,pp. 17-50,1895.
DETAILED STUDY OF VARIOUS POTENTIALS. 31

. The following values were noted with small splinters, said to be 

pyrolusite, from Germany. Constancy was obtained in from half an 

hour to an hour. .

Potential of small splinters said to be pyrolusite.

Electrolyte. 1

Volts.

a 0.52

.74

.72

1.05

1.10

2

Volts. 

0.70

.75

.77

1 79

1.54

a Indefinite.

1. Potential obtained by me.

2. Electrolytic HnOa , Tower, 0. F., op. cit.

In explanation of these results it should be mentioned that pyrolusite is not a well-crystallized species. The crystalline native manganese dioxide is pollianite, which could not be obtained for study. 

A massive specimen of pyrolusite from Rio Apache, N. Mex. (U. S. 

National Museum No. 8424), gave the following potentials:

Potential of massive pyrolusite from Rio Apache, N. Mex.

Electrolyte.

0.00625 normal NaOH . ..........................................................

1

Volts. 

1.22 

.96

2

Volts. 

1.54 

.76

1. Potential obtained by the writer.

2. Potential obtained for MnOa by Tower, op. cit.

This specimen did not come to constancy quickly as did Tower's 

electrodes on platinum. Another difficulty with this mineral was 

due to capillarity as manifested in the tendency of the solution to 

rise up through the mineral and wet the holder.

A very excellent specimen of crystallized manganite from Ilefeld, 

Hanover, Germany (U. S. National Museum No. 45630), proved to 

be too nonconducting to permit measurements to be made. Another 

specimen of the same mineral, however, which had a radiating 

structure, from Markhamville, Kings County, New Brunswick (U. S. 

National Museum No. 45711), was sufficiently conducting. When 

this manganite and the pyrolusite previously mentioned were immersed in the same solution the values shown were as follows:

Potential of manganite and pyrolusite.

Normal KC1,

Normal KC1.

Electrolyte.

0.01 normal MnSO<, 0.01 normal HsSO*. ...........................

Manganite.

Volts. 

1.05

.96

Pyrolusite.

Volts. 

1.33

' 1.11
32 ELECTEIC ACTIVITY IN QBE DEPOSITS.

The above values are at least in the direction which would be expected the higher potential for the higher oxide. The potentials 

obtained in a solution of potassium chloride in the presence of air are 

the highest observed with any of the minerals investigated and indicate that, when coupled with any other mineral in the presence of air, 

the manganese mineral would be the cathode; that is, the other mineral would be oxidized by electrolytic action, if it occurred. It was 

found that pyrolusite is far less easily polarized as cathode than the 

other minerals studied, a fact which is perfectly consistent with the ' 

use of pyrolusite as a depolarizer in the well-known Leclanche' 

battery.

PYRITE.

The preliminary observations on pyrite showed that its potential is 

chiefly dependent on the nature of the solution in which it is immersed. 

In order to determine the effect of changes in the concentration of 

these solutions the following experiments were performed:

A fresh specimen was immersed in tenth normal, hundredth normal, 

and thousandth normal sulphuric' acid successively, each solution 

containing about one-fifth of 1 per cent of ferrous sulphate. The 

three potentials were 0.82, 0.83, and 0.82 volt, respectively, showing 

at most a very .slight dependence on the acid concentration. But 

with similar variations in the ferrous sulphate content of the solution 

the potentials -were 0.78, 0.81, and 0.84 volt, respectively. The 

ferrous sulphate solutions each contained a small amount of sulphuric acid. The differences caused by a tenfold change in content 

of ferrous ion are decidedly greater than those resulting from changes 

in acidity. Similar results were obtained with ferric sulphate, the 

potentials being 1.04, 1.08, and 1.11, respectively. The potentials in 

even dilute ferric sulphate are strikingly high. These results furnish 

valuable suggestions for the interpretation of the phenomena. When 

ferrous and ferric salts are .present together we are apparently dealing, in part at least, with a "ferrous-ferric" potential, as was pointed 

out on page 27.

The potential of pyrite is plainly affected by changes in the 

hydroxide or sulphide concentration of alkaline solutions) the record 

presenting a marked contrast to that obtained by.changes in acid, 

solutions. The following results show the variation caused by 

tenfold changes:

Potential of pyrite in alkaline solutions.

'_ Solution. NaOH.

Volt. 

0.32

.38

.44

.51

Na2Sx.

. Volt. 

 0.07

 .01

-i- no

+ .15
. DETAILED STUDY OF VARIOUS POTENTIALS. 33

Similar effects were noted in solutions of sodium sulphide and 

sodium hydrosulphide.

The question at once arises whether the hydroxide and sulphide 

concentrations affect the potential merely by determining the iron 

content or sulphide content, according to the law of the solubility 

product or some other similar law, or whether they affect the potential in a direct way; in short, whether pyrite can be considered a 

"compound electrode." If it can be so considered, the relation 

between its potential and the concentrations can be expressed by the 

following equation:

E = E0 + ln[Fe] [S] 2 .

There are several difficulties in verifying such an equation. In the 

first place, with access of air some ferric salt is formed in spite of the 

presence of the electrode. With the ferric salt present there is the 

complication of the ' 'ferrous-ferric" potential. Even closed receptacles, boiled solutions, and an atmosphere of carbon dioxide appear 

to be insufficient to eliminate all ferric salt; enough persists to yield 

a relatively high potential. Such a potential might also be due to 

polysulphide ions, which are doubtless formed from pyrite. In an 

atmosphere of hydrogen it appears that enough normal sulphide is 

formed to lower the potential appreciably. With even a small concentration of sulphide ion the potential is considerably lowered, but under 

these conditions a variation of the concentration of ferrous ion (in 

acid solution) appears to have no effect upon the potential. This was 

proved by measurements upon marcasite (see below), but the results 

would probably have been similar for pyrite. The relations of the 

solubility product of these disulphides are unknown. Lastly, it is 

very doubtful whether such electrodes can be considered strictly 

reversible. Whether both atoms of sulphur may function electrically or not has not yet been determined. The presumption is 

that only one does. When the pyrite is anode there is a very strong 

tendency toward oxidation of ferrous ion and 'of sulphur to sulphate 

the last action overstepping the limit of reversibility although as 

cathode soluble sulphide is formed. Neither set of conditions is 

wholly favorable for the maintenance of the pyrite. In view of these 

difficulties any application of the equation must rest upon a good 

many assumptions and will not be attempted at present. At the 

same time it is hoped that the potential will some day be expressed 

by a formula which will be an advance over the present empirical 

statement of the potential in "water."

For the next experiment, instead of adding any of the constituents of 

pyrite to the solution, the attempt was made to obtain definite results 

by letting the pyrite supply its own solution products. If only water

33477° Bull. 548 14 3
34 ELECTRIC ACTIVITY IN ORE DEPOSITS.

is used there will be a considerable liquid potential to be reckoned 

with, and on this account it seemed better to use neutral salt solutions. The following table gives a record of the behavior of pyrite 

dipping in normal potassium chloride in a small closed bottle (fig. 3, 

p. 29) for over a month:

Potential of pyrite in normal potassium chloride in presence of air.

Time. Volt.

0.76

.76

.74

.74

.72

.69

.69

.69

Time.

7 days.... ..............................

9 days.... ..............................

40 days..... ............................

Volt.

0.66

R4

.63

.66

.66

.66

.71

The above results show a slow fall for several hours and thereafter 

a considerable degree of constancy in the period from 2 to 14 days. 

Unfortunately, however, other specimens gave different values. 

Even the same specimen after being dried gave slightly different 

values at different times when first placed in the solution. The 

variations thus noted are illustrated by the following results:

Variations of potential in normal potassium chloride due to accidental causes.

Date.

Apr. 11.................................................................

May 4...... ............................................................

June 14.. .......................... ......................................

June 18.................................................................

June 25......... .........................................................

1

Volt. 

1.01

.99

.93

.94

.97

2

Volt. 

0.96

.92

.88

.88

1.00

3 .

Volt. 

0.94

.88

.87

.85

.83

4

Volt. 

0.72

.71

.68

.83 a.56

o This measurement is evidently erroneous.

1. Pyrite collected by A. A. Julien (No. 39).

2. Pyrite from Elba.

3. Octahedron of pyrite, French Creek, Pa.

4. Marcasite, cockscomb, collected in Missouri by C. E. Siebenthal.

In order to see if the presence of air could account for these irregularities a solution of potassium chloride was boiled to free it from air 

as far as possible. While still warm it was transferred to a flask 

connected with a siphon, through which it could be delivered to the 

tubes containing the specimens under investigation. A platinum 

wire was bound around the specimens and led out between the stoppers 

and the glass. For the sake of comparison a smooth platinum electrode was introduced into one. of the tubes. From time to time 

fresh solution was allowed to flow over the electrodes. Carbon 

dioxide from a generator replaced the solution as it was used. The
DETAILED STUDY OF VARIOUS POTENTIALS. 35

apparatus was the same as described on page 27. The results were 

as follows: .

Potentials of specimens immersed in potassium chloride solution containing C02 at 25° C.

Time.

Pyrite.

A.

Volt. 

0.67 

.69 

.68 

.63 

.62 

.64

B.

Volt. 

0.66 

.65 

.71 

.59 

.61 

.62

Marcasite.

A.

Volt. 

0.66 

.70 

.76 

.70 

.70 

.70

B.

Volt. 

0.78 

.71 

.72 

.72 

.71 

.71

Platinum.

Volt. 

0.62 

.60 

.56 

~ .65 

.65 

.60

Galena.

Volt. 

0.52 

.55 

.51 

.53 

. .52 

.52

The preceding results suggest that in their electromotive behavior pyrite and marcasite function chiefly as "unattackable electrodes," as their 

potentials do not vary greatly from those shown 

by smooth platinum. That the effect was not 

due to the platinum wires was carefully determined in every experiment by attaching long 

pieces of several specimens to platinum wires and 

then completely covering the platinum and a portion of the mineral with paraffin. The results 

were practically identical with the preceding 

ones.

The minerals were examined in potassium 

chloride solution with still greater precautions 

against the presence of air by the "use of tubes 

in which the solution was boiled out directly, 

thus avoiding the necessity .of transfer (fig. 4). 

The results of these measurements were as 

follows:

duced pressure before 

measuring electromotive 

force, the tube to be 

broken off at C.

Potentials of specimens immersed in potassium chloride solution boiled out directly.

Time.

6 days..................................................

Pyrite.

Volt.

0.64

.63

.64

.63

Chalcopyrite.

Volt.

0.64

64

66

.67

Marcasite.

Volt. 

0.60

60

.60

CQ

Platinum.

Volt.

6.69

.69

fiQ

AQ
36 ELECTRIC ACTIVITY IN ORE DEPOSITS.

The conclusion from the preceding experiments is that the exclusion 

of air lowered the potentials slightly. A much better exclusion of 

air was finally accomplished, however, by supporting the specimens 

in a bottle through which a slow current of very pure hydrogen was 

carried as long as desired. This brought the potential of pyrite 

down to values ranging from 0.30 to 0.55 volt according to the length 

of treatment. It seems impossible to avoid the conclusion, therefore, 

that a trace of ferric salt is very persistent unless particular care is 

used to exclude it or reduce it, and that the potentials in air and in 

the absence of air may differ markedly on account of the presence 

of ferric salt. The unavoidable accidental variations shown by 

pyrite in neutral salt solutions are probably due, therefore, to indeterminate traces of fernc salt. The hypothesis that polysulphide 

ions are present in varying proportion to sulphide ions is equally 

plausible and will presently be considered.

Instead of trying to obtain the potential in a neutral solution, a 

state not easily obtained with reference to the ionization of water, 

one may reduce the number of variables slightly by measuring the 

potentials in acid and alkaline solutions of definite concentration and 

taking the mean value for neutrality. Liquid potentials were largely 

eliminated by having the solutions about normal with potassium, 

chloride, and as the value 0.83. volt was obtained in tenth-normal 

sulphuric acid and 0.38 in tenth-normal sodium hydroxide we should 

obtain 0.60 at the neutral point for the potential of pyrite in normal 

sodium sulphate solution in the presence of air, assuming the relation 

to be linear. The values actually obtained in neutral salt solutions 

under these conditions ranged around 0.72. In hydrogen the two 

values obtained for tenth-normal solutions were 0.44 and 0.13 volt, 

respectively, giving a mean of 0.28 volt for the potential of pyrite 

ill a neutral salt solution in the presence of hydrogen.

It was thought that the experiments in hydrogen might have overshot the mark by producing some hydrogen sulphide, and this was 

found to be the case. Some powdered galena was moistened and 

placed in a small wash bottle with a glass stopper and the bottle filled 

with hydrogen. A strip of clean silver suspended in the bottle' 

showed a slight tarnish in a few hours. Evidently, then, hydrogen is 

too vigorous a reducing agent for the purpose of merely excluding air.

The extreme delicacy of the conditions determining the potential 

shown by pyrite in normal potassium chloride solution is further illustrated by the ease of its polarization. With a potential of 0.78 volt at 

the beginning, pyrite was polarized by a very small current,, 0.000005 

ampere, copper in copper sulphate being used as an auxiliary electrode. The polarizing potential was probably not oVer 0.3 volt. In 

15 minutes the potential of the pyrite had fallen to 0.57 and after- >
DETAILED STUDY OF VARIOUS POTENTIALS. 37

ward remained fairly constant at 0.51. Under these conditions the 

pyrite was probably surrounded by a film containing very little ferric 

salt. Upon stopping the polarization the potential rose in half an 

hour to 0..75 again. The pyrite was then polarized similarly as anode. 

The potential rose to about 0.97 and remained fairly constant. This 

probably corresponded to a maximum of ferric salt under the conditions of the experiment. When the polarization was stopped the 

potential soon fell to 0.80. With this specimen, therefore, under

Volts

1.00

0.90

0.80

0.70

0.60

0.50

cathc de

PYRITE

.ation

Polarizati

at 

ano.de

Off

0 12 3 4-5

Time in hours

PICTURE 5. Curves showing rate of polarization. Applied electromotive force about 0.3 volt; current,

0.000005 ampere.

these conditions, the potential certainly lay between 0.75 and 0.80. 

The results are shown graphically in figure 5. We are dealing here 

with a potential which is extremely sensitive to current that is, the 

electrode is very easily polarized. In other words, -pyrite furnishes 

so slight a concentration of its characteristic ions that a very feeble 

current suffices to alter the concentration enormously.

The lag shown by a certain specimen of pyrite in attaining constancy in a new environment is also illustrated by the following 

results: . .
38 ELECTRIC ACTIVITY IN ORE DEPOSITS.

Lag in the potential of pyrite when placed in a new environment.

Substance.

Same specimen stood 9 days in normal NaSH at.. ..................................

Same specimen placed in 0.1 normal H2SO4, in presence of air .......................

Same specimen placed in 0.1 normal H2SO<, air displaced by hydrogen. . ............

Time in 

hours,

f 0.1

1 . 0

I -5

.5 

1.0

2.0 

22.0 

.6 

24.0

96.0

Potential 

in volts.

+0.87 

+ .88

+ .88 

- .03

+ .57 

+ -6.1;

+ .65 

+ .79 

+ .57 

+ .44

+ .44

The arrangement shown in figure 6 is 

believed to be the best one devised for 

mounting pyrite in such a way as to attain 

its own characteristic potential in water. 

A good-sized piece of pyrite was attached 

to a platinum wire and then partly immersed in a block of paraffin. A tube 

about an inch in diameter was then sunk 

in the paraffin, forming a water-tight 

holder. The pyrite in pure water was surrounded with coarsely powdered pyrite

period without access of air. shown by this pyrite was as follows; 

Potential of pyrite in water.

Time.

Start...........!.........................

15'minutes. ..;......:..... . ............

4 HavQ8 days.. ..................................

Volt.

0.90

.89

.87

.83

.80

.80

Time.

13 days..................................

Fresh water added:

6 days.. .............................

Volt.

0.75

.74

.63

.62

Whatever the solution products of pyrite may be at first, the ions 

OH~ and SH~ are probably formed to some extent. It seems reasonable to suppose that these ions will attack the sulphur of the 

pyrite further, and that the progress of these slow, successive reactions 

accounts for the slow fall of the potential shown by pyrite in water.

After reviewing all the preceding results, the most logical conclusion concerning the potential shown by pyrite in water with 

access of air is that its high value is due either to ferric salt, to acid 

produced by the oxygen of the air, or to polysulphideions. Under these 

conditions a single electrode of pyrite is incapable of maintaining the 

iron in the ferrous condition and similarly the sulphide ion is probably 

oxidized in part, thus assisting in the production of a high potential. 

Mere traces of ferric ion suffice to give the high value. This explanation is also consistent with the variable results shown by pyrite in a 

potassium chloride solution.
DETAILED STUDY OF VARIOUS POTENTIALS. 39

When air is excluded, on the other hand, a somewhat lower value 

is obtained, which seems to vary only slightly with the content of 

ferrous salt but is extremely sensitive to changes in alkalinity or 

sulphide concentration and may be reduced to a very low value by 

an excess of alkali sulphide. These low potentials may be termed 

"sulphide " potentials. The true potential of the solution products of 

pyrite must be intermediate between the "ferric-ferrous" potentials 

and the "sulphide" potentials that is, must be between 0.80 and 

0.56. If such a thing were possible, it would appear to be a "ferricsulphide" potential. It seems impossible, however, with any arrangement of apparatus so far devised, to establish this potential 

definitely on account of the slow rate at which the pyrite comes into 

equilibrium with the solutions, and an expression of the potential in 

terms of an equation has therefore not been found.

MARCASITE.

Experiments were made with marcasite to see if it presented any 

difference from pyrite. The marcasite was in. the form of cockscombs, 

collected in the Joplin district of Missouri by C. E. Siebenthal, of the 

United States Geological Survey. By the preliminary method of 

experimentation in air pyrite seemed to give a higher value at first, 

but marcasite averaged slightly higher after a time. The mean of a 

number of determinations for marcasitedn normal potassium chloride 

in the presence of air was 0.80 volt, whereas pyrite gave 0.72 volt. 

Marcasite is ordinarily considered to be more oxidizable than pyrite. 

In other words, it should produce ferric sulphate faster than pyrite, 

and this higher potential, attained after the lapse-of an hour or so, 

seems to agree with the interpretation that the high potential of 

pyrite and marcasite in the presence of air is due to ferric sulphate or 

sulphuric acid in the solution. According to this interpretation the 

real factors in the mechanism of the process would be the rate of 

oxidation and the rate of supply of oxygen.

The variation of the potential of marcasite with changes in alkalinity and in the nature of the solution in general was found to be 

similar to that shown by pyrite, within the limit of error of the observations. Several of these comparisons have already been given. 

The following data form an additional record of the phenomena:

Comparison of potentials oj marcasite and pyrite.

 .

0.1 normal NaOH. ........................".....................................

0.10 normal NaOH, 0.90 normal KC1 ............................................

Marcasite.

Volt. 

0.40

.47

.56

.80

.35

.90

.62

Pyrite.

Volt. 

0.38

.44

.51

.72

.38

.83

.60
40 ELECTRIC ACTIVITY IN OEE DEPOSITS.

On the whole the differences here shown between pyrite and marcasite are of the same order as the error of the determinations; nevertheless, the results indicate a persistent tendency .of the marcasite 

toward a slightly higher value in the presence of air. A difference so 

slight, however, is of a much lower order than the differences caused 

in the potential by a variation in the nature of the solutions.

Measurements were made in 0.005 normal sulphuric acid in an 

atmosphere of hydrogen sulphide, the marcasite being partly submerged and suspended by a fine platinum wire. If the dissociation 

constant 1 of hydrogen sulphide [H] 2 [S] is taken as 1.1X10"23 and 

the hydrogen ion concentration in 0.005 normal sulphuric acid 2 as 

0.0043 we should have

S" =

l.lXlQ--28 

0.000018

= 6.1X10-20

This is certainly not an extreme concentration of sulphide to try, 

although it is insufficient to precipitate ordinary ferrous sulphide. 

The ferrous ion content was varied from 0.0005 molal to 0.1 molal 

without any effect upon the potential, as the following results show:

Potential shown by marcasite in acid solutions containing potassium chloride and ferrous 

sulphate and saturated with hydrogen sulphide.

FormulaconcentrationofFeSO<.

0.0005

.001

.01

.1

.1

. 1 to . 01

Trace.

'

Trace.

Formula concentration of (H+).

0.0043

.0043

.0043

. 0043

.0018

Very little.

Trace.

Trace.

Formula concentration of (S ).

6.1X10-20

6.1X10-20

6.1X10-2C

6.1XHH° 3.4X10-19

Solution saturated with FeS

and H2S.

Solution saturated with PeS

and H2S.

Excess of NaHS.

Potential.

Volt.

. 0.292

.292

.297

.294

.291

.269

.261

.120

The above conditions indicated by the table, some sulphur being 

held in suspension, approach those which are favorable to the development of marcasite.3 In the last measurements the acidity was reduced by repeated additions of sodium hydroxide, so that the solution was plainly saturated with ferrous sulphide and with hydrogen 

sulphide. It was impossible to tell whether or not marcasite was 

forming on the electrode, which in the acid solution retained its characteristic steel-gray appearance, but in the neutral solution was somewhat blackened by adhering particles of ferrous sulphide.

i Knox, J., Zur Kenntniss der lonenbildung des Schwefels und der Komplexionen des Quecksilbers: 

Zeitschr. Elektrochemie, vol. 12, p. 477,1906.

3 Drucker, K., Das Dissociationsschema der Schwefelsaure: Zeitschr. Elektrochemie, vol. 17, p. 398,1911.

»Alien, E. T., Crenshaw, J. L.., and Johnston, J., Mineral sulphides of iron: Am. Jour. Sci., 4th ser., vol. 

33, p. 179,1912.
DETAILED STUDY OF VARIOUS POTENTIALS. 41

The above results bring out the interesting fact that if. the solution 

contains an excess of soluble sulphide a variation in the concentration 

of ferrous salt does not affect the potential, but if the concentration 

of the sulphide is at a minimum higher potentials may be obtained 

by adding ferric salt, the potential being then determined by the 

ratio of ferrous to ferric salt.

PYRRHOTITE.

The pyrrhotite used was collected at Ducktown, Tenn., by F. B. 

Laney, of the United States Geological Survey. Salt solutions and 

acids were found to impart a lower potential to pyrrhotite than to 

pyrite or marcasite. Commercial ferrous sulphide gave a much 

lower value than pyrrhotite, but it contains metallic iron. According to Kichards and Behr,1 metallic iron has a potential of 0.17 

volt in normal ferrous sulphate.

The following readings were made with electrodes of pyrrhotite:

Potential of pyrrhotite.

J Volt.

In normal KC1, 0.54, 0.58, mean................................ 0.56

Normal KC1, 0.001 normal NaOH............................... .45

0.99 normal KC1, 0.01 normal NaOH........................... .34

0.90 normal KC1, 0.1 normal NaOH............................ .25

A tenfold change in the quantity of sodium hydroxide in solution 

produces greater variation in the potential of pyrrhotite than in 

that of pyrite or marcasite, possibly because free sulphur is easily 

dissolved from the pyrrhotite by the sodium hydroxide, the result 

being a low "sulphide" potential. Normal sodium hydroxide gave 

a very inconstant value with pyrrhotite and appeared to convert it 

into oxide and soluble sulphide, but normal acid and normal sodium 

sulphide eventually gave much more constant values.

MAGNETITE.

The magnetite examined was in the form of large crystals from 

Mineville, Essex County, N. Y. (U. S. National Museum No. 47830). 

These crystals were but very slightly fractured. The potentials 

shown by different crystals in the same solution were rather discordant, although each single potential was very definite. Electrodes of magnetite are very easily polarized and respond quickly 

to changes in acidity and alkalinity of the solution. Briefly, the 

mineral behaves very much like an unattackable electrode, but lack 

of time has so far prevented a sufficiently detailed study to determine 

whether or not its potential can be correlated with the concentrations 

of its solution, products. The following preliminary values the

1 Richards, T. W., and Behr, G. E., The electromotive force of iron under varying conditions and the 

effect of occluded hydrogen: Carnegie Inst. Washington Pub. 61,1907.
42 ELECTRIC ACTIVITY IN ORE DEPOSITS. -

averages measured with three different crystals were obtained in 

the ordinary way, in the presence of air:

Potential of magnetite.

Volt.

Normal KC1, with access of air................................ 0. 68

0.001 normal NaOH, 0.99 normal KC1........................... .65

0.01 normal NaOH, 0.99 normal KC1........................... .62

0.1 normal NaOH, 0.90 normal KC1............................ .56

O.lnormal H2S04, 0.90normal KC1............................. . .86

When air is excluded magnetite in contact with water only eventually gives much lower values than those tabulated above. Thus, 

one specimen mounted in paraffin (fig. 6, p. 38) gave 0.55 volt 

after 22 days and eventually 0.46 volt.

GALENA.

Preliminary measurements on galena showed that various solutions alter its potential greatly. Acids do not yield as high values 

with galena as with pyrolusite, pyrite, or magnetite. In fact, 

changes in acidity give irregular results, for reasons which will be 

stated farther on. The potential is not very sensitive to a change 

in content of lead salt, but is sensitive to changes in alkalinity or 

concentration of sulphide.

A specimen that gave +0.49 volt in normal potassium chloride 

fell to 0.24 when hydrogen sulphide was passed into the solution, 

but rose again to 0.47 after being washed with water and replaced 

in potassium chloride. The addition of a trace of lead salt gave 

0.48 volt. ' :

The effect of excluding air was studied with galena as with pyrite, 

and it was found that in tenth-normal sulphuric acid the displacement of air by hydrogen resulted in no appreciable change in potential. The most plausible explanation of this fact is that the acid 

maintains a certain concentration of hydrogen sulphide, which determines the potential both in air and in hydrogen. With pyrite the 

acid generates so little hydrogen sulphide, if any, that air causes 

some oxidation of the ferrous salt to ferric.,. With galena hydrogen 

sulphide is certainly produced, even in air, so that no further effect of 

hydrogen would be expected. In an alkaline solution, on the other 

hand, a difference in potential was noted in the presence, and in the 

absence of air. The value in tenth-normal sodium hydroxide in the 

presence of air was 0.27 volt and in hydrogen 0.12. The lat'ter value 

is practically identical with that shown by pyrite under the same 

conditions, 0.13. Evidently these values are determined by a certain concentration of sulphide, as hydrogen in a platinum electrode 

gave -0.48 volt.
DETAILED STUDY OF VARIOUS POTENTIALS. 43

In normal potassium chloride in the presence of air galena gives a 

lower value than pyrite. The values obtained with different specimens at different times were as follows: +0.49, 0.38, 0.53, 0.58, 0.49, 

mean + 0.49 volt. Pyrite gave 0.63 volt. When mounted in paraffin 

(fig. 6, p.-38) so as to exclude air, and in contact with water only, 

galena gave a potential slightly lower than in air, +0.46 volt, which 

remained fairly constant over a long period.

Bernfeld gives the following potential measurements for artificial 

lead sulphide electrodes:

Potential of artificial lead sulphide (Bernfeld}.

Volt. 

Normal Na2S, with some Pb(C2H3 02)2............ -0.181 to -0.217

0.5 normal Na2S, with some Pb(C2H3 02 ) 2 .......... - .172 to - .188

NaSH, HoS introduced ......................... - . 0069 to - . 0070

Normal Na2S.............................................. - .196

0.5 normalNa2S ............................................ - .179

0.25 normal Na2S......:.................................... - .160

0.125 normal Na2S......... 1................................. - .140

0.0625 normal Na2S......................................... - .114

0 0312normal Na2S.......................................... - .096

00156normal Na2S.......................................... - .076

Normal NaSH.............................................. - .0069

0.1 normal NaSH ........................................... - . 0521

0.01 normal NaSH........................................... - .1070

These results are in essential agreement with those shown by 

natural galena, ranging from 0.1.9 to 0.24 volt in normal Na2S 

and 0.03 volt in normal NaSH. The potentials 0.49 volt in potassium chloride with air present or 0.46 volt in water with air excluded 

appear to be the most characteristic potentials of galena that I have 

noted, as I have found that potentials in sulphide solutions depend 

very much on the way the solutions are prepared.

The above results with galena indicate that a variation in the concentration of the lead salt has very little effect on the potential. The 

latter is greatly affected, however, by the concentration of sulphide. 

As a matter of fact, galena appears to maintain a fairly constant concentration of sulphide ion on its immediate surface, yielding in salt 

solutions a potential of 0.49 volt. In the presence of air galena 

therefore naturally functions as anode against pyrite. In the absence 

of air its potential may be brought down to low values by soluble 

sulphides, being then a "sulphide" potential.

Knox 1 measured the potential of a lead electrode in 0.1 molal 

Na2S. As a mean of seven determinations of the combination

+ Hg / Hg2Cl2 , normal KC1 / normal KC1 / 0.1 molal Na2S / Pb1 Knox, Joseph, A study of the sulphur anion and complex sulphur anions: Faraday Soc. Trans., vol. 4, 

p. 48,1908.
44 ELECTEIC ACTIVITY IN ORE DEPOSITS.

he obtained 0.773. Allowing a diffusion potential of 0.010 volt, 

which is probably better than making no correction at all, we have

Pb / 0.1 molal Na2S = -0.223 

Inserting this value in the equation

E = 0.129 + 0.0295 log CPb++ 

and solving for C, we find, for Pb++

C = 1.2X10-12

Following Khox and taking the concentration of S in 0.1 molal 

Na2S to be 1X10'3 , we have the solubility product [Pb++][S~] = 

1.2xlO~15 and the concentration of Pb++ or S~~ in a saturated 

aqueous solution of PbS at 25° = 3.4 X 10~8 gm. ion per liter and the 

solubility of PbS in water at 25°, assuming complete dissociation, 

would be =3.4 X 10~8 gm.-mole per liter.

SILVEE SULPHIDE.

Bernfeld gives the following potential measurements for silver 

sulphide, H2S having been introduced:

Volt. 

Normal NaSH.............................................. -0.178

0.1 normal NaSH............................................ - .123

0.01 normal NaSH........................................... - .069

Knox measured the potential of a silver, electrode in 0.1 molal 

Na2S. For the combination

+ Hg / Hg2Cl2 , normal KC1 / normal KC1 / 0.1 molal Na2S / Aghe obtained the value 0.880. Allowing 0.010 volt for the diffusion 

potential, as above, we have, for the single potential, the value

Ag / 0.1 molal Na2S - 0.330 

Substituting in the equation

E = 1.06+ 0.059 log C +

we have C = 4.3x 10~24 for Ag+ . The solubility product of Ag2S is 

therefore

(4.3 X 10~24 ) 2 (1 X 10~3 ) = 1.8 X 10'50

and in a saturated aqueous solution of Ag2S we should have Ag+ = 

3.3X 10~17 ; or, assuming complete dissociation, the solubility of 

Ag2S would be 1.6 X 10~17 mole per liter at 25°.

BISMUTH SULPHIDE.

Bernfeld gives the following potential measurements for bismuth 

trisulphide:

Volt. 

Normal NaSH + some antimony solution..................... +0.017

0.1 normal................................................... + .073

0.01 normal.................................................. + .133
DETAILED STUDY OF VARIOUS POTENTIALS. 45 

MERCURIC SULPHIDE.

Behrend gives two measurements on mercuric sulphide,1 namely, for 

mercury in tenth-normal NaSH, 0.12, and in tenth-normal Na2S, 

 0.28, the solution being saturated with the sulphide of mercury. 

These values are very close to those that I obtained from the soluble 

sulphides with a smooth platinum electrode, as described on page 47.

NICKEL AND OTHER SULPHIDES.

The potentials of several metals in solutions saturated with their 

sulphides were measured by Zengelis.2 It is difficult to determine 

from his published results the correct sign or even the values of some 

of the single potentials, but the values stated in the following table 

are believed to be correct :

Single potentials of metals in solutions saturated with their sulphides, as determined by

Zengelis.

Volt.

Nickel in NiS................................................ -0.02

Silver in Ag2S ................................................ - .34

Cobalt in CoS............. ................................... - .04

LeadinPbS......... ............................... +0.20to + .06

Copper in CuS ................................................. - .085

Copper in Cu2S ............................................... - .203

ELECTROMOTIVE BEHAVIOR, OF SOLUBLE SULPHIDES. 

EEDUCING POWER OF SOLUBLE SULPHIDES.

One of the most characteristic properties of soluble sulphides, 

aside from their ability to form insoluble precipitates with many 

metals, is their reducing power. This is shown in the familiar 

reactions with ferric salts, chromates, ferricyanides, the halogens, the 

nitro group, nitric acid, and even sulphuric acid. If we consider this 

property essentially a property of sulphide ions, the reduction of a 

ferric salt, for example, may be indicated by the equation

According to this equation the sulphide ions give up their negative 

charges while an equal number of positive charges are neutralized; 

free sulphur remains; no other ions appear to be directly concerned 

with the reducing action. The tendency of. sulphide ions to give up 

their charges may also be shown by the production of an electric 

current as illustrated by the experiments on page 13. The single

i Behrend, R., Elektrometrische Analyse: Zeitschr. physical. Cheraie, vol. 11, p. 481, 1S93. 

* Zengelis, K., Ueber die elektromotorische Krafte unloslicher und komplexer Salze: Idem, vol. 12, 

pp. 298-313, 1893.
46 ELECTRIC ACTIVITY IN" ORE DEPOSITS.

potentials created by soluble sulphides under these conditions have 

been designated sulphide potentials. Since ordinary hydrogen sulphide made from commercial ferrous sulphide contains considerable 

hydrogen and since in any given solution of acid or alkali a properly 

prepared platinum electrode saturated with hydrogen should yield a 

lower potential than a corresponding soluble sulphide, it appeared 

desirable to ascertain whether hydrogen potentials could be a source 

of error in measurements of sulphide potentials. With smooth 

platinum electrodes it was impossible to obtain correct-hydrogen 

potentials in the concentrated sulphide solutions, so that the presence 

of hydrogen was not considered objectionable, but in the acidified 

sulphide solutions it was found desirable to avoid hydrogen by preparing hydrogen sulphide from sodium sulphide and hydrochloric acid.

The smooth electrodes used had previously been ignited. It was 

found that 1 to 4 hours were required for such electrodes to yield 

constant values in pure sulphide solutions; in the presence of polysulphides, however, constant values were obtained more quickly, and 

the potentials obtained in the course of 15 to 30 minutes were considered the correct values for such solutions.

On the other hand, the behavior of platinized electrodes was 

irregular and somewhat uncertain. It was found that well-platinized 

electrodes which had given correct hydrogen potentials retained a part 

of the electromotive effect of the hydrogen when placed in solutions 

of sulphides. It was impossible to obtain correct hydrogen potentials, 

however, even with the electrodes barely touching the sulphide 

solutions; when the electrodes were wholly immersed the hydrogen 

effect was less, but a part of it seemed to be permanent as long as the 

solutions were saturated with hydrogen. Platinized electrodes 

appeared to be rendered decidedly less responsive, if not wholly 

unresponsive, by use in solutions of sulphides. For the reasons 

noted the use of platinized electrodes seemed highly disadvantageous. 

The variability of platinized electrodes is shown in the table below:

Effect of hydrogen in platinized electrodes used in sulphide solutions.

Electrode.

A freshly platinized electrode, soaked an 

hour in hot water, then touching normal 

NaOH in bottle.

Electrode now touched to colorless normal 

Na2S.

Smooth electrode. ..........................

Time.

1.23

1.32 

1.37

1.40 

3.07 

4.00 

4.02 

4.07 

 5.00 

09.00 

10.00

10.30 

11.00

, Cell 

(Hg+).

0.12

.13 

1.06

1.07 

1.07 

.96 

.90

.88 

.86 

.88 -.'80

.79 

.80

Platinum.

+0.44

+ .43 

 .50

 .51 

 .51 

 .40 

 .36 

 .32 

 .30 

 .32 

 .24

 .23 

 .24

Remarks.

I An oxygen value. 

^Correct hydrogen value.

A variable . intermediate 

value.

Correct sulphide potential.

Next day.
DETAILED STUDY OF VARIOUS POTENTIALS. 47

KELATION BETWEEN POTENTIAL AND CONCENTRATION.

In his measurements on oxidation cells Bancroft cletermined the 

potential of a fifth molal solution of Na2S, which by reference to a 

normal calomel electrode would yield the single potential 0.091.* 

While studying the behavior of polysulphides Kiister obtained the 

value 0.239 for a half molal solution of Na2S.2 Only a few other 

data, most of them incidental, have come to my attention.3 Experiments were therefore undertaken to find out, for one thing, how 

the potential varies with the concentration of sulphide.

For this purpose the potentials of several solutions were measured 

in half cells made up with submerged smooth platinum electrodes 

connected to the normal calomel electrode, by a normal solution of 

potassium chloride. The results are stated in the accompanying 

table:

Potentials of soluble sulphides.

Solution in half cell.

1 mole KC1+1 mole HC1 saturated with H2S. ...................................

1 mole HC1 saturated with H2S.... . .............................................

1 mole acetic acid saturated with HzS ...........................................

KC1 saturated with H2S. ........................................................

1 mole NaSH. ...................................................................

0.05 mole Na2S ..................................................................

1 mole Na2S. . ...................................................................

Potential 

of cell 

(Hg+).

0.15

.16

.27

.36

.70

.72

.76

.83

Sulphide 

potential.

+0.41

+ .40

+ .29

+ .20

 .14

 .16

 .20

 .27

When the solutions were to be saturated with hydrogen sulphide 

the electrodes were supported in the bottles through which the gas 

was passed, but for NaSH 'the excess H2S was expelled with hydrogen. 

In seeking some relation to express the .variation of potential with 

sulphide concentration a trial was made of the following equation, 

analogous to other well-known equations, in which [S~~] represents 

gram-molecules per liter of the sulphide ion:

E = E0 -0.0295 log [S~]. ............ i. ..(1)

This equation may apply to an irreversible electrode. Following 

Knox * we may apply the mass law to the ionization of H2S. Since 

a saturated solution at 25° is 0.1 molal the expression 5

reduces to

±1O

(I)

1 Bancroft, W. D., Ueber Oxydationsketten: Zeitschr. physikal. Chemie, vol. 10, p. 387, 1892. See 

Neumann, Ueber das Potential des Wasserstoff und einiger Metalle: Idem, vol. 14, p. 228,1894.

2 Kiister, F. W., Beitrage zur Kenntniss der Polysulflde: Zeitschr. anorg. Chemie, vol. 44, p. 445,1905.

» Hittorf, W., Zur Kenntniss der elektromotorischen Krilfte galvanischer Kombinationen: Zeitschr. 

physikal. Chemie, vol. 10, p. 612,1892. Two observations yielded 0.10 and 0.04 for 2.77 normal KSH.

* Knox, J., A study of the sulphur anion and of complex sulphur anions: Faraday Soc. Trans., vol. 4, 

p. 47, 1908; also Zeitschr. Elektrochemie, vol. 12, p. 477,1906. 

BAuerbach, Zustand des Schwefelwasserstoffs in Mineralquellen: Zeitschr. physikal. Chemie, vol. 49,' 

p. 220, 1904.
48 ELECTEIC ACTIVITY IK ORE DEPOSITS.

For the secondary ionization Knox gives

FTT+1 TQrr L^l J L^ ' = 1.2X1CT16 (II)

. 2 ~ [SH-] 

whence by combining (I) and (II) we have

rd- i 1-1 XlO"23 

Lb J - [H+]2

an expression giving the sulphide ion concentration for any solution 

of known hydrogen ion concentration. In this way the sulphide ion 

concentrations of the next table were obtained and from them values' 

of E0 calculated by equation (1).

Sulphide potentials observed and calculated by equation (1).

Solution (concentration per liter).

1 mole HC1+H2S ...................................................

1 mole NaSH .......................................................

Sulphide 

ion 

concentration.

1.1X10-23

-.6X10-18

1. 2X10-18

3.6X10-5

.09

E 

observed 

(volt).

+0.40

+ .20

 .14

 .27

EO 

calculated 

(volt).

_ n 07

 .24

- .23

_ 97

_ on

There is a fair approach to constancy in the values of E0 , the 

mean value being 0.26, so that equation (1) must have some significance in view of the enormous range of the sulphide ion concentration and would appear to be applicable to the determination of 

sulphide ion concentrations provided polysulphides are absent. The 

relations shown, however, can not be regarded as a final solution of 

the problem for NaSH and Na2S, as will appear from what follows.

PREPARATION OF SODIUM HYDROSULPHIDE.

In preparing sodium hydrosulphide the best substance with which 

to start is sodium, as recommended by Kuster and Heberlein, 1 but 

for many purposes sodium hydroxide suffices. If it is desired to 

avoid oxidized products it is advisable to pass a current of hydrogen 

through the sodium hydroxide before introducing hydrogen sulphide, 

and at the same time to have a piece Of platinized platinum in the 

bottle to make the hydrogen more effective through electrolytic 

action. The saturation of sodium hydroxide with hydrogen sulphide 

yields a solution containing an excess of hydrogen sulphide. Part 

of this excess may be removed from a solution as concentrated as

1 Kuster, F. W., and Heberlein,' E., Beitrage zur Kenntniss der Polysulflde: Zeitschr. anorg. Chemie, 

vol. 43, p. 55,1905.
DETAILED STUDY OF VARIOUS POTENTIALS. 49

normal by bubbling a stream of hydrogen through the solution for 

some hours. The changes in the solution from the first introduction 

of H2S to supersaturation and expulsion of the excess may be followed 

with advantage by potential measurements. The results of one 

experiment in which the excess of H2S was neutralized by adding 

NaOH from a burette were as follows:

Effect of neutralizing excess H^S in normal NaSH with NaOH.

4 normal 

NaOH 

added.

Co.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Potential 

of cell.

Volt.

0.605

.612

.635

.700

.785

.794

.797

.798

Remarks.

H2S in excess.

Solution becomes alkaline

phenolphthalein.

Solution practically NaSH,

to

About 16 per cent of NaSH now 

converted into Na2S.

It will be observed that the potential changes vary rapidly when 

the solution has a composition near to NaSH. The value 0.70 for 

the cell, or the single potential 0.14, has been selected as the first 

approximation to the correct potential for a normal solution of the 

composition NaSH. Such a solution is plainly appreciably hydrolyzed, as is shown by the fact that the solution already becomes alkaline to phenolphthalein at a potential which clearly corresponds to 

an excess of H2S. The above measurements also afford a very clear 

explanation of the many discrepancies that have been noted in the 

data of observers working with solutions of NaSH; evidently the 

acidity has not been definitely regulated.

CHEMISTRY OF THE SULPHIDE ELECTRODE.

It has been stated that in developing a current the sulphide ions 

probably give up their charges, leaving free sulphur. This appears 

to be the primary action. When a feeble current (0.1 milliampere) 

was passed into.a solution of hydrogen sulphide through a platinum 

anode a white cloud olsulphur was formed on the platinum and'in the 

solution near the platinum; when sodium hydrosulphide or sodium 

sulphide was employed, however, sulphur was not a visible product 

nor was any gas evolved, but after a time the solution turned yellow, 

.indicating the formation of a polysulphide. The formation of a 

polysulphide may be considered a secondary effect due to the solution 

of sulphur freed by the current in the sodium sulphide present, or it

33477°_Bull. 548 14 4
50 ELECTRIC ACTIVITY IN QBE DEPOSITS.

may be that polysulphide ions are an oxidation product of sulphide 

ions, a possibility that will be considered presently.

The pure sulphide electrode appears to be irreversible. When a 

current is passed in the direction to make the platinum a cathode 

hydrogen is set free. The likelihood that this hydrogen will not give 

a proper potential with a smooth electrode has been pointed out. In 

fact Bose long ago concluded that the saturation of even platinized 

electrodes is an extraordinarily slow process of diffusion of the gas 

into the electrode material. There appeared to be no difficulty, 

however, in attaining the sulphide potential from either side by the 

compensation method.

To gain light on the action of the reversed current experiments were 

made to see whether free sulphur in contact with platinum could 

under any conditions be made to ionize. A solution of sulphur in 

carbon disulphide was allowed to evaporate from the surface of a 

platinum electrode, and the electrode, thus coated with numerous 

crystals of sulphur, was allowed to stand in normal potassium chloride 

in a closed half cell for a long period. The potentials observed are 

stated in the first part of the table.below:

Potential of platinum coated with sulphur.

Solution.

KCL.........................................

NaOH... .....................................

Time.

Start. .......................

1 hour.......................

2 days............. ..........

5 days....................:..

12 days............ ..........

Start. .......................

2days. ......................

Cell

(Hg-)O.lO

.05

. (Hg+) .03

.06

.11

.28

.24 .44"

Potential of 

platinum.

+0.66

.61

.53

.50

.45

.28

.32

.12

It seems that the sulphur may be in part the cause of the observed 

slow fall in potential, but there are so many uncertainties about the 

behavior of a piece of platinum in a neutral solution that the results 

are of questionable significance. . They show, however, that the 

effect, if there is one, is produced very slowly at ordinary temperature.

It is well known that sulphur dissolves in alkali hydroxides at an 

appreciable speed. 1 It is not surprising, therefore, that .a piece of 

platinum coated with sulphur as described above and immersed in 

sodium hydroxide gave a lower potential more quickly, as is shown in 

the second part of the table above. Of course the production of a 

soluble sulphide in this way may be regarded as an action that is 

wholly " chemical" and the electromotive effect as secondary. The 

experiments are therefore inconclusive as to whether the platinum 

assisted the ionization of the sulphur at all.

i Kuster and Heberlein, op. cit., p. 80.
DETAILED STUDY OF VARIOUS POTENTIALS. 51

BEHAVIOE OF POLYSULPHIDES.

The electromotive action of polysulphides is important because 

they appear to be formed by the electrolytic oxidation of alkali 

sulphides as well as by the action of sulphur on those salts. Moreover, they may be reduced electrolytically. There appears to be a 

possibility of arriving at a reversible electrode here. Are polysulphide ions the first direct oxidation product of sulphide ions, rather 

than sulphur, and are they capable of affecting the electromotive 

behavior of the sulphide ions ?

Kiister 1 measured the potential of several solutions containing 

polysulphides, and his data are reproduced for reference in the accompanying tables:

Kuster's data on polysulphides.

[Cell: Pt / N^S, / normal KC1 / HgCl / Hg+.) 

Solutions saturated with sulphur.

Formal concentration of NanS^ 

(saturated 

with S).

2

1

0.5

.25

.125

.062

.031

.0155

.0077 

.0038

x in NasSx.

4.47

4.67

4.84

4.98

5.12

5.22

5.24

5.20

5.04

4.45

Potential of cell.

0.621

.609

.600

.592

.584

.576

.568

.560

.552

.541

Solutions with varying sulphur content.

Composition of solution (half formal).

NazSi.oo. ...............................

Potential 

of cell.

0.799

.787

.766

.750

.731

.677

.600

Kiister's data were supplemented by measurements of the effect 

of dilution, given under the next heading.

EFFECT OF DILUTION.

A few measurements were made on solutions of Na2S. The potentials obtained reached constancy very slowly. Solutions containing

1 Kiister, F. W., Beitrage zur Kenntniss der Poly'sulflde: Zeitschr. anorg. Chemie, vol. 44, pp. 439, 445.
52 ELEOTEIC ACTIVITY IN ORE DEPOSITS.

some polysulphide gave, more easily reproducible values, and measurements of such solutions are shown in the accompanying table:

Effect of dilution on potential of cell.

Formal 

concentration.

2

1 

0.5 

.25 

.125 

.062

.031

.015

NasS.

0.851

.828 

.803 

.783 

.760

NasSs.

(0. 785)

.768 

.752 

.734 

.718 

.700

.682 .

(.665)

Na2S3 .

0.748 

.731 

.716 

.700 

-.683

.667

(.650)

NasS4.

0.696 

.683 

.671 

.658 

' .647

.634

(.622)

Apparently the more polysulphide there is present the less marked 

is the effect of dilution. But the potential of a solution of even the 

composition Na2S4>7 varies somewhat with dilution, and at about 

0.015 formal the solution decomposes, with separation of sulphur.

The potential of pure Na2S appears to change by about 0.07 for a 

tenfold dilution, which is not at once reconcilable with equation (1) 

(p. 47); that of Na2S4 about two-thirds as much. Extrapolating, it 

would seem that it would require a solution of rather high sulphur 

content, possibly as high as Na2S6 , to have a constant potential independent of the dilution. Such a solution can not, however,, be 

prepared.

If we try to resolve such a solution into two components it appears 

that the simpler one might be as complex as Na2S4 , which would make 

the higher one Na2S8 . The other possible components would be equal 

amounts of Na2S3 and Na2S9 , or of Na2S2 and Na2S10, or of Na2S and 

Na2Su, or, lastly, different amounts of two or all of these species. 

The properties of the higher polysulphides are such, however, that 

the assumption of much of the species Na2S in them appears unreasonable.

We can gain some light on this perplexing situation by considering 

the changes in potential when the composition of the solution is varied 

from Na2S to Na2Sx. Measurements were made with this point in 

view, and some of the results are given in the following table.

In seeking a mathematical elucidation of the results let us, following custom, see if the potential can be expressed as a function of any 

two molecular species. For the sake of simplicity let us assume that 

the first oxidation product is disulphide, according to the reaction 

2S-=(S2 )-

We should then have a relation for the oxidation-reduction potential like 

E=E + log-E^L---------------~ (2)-
DETAILED STUDY OF VABIOUS POTENTIALS. 53

In testing this equation it is only necessary to resolve solutions of 

compositions between Na2S and Na2S2 into the proper mixtures of the 

two species as shown, in the table below and assume that the ionic 

concentrations are proportional to the molal.

Potential of cell calculated by equation (2). 

[Pt /1.0 molal Na2Sx /1.0 normal KC1 / HgCl / Hg+.]

Composition x in

Na2Sjc.

1.00 

1.20 

1.40 

1.60 

1.80 

2.00

Formal concentration 

ofNa2S.

1.00 

.80 

.60 

.40 

.20 

.00

Na2S2 .

0.00 

.20 

.40 

.60 

.80 

1.00

Potential 

observed.

0.828 

.800 

.785 

.778 

.772 

.768

Eo calculated by (2).

0.782 

.786 

.795 

.810

As the above table shows, equation (2) does not yield a constant 

value of E0 . In fact, the shape of the potential curve suggests that 

the second species must be more complex than Na2S2 . Apparently 

an excess of sulphur forms at once a higher polysulphide than Na2S2 . 

The nearest approach to a uniform slope in the curve falls between 

Na2S2 and Na2S3 , indicating that if Na2S is one member the other is 

nearly as complex as Na2S4 . The electric oxidation would then be

giving for the potential the expression 

 . .059 log (3)

An evaluation of E0 by this equation is shown in the next table: 

Potentials calculated by equation (5).

Composition x in 

Na2Sx.

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

Formal concentration of 

Na2S.

0.83

.75

.67

.58

.50

.42

.33

.25

.17

Na2S<.

' 0.17

.25 '.33

.42

.50

.58

.67

.75

.83

Potential 

observed.

0.780

.772

.766

.761

.757

.752

.748

.743

.735

Eo calculated by 

(3).

0.778

.770

.768

.767

.766

.765

.765

.765

.764

It will be seen on referring to the table that equation (3) gives a 

fairly constant value of E0 from Na2S2 to Na2S3 . No equation as complicated as this has to my knowledge been applied to potential meas-
54 ELECTBIC ACTIVITY IN" OEE DEPOSITS.

urements before, although Fredenhagen suggested such equations. 1 

An equation corresponding to the intermediate possibility, namely,

3S-=(S3 )- + 4e

was found less satisfactory than equation (3).

 There is one other possibility that of writing NaS instead of Na2S2 

for the "disulphide," giving the reaction 

s-=s-+e

The mathematical test of this possibility showed that it does not fit 

the facts, except incidentally, for compositions near Na2Slt3 .

CONCLUSIONS.

The conclusions from the preceding discussion are then as follows: 

The electromotive behavior of the polysulpliides resembles in a general way that of other reversible oxidation-reduction potentials involving anions. Although it is impossible to account for the behavior 

without assuming that several species are present in a rather complicated equilibrium, it appears unnecessary to cpnsider Na2S2 one of 

them or possibly even Na2S3 . The potentials maybe calculated as if 

they depended on the relative concentrations of the two species Na2S 

and Na2S4 for the range from Na2S2 to Na2S3 . On reaching a composition near Na2S4 a rapid shift to a much higher sulphide is suggested.

If polysulphides are absent equation (1) on page 47 is somewhat 

applicable; if they are present equation (3) appears most applicable. 

Future investigation must discover an expression which will fit the 

facts lor all possible combinations.. For acid solutions, however, 

equation (1) is satisfactory over wide ranges of sulphide ion concentration.

The investigation of the behavior of soluble sulphides has proved 

very instructive with regard to that of the insoluble sulphides if one 

can assume that the potentials noted with the insoluble sulphides 

are due to certain concentrations of sulphide ions or polysulphide 

ions. For example, the potential of pyrite would certainly be expected to correspond to that of a polysulphide, and its electromotive 

behavior is entirely consistent with that view, since it gives a higher 

potential than the monosulphide minerals.

CORRELATION OF THE MEASUREMENTS OF POTENTIAL.

The study of the potential of various combinations of solutions 

and minerals has so far'merely opened up a large number of new 

problems, whose complete elucidation will require further study.

1 Fredenhagen, C., Zur Theorie der Oxydations- und Reductionsketten: Zeitschr. anorg. Chemie, vol. 

29, p. 446,1902.
DETAILED STUDY OF VAEIOUS POTENTIALS. 55

Enough has been done, however, to establish the position of several 

minerals in the electromotive scale. It has been shown that oxidizing, reducing, acid, and alkaline solutions in contact with minerals impart widely varying potentials to the minerals. Neutral 

salts, however, have less effect and it seems reasonable to conclude 

that the potentials shown in neutral salt solutions which are not 

markedly oxidizing or reducing are practically identical with the 

potentials that the minerals would assume in pure water that is, 

they are due to the solution products of the minerals. The chief 

sources of error in obtaining such potentials are: The presence of 

impurities in the minute fractures and pores of the minerals; the 

fact that the atmospheric environment is not, in general, in equilibrium with the minerals and their solution products; and the extreme slowness with which the very insoluble minerals 'attain equilibrium with solutions in contact with them.

It is obvious that the ions formed by the solution of most minerals 

in water-fall into the class of easily oxidizable or reducible substances. 

The mineral potentials are therefore "oxidation and reduction potentials." -In order to present a general view of the results of all 

possible combinations in cells of different types so that the values 

may be compared, it will be advantageous to assemble in one table 

the available data showing single potentials. Such a table has been 

prepared and is presented on page 56, all potentials being stated on the 

assumption that the normal calomel electrode has a potential of 0.56 

volt. The concentrations are normal unless otherwise stated, except 

that several values for oxidizing and reducing solutions are taken 

from Bancroft's data, in which the concentrations were about onefifth normal. An attempt will be made to apply this table to elucidate combinations found in nature.

The table on page 56 shows the oxidizing or reducing potentials of 

a number of solutions, as well as the positions of solid conductors in 

the series when these conductors supply their own solution products. 

It will be noted that-the sulphides cover a relatively narrow range, 

so narrow, in fact, that the potentials which metallic copper might 

show in solutions of different concentration of copper sulphate would 

embrace nearly the whole range. It is this fact, coupled with their 

great insolubility and the slow rate at which they attain equilibrium, 

that has made an exact determination of the positions of the minerals 

difficult. The possible usefulness of a new method of presentation, 

aiming to indicate the direction in which certain chemical reactions 

will proceed, is the main justification of the table. I am well aware 

that future study may alter the positions of the.minerals slightly if 

it shall prove possible to refer their electromotive behavior to reversible equilibria. Nevertheless the table shows a natural association
56 ELECTRIC ACTIVITY IN OEE DEPOSITS.

between certain solutions and minerals, whether one regards the solutions as produced from the minerals or the minerals as produced from 

the solutions. The principal qualification necessary is the great effect 

of varying concentration, which can not easily be shown by such a 

table as that given below. For example, soluble sulphides may impart potentials anywhere from 0.30 to +0.60, according to their 

concentration.

Single potentials of solutions and solid electrodes.

Solution (unattackable electrode), a

Gold solution (aurous) & ................

Sulphuric acid saturated with oxygen...

Acid ferric sulphate. . ...................

Acidified ferrous sulphate /.............

Antimoay solution d. ...................

Neutral ferrous sulphate. ...............

Sodium sulphite /......................

Thousandth-normal sodium hydroxide. .

Potential.

VoUs. 

+ 1.8

+1.47 

+ 1.14

+1.11

+1.09

+1.07

+1.06

+1.03

-u Q";

+ .79

+ .74

+ .68

+ .63

+ .58

+ .56

+ .51

+ .50

+ .41 

+ .29

- .14

- .14

- .27

- .30

- .51

Solid conductor (in water or solution 

stated).

acid.c 

Manganese dioxide in sulphuric acid...

Pvrolusite in water

Potential.

Volts. 

+ 1 84

+1.72

+1.11

, + .96

+ .68

+ .62

+ .60

+ .56

+ .46

+ AA

 ftd.

- .12

- .31

- .32

 66

- .97

o Under certain conditions minerals may function'as unattackable electrodes for many of these solutions, 

but for hydrogen and oxygen platinized electrodes are essential.

6 Abegg, Handbuch der anorganischen Chemie, vol. 2, pt. 1, pp. 674,788,1908.

cTower, 0. F.. Studienuber Superoxyd-Elektroden: Zeitschr. physikal. Chemie, vol. 1?, pp. 17-50,1895.

d Wilsmore, N. T., Ueber Elektroden-Potentiale: Idem, vol. 35, p. 31£, 1900. Solution normal with 

respect to metallic ion.

«Calculated as shown on a succeeding page.

/ Bancroft, recalculated by Neumann, Ueber das Potential des Wasserstoff und einiger Metalle: Zeitschr. 

physikal. Chemie, vol. 14, p. 228,1S94.

9 The value stated is the "electrolytic potential" (Wilsmore) minus (6X0.029),corresponding to a supposed solution of a millionth normalionic concentration.

ft Richards, T. W.. and Behr, G. E., The electromotive force of iron under varying conditions and the 

effect of occluded hydrogen: Carnegie-Inst. Washington Pub. 61,1907. The value stated is that of Richards 

and Behr, 0.15 volt against normal ferrous sulphate, reduced by (6X0.029), as for the other metals.

It is well known that certain oxidation-reduction potentials 

depend on the ratio of the concentrations of two ions of higher and 

lower valence concerned. 1 But if a single pure reagent is taken in 

water an adjustment immediately occurs with the other ions present, 

fixing a concentration of the second member, so that as a rule the 

potentials of single reagents have a perfectly definite meaning. For

i See Peters, R., Ueber Oxydations- und Reductionsketten: Zeitschr. physikal. Chemie, vol. 26, p. 201, 

1898.
DETAILED STUDY OF VARIOUS POTENTIALS. 57

example, such reactions with ferric and ferrous salts may be represented as follows:

+ 2H+ =2Fe+++

Theoretically these reactions must occur and modify the character of 

the water, although the modification of the water is usually neglected. 1 

Except for this action the potentials of solutions of single pure salts 

are not reversible. The oxidizing and reducing solutions are perfectly 

available, however, for furnishing current in one direction, and the 

potentials thus manifested by solutions of single salts are shown by 

the table. In view of the above facts all oxidation and reduction 

. potentials might be regarded as due to certain concentrations of oxygen and hydrogen. Such concentrations would of course usually be 

less than those of saturated solutions; moreover, platinized electrodes 

are necessary to secure a proper electric action of oxygen and hydrogen, whereas, fortunately, .the salts may enter into electromotive 

action directly.

A very interesting distinction has been brought to light as a result 

of the study of mineral electrodes. The solution products of many 

minerals in water are so dilute that their potentials, even with an 

''unattackable electrode," would correspond to very small concentrations of oxygen or hydrogen. This fact led to the thought that 

water itself, in the presence of platinum, may be assumed to have 

certain concentrations of free oxygen and hydrogen in equilibrium 

with it, and that it would be interesting to calculate the potential 

which would be shown by hydrogen or oxygen electrodes, with the 

gases at the pressures possible in pure water. With an ideal inert 

conductor the hydrogen or oxygen might not need to be at atmospheric pressure in order to establish a definite potential. Such an 

electrode in pure water should have the potential 0.676 volt. This is 

calculated as follows:

Lewis 2 has shown that the electrode [normal OH~,O2] has the value 

+ 0.674 volt at 25° C., and according to Wilsmore the electrode 

[normal H+,H2] has the value + 0.277 volt. We may calculate the potentials which would be obtained in pure water in which the hydrogen and hydroxyl ions have the concentration 1.05 XlO~7 by the 

following equations:

E02 = 0.674 + 0.059 log 

 ___________ EH2 = 0-277 -0.059 log 1

'A similar modification of water by silver and copper was established by F. Flscher (Zeltschr. 

physikal. Chemie, vol. 52, p. 55, 1905). 

a Lewis, G. N., The potential of the oxygen electrode: Am. Chem. Soc. Jour., vol. 28/p. 170, 1906.
58 ELECTRIC ACTIVITY IN QBE DEPOSITS.

This would give +1.087 volt for [water, 02] and -0.136 volt for 

[water, H2 ] both gases being at atmospheric pressure.

In figure 7 the two single potentials 1.087 and -0.136 are indicated by the points A and C. We may now calculate the potentials 

of oxygen and hydrogen electrodes at partial pressures less than one 

atmosphere by the following equations:

E0 = 1.087- 0.0592 log 

,og

D

Water

Volts

1.00

0.5

10-30 iQ-20 iQ-io

Atmospheres 

FIGURE 7. Single potentials of oxygen and hydrogen electrodes in water.

If the oxygen is at a lower partial pressure than atmospheric, the 

potential will be reduced by 0.0148 volt for every tenfold decrease in 

pressure. This decrease is shown by the line AB. With a tenfold 

decrease in partial pressure the hydrogen potential rises 0.0296 volt, 

as shown by the line CD. At the point N both gases would have the 

same pressure and the potential, indicated by P, would be 0.679 volt.

The potential P is not exactly the potential which we are seeking, 

since in the purest water conceivable the partial pressure of the hydrogen will be twice that of the oxygen, on account of the dissociation of 

water into hydrogen and oxygen according to the equation

2H20=2H2 + O2
DETAILED STUDY OP VARIOUS POTENTIALS. 59

. It is true that the concentrations of hydrogen and oxygen in water are 

extremely small, but they appear to be related to the oxidizing and 

reducing power of solutions. The correction on account of the slight 

difference in the partial pressures of oxygen and hydrogen at pressures so low would reduce the above potential only three or four 

thousandths of a volt and is therefore almost negligible when compared with the other possible uncertainties in the value. The final 

corrected potential sought is 0.676 referred to the normal calomel 

electrode.

If one could imagine an electrode of oxygen at a lower pressure 

than that corresponding to the point N, it would have a lower potential 

than a hydrogen electrode at the same pressure, and under such conditions water would decpmpose spontaneously. This decomposition 

could in fact occur until the partial pressure of the hydrogen became 

twice that of the oxygen, but no longer, a fact that shows the reasonableness of considering a platinum electrode in a neutral solution at a 

lower potential than 0.676 a hydrogen electrode, following the line 

NO instead of NB. A lower potential in a neutral solution must 

correspond to an excess of hydrogen.

Preuner 1 has calculated the equilibrium constant of the reaction 

2H2 + 02 =2H2O at 20°, finding

(H2 ) 2 (02 ) -78

(H20) 2

This becomes 1.6 X 10~76 at 25°, at which temperature the vapor pressure of water is 0.0031 atmosphere, so that in the presence of liquid 

water we have

(H2 )M02 )_ (0.0031) 2 -i -DXlu 

or

(H2 ) 2 (02 ) = 1.5X10-81

Now in pure water there will still be two volumes of free hydrogen 

for every volume of free oxygen, so that we may write

[2(02 )] 2 (02 ) = 1.5X10-81 

whence

(02 )=7.2xlO-28 

and

(H2 ) = 1.4x10-"

the last two values being the partial pressures of oxygen and hydrogen 

in pure water.

It is a surprising fact that at the partial pressures of oxygen and 

hydrogen thus calculated the potentials of the two electrodes would be

1 Premier, G., Ueber die Dissociationskonstante des Wassers und die elektromotorische Kraft des Knallgaskette: Zeitschr. physikal. Chemie, vol. 42, p. 54,1903.
60 ELECTEIC ACTIVITY IN ORE DEPOSITS.

0.670 and 0.688, if the calculation is made as usual from the values 

1.087 and -0.136 for the two electrodes in water and at atmospheric 

pressure, with a mean value 0.679 volt. 

We now have two values for the potential sought.

From Lewis's and Wilsmore's values when E02 = EH2 __...... 0. 676

From Preuner's data for pHa and p02 ..._..-............... 0. 679

For the present the value 0.676 volt will suffice.

Several interesting distinctions may be drawn in regard to this 

potential. Bearing in mind the fact that it refers to a neutral solution 

we obtain a criterion by which we may decide whether, a solution 

contains an excess of oxygen or hydrogen over the amounts necessary 

to form water. Thus all neutral solutions imparting a higher potential than 0.676 to an unattackable electrode should contain an excess 

of free oxygen, whereas those giving a lower potential should contain 

an excess of free hydrogen. The various oxidizing and reducing 

solutions may contain almost every concentration of free oxygen and 

hydrogen. Thus waters at the surface of the earth will contain 

oxygen at a pressure of about one-fifth of an atmosphere. The 

oxygen in the ground water, however, will be considerably reduced, 

passing through very small concentrations until at a certain depth, 

differing according to the geologic structure, there will be practically 

no excess of oxygen or hydrogen. Below this depth hydrogen will be 

in excess at concentratio.ns increasing with the depth. The differentiating potential 0.676 will be higher in acid solutions and lower in 

alkaline solutions, but it is most convenient to discuss its application 

in neutral solutions. It is a sort of natural zero of potential.

Salts of the metals more noble than copper will behave like oxidizing 

agents that is, an indifferent electrode immersed in them will 

become a cathode with respect to the imaginary electrode of 0.676 

volt. Combinations such as water / metal of the metals less noble 

than copper will act as reducing agents; that is, will function as 

anodes against the reference electrode. According to the same 

criterion the mineral sulphides would be feeble reducing agents, 

although their solution products are probably hydrolyzed slightly, 

yielding feebly alkaline solutions. Considering this fact they may 

indeed be said to lie very near the point where the solution contains 

no excess of oxygen or hydrogen.

The potential shown by an indifferent electrode in a given solution 

may be ascribed, then, to two factors, one of them a certain concentration of oxygen or hydrogen, the other a certain concentration of 

acid or alkali. A potential may be the resultant of many properties 

in a solution, such as acidity, oxidizing power, degree of ionization or 

hydrolysis, and the nature of the cations and anions present, but the 

potential can be interpreted only through the two factors just men-
DETAILED STUDY OF VARIOUS POTENTIALS. 61

tione.d. The line of demarcation drawn above, 0.676 volt, applies 

only to neutral solutions, but every other solution would have, a 

similarly characteristic potential which could easily be calculated 

and which would determine its oxidizing or reducing power. It is 

interesting to observe in this connection that, according to Bancroft's 

original measurements, neutral ferrous sulphate has a single potential 

of 0.63 that is, it is feebly reducing according to the above criterion 

of potential whereas an acidified solution shows a potential of about 

0.79, NaHS03 is given the value 0.66, and the next neutral oxidizing 

agent has a considerably higher value. In other words the potential 

0.676 fits in very well as a dividing line between "oxidizing" and 

"reducing" solutions in Bancroft's data although the point of division 

can not be located exactly, as his measurements included acid and 

alkaline as well as neutral reagents.1 

It is a general rule of wide application that if two reactions c^n

occur simultaneously at the same electrode one will proceed until 

the potential of the other has been reached. A-solution that tends 

to make an indifferent electrode a cathode combined with a solid 

that tends to function as anode in water will produce spontaneously a 

chemical reaction.

By keeping this rule in mind and considering the several possibilities in any given combination we may gain some idea of the 

nature of the reaction that will occur when a given mineral and 

solution are brought together. Consider, for example, a piece of 

iron and a solution of gold. The table on page 56 shows that a 

gold solution is an oxidizing solution (+1.8 volts) and tends to make 

any conductor a cathode. Iron ( 0.32 volt), on the other hand, tends 

to function as anode. On bringing the two together the iron acts 

a moment as cathode until some gold is deposited, then as anode, 

gold being precipitated upon itself and iron going into solution. 

Since neither action alone is polarizable the combined action is not 

polarizable and practically all the gold will be precipitated at the 

expense of the iron.

Now take the combination of pyrite and an acidified ferric sulphate 

solution. Ferric sulphate solution tends to make any conductor 

a cathode with very little polarization. On the other hand, if pyrite 

is made anode in water it is easily polarized and its constituents 

are partly oxidized. The combined result of pyrite acting simultaneously as cathode and anode in ferric sulphate is that ferric 

sulphate is reduced to the extent required to polarize the pyrite 

anodically, a little of the pyrite thereby being oxidized. That this 

occurs is shown electrically »by the electromotive force of the cell:

Platinum / acid ferric sulphate / water / pyrite

i Single potentials compiled from Bancroft's data by Neumann, Ueber das Potential des Wasserstofl 

und einiger Metalle: Zeitschr. physical. Chemie, vol. 14, p. 228,1894.
62 ELECTRIC ACTIVITY IN ORE DEPOSITS. ,

that is, by separating the reacting substances, for otherwise no 

current would be evident. Or it may be shown chemically as follows: 

If pyrite is placed in a very dilute solution of potassium ferricyanide 

at the right concentration a precipitate of Turnbull's blue willbe 

formed slowly from ferrous salt dissolved from the pyrite and the 

ferricyanide. If, however, some acidified ferric sulphate solution 

is added the precipitate forms rapidly a thin adhering blue film on the 

pyrite. The explanation of this action is that the ferric sulphate 

is slightly reduced and the pyrite is oxidized, so that a reaction 

occurs in the boundary layer. It is possible to obtain such coatings 

quickly on many minerals by using solutions well separated from them 

in the potential series.

A combination of any pair of the solutions or conductors whose 

single potentials are given in the table on page 56 will constitute.a 

cell that will yield an electric current on completing the circuit. 

Some of these cells will be very easily polarizable and others less so, 

as was shown in the earlier pages. An extended study of polarization 

would undoubtedly disclose interesting relations between rates of 

oxidation, diffusion, porosity, and other factors, but this study must 

be reserved for another time. The table plainly indicates, however, 

in general, the degree of compatibility of various solutions and minerals with each other; the nearer equal their potentials the greater 

will be the chance of compatibility.

APPLICATION TO ORE DEPOSITION.

GENERAL CONDITIONS.

It is evident from the preceding discussion that ore deposits 

are likely to be the seat of countless differences in electric potential 

between both ores and solutions, the whole system being Unstable 

and subject to continuous change. Geologic changes expose large 

areas to chemical attack. Chemical differences thus produced 

will generate electromotive forces, which, if opportunity is afforded, 

will produce currents operating to equalize the differences in potential. Diffusion in and through the solutions will also generate 

feeble electric currents and, conversely, electric currents may influence the diffusion and transference of matter. The greater electric 

conductivity of solutions as compared with conducting minerals 

will probably restrict the currents to solutions wherever two courses 

may be possible.

If a particle of ore without electric potential could be transferred 

in that condition to a solution it would at once assume an electric 

charge. Without attempting to explain this phenomenon, one 

may say that the particle of ore has become slightly polarized and 

will remain polarized until the system is reduced to electric uni-
APPLICATION TO OEE DEPOSITION". , 63

formity by chemical action or until there is an addition or subtraction of electricity. While it is in the solution the ore will be protected somewhat from chemical attack or, on the other hand, it 

may be made more liable to attack, according as the polarization is 

positive or negative. If a current is generated a chemical change 

will occur either in the ore or in the solutions bathing the ore. It 

has been shown that when the current is in the direction "solution to 

ore" the chemical change is a "reduction." Conversely, a current 

from ore to solution must be accompanied by oxidation. Whatever 

may be the course of the current, secondary reactions may cause the 

precipitation of insoluble films on the minerals that form the electrodes.

It is conceivable that differences in degree of polarization might 

affect the growth or solution of minerals and thus lead to variation 

in crystal form, distortion, unequal development of the different 

faces, and like effects. The polarization need not be caused wholly 

by the solution immediately in contact with an ore; it might be the 

result of electric activity at another point, the current being transmitted through the conducting ore. Though crystallization and 

solution are affected by foreign salts in solution no attempt has 

been made, so far as I am aware, to correlate these effects with 

electric conditions, partly, no doubt, for the reason that the crystallization of electrically conducting minerals can not be studied 

under conditions as favorable as those available for the study of the 

crystallization of more soluble substances. If it is granted that 

different crystal faces have different solubilities this fact would 

demand as a corollary that there should be different degrees of 

electric polarization on the different faces.

Electric activity is most likely to come into play in ore deposits, 

however, when a single mineral, or body of ore is bathed by different 

solutions at different places. The electric circuit afforded by such 

conditions would be like the following cell:

+ Pyrite / acid ferric sulphate / potassium chloride / sodium sulphide

/pyriteIf, on the other hand, two different minerals are involved, the cell 

may be of a simpler type, like the following:

4- Marcasite / potassium chloride / galena 

or

+ Marcasite / potassium chloride / sodium sulphide / pyrite 

At each of the above electrodes some local action, either chemical 

or electric, would doubtless occur immediately; there would remain a 

residual potential, however, which would be available for producing 

further effects, possibly at more or less remote points.
64 ELECTRIC ACTIVITY IN ORE DEPOSITS.

. If a considerable mass of ore is in contact near the surface with 

an oxidizing solution for example, acidified ferric sulphate and 

at depth with a less oxidized solution as ferrous sulphate (there 

being also any circuitous liquid connection) electric action should 

result in the oxidation of the lower solution and reduction of the upper 

solution until equilibrium is attained. The current would pass 

downward in the solid conductor and upward in the electrolytic 

conductor a vein solution, for example in which the current 

would consist in the migration of cations upward and of anions downward. Practically every kind of solution could function to some 

extent at either one end or the other of the cell suggested. Cathodic 

effects, such as those described on page 21, would occur chiefly at 

the upper levels; anodic effects would predominate at the lower 

levels.

By such "chemical action at a distance" 1 it would seem that the 

oxidizing and reducing zones would be extended either in one direction or the other somewhat faster and possibly further than by 

diffusion alone.2 As a matter of fact the process should probably be 

conceived not as extending from a given point to a remote point 

but as creeping along veins by local action, producing an extended 

zone at an intermediate stage -of oxidation or reduction.

In the action suggested above the prime mover, of course, is the 

ferric sulphate produced by the oxidizing power of the atmosphere, 

and this oxidizing power of the surface solution is available through 

electric action at points in front of or below the oxidizing, solution. 

In the same way a reducing solution at depth could exert an influence above it through electric action. As coming events cast their 

shadows before, a solution of sodium sulphide rising through a 

metalliferous vein system would by electric action tend to precipitate gold, silver, and copper on the minerals above. Of course, 

there would be a limit to the extent of either action, a limit set by a 

lack of proper electric circuits and by polarization, but the essential 

point is that electric action may cause " chemical action at a distance."

DEPOSITION OF ORES.

Ores are doubtless formed in a great many ways. Some sulphides 

appear to have been segregated during periods of igneous activity; 

others occur where obviously there was no igneous activity. In all the 

varied changes to which the compounds of the valuable metals are 

subjected fractional solution and precipitation seem to predominate.

1 Ostwald, Wilhelm, Das Chemometer: Zeitschr. physikal. Chemie, vol. 15, p. 399,1894.

2 Percolation is entirely another matter and may be fast or slow according to the geologic structure. The 

electric effects must be of approximately the same order as those of diffusion, yet they may be superposed 

on diffusion effects and also may set up a selective diffusion of certain ions.
APPLICATION TO OEE DEPOSITION. . 65

According to Lindgren 1 the chief causes that produce ore precipitation from aqueous solutions are (1) mingling with other waters and 

(2) reducing agents. It is in the course of these changes that electric 

activity becomes a factor demanding consideration. One way in 

which electric action may effect at least ore solutions, and presumably 

the deposition of ores, by equalizing chemical differences along veins 

has been mentioned. Another effect of electric activity would be to 

keep the ores in a polarized state. The citation of known facts to 

explain the deposition of sulphides must necessarily be conjoined 

with qualifications due to uncertainty about the solutions. In 

general, however, the phenomena of deposition would probably be the 

reverse of those of solution. Other things being equal, an electric 

potential at which one mineral would be stable might differ from that 

at which another mineral would be stable. The several sulphides are 

by no means equally precipitable from solutions containing two or

more metallic salts, and though that fact has heretofore been ascribed 

wholly to specific differences in solubility, there may be reciprocal 

relations between the electro-affinities of the metals and their precipitation by a sulphide which should be elucidated.

In considering the protection from chemical action afforded by 

electric polarization, it may be recalled that some metals are rendered 

"passive" by making them anodes for a short time. Similarly, the 

polarization of sulphides by positive electrification would undoubt^ 

edly preserve them by maintaining around them a film of hydrogen 

sulphide. Some years ago A. N. Winchell 2 studied the rate of solution of pyrite under certain conditions. His results are almost 

valueless for our present purposes because, unfortunately, he placed 

the pyrite upon a screen of metallic aluminum, which must have 

formed an electrolytic couple with the pyrite and to some extent 

protected it. Winchell noted the presence of aluminum salt in the 

solution. Recently Graton and Murdoch 3 have called attention to 

the fact that pyrite is one of the last of the sulphides to succumb to 

alteration but note that in decomposing it maintains a bright, clean 

appearance. The measurements of its potential are consistent with 

these facts and the brightening effect of a very feeble cathodic polarization of pyrite was noted in the electrolytic experiments.

With a large current almost any metal may be plated out on sulphides; with a smaller current and potential, however, a sulphide 

may be formed instead of the free metal, just as a forced electrolysis of an acid or of an alkali salt with a mineral sulphide electrode

i Lindgren, Waldemar, Econ. Geology, vol. 1, p. 40,1905. 

'Winchell, A. N., The oxidation of pyrite: Econ. Geology,vol. 2, p. 291,1907.

$ Graton, L. C., and Murdoch, J., The sulphide ores of copper, some results of microscopic study: Am. 

last. Min. Eng. Trans., Feb., 1913.

33477° Bull. 548 14 5
66 ELECTRIC ACTIVITY IN ORE DEPOSITS.

evolves hydrogen, whereas a feeble current generates a soluble sulphide. Even feeble currents ordinarily precipitate only free gold or 

silver, but the slow precipitation of copper on even pyrite might 

produce some cuprous sulphide. So far, however, cuprous sulphide 

has not been produced electrolytically under conditions that would 

afford unquestionable results. In the electrodeposition of copper for 

analytical purposes, in the presence of sulphates, the copper is sometimes tarnished by sulphide, but it is doubtful whether this observation is applicable in a theory of ore deposition. If, however, electric 

currents might direct metallic ions toward the conducting sulphides, 

the metathetical reactions which would then occur are clearly indicated by Schurmann's series.1 The suspicion that electric factors 

are not wholly negligible, even in metathetical reactions involving 

metallic ions, is greatly strengthened by the fact that there is a rough 

parallelism between Schurmann's series and the "electrolytic potential" series of the metals to be considered below. At present, however, there are absolutely no data on the subject.

More definite statements can be made about the metals than about 

the sulphides, as their behavior has been shown by many experiments 

in electroanalysis and related chemical processes. From a mixture of 

metallic salts in solution the metal of lowest solution tension will tend 

to deposit first in electrolysis, in accordance with the well-known 

"deposition potentials." The following table of electrolytic potentials gives the value assumed by the metal in a solution normal with 

respect to the metallic ion: 2

Electrolytic potentials.

Au +1. 356 

Pt +1-140 

Pd +1.066

Ag +1. 048 

Hg +1. 027

Sb +0.743 

Bi + . 688 

As + .570 

Cu + . 608

H +0.277 

Pb + . 129 

Sn + .085 

Ni + .049 

Co + .045

Tl -0.045 

Fe - .063 

Cd - .143 

Zn - .493 

Mn - . 798

Some of the above values have been changed from time to time as 

better determinations of the ionic concentrations have been made, the 

latest value for silver being 1.06 volts, and for gold, about 1.8.3

It will be noticed that gold, platinum, palladium, silver, and mercury show higher potentials in normal solutions of their respective 

ions than most of the minerals that have been studied show in salt 

solutions. Therefore, in cells constructed as follows:

+ Metal / metal solution / potassium chloride / conducting mineral 

these metals deposit spontaneously until polarization puts a stop to 

the action. In such a cell the mineral is oxidized simultaneously

1 Eor the latest discussion of the application of this series see Emmons, W. H., The enrichment of sulphide 

ores: U. S. Geol. Survey Bull. 520,1913.

2 Wilsmore, N. T., Ueber Elektroden-Potentiale: Zeitschr. physikal. Chemie, vol. 35, p. 318,1900.

3 Abegg, Handbuch der anorganischen Chemie, vol. 2, pt. 1, pp. 674,788,1908.
APPLICATION TO ORE DEPOSITION. 67

with the deposition of the gold, silver, or other metal. In speculating on the starting of such action in nature it may be noted that 

most specimens not only have "points of weakness" but that in 

nature several sulphides are likely to be in association so that 

deposition seems likely to start unequally. No specimens of commercial iron and steel are so uniform throughout in composition 

or texture that some local electric action does not occur when they 

are immersed in solution. 1 In such specimens there are always 

some points of greater "solution tension" than others. We may 

perhaps assume that ores will also vary somewhat in structure or 

composition. When deposition has started, the metal deposited is 

very likely to serve as cathode for the deposition of more, and the 

principal factor limiting electric action is then the polarizability of the 

oxidizing sulphide. (See p. 18.) In this way the natural occurrence 

of filaments and nuggets may be very readily explained as due to 

electrodeposition in which the oxidation of a mineral more or less 

removed from the metal, if only still connected by the thinnest 

metallic thread, is an essential part of the process. That silver 

deposited by electrolysis is prone to form beautiful crystals is well 

known, although I am not aware that gold crystals have been so 

produced in the laboratory. I have observed nuggets of gold, however, which possessed a crystalline "treelike" structure exactly like 

that so well manifested by silver.

In order to see how solutions of the above-mentioned metals would 

affect pyrite, the following experiments were performed: Small 

weighed pieces of pyrite were placed in solutions of gold, platinum, 

and silver. It was found that 5.2 milligrams of gold were precipitated on a gram of pyrite in two days from a chlorauric acid solution. 

Platinum was not visibly deposited in the same time from a chlorplatinic acid solution, but after several days 0.4 milligram was 

deposited. A piece of pyrite gained 0.3 milligram after standing in a 

silver nitrate solution for 10 days. 2

A number of qualitative experiments of a similar character are 

described by Skey.3 One or two quotations will show the nature of 

his experiments:

I therefore agitated a little finely powdered galena with a weak solution of terchloride of gold, omitting the addition of organic matter and taking every precaution 

against its presence accidentally, when I found, after a little while, the gold solution 

had become quite colorless, and on testing it not a trace of this metal could be found;

1 Cushman and Gardner, Corrosion and preservation of iron and steel, p. 51,1910.

3 Very interesting and much more extended experiments of a like nature have since been made by 

Chase Palmer and E. S. Bastin, who investigated the behavior of a large number of metallic minerals as 

procipitants of silver and gold and classified them with regard to their activities. See Econ. Geology, 

vol. 8, pp. 140-170, 1913. See also the results of the work of A. C. Spencer, in Econ. Geology, vol. 8, 

p. 629,1913.

a Skey, W., On the reduction of certain metals from their solution by metallic sulphides, and the relation of this to the occurrence of such metals in the native state: New Zealand Inst. Trans. and Froc., vol. 3, 

p. 225, 1S70.
68 ELECTRIC ACTIVITY IF ORE DEPOSITS.

it had evidently been absorbed, as it were, by the galena, and, in fact, a careful 

inspection of the mineral showed it to be feebly gilded. * * *

Chloride of gold was also found to be reduced by contact with the following sulphides sulphides of iron, copper, zinc, tin, molybdenum, lead, mercury, silver, 

antimony, bismuth, arsenic, platinum, and gold; and among the arsenides, mispickel 

and arsenide of silver; cubical iron pyrites is rather slow in its action upon this 

solution of gold, while sulphide of antimony scarcely affects it at all at first, but after 

some hours contact with it reduction goes on rapidly, perhaps by aid of some voltaic 

action. All these effects were produced at common temperatures (with the exception 

of that with sulphide of bismuth), while other experiments with iron and copper 

pyrites prove that similar effects are produced when all kind of light is excluded, 

BO there is no reason to suppose that light has been concerned in any of these reactions. * * . * .

A portion of the metal of the sulphide operated upon was uniformly found in the 

solution afterward, and also sulphuric acid; the mode, therefore, in which these 

effects were produced was evidently by the oxidation of both the constituents of the 

nucleus employed at the expense of the chloride of gold. * * * Enough has 

been discovered to show that silver and one or more of the metals of the platinum 

series are reduced from their soluble salts by these substances generally. * * *

When common iron pyrites and galena are placed in dilute acids or saline solutions 

within a short distance of each other and connected by platinum wires with a solution of gold chloride contained in a separate vessel, it will be found, after the expiration of a few hours, that the wire connected with the galena has been well gilded 

over that end of it submerged in the gold solution * * *.

These results, taken in connection with the abundance of metallic sulphides in 

many of our mineral veins and rocks, make it appear very probable that much of 

our native gold, silver, and platinum have been electrodeposited from saline solutions by voltaic action set up by the contact of dissimilar sulphides, or sulphides 

with more negative substances, such as hematite, magnetite, or ferruginous rocks.

The tendency of metals to precipitate by electrodeposition may be 

expressed quantitatively by the well-known formulas for the potentials of concentration cells. Since this tendency decreases with 

diminishing concentration, it should be possible to calculate the 

degree of dilution at which solutions of gold, copper, or silver would 

cease to deposit the metals at a given potential. The calculation is 

as follows: Let the given potential be +0.50 volt, a value about equal 

to that found for galena in dilute salt solutions: How dilute must a 

solution of a metal be to give this potential against the metal ?

The following equations express the-relation between the potentials 

of gold, silver, and copper in solutions of their salts and the concentrations of the salts:

ECB-H- = 0.61+0.029 log Cou++ 

EAg + = 1.06 + 0.059 log CAg+ 

EAU + = 1.8 + 0.059 log CAU+

in which C, the concentration of the respective ions Cu**, Ag+, and 

Au+, is expressed in gram equivalents per liter. Substituting 0.50 

for E in each equation and solving for C we have:

Ccu4* = 4.2X10-* 

(V =1.1X10-10 

CAU+ =0.9XiO~22
APPLICATION TO ORE DEPOSITION. 69

Solutions whose normality is lower than the numbers just given 

would have no tendency to overcome a voltage of +0.50. It is seen 

that such a solution would be exceedingly dilute for gold (not quite so 

dilute for the metal as the number given for the aurous ion because 

gold solutions are not completely ionized into aurous ion, but still very 

dilute), very dilute for silver, and rather dilute for copper. Similar 

calculation could be made for other potentials than 0.50 volt.

Deposition of the valuable metals could be brought about not only 

at the expense of an oxidizable mineral in the manner just considered 

but also by a combination of a rather inert mineral like pyrite with 

any reducing solution. The cell would be of this form:

+ Pyrite / metal solution / reducing solution / pyrite 

As a very simple demonstration of this mode of action in the deposition of silver from silver sulphate by sodium sulphide the following 

experiment was performed: A few drops of silver sulphate solution 

were placed on the smooth face of a large crystal of pyrite. Some 

distance away, on the same crystal, a few drops of sodium sulphide 

were placed. The two solutions were then connected by a small 

U-shaped capillary tube filled with water. In the course of an hour 

crystals of silver could be distinguished on the pyrite in the silver 

sulphate solution. This experiment shows how associations of the 

valuable metals with conducting minerals may be produced in nature 

by the electromotive power of reducing solutions at another point. 

This mode of action was mentioned on page 64 under the heading 

"General conditions."

The present study is not to emphasize the fact that sulphides, 

either soluble or insoluble, are capable of reducing solutions of gold, 

silver, and certain other metals, for that has long been known, but to 

show that electric action may produce forms and associations of 

those metals which can not be explained otherwise.

SOLUTION OF ORES.

The solution of ores may be the result of many different chemical 

agencies, but the facts presented in the preceding pages indicate that 

electric action may be one of the factors where conducting minerals 

are present. The evidence afforded by experiment shows that a current passing from mineral to solution will produce oxidation of the 

solution first if it is oxidizable, otherwise of the mineral. The order 

in which the various solutions would be oxidized is shown by the 

table on page 56. Alkali sulphides would go first, then alkali hydrosulphides, if present, and finally the ores, if no further reducing agents 

were present in the solution. Of several ores, the one of lowest potential would tend to be oxidized first.
70 ELECTRIC ACTIVITY IN ORE DEPOSITS.

Although the order in which the substances other than ores would 

be oxidized in ore deposits is here stated, it is believed, for the first 

time, the fact that this principle was applied to ores long ago by Skey 

seems to have been generally overlooked. Skey says: 1

In a natural way, therefore, the contact of dissimilar sulphides generally should set 

up galvanic action and chemical decomposition, and by setting up this action we might 

have an easily decomposable sulphide preserved by the association with it of one still 

more ready to decompose. -

The increased oxidation of one sulphide in contact with another 

was noted in chemical studies by Gottschalk and Buehler before they 

developed an explanation in terms of electric action.

The results of experiments suggest that magnetite, marcasite, 

pyrite, and chalcopyrite should be more resistant to oxidation, and 

that chalcocite, pyrrhotite, and galena should be oxidized more 

rapidly than the former when in contact with them. A very important factor in any electric action, however, is the polarizability of the 

electrodes. The study of their polarizability showed that certain 

minerals were oxidized in the following order, the series beginning 

with the one which would be oxidized most rapidly: Iron, copper, 

silver, chalcocite, galena, pyrrhotite, covellite, marcasite, chalcopyrite, 

pyrite, and magnetite. It should be emphasized that this series shows 

speed effects, which may constitute an important factor in the alteration of the ores. The series by itself is probably not sufficient to 

indicate an order of occurrence in nature, nor does the order correspond exactly with, the electromotive series; both series are significant, but their application must depend on the nature of the conception of the phenomena taking place in ore deposits that is, whether 

they are viewed as equilibria or as effects of varying rates of oxidation. With the principle that the mineral of lowest potential will 

tend to be oxidized first must therefore be linked a second principle 

that every electrolytic process is accompanied by corresponding polarization. For example, pyrrhotite shows a lower potential than pyrite in a salt solution, but if a current passes from the pyrrhotite to 

the pyrite through the solution it forms alkali sulphide around the 

pyrite and the effective electromotive force falls to millivolts or less. 

There are countereffects in electrolytic action at every stage," and the 

speed of such action would under many conditions be limited by processes of diffusion.

Electrochemists are familiar with a class of feeble currents known 

as "residual currents." For example, the application of a low electromotive force to the electrolysis of water with platinum electrodes 

soon "polarizes" the electrodes with hydrogen and oxygen; further 

current flows only as fast as the gases dissolve in the solution and dif1 Op. cit., p. 224. See Hunt, Geol. Survey Great Britain, Mem. vol. 1.
APPLICATION TO QBE DEPOSITION. 71

fuse out of it into the air! The disappearance of an unstable mineral 

might be the supposed ultimate result of electric action between two 

or more ores, but practically this would occur very slowly on account 

of the accumulation of certain products of the reaction; in short, an 

equilibrium would be attained which would by no means be incompatible with the existence of two or more minerals. The disappearance of one would require a very long continued supply of fresh 

solution and the removal of the solution products. Specially rapid 

action, however, would be developed by the alternate drying and 

wetting of particles of different ores in contact. Traces of ferric salts 

that would form on iron minerals when dry would exert high potentials when moistened. This seems to be the condition under which 

Gottschalk and Buehler obtained the most notable effect of acceleration in the oxidation of one sulphide by another in the laboratory. 0

It is hardly necessary to point out that the order of solution of 

metals under electric action would be the opposite of the order of 

their precipitation that is, when in contact the base metals would 

oxidize and dissolve first, then copper before silver, and finally silver 

before gold. Speaking broadly, the base metals would also protect 

most sulphides from oxidation if in contact with them in the presence 

of water, and the sulphides would protect silver and gold. Native 

silver and gold, as we now find them, may therefore have been held 

in their present position for a long time partly by the presence of 

sulphides which have now just about disappeared.

To summarize the electric activity of ores very briefly: Contact 

with solutions as well as certain other conditions impart. electric 

potentials to conducting minerals. When local or electrolytic action 

is possible the chemical and electric differences will proceed toward 

equalization by processes of diffusion, decomposition, solution, oxidation, and reduction until the system reaches electrochemical equilibrium. Measurements of potentials are helpful in indicating the 

direction of possible changes and in giving a quantitative statement 

of the intensity by which the differences tend to become equalized.

All the experimental results on electric potentials that are here 

reported or that may hereafter be presented must necessarily be in 

harmony with natural occurrences, for electric potentials are a quantitative expression of the intensity factor of the "available" energy 

in chemical systems, and every change that takes place spontaneously 

in a system in nature can do so only with a diminution of the " available" energy. Where electric conductors are present, electric activity therefore goes on simultaneously with chemical activity and is 

a means by which chemical differences can frequently be adjusted 

more rapidly than otherwise.
72 ELECTRIC ACTIVITY IF ORE DEPOSITS.

SUMMARY.

A large number of metalliferous minerals are capable of conducting 

electricity and could therefore function as electrodes and as conductors for electric currents in ore deposits. In this paper only bare 

mention has been made of the possible existence of induced earth 

currents and thermoelectric currents, inasmuch as the field results 

of Barus on this point were almost wholly negative. It has been 

shown, however, that the energy which ordinarily manifests itself 

in chemical reactions may be, and in fact will be, manifested to some 

extent in electric action whenever the proper circuits are present. 

Conditions for electric action in ore deposits are by no means unusual. 

One of the simplest possible combinations by which electric action 

could occur would consist in the presence of two different active 

solutions in contact with a single body of ore, the two active solutions 

being united by any "indifferent" electrolyte. The combination 

suggested would have many ramifying variations, depending on the 

amounts of solution available, the position of the ores, and otherlike 

factors. The effects of electric action are naturally somewhat different from those which would result from direct admixture of solutions 

and entirely different mineral associations might be produced through 

such action.

The chemical difference producing the largest electric effects appears 

to be that existing between oxidizing and reducing solutions.' All 

solutions may be arranged in an electromotive series grading, speaking broadly, from the strongest oxidizing solutions, which will charge 

an unattackable electrode most positively, to the strongest reducing 

solutions, which will charge it most negatively. It has been found 

that pyrite, and, to a less extent, several other minerals, are so inert 

to many solutions as to function electrically like u unattackable" 

electrodes for long periods, thus making oxidizing or reducing solutions available for producing electric currents in ore deposits. The 

solution products of minerals themselves may take part in producing 

currents in the absence of more active substances; accordingly, 

different minerals show different electromotive forces in " water.''

The currents generated in any of these ways may cause effects at 

more or less remote points. Electric action on a large scale would 

tend to maintain a common level of oxidization along veins and large 

bodies of ore; where it is effective, the zones of oxidation and reduction would depend on the structure of the ore rather than on depth. 

Electric action on a small scale would polarize ores feebly, favoring 

the disappearance of unstable ores and protecting stable ores up to the 

point of equilibrium. The cathodic and anodic phenomena attending 

the passage of a current from a solution to a mineral, or vice versa,
SUMMARY. 73

have been studied in some detail. It has been found that mineral 

electrodes possess various degrees of polarizability. The polarization 

of minerals would doubtless to some extent influence metathetical 

reactions in ore deposits. Feeble currents might also give a directional trend to metallic ions that would carry them toward the stable 

minerals. Under many conditions the valuable metals would be 

deposited from solutions by electrolytic action and they would 

thereafter be protected from redissolving by contact with any of the 

more oxidizable ores.

INDEX.

A. Page.

Abt, A., thermoelectric series of.............. 20

Air, apparatus for boiling out, figure showing. 35

exclusion of, effect on potential........ 36,39,42

Alkalinity, development of, by electric action. 22

effectof,onoxidizingpower.............. 60

Alien, E. T.,cited............... % ........... 9

Antimony, potential of...................... 56

B. 

Bancroft, W. D., experiments of........... 11,47,61

Barus, Carl, work of....................... 5,6,8,72

Bastin,E.S.,workof........................ 6

Batteries, natural, occurrence of.............. 20

Becker,G.F., cited.......................... 20

work of.................................. 5

Becquerel,A.C.,cited....................... 8

Behrend,R., cited........... -i............... 45

Bernfeld,Incited.................... 9,22,25,43,44

Bismuth, sulphide, potential of.............. 44

Braun, -F., cited............................. 8,25

Brokaw, A. D., work of...................... 5

Buehler, V. A., and Gottschalk; H. A., work

of......................... 5,6,9,70,71

C.

Cadmium, potential of...................... 56

Capillary spaces, deposition in............... 8

in galena, polarization in................. 22

Cathode combination, definition of........... 21

Chalcocite, polarization of.................... 16,18

Chalcocite, potential of....................... 15,29

Chalcopyrite, polarization of................. 17,18

potential of.............................. 35

Chemical action, possible range of............ 64

Chemical differences, electromotive force generated by............... 19-20,62,71,72

Chemical reduction, relation of, to electric reduction ......................... 19-20

Cobalt, potential of, in a soluble sulphide.... 45

Compatibility of minerals and solutions... 55,61,62 

Comstock lode, electric activity in........... 8

Conductivity of minerals, irregularities in

data on.......................... 25

scale of.................................. 24-25

Conductiv ity of solutions.................... 21

Copper, electro deposition of, on pyrite...... 21,66

polarization of........................... 18

potential of, in a soluble sulphide....... 45

in water............................. 56

in solutions of cupric salts........... 68-69

Corrosion by electric action, occurrence of.... 9,21

Covellite, polarization of.................. 16,17,18

Crystals, formation of, possible electric influences on...................... 63

Cupric chloride, electric reduction of......... 23

Cuprous chloride, electrolytic production of.. 22 

potential of.............................. 56

Currents, earth, residual, origin of........... 14-15

production of, by a difference in the

electrode........................ 15

by a difference in the solution....... 14

D. 

Deposition, electric, conditions of............ 65-66

of metals on minerals, occurrence of..... 21

Depth, relation of, to electric potential..... 7,12,64

Diffusion, electromotive force developed by.. 20,62 

Dufet.H.,cited............................. 25

E.

Earth, electric currents in.......... 19-21,26,62-73

electric currents in, intensity of.......... 20,72

free oxygen and hydrogen in solutions in. 60 

See also Ores; Depth.

Electric reduction, relation of, to chemical reduction. ..................... 19-20,71

Electrodes, polarization of................... 12-18

Electrodes, mineral and platinum, comparison of............................... 10

supply of solution to, without air, figure

showing......................... 27

Electrolysis, alteration by................... 21-23

of minerals, osanodes..................... 23

as cathodes.......................... 21

Electrolyte, variation in, effect of............ 9-11

See also Solutions. 

Electrolytic potentials, table of.............. 66

Electromotive force, direction of, between

minerals......................... 15

magnitude and continuance of, from minerals. ............................ 12-19

experiments in, figure illustrating... 13 

measurement of, method of.............. 26

relation of, to chemical differences..... 11,71,72

Electromotive scale of minerals, investigation

of............................... 8,56

Electrostatic separation, theory of........... 24

Emmons, W. H., work of.................... 6

Error, source of, in measurements involving

sodium hydrosulphide........... 49

Eureka, Nev., electromotive forces at........ 8

F. 

Faraday, M., cited........................... 25

Ferric-ferrous sulphate solution, effect of, on

potentials....................... 27

reduction of.......................... 19-20,27

effects of............................. 21-23

Ferric sulphate solution, effect of, on potentials ........... 13,13-14,17-18,28,32,36

effect of, on potentials, apparatus for ascertaining........'............... 28

apparatus for ascertaining, figure

showing......................... 27

potential of.............................. 56

75
76 INDEX.

Ferrous sulphate solution, effect of, on potential..................'......... 14,32,40

potential of.............................. 56

Fisher, F., cited............................. 57

Fox, R.W., cited......................... 7,12,24

G.

Galena, behavior of, with acids.............. 42

electrodeposition on..................... 68

polarization of........................... 16,18

potential of......... 10,14-18,28,29,35,42-44,56

effect of solutions on................. 42

Gold, electrodeposition of.............. 21,22,66-68

polarization of........................... 17,18

potential of........................... 56,68-69

protection of, by sulphides.............. 71

Gottschalk, H. A., and Buehler, V. A., work

of........................... 6,9,70,71

Graton, L. C., work of....................... 6

Graton, L. C., and Murdoch, J., cited......... 65

H.

Heberlein, E., and Kiister, F. W., cited..... 48

Henwood, W.J.,cited....................... 7

Hittorf,W., cited........................... 21,47

Hydrogen, action of, on galena.............. 36

evolution of, on a mineral electrode....... 21

potential of........................... 56,57-60

chartshowing....................... 58

Hydrogen sulphide, neutralization of excess,

in NaSH........................ 49

relation of, to potential..... 36,40,42,46,47,49

I.

Ions, migration of, in electric activity......... 21

Iron, electric deposition of, on pyrite........ 21

polarization of........................... 18

potential of.............................. 56

K. 

Knox, Joseph,cited................... 43-44,47-48

Kiister, F.W., cited.'........................ 47,51

Kiister, F. W.,and Heberlein, E., cited...... 48

L. 

Lead, potential of........................... 45,56

Lead dioxide, potential of................... 56

Lead sulphide, artificial, potential of........ 43

See also Galena. 

Lehner, Victor, work of.......:........,..... 5

Leith, C. K., and Van Hise, C. R,, work of.. 5 

Lewis, G. N., cited.......................... 57,60

Lindgren, W., cited......................... 65

Luther, R., cited............................ 12

M. 

Magnetite, polarization of................ 17,18,41

potential of........................ 10,15,41-42

reduction of............................. 22

Manganese, potential of...................... 56

Manganese dioxide. See Pyrolusite. 

Manganese nitrate, effect of, on potential of

pyrolusite....................... 30-31

sulphate, effect of, on potential of pyrolusite. .............................. 31

Manganite, potential of...................... 31,56

Page.

Marcasite, influence of, on oxidation and solution............................. 9

polarization of........................ 16,17,18

potential of............ 15,27,28,29,35,39-41,56

reduction of electrolyte by............... 22

Membranes, semipermeability of............. 8

Mercuric sulphide, potential of............... 45

Mercury, electric deposition of............ 22,66-67

potential of............................... 56

Minerals, combinations of solutions and,

effects of................ 12-19,60-62,72

currents flowing from solutions to. See

Solutions.

disappearance of, conditions governing.. 70,71 

electric action of, in solutions of themselves.'.... ................. 29-30,38,72

electric and chemical oxidation of, relations of........................ 19-20

electromotive scale of................. 24-25,56

investigation of...................... 8,9

observation of, in solution, cell for, figure

showing......................... 29

oxidizing powers of...................... 16,17

polarization of........................... 12-18

potentials of............................. 55-56

effect of earlier solution on........... 30

effect of different solutions on.. 10-14,19,26 

apparatus for testing, figure 

showing....................... 13

in water............................. 29

thermoelectric scale of................... 20

See also Ores. 

Minerals, different, potentials of, in the same

solution.......................... 15,19

Murdoch, J., and Graton, L. C., cited........ 65

N.

Nickel, potential of.......................... 45,56

Nuggets, production of...................... 67

O.

Ores, combinations of, resemblance of, to

galvanic combinations........... 7

deposition of............................. 64-69

application of electric activity to..... 62-73

beginning of......................... 67

electric currents in, effect of.. 6,65,71,72-73 

generation of........................ 62-64

electric oxidation of................... 23,69-70

solution of............................... 69-71

order of............................. 71

See also Minerals. 

Ostwald, W., cited.......................... 8,64

Oxidation, order of, relation of potentials and. 69 

Oxygen, potential of..................... 56,57-60

potential of, chart showing.............. 58

P.

Palladium, electric deposition of............ 66-67

Palmer, Chase, work of...................... 5

Peters, R., cited............................. 11

Platinized electrodes, effect of, in sulphide

.solutions........................ 46
INDEX. 77

Page.

Platinum, electrodeposition of ......... 21, 22,

polarisation of ..................... 13-15, 16, 17

potential of ......... 10, 13-14, 16-17, 28, 29, 35, 56

Polarization, possible cause of, in ores ........ 63

effect of, on ore deposition .............. 65, 70

of minerals, as anodes ................... 16

as cathodes ......................... 18

of pyritein detail.. ...................... 36

Polysulphides, soluble, decomposition of, by

water ........................... 52

electromotive behavior of. ............... 51-54

effect of dilution on... ............... 51

probable complexity of .................. 53-54

Potassium arsenite, potential of ............. 56

Potassium chlorate solution, effect of, on potential .......................... 14

Potassium chloride solution, effect of, on potential... 10,14-18,31,34-36,39,41,42,50 

Potential, apparatus for obtaining, of mineralsin water... ................. 38

constancy of .................... 10-11, 26, 29, 34

effect of different solutions on ............ 10, 55

oxidation and reduction, variations of . . . 11 

relation of oxidation and. ................ 69

variation in, relation of, to oxidizing

power of solutions ............... 11-12

Potential, single, measurement of. ........... 26-45

measurement of, correlation of ........... 54-62

errors in, cause of. ................... 55

See also particular minerals. 

Premier, G., cited. .......................... 59-60

Prospecting, relation of electric activity to ... 6, 8 

Pyrite, anode and cathode changes in. ....... 23

electric deposition on ................. 22, 23, 67

influence of, on oxidation and solution. . 9, 72 

oxidation of ............................. 19

polarization of ................... 16, 17, 18, 36-37

chart showing ....................... 37

potential of. ..... 10, 12-18, 27, 28, 29, 32-39, 54, 56

lag in. ............................... 37-38

relation of, to electrolyte .... 32-33, 38-39, 72

reduction of ferric sulphate by ........ 19, 21-23

saturated solution of, apparatus for, figure

showing. . ....................... 38

solution of. .............................. 65

Pyrolusite, polarization of ................ 16, 17, 32

potential of. .......................... 30-32, 56

reduction of ...............:.........'.... 22

Pyrrhotite, anode and cathode changes in .... 23

polarization of. ..................... .\ 16, 17, 18

potential of. ................. 10, 15, 27, 29, 41, 56

reduction of electrolyte by. .............. 22

R.

Reductions, effects of electric. ............... 21-23

Reich, F., cited........ ...................... 7

Residual currents, origin of ............. 14-15, 70-71

S. 

Schurmann's series, application of, possible

qualiflcationin.................. 66

Silver, electrodeposition of ......... 21, 22, 66-67, 69

potential of.. ...................... 45,56,68-69

protection of, by sulphides .............. 71

by copper........................... 71

Silver sulphide, potential of................. 44,45

Skey, W., cited............................. 7-8

quoted............................... 67-68,70

Smith, G. 0., preface by.................... 5-6

Sodium hydrosulphide solution, potential of. 56 

preparation of.........-............-.'..... 48-49

source of error in measurements involving...........................'... 49

Sodium hydrosulphide, relation of, to potentials................. 28-29,33,43,44,45

Sodium hydroxide solution, potential of..... 56

relation of, to potential.................. 11,

14-15,30-31,32-33,39,41,42,50

Sodium polysulphide solution, reduction of.. 23

relation of, to potential.................. 32

Sodium sulphide solution, dilution of, effect

on potential..................... 51-54

potential of............................. 56

electric formation of, from minerals...... 22.23

relation of, to potential............ 10,14,33,43

Sodium sulphite, potential of................ 56

Solutions, character of, importance of, in determining electromotive behavior .............................. 26, T2

combinations of minerals and, effects

Of......................... 12-19,60-42

currents flowing from minerals to........ 23

currents flowing to minerals from, effects

Of............................... 21-23

effects of, on mineral potentials.......... 6,

10,12-15,20,26,29-30

 apparatus for testing, figure showing. 13 

observation of minerals in, cell for, figure

showing......................... 29

oxidizing power of, relation of, to potentials............................. 11-12

supply of,- to electrodes, without air,

figure showing................... 27

variation in, effect of.................... 9-11

Spencer, A. C., work of...................... 67

Stannous chloride, potential of.............. 56

Stromberg, A. von, cited.................... 7

. Sulphide electrodes, behavior of............. 49-50

Sulphide potentials, definition of............ 39,46

Sulphides, iron, potentials of................ 27

Sulphides, solid, thermoelectric action

between......................... 20

Sulphides, soluble, bearing of, on behavior

shown by minerals.............. 54

concentration of, relation of, to potential.. 47-48 

electromotive behavior of................ 45-54

potentialsof, relation of, to concentration. 47-18 

reducing power of....................... 45-46

Sulphides as anodes, effect of current on..... 23

Sulphides as cathodes, effect of current on... 21-23 

Sulphur, possible ionization of............... 50

potential produced by presence of....... 49-50

Sulphuric acid, effect of, on potentials. 10,14,30-31,32 

potential of.............................. 56

T.

Temperature, effect of, on conductivity...... 25

Thermoelectromotive forces, development of. 20 

Tower, O. F., on potential of pyrolusite..... 30-31
78

v.

INDEX.

Page.

Van Hise, C. R., and Leith, C. K., work of.. 5

W. 

Water, potential of minerals in........ 29-30,38,72

modification of, by dissolved salts....... 56

by metals........................... 57

by minerals......................... 56,60

free oxygen and hydrogen in. 59

Wentworth, H. A., on conductivity of minerals............................. 24

Wilsmore, N. T., cited.... . .................. 60,66

Winchell, A. N., cited......!... ............. 65

Z. 

Zengelis, K., cited. .......................... 45

Zinc, potential of . 56

o