Sulphate Minerals of the Comstock Lode, Nevada

SULPHATE MINERALS OF THE COMSTOCK LODE, NEVADA. •

CHARLES MILTON AND W. D. JOHNSTON, JR.

ABSTRACT.

Seventeenrepresentative samples of supergenesulphates from old workings on the ComstockLode are described. They range from simple minerals such as gypsum and epsomiteto complex aggregates of four or more distinct species. All are well known speciesexcept a mineral of the copper (chalcanthite) or magnesium sulphate pentahydrategroup, with about half the magnesium replaced by copper, zinc, ferrous iron, and manganese. Four new analysesof ComstockLode mine waters are given.

CONTENTS. Introduction ...... 749 Sulphate minerals ...... 75: •I. One-mineral samples ...... 753 Gypsum ...... 753 Epsomite ...... 753 Melanterite ...... 756 Goslarite ...... 756 B. Two-mineral samples ...... 759 Pickeringite and epsomite ...... 759 Pickeringite and RO pentahydrate ...... 762 C. Complexsamples with ferric sulphates...... 764 Pickeringite,copiapite, and epsomite...... 764 Pickeringite,copiapite, alunogen, and coquimbite...... 764 Pickeringite,epsomite, copiapite, and voltaite ...... 766 Alunogen,epsomite, gypsum, copiapite, and rhomboclase... 767 Compositionof the samples...... 769 Mine waters ...... 769

:NTRODUCT:ON. IN Tr:•. courseof a recent investigationof the accessiblemine workingsof the ComstockLode, all of the membersof the party were impressedby the varietyof form and colorof the supergene : Publishedby permissionof the Director, U.S. GeologicalSurvey. 749 750 CHARLES MILTON AND W. D. JOHNSTON, JR. sulphatesthat formed stalactites,crusts, and efflorescenceson the walls and timbers of the old workings. To determinethe compositionof thesedeposits, Dr. V. P. Gianella,of the Nevada State Bureau of Mines, and Mr. Johnston collectedseventeen representativesamples of the sulphates,sealed them underground in tin cansby meansof friction tape and shippedthem for ex- aminationto the ChemicalLaboratory of the GeologicalSurvey. At the sametime four half-gallon samplesof mine waters were collectedand sent to the laboratory. The waters were analyzed very soonafter opening. The sulphateswere exposedto the air of the laboratory for variable periods of one to eight months; there appeared,however, to be only minor changesin composition, if any, resultingfrom the exposure. The sampleswere obtainedin the following workings: •. The Central Tunnel and connectedshallow workings in Con- solidatedVirginia, California, and Ophir ground. 2. The tunnel of the Arizona Comstock and connected shallow workings in Hale and Norcross ground. 3- The Silver Hill mine, shallowworkings. 4- The Sutro Tunnel and its north and south laterals. 5. The Alta shaft; old workingsapproximately 50 ft. abovethe •,•oo or Sutro Tunnel level. Fig. • is a sectionthrough the Sutro Tunnel showingthe ap- proximateposition of someoœ the samples.

•. M•if;ude •o,ooo' •,ooo' •P,c • ANDE$ITE$ Epsom•te a$oo' aooo Section thru :the Sutro Tunnel FIG. I. A sectionthrough the Sutro Tunnel showingthe approximate positionof someof the samplesdescribed. SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 751

Thanks are due to Dr. Gianella who aided in collectingthe specimens;to Miss Margaret Foster, of the Water Resources laboratory of the Survey, for assistancein the determinationof fluorinein the waters; to Miss J. J. Glass for preciserefractive index determinations;and to Dr. W. T. Schaller, of the Chemical Laboratory of the Survey, for unfailing advice whenevermin- eral0gicaldifficulties appeared.

SULPHATE MINERALS.

The seventeenrepresentative samples of the incrustations, stalactitic formations, and similar depositsin the mine are all sulphates,varying from simple minerals, such as gypsum, to complexmixtures of four or more distinct species. The individual crystalsare, for the mostpart, of microscopicsize and the specimensare not suitablefor any precisegoniometrical determinations. With one exception,all of the samplesconsist of well-knownmineral species, the exceptionbeing a mineralof the copper or magnesiumsulphate pentahydrate group, with about half the magnesiumreplaced by copper,zinc, ferrous iron, and manganese,in the order named. The identificationof the different mineral specieswas basedon the usualpetrographic and analyticalprocedures, supplemented by microchemicaltests where indicated. The object of the investi- gation being the identificationof the mineralspresent, exact and refined analytical methods were not called for. The hetero- geneousand varying nature of the material availablefor analysis would have made preciseanalyses of no greater value than less exact, though sufficientlyaccurate, procedures. For most of the samples,therefore, .the different metallic oxides and the acid (sulphuricin eachcase) were determinedby the usualmethods, and, after determination of the insoluble fraction, water was reckonedby difference. A fair degreeof correspondencewas obtainedbetween the compositionof the sample,as computed from the analysis,and the observedminerals, shown by the microscope. 752 CHARLES MILTON AND W. D. JOHNSTON, JR.

A test for silver was made on a compositesample of the seventeen sulphatespecimens with negativeresults. The following specieswere identified: Gypsum CaO'SO3' 2H20; EpsomiteMgO' SO•' 7H20; MelanteriteFeO' SO3'7H20; Goslarite ZnO'SO•'7H20 including magnesian and cuprian varieties; Magnesium-copper-zincsulphate pentahydrate; Picker- ingite MgO'Al.•'4SO•'22H. includingzinc, iron, and manganese varieties; Alunogen AI.•'3SO•'•6H•O; Copiapite 2Fe.Oa'5SO•'•9H20 (?); Coquimbite Fe.•'3SO•'9H20; Voltaite 3(K.e)O'2(A1, Fe)20•-6SO•'9H. (?). Of uncer- tain determinationwere: RhomboclaseFe.•'4SO•'9H.; and somepoorly crystallizedbasic sulphates. The seventeensamples may be groupedinto three divisionsas follows:

A. Simple one-mineraldeposits. There are eight of these. Gypsum (S.S. 6200) and (36J45) Epsomite (36JI7), (36J2x) and (36J36) Melanterite (36J9x) Magnesiangoslarite (36.•x6) Magnesian cuprian ferroan goslarite (36.•I5) B. Two-mineral deposits. There are five of these. Pickeringite-epsomite(36JxI), (36JI8), (36J3I), and (ST. 2260) Pickeringite--RO pentahydrate (36Jx2) C. Complexdeposits with ferric sulphates. Four of these. Copiapite-pickeringite with a little epsomite and basic ferric sulphate (36JI9) Copiapite-pickeringite-epsomite-voltaite(36J2o) Copiapite-pickeringite-alunogen-coquimbite(SN. x9oo) Copiapite-alunogen-epsomite-rhomboclase(36Jio) The first group includesfairly water-solublesulphates which, except possiblymelanterite, neither hydrolyze to basic salts nor oxidizeto insolublecompounds. They are.thus easilycarried in solutionand effectivelyremoved from the place of their origin and associationof contemporaneously-formedminerals. The secondgroup is a mixture of pickeringirewith the hepta- or penta-hydrateof magnesiumsulphate; these are also quite soluble, and their solutionshould be fairly stableunder a variety of SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 753 conditions. In general, conditions favoring precipitation of pickeringiteshould favor precipitationof the accompanying minerals. The third group is more complex, some of the constituent minerals being of low solubility. It is characterizedby the presenceof the ferric sulphateminerals. It is unlikelythat these have traveledfar from their pointsof origin; they indicatecom- plex equilibria betweenoxidizing acid solutionsand solid rock, whichsupplied the aluminum,potassium, magnesium, etc., present in theseminerals. The oxidationof pyrite would furnish the acidferric-iron sulphates characteristic of thisgroup.

,4. One-mineralSamples. Gypsum.--Specimenno. S.S. 62oo, from the southlateral of the Sutro Tunnel, 6,200 feet from its junction with the main tunneland specimenno. 36J45 from a stopeabove the I,Ioo-foot level of the Alta shaft. Radiating blades,several millimeters long, and in more finely crystallizedaggregates. Epsomite.--Specimenno. 36J36, from the Arizona-Comstock 365 foot level. Abundantbeard-like masses pendant from the roof of the drift. In placeslarge masseshave fallen from their supportand collectedon the floor or on old timbers(Figs. 2 and 3). White and finely fibrous almost like asbestos,with a little water-insolublebrownish clay. Fairly puremagnesium sulphate, with a little zincand manganese,probably less than oneper cent. Under the microscopethe fibersare seento be not homogeneous; undercrossed nicols they are seento be madeup of a fine-grained mosaicof variouslyoriented particles. This indicatespartial dehydrationof the originalheptahydrate to a lowerhydrate. Specimensnos. 36JI 7 and 36J2I from the 23I foot levelof the CaliforniaMine, accessiblein I936 throughthe CentralTunnel. Beardlikemasses pendant from the roof of the drift, and composedof a felt of delicatewhite fibers with the propertiesof epsomite. Sampleno. 36J2I containedabout I2 per centof water insolubleearthy material and analysis of the watersoluble portion gave F•G. 2. "Feathers" of epsomite on quartz vein. Arizona Comstock 365 level. F•o. 3- Epsomire growing behind and falling on lagging. Arizona Comstock365 level. F•G. 4- Calcite deposited on lagging and coating a barrel. Sutro Tunnel. x4,2oo feet from portal. Analysis of depositing water is given on page 77o. Fro. 5. Pickeringite and epsonritegrowing between lagging. Sutro Tunnel, north lateral. Fro. 6. Pickeringite and epsomireaggregate breaking lagging in roof. Sutro Tunnel, north lateral. Fro. 7- Light colored cauliflower-like masses of pickeringite and epsomireand dark stalactite of complex ferric sulphates. Sutro Tunnel, north lateral. Fro. 8. Pickeringite and epsomiteshredding lagging in zone of pyritized andesireremote from the Lode. Sutro Tunnel, 2,xoo ft. from portal. 756 CHARLES MILTON .AND W. D. JOHNSTON, JR.

Ratios

A12Oa, Fe2Oa, Mn2Oa ...... CaO ...... MnO, ZnO ...... MgO ...... x4.35 356 I .OO SOa ...... 28.77 359 I .OI H20 (by diff.) ...... 55.88 3IO4 8.72

IOO.OO

TheRO: SO3 is almost exactly •: •; theratio of H20exceeds the 7:I c;f epsomite,but thismay be dueto moistureof the sample,and to the water contentof the insolubleportion (which was weighedafter ignition). Melanterite.--Specimenno. 36J9I. from the footwall gougeof the 60 foot level of the Silver Hill mine. Leek-greenor bluish- green crystals,a centimeteror so across,partly oxidizedto a yel- lowish-brown pasty mass. The sample was moist, yielding a small excessof water. The analysisgave

Ratios

Insoluble ...... 1.5 ø AI•Oa ...... 1-09 II Fe•Oa ...... 66 FeO ...... 23.4X I.OO MgO ...... ßI 4 3263 SOa ...... 27.15 339 1.03 H•O (by diff.) ...... 46.05 2555 7.77

IOO.OO

The analysisindicates that the sampleis about 92 per cent melanterite,FeSO•' 7H20. Goslarite.---Twosamples of goslaritewere analyzed. Both samplescontained several per centof MgO and onesample con- tained,in addition,appreciable quantities of both CuO and FeO. In both samples,however, ZnO is the dominantbase. Specimenno. 36J•6, from the x45 foot or Centraltunnel level of the Californiamine. Vitreousand clear, very pale bluish- white crystals,in a massivecrust. The exterior is altered to a ßSULPHATE MINERALS OF C03ISTOCK LODE; NEVADA. 757 dull white powdery substance;however, optical examination showslittle differencebetween the fresh and altered portions of the crystals,except that the dull exterior coating consistsof a mass of randomly oriented brightly polarizing particles. The clear grains yield good negative interferencefigures of small optic angle; the refractive indices are close to X = •.447, Z --- •.47o. Neglectingthe minor constituents, the analysis indicates a composition of about 54 per cent ZnSO4'7H20 and 46 per cent of MgSO4'7H20. The calculatedindices of refraction are: a---•.446, y--: •-473, agreeing closely with the determined values. A very small quantity of a fibrousmineral resembling pickeringiteis also present. An analysisof the white crustgave

Ratios

Insoluble (mostly FelOn) ...... •6 CuO ...... •9 • ) MnO ...... 2o 3 [ ZnO ...... •4.o5 •73 ( 349 •.oo MgO ...... 6.9o • 7• J AI:Os, Fe•Os ...... 3• 3 SOs ...... 30.o0 375 •.o7 H:O (by diff.) ...... 48.•9 2674 7.66

I OO.OO

This analysiscomputes reasonably close to (Zn, Mg)O'SOa' 7H_oO98 per cent and pickeringite2 per cent. Although the percentageof ZnO is more than twice that of MgO, the molecularratio of the two oxidesis nearly • :I, with, however, the ZnO in slight excess. Hence the material is re- ferred to goslarite,and may be calleda magnesiangoslarite. Specimenno. 36J•5 from the Central tunnel level of the Cali- fornia mine. Pale bluish-greenstalagmitic masses and incrustations, slightlycoated with a dull white material. The distribution of the color is irregular,the greenishcrystals grading into color- less or white. 758 CHARLESMILTON AND W. D. JOHNSTON,JR.

The measuredindices of refractionof the bluish-greenmaterial are a: x.463, fi--x.475, Y = x.48o, all q-.003. Optically negative, 2V moderate. _Ananalysis of the blue-greenmaterial gave

Ratios

Insoluble in water (mainly Fe2Os)... Fe2Os, AI•Os ...... CuO ...... 4.27 PbO ...... None FeO ...... 2.85 364 1.00 MnO ...... 30 ZnO ...... xx.77 I45 [ MgO ...... 4.86 121 ) SOs ...... 29.41 367 1.0I HiO (by diff.) ...... 46.xx 2562 7.04

IOO.OO

The ratios agree very closelywith the general formula RO: SO•. 7H20. Accordingto the componentcomposition calculated from the analysis,as shownbelow, the calculatedindices of re- fractionare: a: 1.454,/• = x.472, Y•--- 1.478- a as determined is x.463, the differenceof .0o9 being greater than the error of determination,/• and y as determinedare slightly higher also, but not more than the possibleerror of determination. The quantity of water present is almost exactly that required for the heptahydrate; it may be however that a slight degree of dehydration exists, which would tend to raise the indices; the MgSO4'7H20 on dehydrationto MgSO4'5H.oO changesin indices,a from x.433 to x.482, /• from 1.455 to x.492, y from 1.46• to x.493. The greatestchange is in the 'a index. Zinc oxide is the dominantbase, both in percentagevalues and ratio, hencethe material is goslarite,with MgO, CuO, FeO, and MnO isomorphouslyreplacing a portion of the ZnO. The com- ßposition of the samplemay be explainedas an isomorphousmix- ture of the following components,in weight percentage,as calculatedfrom the analysis: SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 759

ZnO. SOa. 7H•O ...... 42 MgO. SOa- 7H•O ...... 30 CuO. SOa. 7H•O ...... x6 FeO. SOa- 7H•O ...... x x MnO. SOa. 7H•O ...... x

IOO

Of the analysesof goslarite listed by Hintze 2 several have about the samequantities of either MgO, CuO, FeO, or MnO, as the material here described;but none of the ten analysescon- tains suchquantities of all thesebases.

B. Two-mineral Samples. Pickerin#iteand Epsomite.--Fourof the five specimensin this groupare mixturesof pickeringiteand epsomitewith pickeringitc dominant,forming from 70 to 92 per centof the specimen. The four analysesare grouped together in Table I in the order of increasingpercentage of pickeringite. Specimenno. 36J•8 from the 23I foot level of the Central tunnel.Tough matter felt 6f fibers,generally white but in places paleblue, encrustingold timbers. Small irregulargray particles (up to 2 mm.) of quartz are disseminatedthrough the crust. Someof theepsomite is in relativelylarge clear masses and some of it in the mosaic-like"pseudomorphous" aggregate. Specimenno. 36J•, from the 242 foot level of the Central Tunnel. A heavycrust deposited behind the lagging,generally of a Whitecolor, but locally very pale blue. Microscopically,it is seento consistof pickeringiteand epsomite,with muchof the latter apparentlytransformed to a mosaicof brightlypolarizing particles,possibly a lower hydrate. The pickeringitegave a strongtest for zinc with mercuricthiocyanate. However,only 0.99 per cent of zinc oxide was presentin the compositesample analyzed,and the indicationsare, therefore,that the epsomiteis substantiallyfree from zinc. Specimenno. 36J3• , fromthe Central Tunnel level. Stalac- titic, very paleblue crustson timbers. A massof radiating fibers,mainly pickeringite in finelytwinned fasicules. A very v.Hintze, Carl: Handb.d. Mineralogle.•: pt. 3, 4353, •93o. 760 CHARLES MILTON AND W. D. JOHNSTON, JR. little quartz is present. Another undeterminedmineral, resem- blingvery thin flakesof muscovite,is sparselypresent. The two observedindices are closeto •.46; the mineral is opticallynega- tive with a moderateaxial angle. A lower hydrateof magnesium sulphateis a possibility. Specimenno. ST 2260 from the Sutro Tunnel is a white fibrouscrust slightly coated with a yellowishor light brownfilm. The analysesof thesefour samplesare givenin Table I.

TABLE I.

ANALYSES OF FOUR SAMPLES OF MIXTURES OF PICKERINGITE AND EPSOMITE.

Sampleno ...... 36Jt8 36Jxx 36J3x ST 2260

. --

Composition: Pickeringite ...... 70 80 84 92 Epsomire...... 30 20 i6 8

Insoluble ...... 2.22 0.28 0.26 0.I 7 MgO ...... 6.28 6.75 6.I5 5.54 MnO ...... 72 .66 .63 .I3 FeO ...... None None .34 . I I CuO ...... 52 .I7 .87 ' .... ZrrO ...... x.57 .99 .IX .... A12Oa...... 7.42 8.33 9.82 9.9 ø Fe2Oa...... 75 .x8 None x.I I SOs...... 35.78 36.02 36.07 36.83 H20 (by diff.) ...... 44-74 46.62 45.75 46.2x

IOO.O0 IOO.OO IO0.00 I00.O0

Pickeringiteand epsomite appear to markzones in a highly pyritizedandesite. They are abundant in bothnorth and south lateralsof the Sutro Tunnel and occurin many placesalong the maintunnel. Approximatelyone-half mile from the portalof thetunnel (specimen ST 2260) andremote from the lodeitself is a zoneof pyritizedandesite conspicuously marked by theabun- dantsulphates in andbehind the lagging(Fig. 8). Here py- ritizationand subsequentoxidation are laterthan the precious metalmineralization, and probably later than the pyritizationof the wall rocks of the Comstock Lode. The depositionof pickeringiteand epsomite causes much dam- ageto timbersunderground (Figs. 5 to 8). Laggingis dis- SULPHATE MINERALS OF COMSTOCKLODE; NEVADA. 761 placed,broken, and even shreddedby the growingsulphate masses. The fifth specimenof groupB, specimenno 36J•2, from the 242 foot levelof the Centraltunnel, is a massivecrust, deposited on the lagging. It is composedof two distinctsubstances, the principalone beinga light greenish-blue,massive mineral; the other,coating this, a palebrown finely fibrous and radiallycrys- tallized druse. The bluish mineral can be obtained in sufficient quantityfor analysis,but the brownfibers cannot be separated from the blue sufficientlywell, and themselvesare not homogeneous. From opticaland chemicalstudy, however, they are essentiallypickeringite mixed with a brown ferric sulphate. The blue mineral belongsto the triclinic chalcanthitegroup, with the generalformula RO'SOa'5H20. The analysisshows that it is dominantlymagnesium sulphate but copper,zinc, and in minor quantity, iron and manganese,are also present. Ex- pressedas sulphatemolecules with 5H,oO,these last four com- ponentstogether constitute 50 per centof the mineral. The opticalproperties, determined by Miss J. J. Glassof the GeologicalSurvey, are as follows: a• •.495, /• = •.5•2, ¾• t.5t8, negativewith 2V--55 ø, dispersionr

Analysis Ratios

Insoluble ...... 0.20 MgO ...... 9.40 o.233 MnO ...... ß3 ø .004 FeO ...... i .36 'øI9 .438 I .OO CuO ...... 9.00 .I•3 ZnO ...... 5.6o .069 A12Oa ...... i ...... Trace SOa ...... 35.07 .438 1.00 H,O (by diff.) ...... 39.07 2.I68 4.95

IOO.OO

The mineral has the following componentcomposition, by weight percentage' 762 CHARLESMILTON AND W. D. JOHNSTON,JR.

MgO' SOa' 5H20 ...... 50 CuO' SOa- 5H20 ...... 27 ZnO'SOa- 5H20 ...... x7 FeO- SOs- 5H•O ...... 5 MnO. SOs' 5H20 ...... x

ioo

Basedon the componentcomposition given. above, the measured and calculatedindices of refraction show good agreement.

Measured Calculated oe•.495 ...... x.498 fi x.5x2 ...... x.5x3 'y x.5•8 ...... x.5x? 'y -- (z .023 ...... ox9

The ratios obtainedfrom the analysisare very closeto •: • :5, indicatingthat the mineralis a pentahydrate,with MgO the prin- cipal base. Although not recognizedas an establishedspecies and hencenot named,magnesium sulphate pentahydrate has been describedseveral times in the literature. An analysisof apparently the pentahydrateof magnesiumsulphate from the Cripple Creekdistrict, Colorado,was published• by Hobbs in •9o 5. The analysis,by W. O. Hotchkiss,is given in column• of Table II. The material was called "epsomite." Three years later, Keller published4 an analysisof a copperbearing magnesiumsulphate pentahydrate. His material occurredas earthy, friable, bluish- white masses,associated with chalcanthite,from Copaquire, Provinceof Tarapaca, Chile. From his analysis (no. 2, Table II), the ratios of RO:SO•:H20 are calculatedas •.o5:•.o3: 4.92, or essentially•:•:5. Expressed in componentcomposition the Chilean mineral consists of:

MgO-SOs- 5H•O ...... 58 CuO' SOs' 5H•O ...... 38 FeO.SOs' 5H•O ...... 3 MnO-SOs' 5H20 ...... x

ioo

• Hobbs, W. H.: Contributions from the mineralogical laboratory of the Uni- versity of Wisconsin. Am. Geol. 36: •84, x9o5. 4 Keller, H. F.: Notes on some Chilean copper minerals. Am. Philos. Soc. Proc., 47: 8x-82, I908. SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 763

The mineral from Nevada (no. 4, Table II) is similar in com- positionto the Chilean mineral exceptthat considerablezinc is

present. ß The cupromagnesitefrom Vesuvius,a sulphateof copperand magnesium,is of unknown hydration. From its water solution, crystalsof the heptahydratecrystallized out. Practicallypure magnesiumsulphate pentahydrate, a natural dehydrationproduct of epsomiteand showinga pseudomorphous. structuremicroscopically, being an aggregateof minutevariously oriented grains, was described5 by Allen and Day from The Geysers,near Cloverdale, Sonoma County, California. The analysis(no. 3, Table II), showsthat the materialwas almostof ideal purity. The availableanalyses of magnesiumsulphate pentahydrate from four localitiesare broughttogether in Table II.

TABLE II.

ANALYSES OF NATURAL MAGNESIUIV[ SULPHATE PENTAHYDRATE.

x • 3 4 $ Colorado Chile •2alifornia Nevada Calculated

MgO ...... 19.35 11.39 17.9I 9.40 19.15 CuO ...... 13.43 .... 9.00 .... ZnO ...... 5.6o .... FeO ...... i.oi .33 1.36 .... MnO ...... 33 .14 .30 .... NiO ...... 06 .11 ...... CaO ...... i3 .... SOs ...... 38.5I 35.70 38.I3 35.07 38.07 H20 ...... 43.03 38.38 42.97 39.07 42.78 Insol ...... 3o ....

-- 99.89 99.39 99.63 lOO.OO Ioo.oo

Although copperis presentin two of the analysesto a con- siderableextent and zinc and iron in smaller but appreciable quantities,magnesium is the principalbase of all. Nevertheless, the writers are hesitantabout proposing a mineralogicalname for the pentahydrateat presentchiefly becausethe mineral from Nevadais only half magnesiumsulphate and alsobecause other • Allen, E. T., and Day, A. L.: Steam wells and other thermal activity at "The Geysers," California. Carnegie Inst. Wash., Pub. 378: 4•, 19•7. 764 CHARLESMILTON AND I/V.D. JOHNSTON,JR. properties,such as the opticalones, are not clearlydefined for the pure magnesiumsalt. Accordingto Merwin6 theremay be threemodifications of MgO'SO•'5I-I20. The relationsof hexahydrite,described as the hexahydrateof magnesiumsulphate, to thepenta- and hepta- hydrates,are not yet clearlyestablished.

C. CompleteSamples with Ferric Sulphates. All four samplesof this complexgroup contain copiapite mixed with other ferric sulphatesand other iron-free sulphates. Pickerin#ite,Copiapite, and Epsomite.--Specimenno. 36J•9 from the 23• foot level of the Central Tunnel. Massive crust, part of it like wet snow (pickeringite),the other part like finely dividedsulphur (copiapite), with a smallsprinkling of a saffron- coloredmaterial. A little epsomitewas also noted. 'Thepickeringite is crystallizedin the usualhair-like needles which gavedecided microchemical tests for aluminumand magnesium, with minor reactions for zinc and manganese. The copiapiteshows the usual aggregateof microscopicplates, with good crystallographicdevelopment, and refractive indicescon- siderablyabove and below 1.55. The saffron mineral is indis- tinguishablefrom the yellow copiapite,except by a somewhat deepercolor under the microscope,and is a somewhatmore basic (?) ferric sulphatederived by a slighthydrolysis of the yellow copiapite. Pickeringite, Copiapite, Alunogen, and Coquimbite.--Speci- men no. SN x9oo from the north lateral of the Sutro Tunnel •,9oo feet from its junction with the main tunnel. Lumps of white crystallinematerial, heavily coated with scalesof copiapite. Massesof this mineral aloneare also present. Also noted were dark brown or greenishblack particles, associated with fragments of quartzitic rock. The white material consistsof relatively coarsely-crystallized alunogen,fibrous crystals of pickeringite,and a finely crystallized 6 Personalcommunications to W. T. Schaller,May •6, x938,and June 6, •938. SULPH.4TE MINER.4LS OF COMSTOCK LODE; NEVADA. 765 coquimbite,resembling wet snowin appearance..All of theseare fine-grained,no crystalsbeing larger than half a millimeter. The alunogenconsists of glistening white crystalline masses, colorlessunder the microscope.The meanindex of refractionis closeto •.47, the birefringencearound o.o•. It is readily soluble in water, givesno test for iron, and a strongtest for aluminum (using c•esiumchloride). The alunogencrystals resemble in outlinethose of the copiapite,having a similar platy habit, but are coarserand lessperfectly formed. The fibersof pickeringite.are similar to thosedescribed else- wherein this paper. The coquimbiteis colorlessunder the microscope,short pris- matic, with positiveelongation. The mean index of refraction is closeto 1.55, the birefringence.low, the interferencecolor an abnormalblue. It is not readilysoluble in coldwater; the acid solutiongives good tests for ferric iron and sulphate,a slight test for magnesium,and none for ferrous iron. Larsenand Berman7 list coquimbiteas havinga composition Fe2Oa'3SO3'9H20,uniaxial positive, with indices1.55o and •.556,and being soluble in water. Anotherlisting (page 69) of a questionablecoquimbite has a slightlydifferent composition, and themineral is describedas solublein boilingwater, and has much greaterbirefringence. Dana 8 sayscoquimbite is whollysoluble in coldwater, and is decomposedon boiling. Thecopiapite is wellcrystallized, showing under the microscope a generaltabular habit, with excellentdevelopment of the faces m, o, s, and b as figuresin Dana's System.9 No interference figureswere obtained on the plates. Pleochroismis distinct from practicallycolorless to yellow. The two measuredrefractive indiceson the plates were •.535 and•-59. Larsenand Berman •o give a: •.53o, fi: •.55o,•: •.592. 7 Larsen,E. S., and Berman,Harry: The microscopicdetermination of the non opaqueminerals. U.S. Geol. Surv., Bull. 848: 69-70, •934. 8 Dana, J. D., Systemof mineralogy,x9x5, p. 956. • Op. cit., p. 965. •o Op. cit., p. xo4. 766 CHARLESMILTON AND W. D. JOHNSTON,JR.

The very small deepgreen or brown or blackparticles and patchesseem to be opaqueunder the microscope. It is impossible to separatethem from the other constituentsclearly enough for chemicaldetermination. They may be alteredvoltaite. Pickeringite,Epsomite, Copiapite, and Voltaite.--Specimen no. 36J2o from the CentralTunnel, 23• foot level. Crust on drift wall, composedof variouslycolored minerals, all sulphates,except for somefragments of quartz. Hydrolysishas occurred, resultingin the formation of free sulphuricacid, which has at- tackedthe paperbag in whichthe sampleswere wrapped. The mineralsrecognized are white pickeringiteand epsomite, yellowcopiapite, and greenish-blackvoltake; in fairly equalpro- portions,with perhapsleast of the epsomite. Thepickefingite and epsomite probably deviate in composition from the simple formulas MgO'A120•'4SOa'22 H20 and MgSO•' 7H•oO,respectively, for testson the bulk sampleshowed both zinc and manganeseto be presentin minor quantity. The copiapiteoccurs in the usualexcellent microscopic platy crystals. In part, it seemsto have been altered to a darker reddish-brown substance,of variablebut higher indicesof refraction, suggesting a mixture of poorly definedsubstances. The most interestingmineral in this aggregateis the voltaite. It forms rather conspicuousdark greenmasses, without any defi- nite boundaries,intimately mixed with the other minerals. It is practicallyimpossible to isolate it pure enoughin quantities sufficientfor accurateanalysis, but by washingwith cold water most of the other mineralsmay be removed. An excellenttest for potassiumwas obtainedfrom the material thus cleaned,also testsfor ferrous and ferric iron. The larger massesdo not show clearly-definedoptical char.acters, some parts being apparently isotropic. Other partshave a vagueradial crystallization. Good crystals,however, of microscopicsize, can be found enmeshedin the pickeringiteand especiallythe copiapiteaggregates. These are dark-green(lighter-green by transmittedlight), of high index, near •-59, and low birefringence. Under crossednicols some appearto be isotropic;the variously-birefringentsegments of SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 767 otherssuggest fourlings or fivelings,as have beenpreviously described for voltaitc. In view of the complexand uncertainformula for voltaite,and the heterogeneityof the sample,an analysiswas not undertaken, as its interpretationwould rest on too many variableassumptions. Alunogen,Epsomite, Gypsum, Copiapite, and Rhomboclase.-- Specimenno. 36Jto from the Central Tunnel level. Stalactitic crusts, the individual stalactitesaveraging t to 2 centimetersin length and about a centimeterthick, showingconcentric banding on a very fine scale, with the interior portion pale-yellow,the outer brick-red. With a hand lens the pale-yellow interior is seento consistof two materials,one granular like tiny grains of sulphur, and the other glistening colorlessmicaceous appearing scales. Opticalexamination, supplemented by qualitativetests, showed the outer brick-red zones to be of rather variable nature, both in the indicesof refraction,ranging below and above •.57, and in intensityof color. Some of it appearsuncrystallized and somean exceedinglyfine-grained aggregate of brightly polarized particles. From the bulk analysisgiven belowit is inferred that the material is a basic ferric sulphate,related to rhomboclase, Fe•O•'4SOs'9H•O. Its precisenature cannot be determined, owing to its complexity. Scatteredabout the outermostbrown shell are numerousbright red or scarletmasses of microscopic size. They have rounded contours,rather than angular, and appearto be hygroscopic,to somedegree liquid. They may be a saturatedsolution of ferric sulphatein equilibriumwith a crystalline phaseof somebasic ferric sulphate?* The sulphur-yellowparticles are too fine-grainedfor any defi- nite microscopicstudy. They appearto haverather high indices and birefringence,and may be copiapite. • Concerningthis, Hintze, (op. cit., p. 4394) statesthat all the mineral ferric sulphates,including both the truly basicand the so-called"normal '; sals,are ferric hydroxy-compounds.Even the "acid ferric sulphate" is considered to be a hydroxy-compound,derived from complex ferrisulphuric acids, of which botryogen and roemeriteare salts. Hintze further classesthe basic ferric sulphatesaccording to theSO s :Fe20 a ratio,varying from 3 :I in coquimbiteto less than i :i in amor- phousgels such as gloekeriteand pissophane. 768 CHARLESMILTON AND W. D. JOHNSTON,JR.

The colorlessmicaceous-appearing material resolves itself under the microscopeinto two distinctminerals, one having the char- acters of epsomite,the other, presentin about equal quantity, consistingof minute flakes of very low birefringence,with the lowest observedindex near 1.46, and the highest near 1.465. From the mixture of the two minerals(it being impossibleto separatethem) a fair test for ferric iron and a strongtest for aluminumwere obtained; it is thereforeinferred that the weakly- polarizingflakes are a basicaluminum sulphate, with someferric iron isomorphouslypresent. Much difficultywas found in de- terminingthe opticalcharacter; the plateswere very small,and gave a vague figure of possiblynegative character. These char- actersaccord with thoseof alunogen,which has a platy develop- ment parallel to (oxo), to which the obtusebisectrix is normal. The small optic anglecan only be observedon the acutebisectrix, whichis nearlyparallel to the plates. Besidesthe epsomiteand alunogen,minute quantitiesof sharp prisms, extinguishingaround 45 ø, and showing "swallow-tail" cleavageand terminal re-entrant angles, were seen; they have indicesnear •.52, and are probablygypsum. It is also possible that a little pickeringiteis also present. It thus appearsthat in addition to the identifiedepsomite and alunogen,there are less certainly determinedbasic ferric sulphates,as rhomboclaseand copiapite,and, in very minor quantity, gypsumand pickeringite. An analysisof one of the stalactitesgave: Ratios Insoluble ...... x3 CuO ...... I9 ß 2 FeO ...... 83 x x MgO ...... 4.3x xo7 CaO ...... Trace MnO ...... 65 9 ZnO ...... Trace AlxO3 ...... 6.74 66 FexOa ...... 8.49 54 SOa ...... 35.63 445 H20 (by diff.) ...... 43.03 2388

IOO.OO

This analysismay be computedinto 42 per centalunogen, 3o per centepsomite, and the remainderbasic ferric sulphatessuch as rhomboclaseand copiapite. The computation,however, is SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 769 merelyan approximationto the actual composition of thecomplex stalactites.

Compositionof the Samples. Table III recapitulatesthe mineralogicalcomposition of the seventeen samples. The figures represent approximate percentageof the entire sample. X indicatesthe mineral is abundant in the sampleand x that it is presentin small quantities.

TABLE III.

Sample number

Mineral • g g

Gypsum ...... • •oo •oo Epsomire ..... 88 88 99 2o 30 I6 8 x X X Goslarite ...... 98 99 Melanterite... 92 Magnesium sulphate pentahydrate ..... X Pickeringire... 2 x 8o 68 84 92 X X X ? Coquimbite... X Alunogen ..... X X Copiapite ..... X X X ? Voltaire ...... ? X Rhotnboclase.. ? Basic ferric sulphates ...... x x Clay, etc ...... 8 x2 x2 x x 2

MINE WATERS.

Analysesof eight samplesof mine waters from the Comstock Lode are given in Table IV. Of these,nos. x to 4 are new and nos. 5 to 8 have been taken from publishedreports. With the exceptionof no. 8, the waters are all of the same generaltype beingessentially solutions of calcium(magnesium, sodium, potassium) sulphateand bicarbonateor carbonate. Nos, 2, 3, 4, and 5 are cold water of undoubtedmeteoric origin that have des- cendedthese fissures and old mine workings and have dissolved mineral matter en route. They differ very little from the hot waters representedby analysesx, 6, and 7 exceptthat the two 770 CHARLES MILTON AND W. D. JOHNSTON, JR.

samplesfrom deepworkings (nos. 6 and 7) containless carbonate in solution.

TABLE IV. MINE WATERS OF TH]• COMSTOCK LODE, NEVADA. (Parts per million.)

x 2 3 4 5 6 7 8 Yellow Haleand C.& C. Union Jacket Central Sutro NorcrossSutro Savage6oo t Shaft Consol.Central Tunnel 2250t 265ot Tunnel Sutrolevel Tunnel Tunnel Tunnellevel level level level

Temperature ...... 13oø F. Cold 80ø F. Cold ... Hot 13oø F. Cold

COa ...... None None None None ...... 9.3 .... HCOa ...... 387 184 3Ol 307 236 41.7 None .... SO4...... 972 725 320 1,3o2 380.7 474.8 752 I93,45o C1 ...... 8 34 1o 14 1.3 19.o 12.o 128 HS • ...... 0.3 None None 0.8 ...... F ...... 0.8 0.4 None 0.8 ...... Ca ...... 334 217 141 218 148.3 lOO.4 204.0 1,244 Mg ...... 82 86 31 xx3 20. 9 5.8 4.0 6,519 Na ...... 55 32 54 45 57.1 13o.6 145.o 535 K ...... to x5 9 9 8.4 53.4 8.4 .... AV" ...... 9,638 Fe "• ...... 5,o25 Mn ...... None 9oo Cu ...... 148 SiO2 ...... 49 13 34 24 30.5 133.4 61.o 616 A12Oa...... x.9 3.2 0.8 1.9 .... 2.5 ...... FeeO3 ...... 0.7 0.6 0.4 0.6 .... 9.x None .... OH ...... 7.5 .... H ...... 2,822

Total ...... x,9oo.7 1,31o.2 9Ol.2 2,o41.6 883.2 970.7 1,2o3.2 221,o25

x. Water flowing from cross-cutto Yellow Jacketmine on the SutroTunnel level. Rate of flow, IOOnu gal. per minute. Tem- perature,I3 oø F. (U.S. G. S. 8377). CharlesMilton, analyst. 2. Central Tunnel. Descendingmine water flowing one-half gal. per minutefrom chuteon 23I ft. level (U.S. G. S. 8826). Charles Milton, analyst. 3- Drip in Sutro Tunnel about14,2oo feet from portal. Drip approximately29 gals.per minute,80 ø F. (U.S. G. S. 8829). CharlesMilton, analyst. 4. Water flowing from cross-cutto the Hale and Norcross mine, Sutro Tunnel level. Rate of flow approximately25 gal. per minute(U.S. G. S. 8876). CharlesMilton, analyst. 5- Savagemine, 6o0 ft. level. S.W. Johnson,analyst. Ex- SULPHATE MINERALS OF COMSTOCK LODE; NEVADA. 771 plorationsof the Fortieth Parallel, vol. III, p. 87. (Analysis recalculated.) 6. C. & C. Shaft, 2250 ft. level. N. E. Wilson, analyst. John A. Reid, Bull. California Univ. Dept. of Geol., vol. 4, PP. I77-I99 , I9O5. (Analysis recalculated.) 7. Hot water (•3 oø F.) from Union Consolidatedmine, 265ø ft. level. ChasePalmer, analyst. Edson S. Bastin, Bull-.U.S. G. S. 735-C, pp. 4•-63. 8. V•adosewater, Central Tunnel. N. E. Wilson, analyst. Seepaper by J. F. Reid abovecited, p. I92 (analysisrecalculated and adjusted). However, Wilson/2 who analyzedno. 6, notes that his CO2 determinationis low becausethe samplewas exposedto air for some days before analysis. Wilson also evap- oratedIO liters of this water and by microscopicmeasurement of the assaybutton determined the preciousmetal content to be 0.298 milligramsof goldand 2.92 milligramsof silverper ton of water. This silver content--about 3 parts in a thousandmillion--lies very near the limit of analyticalskill. Even though the results are accurate,it shouldbe noted that sea water, from the Kristiana Fjord, Norway, was found to contain5 to 6 milligramsof gold and •9 to 20 milligrams of silver per ton.•s In the analysesof Comstockwaters listed, the writers see no internalevidence to supporta magmaticorigin for them. AnalysesI to 7 representwaters too dilute to depositsulphates altho.ughsome of themare depositingcarbonates. Fig. 4 shows a depositof calciumcarbonate from water no. 3 that coatslagging on the wall of the tunneland a barrel restingon the tunnel floor. The only analysisof a highly concentratedsulphate water of the type from which the saltsherein described might be deposited is no. 8. In addition to the metallic constituents determined it is probablethat severalhundred parts per million of zinc are also present. U.S. G•.o•.o•ic.•. SURVEY, WASaI•GTO•, D.C., Aug. 9, •r938. x• Reld, J. A.: The structureand genesisof the ComstockLode, Univ. of Calif. Pub., Bull. of the Dept. of Geology,4:x9 o, x9o$. xsClarke,.F.W.: Data of geochemistry.U.S. Geol. Surv. Bull. ??o: x•4, x9•4.