A merican M ineralogisl, Volume63, pages I I 53-l I 6l , 1978

Polhemusite,a newHg-Zn sulfidefrom Idaho

B. F. LnoNeno.Groncs A. Dsssonoucg U.S. GeologicalSuruey, Denuer, Colorado 80225

eNo CvNrsll W. Mnnn Rockuille, M aryland 2085 2

Abstract

Polhemusite,empirically Hgo.ruZno..7Feo.o,Sorr, is a microscopic constituent of mercury- bearingantimony ore from the B and B deposit,Big Creek district,Valley County, Idaho. Polhemusiteoccurs as stubbytetragonal prisms, dipyramids, and irregulargrains associated with stibnite, cinnabar,mercurian sphalerite, and zincian metacinnabarin a quartz lode. Polhemusiteis black, resinousto adamantine,locally with dark red internalreflection. X-ray diffractiondata for polhemusitecan be indexedaccording to a primitivetetragonal cellhaving a: 8.71,c: 14.74A.A pseudocubicsubcell, a: 5.33A,is alsopresent. Possible spacegroups areP4/n, P4r/n, P4/nbm,P4/nmm, P4r/nnm, and P4"/ncm. Single-crystal data and densityare lacking, owing to the minute sizeof the particles.The cell content,Z, is probablyabout24 to 32.Stronger lines ofDebye-Scherrer and Gandolfiphotographs are 3.16 ms,3.08 vs, 1.888s, 1.608ms,1.222 m, 1.086m,1.024 m. Polhemusiteis variablein composition.Microprobe analysis gives, in weightpercent, Hg 18.0-34.7,2n42.6-54.7, Fe0.3-0.7, S 24.0-28.9,sum 101.3-102.8.The meanof l5 analyses, eachon a separategrain, is Hg 25.8t 5.5,2n49.1 + 3.8,Fe0.5 + 0.1,526.7+ 1.6,sum 102.1+ 0.7. Compositionalvariation resultsfrom zoning, as well as from grain-to-grain variation.Limiting compositionsare equivalentto Hgo,oZno.r2So",and Hgo22Zno.6rSo.ru. In polishedsection, polhemusite looks gray in air, much darkerin oil. Againststibnite, it is lavendergray to bluish gray. Polishinghardness )stibnite. Reflectionpleochroism in aggre- gates:distinct in air, very slightlybrownish gray to slightlylavender gray; conspicuous in oil, light brownishgray (almostyellow) to purplishgray, but partly obscuredby internalreflec- tion. Where unobscuredby internal reflection,anisotropism is distinct in air, strongin oil. Internalreflection, not everywheredetectable in air, is strong-lavender,pink, reddishorange, dark red;in oil it is strong,red-orange, not visiblein all grains.Reflectance at470,546,589, and 650nm is (R1,Rri (R, + R2)/2,in parentheses)20.1-24.4(21.8), 17.1-18.3 (17.6), 16.5- 17.7(16.9), -17.9 percent.Bireflectance decreases with increasingwavelength and is effec- tively absentat 650 nm. HVzu: 220-333,mean 262. Knee-shaped twins resemblingthose of rutile are common; other simple twins and some lamellar twins are present.Cleavage or parting is rare. Zoning is conspicuousin someareas. Polhemusite,except for its reflectionpleochroism and anisotropism,somewhat resembles sphalerite,but its quantitativeore-microscopic properties, twin habit, and X-ray diffraction data are distinctive.Its compositionoverlaps that of the associatedmercurian sphalerite. Polhemusiteis inferredto be a metastable,disordered form of (Zn,Hg)S-a polymorphof mercuriansphalerite.

Introduction and B deposit, Big Creek district, Valley County, Polhemusiteis a noncubicHg-Znsulfide. Though Idaho. The deposit,discovered in 1964,was described it is optically anisotropic,it looks somewhatlike by Leonard(1965). sphalerite,and its compositionoverlaps that of mer- The distinctiveoptical propertiesof polhemusite curian sphalerite.Polhemusite is a microscopiccon- led to its tentative assignmentas a new speciesin stituentof mercury-bearingantimony ore from the B 1965.This tentativeassignment was confirmedin 0003404x/7 I / I I l 2-l l 53$02.00 I 154 LEONARD ET AL.: POLHEMUSITE

1966by Mrs. Mead'smicroprobe analysis of several The deposithas been mined in a smallway, and some grains,all of themtoo smallto beextracted for X-ray tonsof cobbedstibnite have been sold. powderdiffraction analysis. In 1970,Mr. RichardB. Gangueminerals of the depositare quartz,barite, Tripp concentratedseveral grams of stibniteand as- muscovite,illite, zircon, and fluorite. sociatedsulfides by digestingthe quartz-richgangue Quartz of severalgenerations makes up more than in hydrofluoric acid. He then picked out cinnabar 95 percentof the gangue.An earliergeneration of grainscontaining black specksthat werethought to quartz is chalcedonic;a later generationis moder- be metacinnabar.The powderphotograph of threeof atelyto coarselycrystalline and comb-like;some very the blackspecks, each about 20 micronsin diameter, late quartz is chalcedonic,forming thin veinletsthat was not that of metacinnabar;it was, by a lucky cut the combquartz. The varietiesand apparent ages accident,a photographof a pure sampleof the only are far more complexthan this arbitrary division other black mineral in the specimen-very spotty suggests.All the quartz contains microscopicin- becauseof the sizeof the particles,but lackingthe clusions,both solid and fluid, that give it a dusty stronglines of the otherminerals already identified in appearancein thin section.Some comb quartz forms the specimen.Mr. Tripp then mountedthe particles lenticlesin chalcedony,the medial area betweenthe in a Gandolfi camera.The excellentGandolfi record quartz teeth being filled with stibnite.Some quartz provedto be both uniqueand indexable on the basis crystalswithin the comb are twinnedaccording to an of a tetragonalcell assumedfrom the morphology. undeterminedinclined-axis law. To confirmthe assumedcell, the particleswere dis- Baritemakes up 5-10 percentof somespecimens, solvedoff their gelatinesliver for remountingof the but it cannot be detectedmicroscopically in other largestparticle in a precessioncamera, but the only samples.Barite crystals-some of themlightly dusted particlelarge enough for precessionphotography was with cinnabar,others coated with stibnite-partly fill lost in the attempted remounting. No other pure small,irregular fissures in quartz.Beyond the ends of particlesofsuitable size have been found in searching somemicroscopic fissures of this sort,discontinuous alargesuite of samples.That the threeparticles origi- replacementveinlets of barite are presentin unfrac- nallyphotographed do indeedcorrespond to themin- turedquartz. Fluid andsolid inclusions are visible in eral optically and chemicallycharacterized as pol- someof the largerbarite crystals. hemusitehas been confirmed by X-ray powder Muscovite can occasionallybe found in barren photographyof severalsamples of mixtures,all of quaftzas a fist-sizedaggregate ofcoarse flakes, or as whoseconstituents were known. a singlesmall flake in a thin section.Muscovite is The mineral is named polhemusitefor the late little more than a traceconstituent of the deposit,but ClydePolhemus Ross, born ClydePolhemus, a stu- this trace constituentcan be found as hand speci- dentofmercury deposits and ofthe geologyofldaho, mens. and a memberof the U.S. GeologicalSurvey for Illite, in contrast,has beendetected only micro- morethan 40 years.The mineraland the namehave scopically.It occursas isolated wisps and asirregular been approvedby the IMA Commissionon New nets and clouds of flakes within and next to large Mineralsand MineralNames. crystalsof quartz.In otherspecimens, illite andchal- cedony are the matrix for angular fragments of strainedquartz in microbreccia.The total quantity Settingand mineralogy of the deposit of illiteis small,its occurrenceis sporadic,its concentra- The B and B depositis a replacementdeposit of tion is too slightfor X-ray diffractionanalysis, and its stibniteand associatedmercury minerals in a quartz identificationis thereforetentative. In thin section. lode. The lode is part of a large silicified zone in the if lite looks like sericitebut has 2V, rather small; granodioriteof the Idaho batholith.The silicified spectrographicanalyses of samplesthat contain sev. zone,accompanied by a swarmof rhyoliteand latite eral percentof this mica show no K and only 0.05 dikesof Tertiaryage, is now knownto occupyone of percentNa. The absenceof K eliminatesmuscovite; the systemsof ring fracturesof the Thunder Moun- scarcityof Na, as well as geologicoccurrence, elimi- tain caldera,a major collapsestructure whose center natesparagonite; a sufficiencyof Al, Fe,and subordi- is 14 miles(23 km) southeastof the deposit.The nate Mg permitsthe inferenceof illite; and illite is a hypogenemineral assemblagenoted below is Ter- commonclay mineral in someother quartzlodes of tiary. Locally it has beenaltered to stibiconiteand the district. other supergeneminerals, including metastibnite. Zircon is ubiquitousin traceamounts; a few small LEONARD ET AL.: POLHEMUSITE I 155 crystals,commonly but not everywherewithin areas mineralswere precipitated together, only stibniteand containingillite, are visiblein everythin sectionof cinnabarlocally continuing to grow in late veinlets ore. Optical identificationof the zircon is confirmed and as perchedcrystals. An alternativeinterpretation by spectrographicanalyses (Leonard, 1965) that is that all but the sparsestore mineralsrepresent show20-50 ppm Zr in samplesof the ore. multiplegenerations. The numberof generationsor A singleparticle of colorlessfluorite intergrown redistributionsrequired to accountfor the complex with brown sphaleritewas recoveredfrom the HF- patternof inclusionsseems improbably large; this, as insolubleresidue of onesample. The sizeof thefluor- well as the scarcityof overgrowthsand consistent ite particle(-l/5 mm) andthe presencein it of fluid sequentialreplacement relations, leads us to favorthe andsolid inclusions indicate that the particlewas not hypothesisof dumping-the rather suddendeposi- formed during acid treatmentof the sample.(The tion of a disequilibriumassemblage in whichthe ele- CaF, that occasionallyforms during HF decomposi- mentsHg and Zn enteredinto cinnabar,metacinna- tion of mineral samplesis extremelyfine-grained, bar, polhemusite,and sphaleritein quite variable accordingto GeorgeJ. Neuerburg,oral communica- proportions. tion.)The cell edge of thefluorite is 5.51* 0.01A;the Appearanceand polishingqualities of polhemusite refractiveindex is 1.437+ 0.001. The ore mineralsof the deposit,listed alphabeti- Grains of polhemusiteare too smallto be recog- cally, are arsenopyrite,chalcopyrite, cinnabar, cop- nizedwithout a microscope.In incidentlight, under a per, metacinnabar,polhemusite, pyrite, sphalerite, stereosccipicmicroscope, polhemusite Iooks black. Its stibnite,tetrahedrite, and unidentifiedbright miner- lusteris resinousto adamantine.A dark red internal als.A roughestimate of the abundanceof oreminer- reflectionis visiblein somegrains. Accordingly, pol- als in representativesamples is: ) l0 percent-stib- hemusiteresembles livingstonite and some of the nite; 0.I - I percent-cinnabar, pyrite, sphalerite; ruby silverminerals viewed under similar conditions. 0.001-0.01percent-metacinnabar, polhemusite, tet- Polhemusiteis difficultto distinguishfrom grainsof rahedrite;trace-arsenopyrite, chalcopyrite, copper, stibniteflecked with cinnabar,and it cannotbe distin- and unidentifiedminerals. guishedfrom metacinnabarin incidentlight. The compositionof someof the ore mineralshas Specimensof the ore are difficult to polishbecause been investigatedin a preliminaryway. Cinnabar they containminute vugs, not readilysealed by the containsl.l-2.7 weight percentZn. Metacinnabar usualimpregnating agents, as well asfrangible aggre- contains10.0-17.7 weight percent Zn. Somesphaler- gatesof cinnabarand stibiconite.Lead-lap polishing ite is nearlypure, some contains a littleFe, and some with magnesiacan seldombe completedwithout containsas much as 20 weight percentHg. Some some plucking of these fine-grainedbrittle aggte- tetrahedriteis argentian,some is mercurian,and gates.Slow, low-pressure polishing with aluminaon somecontains both Ag andHg. Analysisof a random cloth producesthe bestsurfaces and makesthe pol- grain of the lastvariety gave, in weightpercent, -2-4 hemusiteconspicuous by heighteningits reliefagainst Ag (distributedinhomogeneously) and - 10Hg. the softerstibnite. Ultimately polhemusite takes an Stibnite,pyrite, and sphalerite range from veryfine excellentpolish. to coarse.All otherore mineralsare fine-grainedor Polhemusitedoes not tarnishin the dry climateof extremelyfine-grained. Only stibniteand cinnabar Denver,and it scarcelydarkens after a year'sex- form discreteveinlets. Both minerals,as well as posure to the humid atmosphereof Washington, sphalerite,form replacementpatches. Pyrite and ar- D.C. senopyrite,and somestibnite and polhemusite,are euhedral. Microscopicproperties An orderly parageneticsequence cannot be recog- itatiue prope rt ies nized. Except for the sparsestore mineralsand re- Qual gardlessof the veinor replacementhabit of some,all We thoughtwhen we first observedpolhemusite in the oreminerals occur somewhere as inclusions in all polishedsection that its qualitativeproperties were the others,the pair polhemusite-sphaleritedoubt- distinctive.They are, but only underfavorable condi- fully excepted,owing to the difficulty of telling one tions. Here a favorablecondition is the prevailing mineral from the other under certaincircumstances. occurrenceof polhemusitewithin stibnite.Against We interpret the gross fabric as indicative of a stibnite, polhemusiteis slightly lavendergray to dumpedassemblage in which virtually all the ore bluishgray. The lavendertint, asopposed to pink or I156 LEONARD ET AL.: POLHEMUSITE brown, is better seenat low and mediummagnifica- reportingsuggests. (2) The singlebipyramid repre- tion than at high. Againstcinnabar and metacinna- sentsa specialcase and thus tells only part of the bar,polhemusite looks less lavender and more bluish. story;the general case is that c is alwaysthe direction Polhemusiteis considerablydarker and harderthan of elongation, and the effectsobserved on prism thesethree associated minerals. Viewed against stib- cross-sectionshave been distinguished with certainty; nite under a comparisonmicroscope, polhemusite accordingly,the sign of the white-lightbireflectance resemblesrealgar in lavenderto bluishgray tint, but changesfrom (*) to (-) within the compositional the red internal reflectionof polhemusiteis lessper- rangerepresented by our samples.At present,we do vasiveand lessconspicuous than that of realgar.The not havematerial suitable for the kind of measure- carbon coating applied to specimensprepared for mentthat wouldsuggest the better of thesetwo inter- microprobeanalysis greatly enhances the lavender pretations. tint and positiverelief of polhemusite,making this With polarizerand analyzerslightly uncrossed, the mineralreadily distinguishable from all its associatesanisotropismof polhemusiteis moderatein air, but exceptsphalerite. Under a comparisonmicroscope, commonly obscuredby internal reflectionand diffi- polhemusitelooks very slightly lavender gray or very cult to detectif polhemusiteis completelysurrounded slightlybrownish gray against sphalerite, but thetwo by stibnite.In oil, anisotropismis strongbut largely mineralsare difficult to distinguishin brightnessand obscuredby internal reflection.Internal reflection, tint. The sphaleriteused as an externalstandard of not everywheredetectable in air, is locallystrong- comparisonwas a ferroanvariety, R(air) : 17.l per- lavender,pink, reddishorange, dark red; in oil it is centat 546nm. strongin red-orangebut not visiblein somegrains. Sphaleriteis presentwith polhemusitein our sam- Polhemusiteoccurs mostly in stibniteas minute ples,but we havenot found the two mineralsinter- stubbyprisms, as trains and aggregatesof prisms,as grown. Avoiding aggregates,we have scrupulously dipyramids,as droplets, as forms resembling weevils, selectedfor descriptionand analysisas polhemusite andas an int€rgrowthresembling myrmekite (Figs. I, only the mineralthat waspositively distinguishable 2). Extremelythin rimsof polhemusiteon stibniteare on the basisof all its observedproperties, beginning presentbut rare.Small granules of polhemusiteare with crystalhabit (see below). Polhemusite does have intergrownwith cinnabar,metacinnabar, and quartz. propertiesthat are thoroughlydistinctive, but unless The prisms,dipyramids, and irregularindividuals of theseare sought,the mineralcan be mistakenfor polhemusiteare lessthan 25 micronslong. Someof manyanother-notably for sphalerite. the prisms are combinedwith dipyramids,others The reflectionpleochroism of polhemusitein aggre- with the basalpinacoid. Knee-shaped twins resem- gatesis weak in air, very slightlybrownish gray to blingthose of rutileand cassiterite are common. The slightlylavender gray; distinct in oil, light brownish anglebetween the polesof the prism "faces"is 37o gray (almostyellowish) to purplishgray but partly for some twins and about 54o for others,the first obscuredby internal reflection.Some elongate crys- angleperhaps representing only a foreshortenedview tals having a rectangularcross section are, in air, of the secondas the twins are seenin the polished darkerand more bluish ll/, lighterand morepinkish section. Referredto the X-ray cell provisionally or brownishI/. Other elongatecrystals show these adoptedfor polhemusite,the compositionplane of relationsreversed. A single perfectly-developedbi- the knee-shapedtwins is {605}.Contact twins in pyramidshows R6 slightlypinkish, Rr slightlybluish, which the prismsmeet at right anglesare seenless Ro ) Rr. Crosssections of prismslook gray-either often; twinning here is presumablyon {ftkl}.Some slightlybrownish or slightlybluish-and whereun- trainsof prisms,their long axes parallel, are perhaps twinnedare probably optically isotropic, but compo- lh0ll contact twins. Broad, discontinuouslamellae sitional zoning makes the latter observationuncer- seenin one prism cross-sectionmay representtwin- tain. The orientation of the optical effectscan be ning on lh)ll. Cleavageor partingis rare:(l) trace interpretedseveral ways, two of which deservecon- perpendicularto the prism axis-perhaps{001}; (2) sideration:(l) The relationsobserved on oneperfect two perpendiculartraces parallel to the diagonalsof bipyramidare valid for all crystalsof polhemusite; the prism cross-section-perhapseither {ft0l} and the signof the bireflectanceof the mineralis therefore {0kl}, or lhhll, dependingon the boundingforms of (-) in whitelight, and it followsthat c is not always the prism.The planesof cleavage(2) look inclinedto the directionof elongationof the lathy sections,and thec axis;their relation eliminates {100} and {010},or that the tint detectedin cross-sectionsof prisms can- {ll0}, aspossible forms. not be distinguishedwith as much certaintyas our Zoning is present,both in euhedralcrystals of pol- LEONARD ET AL.: POLHEMUSITE I 157 hemusiteand in granules.The vaguelydefined, ellip- tical, generally brighter core is envelopedby a nar- row, generally darker rim. Some individuals are slightly mottled, rather than zoned. In red light (656 nm), zoning is conspicuous,and the brighter area (core,generally) shows strong internal reflection.The possibilitythat the zoning is insteadan overgrowth of polhemusite on sphalerite has been consideredand rejectedbecause at shorterwavelengths both core and rim are optically anisotropic,the transition from one to the other is gradational,and thereis no perceptible difference in polishing hardness between core and rim. Similar evidencedistinguishes mottling from in- tergrowth, but with greater difficulty. In thin sectionscut as mirror images of the pol- ished sections,polhemusite ranges from translucent yellow through dark orange-redto highly absorbing brown. Pleochroism is weak; refringenceand bire- fringenceare very high; mottling and zoning are con- spicuous.The stubby prismatichabit of the mineral is easilyseen in some myrmekite-likeintergrowths with Fig. 2. Train of polhemusite prisms in stibnite. Bluntly the stibnite host, and a few slender prisms of pol- terminated knee-shapedtwins and linked prisms are faintly gray. Stibnite-light gJay, hemusite are identifiable in the quartz matrix. Ab- motrled. Polhemusite-medium bireflectant.Medium gray matrix-mostly quartz Single polar, sorption is stronger ll/ in some prisms and weaker in green filter. others. Optic sign and orientation of the optical in- dicatrix are not determinable,owing to the small size Quantit at iue propert ies of the crystals. Quantitative optical data for polhemusiteare given in Table l. The reflectanceand bireflectance at 470 nm look suspiciouslyhigh relativeto the valuesmea- sured at longer wavelengthsand to the qualitative observationsmade in white light. Two testssatisfy us that there is no reasonto supposea lower reliability for the blue-light measurements.The slightly ferroan sphalerite(Runu : 17.1percent) previouslyused with the comparison microscope gave Rnro= 17.7 percenr when measured in serieswith polhemusite and the carborundum standard.This value is closeto the one

Table l. Reflectanceof oolhemusite

R(air),Z

Wavelength, m 410 546 589 650

76.5-I7 .7 \I7 .9 -2R 21.3-24.4 17. 3-18 . 3

R 20.7-22.1 17.1-18.1 16,5-L7.5 NI7.9

R^ (mx. in 2.3 1.0 0.4 r0 \ - indivldual secE ion)

(R. + R^)/2 21.8+1.0 17.6+0.3 16.9+0.5 &18

Reflectance of three individuals neasured by B. I'. L. wlth a Fig. l. Aggregatesof polhemusite in stibnite. Polhemusite- Reichert photoelectric Eicrophotometer fitted wlth a contitruous medium gray, high relief,locally resembling myrmekite. Stibnite- interference f11ter whose half-uidth at half-helght 16 13-15 m. Calborundum standard N2538.29 callbraEed by Natiooal Physlcal white, distinctly bireflectant. Cinnabar-light gray patches near Laboratory, Teddlngton, England. l"tount Press-leveled on center. Dark gray matrix-quartz. Black-vugs and pits. Single plasticine. The + value folLowlng the mean leflectance is the staildard deviallon. polar, green filter. I 158 LEONARD ET AL.: POLHEMUSITE obtained by adding the averagedispersion of iron- diameterof the indentations-lessthan ideal but poor sphalerites(Rnro - Run"= 0.6-1.0percent; see, without detectablebias from the matrix of the indi- for example,COM Tables,1970) to the measured viduals.The indentationswere made with theVickers Runuof 17.1percent. The checkprovided reasonable diamondpyramid of aLeitzDurimet hardness tester, assurancethat the measuringand recordingappa- descent-and-indentperiod 30 sec,and were measured ratus was not yieldingerratic resultsin the blue. in nonpolarizedgreen light. Severalareas of polhemusitewere then examinedvi- sually,from the blueto the red,in light from individ- Chemicalcomposition ual narrow-bandinterference filters. This examina- Electronmicroprobe analyses (Table 2), madein- tion showedthat the bireflectanceof polhemusiteis dependentlyby G.A.D. andC.W.M., show that pol- rather strong at 482 nm, distinct at 588 nm, and hemusitevaries in composition,as one wouldexpect barelydistinguishable at 658nm. The visual observa- from the zoning,mottling, and variablereflectance tion at 658 nm alsoconfirms the photometricmea- alreadydescribed. Within the limits of precisionof surementsat 650 nm, showingthat polhemusitebe- the analyses,the variationin compositionis continu- haves virtually as an isotropic substanceat ous betweenthe observedextremes numbered I and wavelengthsnear 650-658 nm. 2, Table 2. The quantitativecolor designationof polhemusite The analyseswere made on separategrains, each is I/ : 18.0percent, x : 0.313,y : 0.304,pe : g grainrepresenting a polhemusite individual, so far as -536 percent,tra : nm. The valuesare derived from that could be determinedmicroscopically. Mrs. the meanreflectance according to the methodapplied Mead's analyseswere made in 1967and 1968by to ore mineralsby Piller(1966). Without ambiguity, meansof a MaterialsAnalysis Company microprobe the brightnessvalue, trichromatic coefficients,ex- with an effectivetake-offangle of 41" at an accelera- citation purity, and complementof the dominant ting voltageof 25 kV. The standardsused were ZnS wavelengthstate in a formal way what the eye has and HgS. LiF crystalswere used for the determina- alreadyseen: that polhemusiteis not bright,that it is tion of Zn and Hg. A PET crystalwas usedfor the almost neutral,but that it is definitelypurple, the determinationof S. The results,corrected for back- purplecharacter requiring formal expressionas the ground, drift, and absorption,clearly showedthe complementarygreen of wavelength536 nm. natureof polhemusitebut raisedsome doubt about The microindentationhardness of polhemusiteat the limits of compositionalvariation. Additional 25-gramload is 220-333,mean 262, for 13 indenta- analyseswere needed. Mr. Desboroughmade these in tions.The indentationshave straight or slightlycon- 1972by meansof an Anl-Errlxelectron microprobe. cavesides and areperfect to slightlyfractured. Owing Threeanalytical conditions were used with operating to the smallsize of the polhemusiteindividuals, the conditions,spectral lines, and analyzingcrystals as diameterof the areasindented is about twice the follows: 15 kV, 20 nA specimencurrent, HgMa-

Table 2. Electron microprobe analysesof polhemusite(weight percent)

I 2 3 4

Hg 18.0 25.8+5. 5 27.2

Zn >4.t 42.6 49. 1+3.B 4t.) Fe 0. 5l{. I't S 28.9 24.0 26.7+I.6 24.6

101. 6 101.3 IO2.L+O.7 99.3 (Hgo.roz'o.sz)so.gg (Hso.z2z'o.a3)so.ss(HBo.rs,'o.87F.0.0r)so.ez (HBo. so.9+ ,.::::l;"* tz"o.8g)

* Range 0.3-0.7, not paired with analyses L and, 2

1. Grain conlaining least ltg. L-64-sR7.2, area B. George A. Desborough, analyst.

2- Graln contalning most Hg. L-64-sR7.2, area A. George A. Desborough, analyst. 3. l"lean, 15 gralns. George A. Desborough, analyst.

4. Mean, 6 grains. Cynthia W. Mead, analyst. LEONARD ET AL: POLHEMUSITE I 159

ADP,ZnKot-LiF,SKa-ADP; l5 kV,20 nA specimen Table 3. X-ray data for polhemusite current,FeKo-LiF, Sbrd-ADP, SKa-ADP; 6 kV, a-8.71' c=14'74L 40 nA specimencurrent, HgMa-ADP, SKa-ADP, o A r canera bEl 4u"' 4.t"' simultaneouslyon separate SKa-ADP, the last two 110 6.19 6.L6 w D spectrometers.The followingcompounds were used 1II 5.68 5.68 w D as standards:ZnS, HgS, Ag2S,HgTe, CdS, PdS, Lr2 4.74 4.73 w D BizSs,InS, FeS,SbzSs, (Fe,Zn)S. Values fot Zn and 113 3.83 3.84 w D Fe wereobtained from the l5 kV analysesand values 004 3.60 3.68 m D 2L2 3.40 3.44 w D for Hg and S from the 6 kV analysesin order to 114 3.13, 3.19 3.16 ms D' G eliminatethe needfor absorptioncorrections. The 220 3.09,3.08 3.08 s' vs D' G precisionof the individualanalyses is high: Hg 1.0, 301 2.85 2.85 w G Zn 0.5,Fe 0.1,S 0.3,in weightpercent. 115 2.66 2.66 mw G Minor elementsdetected in polhemusiteare Cu, Fe 3O4, 3r4 2.24 2.28' 2,21 w G 2.Oi w G (Table2), and Sb.The concentrationof Cu andSb is 305 2.O7 403 1 989 1.991 w G too low to reportin numericalform. 333, 422 1.888 1.894,1'883 s G The empiricalformulae given in Table 2 are dis- 334 L.79L I.793 w G cussedin the sectionon X-ray data. 52r I.608 1.608 ns G The chemicalcomposition of polhemusiteoverlaps 2O9, 523 1.535 1.535,1.536 w G 1.335 mw G that of mercuriansphalerite and approachesthat of 614 1.335 6t5 1.288 1.288 w G zincianmetacinnabar. 4.0.r0 1.222 L.221 n G 642 1.19I L.192 w G X-ray diffractiondata and notes 801 1.086 1.086 m G X-ray diffractiondata for polhemusite,obtained 2.0.14,558,661 1.024 L-024 m G '9414 w G by combiningthe recordsfrom Gandolfiand Debye- 836? A44? .9405 .94L5, ... .8996 .8996 mw G givenin Table3. The datahave 765, Scherrercameras, are 775,... .8435 .8430 v G beenindexed according to a primitivetetragonal cell 8.r.12, ... .8114 .aLIz w G : = groups havinga 8.71,c 14.74A.Possible space D--Debye-scherrer. Film D-8167; reflections very sPotEy. are P4/n, P4zfn, P4/nbm, P4/nmm, P4z/nnm, and c--cando1f1. II1m D-8174i ieflections good.. P4r/ncm.The provisionalcell and the spacegroup cameias of dianeter 57.3 m. cuKa = L.54l7aA' Nl filter; Straunanis film arlangement; film shrinkage negligible' RelaEive remainto be confirmedby single-crystalstudy, but inEenslties estimted visually--s, strong; v' very; m, nedium; of 3 parElcles. the largevalue of c is corroboratedby a very spotty w, weak. GelaElne-tiP nount photographtaken in a Debye-Scherrercamera acting as a cruderotation camera. The photographshows polhemusitemay containapproximately 8 X 6 : 48 that the repeatdistance along one crystalaxis is atoms. If the total number of atoms is 48, the >l2A il as seemslikely, the spots are a blurred complete formula of polhemusite becomes recordof the zerolevel and first levelof a favorably- -24(HgoruZno."rFeoo'So.rr). The density of polhemu- orientedcrystal fragment. sitecould not be measured.We supposeit is greater The provisionalcell of polhemusiteis geometri- than that of sphalerite(4.09 for syntheticcubic ZnS) callyplausible in relationto the cellsof cinnabarand and lessthan that of metacinnabar(7 .7 | for synthetic metacinnabar: cubic HgS). The calculateddensity of polhemusite, basedon cell volume,empirical composition, and Z N *{2u^.n,rorn - = apn 2acn * c.n, Vprn 8V"n = 24,is 4.23g/cms. This valueis plausiblebut per- hapsless than the true density.lf Z : 28-32(based In relation to the cell of artificial T-HgS, on the relationVE1,^g./Vp..uu : 7+), the calculated density of polhemusitebecomes 4.93-5.63-also ao6: yQa, of cprrr- c of ?-HgS, t-HgSt plausibte.We conclude,therefore, that Z is probably The unit cell of cinnabar contains 6 atoms- somenumber between 24 and 32. 3(HgS)-and the relatiorTVpn - 8V.n suggeststhat Polhemusitehas a pseudocubicsubcell with a : 5.334(Drs. D. C. Harrisand Akira Kato,1972,oral 'The cell edgesof cinnabar, metacinnabar, and artificial 7-HgS and writtencomments on the IMA abstractpropos- (1970). are from Strunz The data recently reported by Pro- new mineralpolhemusite; subcell edge and tod'yakonova et al. (1971) for natural 'y-HgS do not include cell ing the edges. indexingconfirmed by us). Multiplesof the subcell I 160 LEONARD ET AL.: POLHEMUSITE

cannot be conventionallystacked to form a tetra- the proposed hexagonalcell, but the cell must have gonal cell of the requireddimensions. Instead, the hexagonal symmetry, not rhombohedral, because fundamentalgeometric relation is this:two corner-to- somereflections of the typeh k i lhave -h + k + I 13. cornercubic subcellsof polhemusitesit within the Kato's proposed hexagonal cell for polhemusite tetragonalcell with bodydiagonals of cubesand par- doesnot agreewith the observedtetragonal morphol- allelopipedcoinciding. When the subcellsare so posi- ogy. If polhemusite is structurally hexagonal but tioned,they can be rotatedabout their body diago- morphologically tetragonal,the implication is clear: nals to bring an appropriateface diagonal d of each the tetragonal form that we observein polished sec- subcellclose to coincidencewith thec directionof the tions is a relict form, sometetragonal precursor hav- tetragonalcell. The body diagonal ofthe subcellhas a ing inverted to a mineral that is structurally hexago- lengthI : atB : 9.23A.The body diagonalof the nal. The alternativeto inversion is pseudomorphism tetragonalcell has a lengthI of 19.2lA.Thus 2/p"",u. without preservationof a trace of the original tetra- N lt"t^s.l 18.46Ax l9.2lA, difference0.75,{. The gonal host. Until firm structuralevidence for a hexag- face diagonal dps",n.: a\/Z : 7.54A; 2dp.",t. N onal cell can be obtainedand a tetragonal-to-hexago- ctetras.l15.084 x 14.74A.However, the verticalpro- nal inversion demonstratedexperimentally, we are jectionof dps"uu.on c6s,,6g.is7.29A,twice it is l4.5gA, inclined to acceptthe simpler solution provided by a and the fit parallelto c is good.We emphasizethat provisional tetragonal cell that is consistentwith the this is the geometricrelation bptween subcell and observedmorphology. tetragonalcell; it is not necessarilythe fundamental We speculatethat polhemusitemay be a disordered structuralrelation. high form, metastablein the mineral assemblageof The existenceof the subcellin polhemusitesug- the B and B deposit. Mercurian sphalerite having gestedto Dr. Kato (writtencommunication, 19j2. almost the samecomposition as polhemusiteand as- 1973)that the fundamentalstructure of polhemusite sociatedwith it in the B and B assemblagemay repre- is relatedto the sphaleritestructure, though as he sent the ordered, low form. The slight metal excess pointedout the edgeof the subcellof polhemusiteis indicated for polhemusiteby the microprobe analy- unexpectedlyshorter than that of pure ZnS (a : ses,if real and not due to excessiveabsorption ofS by 5.41A).In pure ZnS, the Zn-S distancecalculated Zn, might explain the persistenceof the disordered from atomiccoordinates is 2.34A;in pureHgS, iso- form in the assemblage. structuralwith sphalerite,the Hg-S distanceis 2.53A. But in the polhemusitesubcell, the interatomicdis- Acknowledgments tanceis only 2.31Aif we assumethat the subcellhas The following members of the U.S. Geological Survey con- the sphaleritestructure. Therefore, we aredisinclined tributed substantially to this study: Richard B. Tripp undertook to believethat the polhemusitesubcell has the sphal- the HF decomposition of samples, isolated three polhemusite eritestructure. particles, and mounted the particles for X-ray photography; Keith Robinson listened with patient disbelief to a discussionof the After our manuscriptwas submitted to thejournal, geometric relation of the provisional cell and pseudocubicsub- Kato (writtencommunication, 1975) proposed a hex- cells; he and John W. Adams helpfully reviewedthe manuscript; agonalcell for polhemusite,a : 25.22,c : 9.28A,cell Arthur J. Gude III provided some simple crystal models; Rich- contentl5(MerSr), Me:S 1.125.The densitycalcu- ard B. Taylor, Mrs. William Brennan, and Mrs. Joseph M. latedaccording to his formula Hedricks guided the photographing of polished sections used in is 4.98g/cms, and his microprobe hexagonal analysis; Mrs. Hedricks made copies of X-ray photos cell is relatedto our tetragonalcell as for study by Dr. Kato. Dr. D. C. Harris and Dr. Akira Kato, follows: acting as members of the Commission on New Minerals and 558 Mineral Names, called attention to the pseudocubic cell of pol- aher.- or.rr^".,cher.- a,",r"", Vn"*.* hemusite, and Dr. Kato stimulated much thought on the nature A f *Vr"rr^". VJ VJ of polhemusite. Professor J. J. Finney, Colorado School of Mines, made the attempt at precession photography of a hope- Kato's elegant argument for the hexagonalcell and lessly small particle. Dr Horst Piller, Carl ZeisslOberkochen, its content is basedon the assumptionthat a sphaler- provided templates that simplified the calculation of color values. ite-like structure of the pseudocubicsubcell remains William C. Leonard made skeletonmodels of the tetragonaland very likely, and on the analogy betweenpolhemusite cubic cells for his father's use. We are grateful to all these people for their generous and nowackiite, hexagonal (rhombohedral) help. CurZnrAsoS,,(Marumo, 1967),which has a sphaler- References ite substructureand Me:S : 1.083.The powder data CommissiononOre Microscopy, Internat. Mineral. Assoc. (1970) for polhemusite can be indexedvery well accordingto International Tables for the Microscopic Determination of Crys- I 16l LEONARD ET AL.: POLHEMUSITE

Fedorchuk talline SubstancesAbsorbing in VisibleLight' Batcelona,Spain, Protod'yakonova,Z. M., T. S. Timofeeva,and V. P. rtuti v rtutno- Dept. Cristalogr.y Mii,reral.,Univ. Barcelona.[provisional is- (1971)O nekotorykhmodifikatsiyakh sul'fida modifica- sue]. sur'myanykhmestorozhdeniyakh Srednei Azii [Some depositsof Cen- Leonard,B. F. (1965)Mercury-bearing antimony deposit between tions of mercurysulfide in mercury-antimony Big Creekand Yellow Pine,central ldaho. U.S. Geol. Sura.Prof. tral Asial. Vses.Mineral. Obshch.Zap., IAO'731-738. ed' Akade- Pap. 525-8,B'23-B28. Strunz, Hugo (1970) MineralogischeTabellen, 5th Marumo, F. (1967) The crystal structure of nowackiite, mischeVerlagsges. Geest und Portig K.-G. CueZngAs.Srz.Z. Kristallogr., 124, 352-368. accepled Piller, H. (1966)Colour measulementsin ore-microscopy.Min- Manuscript receiued,January 6, 1975; eral. Deposita,I, 115-192. for Publication,June 14, 1978.