Download the Scanned
Total Page：16
File Type：pdf, Size：1020Kb
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