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Home , Getchellite, Getchell Mine, Orpiment, Stibnite

Tun AUERTcANM rxERALocrsr JOURNAL OF THE MINERALOGICAL SOCIETY OF AMERICA

Vol. 50 NOVEMBER_DECEMBER, 1965 Nos. 11 and 12

GETCHELLITE, AsSbS3,A NEW MINERAL FROM HUMBOLDT COUNTY, B. G. Worssnx,xc, Chemistry Dhision, Department of Scientif,c and Ind,wstr.ial,Research, Lower Hutt, New Zealand.

ABsrRAcr is a new mineral, AsSbS:, from the , Humboldt County, Nevada. It occurs in an epithermal arsenical deposit intimately associated with , , , and . Getchellite forms transparent, dark red, sectile crystals with a perfect {001} (yielding flexible, inelastic cleavage iamellae). H:1* to 2; G:3.92 (obs.), 4.01 (calc., synthetic); it melts in the range 340o to 355" C.; its luster is pearly to vitreous on cleavage surfaces, otherwise resinousl its is -red. Getchellite is monoclinic, biaxial (f); 2Y2146";q)2.11 (wbite light), B>2.72 (Lilight);Z:b,.Yla:15o * 5"; Ylc:101" * 50; dispersion crossed rlv strong. In reflected white light getchellite is grayish-white with a blue tint; it is anisotropic and shou's strong blood-red internal reflections. Its reflectivity is in the range 25 30 (by visual comparison). Chemical analysesgive 25.09 wt. 0/6 As, 42.04 ol rvt. Sb, and 32.82 wt. /6 S; the corresponding formula is Aso seSbr.orSg.The sttongest linesinther-raydifiractionporvderpattern are2.89(100),4.M (80),3.63 (70),2.54(60) and 2.33 (60) A. Weissenberg eq-r'i-inclination photographs of synthetic single crystals give o:11.85, b:8.99, c:l}.16 A, B:tt6' 27', spacegrotp P21fa, cell volume 969 43, ceil content 8 [As SbSa]. The physical properties and x-ray data of synthetic getchellite, crystallized hydro- thermally from As3SbS6glass in sodium sulfide solutions at 2600 C. and 1,000 bars pressure, those of the natural material. Getchellite is named in honor of the Getchell Mine, at which it was discovered.

OccunnnNcn AND ORTGTN Getchellite(AsSbS), is a new mineral from the Getchell Mine, which is about 20 miles north of Golconda,Humboldt County, Nevada. Ore specimenscontaining getcheilite were collected by the author during a visit to the mine in August 1962 as part of an investigation of the para- genetic relations between the of iron, arsenic, antimony and mercury, and associatedgangue minerals, but its presencewas not recog- nized until the specimenswere examined nearly a year later. The mineral is named in honor of the GetchellMine, at which it was discovered.The name has been approved in advance of publication by the Commission on New Minerals and Mineral Names. IMA.

1817 B. G. IYEISSBERG

The geology of the Getchell Mine, previousl,vdescribed by Hardir (1941) and Joralemon (1951), consistsessentialiy of a narrow steeply dipping fault zone cutting interbedded Paleozoic(?) shales,argillites, and limestonesnear a granodiorite intrusive. Mineralization is mainll'' confined to the shearedrocks in the fault zone and consistsprincipalll' of quartz, ,realgar, orpiment, stibnite, , ,cinnabar, barite, , gy-psumand very fine-grainedgold. Structures in the shearedrocks are complex,open spacesare common,and open-spacefilI- ing is the dominant mode of mineral emplacement. Getchelliteoccurs near the southern end of the ore body on the west side of the "south Pit Extension" where it is intimatelv associatedi,vith abundant orpiment and realgar and with lesseramounts of quartz, stib- nite and cinnabar,all of which occurin the f ault zonein veinsup to eight inchesthick cutting shearedand brecciatedcountry rocks.In theseveins getchellite was apparentll' one of the first hydrothermal minerals de- posited after quartz. It completetl-fills some vugs lined with euhedral crvstals of comb q'Jartz, but more commonlf it onlv partl1''fills such spaces,the remaining space being occupied by orpiment and realgar. Both orpiment and realgar commonly are molded around subhedral cr1-stalsof getchellite,but in some placesanhedral grains of getchellite and a few rare euhedral crl.stals of getchellitewere observedas inclu- sionsin orpiment, suggestingthat somegetchellite and orpiment was de- posited simultaneously or nearlv so. Realgar is commonly molded around euhedral crvstals or orpiment and also contains some euhedral crystalsof orpiment. Minute doubly terminated crystalsof quartz occur as inclusionsin getchellite,orpiment and realgar,although most of the quartz in vugs and along vein walls underlies these minerals. Minute acicularcrystals of stibnite are includedin getchellite,orpiment and real- gar, and thus stibnite was presumably depositedmore or Iess continu- ously, but in small amounts, throughout the period of ore deposition. Cinnabar occursin very minor amounts as very small, dark-red,pseudo- cubic rhombohedralcrystals along the grain boundariesbetween realgar and orpiment and presumably was among the last of the minerals de- posited. Small veinlets of realgar transect fragments of country rock, qnartz, getchelliteand orpiment, and indicate depositionor redistribu- tion of realgar after some movement along the fault subsequentto the main sequenceof ore deposition.Cr,vstals of getchelliteand orpiment are commonlv bent or otherwisedeformed bv movementssubsequent to ore deposition. From these textural relations it is concluded that qttartz, getchellite,orpiment, realgar,stibnite and cinnabarare verv closelyasso- ciated both in time and in spaceand ivereprobabiv transportedby, and GETCHELLITE depositedfrom the sameor very similar solutions.The closeassociation of the arsenic and antimony sulfides in manv places throughout the world raisesthe questionof why getchelliteshould be so rare a mineral. With its unusual physical properties, it is unlikely to have been overlooked.

Pnvsrcar- AND OprrcAL PRoPERTTES Getchellite,apart from its dark blood-redcolor and orange-redstreak' closely resemblesorpiment. In places, crystal faces and cleavagesur- faces of getchellite display a purple to green iridescent tarnish. Its luster is pearly to vitreous on fresh cleavagesurfaces, but elsewhereit is resin- ous. Getchellitehas a perfect micaceouscleavage parallel to {001} yield- ing flexiblebut inelasticcleavage lamellae. Its is splintery which suggestsa secondless perfect cleavage.Getchellite is sectile,and has a Mohs scalehardness of 1] to 2.The specificgravity of getchellite,deter- mined bv meansof a Berman torsion balance,is 3.92+0.031and the cal- culatedspecific gravity is 4.01 (s1nthetic). Getchelliteturns darker red when heated.Above 320" C. it gradually turns black, sublimes, and minute black acicular crystals are formed on the coolercover plate of the heating stage.In the range 340 to 355oC. getchellite apparently melts to an extremely viscousblack mass which re- tains the grossshape of the original crystal fragments.Near 4700C. the melt losesviscosity very rapidly and begins to boil, and acicular metallic gray crystals, similar in appearanceto stibnite, begin to form at 490oC. In transmitted light getchellite is blood-red in colorl it is biaxial posi- tive with a)2.11 (white light) and A>2.72 (lithium light); its bire- fringenceis very strong to extreme with 2Hz:80"*3"2 and 2Y2146". The orientation of getchelliteis Z: b, YAo: 150+ 5o, and Y,\c: 1010 f 5o, and the dispersionis crossed,r)v strong. Pleochroismand differ- ential absorptionwere not detected. In vertically reflected white light the color of getchellite is grayish- white with a slight blue tint. Its reflectivity in white light is greater than that of the associatedorpiment and realgar,but lessthan that of stibnite, and was visually estimated to be in the range 25-30. The weak anisotro-

1 The actual measured vaiue o{ the specific gravity, determined lrom 27 observations on three crystals, is 3.88 + 0.02 (probable error). This was corrected to 3.92 + 0.03 to account for 2 0 wt. 16 qtartz as inclusions within the crystals indicated by the chemical analyses, and also includes an additional *0.01 error to allow for *0..5 wt a/6 variation in quartz content. 2 The optic-axial angle in oil (1-bromonaphthalene, n:1.661), 2H7, was measured di- rectly using a needle stage and a synthetic crystal, the optic plane of which was mounted normal to the needle axis. 1820 B. G TVEISSBERG

pisn tends to be obscuredb1' strongblood-red internal reflections.Nearly all the crvstals of getchellite examined with the reflecting microscope were bent, so that intern:rl reflectionsarising from slip surfacesalong cleavagelamellae r,vere readily apparent. Getchellitepolishes better than realgar and about the same as or slightly better than orpiment. fts polishing hardnessappears to be slightl)- greater than that of orpiment, and it scratchesand exfoliatesreadily asdoes orpiment.

CnBMrc,c.rANarvsrs Samples of getchellite were prepared for analysis by crushing ore specimensin a jaw crusher,sieving, and then hand picking cleancrystals and cleavagefragments of getchellitefrom the 7 to 18 mesh fraction, after rvashingthe sievedfraction with distilled water and drf ing in air. The handpickedmaterierl was re-washedin distilled water, dried in air at 105" C. and storedin a desiccatoruntil used. Preliminarv analysis of finely-ground getchellite bv ;r:-rayfluorescence indicated the presenceof only arsenic,antirnony, and a trace of selenium. The arsenicand antimony in getchellitelvere determined in three sam- ples taken into solutionby a mixture of sulfuric and nitric acids.After re- rnoval of the nitric acid by evaporation and reduction of the solution with h.vdrazinesuifate, arsenicwas separatedfrom antimonl' as the sul- fide b1'the method given by Hillebrand et al. (1953).After re-solution, the arsenicand antimonv were determinedseparately by titration with potassiumbromate using the method of Smith and Ma1' (19.11). Su1furin getchellite was analvsedseparately in three samples.The samples lvere dissolved with bromine in carbon-tetrachloride (the method of Hillebrand et al., 1953), and sulfur was determined gravi- metrically as barium sulfate. The results of the analysesare given in Table 1. The formula of getchellitecalcuiated from the analvsesis As6esSb1.s1S3. The excessof erntimonyin relation to errsenicindicated by the formula can be at least partl-v accountedfor by an incomplete separation of arsenicfron'r antimony in the analytical procedureand subsequentinclu- sion of some arsenicwith antimony in the determination of antimony. Previous checkson the analytical procedureusing known standard mix- tures of AszSrand SbzSaindicated that arsenicconsistently ran from 0.4 to as much as 1.0per cent low and that antimony ran from 0.1 to 0.5 per cent high. Inclusionsof stibnite within the getchelliteanall'sed probably also contribute to the high antimony content indicated by the analyses. The trace elements in getchellite, determined by spectrographic analysis,are listed in Table 2. GETCHELLITE 1821

Telr,r 1. Cnrlrrcer- Axer-vsrs ol Getcnlr-lrtr

InsolubleResidue Wt. Wt Wt Wt Analysis Sample -- wt.. _ . A;;i_.. ..' ^Tj % % % Number Weight W;rcht Anenic Sulfur Weicht As Sb S

1 0.2240 0 0051 2 3 0.0548 0.0917 25.03 4r.89 2 0.2t51 0.0070 3.2 0 0527 0 0869 25 32 41 76 3 0.1933 0.0035 1.8 0 0473 0 0806 24 92 42.+6 4 01576 00023 15 0.05rj8e 32'74 5 0.1448 0.0031 2.1 0 0465s 32.87 6 01678 ooo3o 18 o'o54lr 32'85

Total Average 25 09 42 04 32 82 99.95

Theoretical composition of AsSbSa 25 59 41 57 32 84 100.00

Norn: The insoluble residue was composedof minute doubly-terminated crystals o{ quartz, the rveight of which was subtracted from the initial sample weight in calculating the weight percentage oI As, Sb and S in getchellite Analyst, B. G. Weissberg.

SvNurpsrs Early in the investigation the composition of getchellite was incor- rectly estimated as AssSbSofrom the peak height ratios of arsenic to antimony determined by r-ray fluorescence,and glassof this composition was preparedby air quenchinga melt of AszSeand Sb2SBmixed in a 3:1 mole ratio. Portions of this glass were sealedin gold capsulescontaining 2 weight per cent sodium sulfide solution and held at 2600C' and 1,000 bars pressurefor 5 days, 14 days and 50 days. The product in eachof the runs was composedof both orpiment and getchellitein roughly equal

TlreLn 2. Eurssrom Sppcrnoonapnrc ANAr-YSrsol GETcrIEl,LrrE

Constituent Percentage

As, Sb, and S Major Si o2 Mg <0.003 A1 <0.01 Ca o02 Cu 001 Ag 0.002 Pb 001 T1 0.000s Hg -0.0001

No other elementswere detected. Hutt, Analysts: H. J. Todd and W. C. Tennant, Chemistry Division, D.S.I.R', Lower New Zealand, 1822 B. G. II.EISSBERG quantities. In the run of only 5 days duration about 50/6 of the initial glassremained unreacted, but in the runs of longer duration no trace of the initial glassremained. Crystals of the synthetic getchellitethus pro- duced were usedin the crystallographicinvestigations that follow.

Cnvsrer-r-ocRAPrrY Getchellite was found variously as small equant subhedral crystals less than 0.5 millimeters in diameter up to anhedral grains as large as 4 milli- meters in maximum dimension.All the getchellitefound was badly de- formed, and the author located only one crystal fragment which yielded results with the two-circle reflecting goniometer and the single crystal r- ray camera.This crystal fragment was sub-equant,approximately 0.15 millimetersin diameter,and although the crystal was bent through about ten degreesof arc in the oc plane about D as an axis, it was possibleto de- termine the Miller indices of the better developedcrystal faces and to match the single crystal r-ray photographs with those of synthetic getchellite.The crystal was boundedby {001} cleavagesurfaces and the faces (010), (201), 111), (2lI), all equally well developed,and by (2I2), (212), (211) and (4I2) smaller and lesswell developed.It was not possible to index severalother smallerfaces on the crystal becauseof the largeun- certainty in their positions. A rotation photograph about the crystallo- graphica-axis and a normal beam Weissenbergzero Iayer-line photograph about the o-axis of this crystal matched perfectly with similar photo- graphs of synthetic getchellite in spite of the streaky nature of the ro-ray reflectionsof the natural crystal. Singlecrystals of svnthetic getchellitewere up to 1.0 millimeter iong and wereprismatic parallel[100] and slightly tabular with the form 1001f predominatingover {011} and with {010} very small and commonly ab- sent. Terminating forms were l2lll , 1111|, and more rarely 12011, {101}. Simple and polysynthetic twins were commonly observedin the synthetic getcheilitewith the twin plane and compositionplane parallel to {0011. The r-ray powder diffractionpatterns of natural and synthetic getchel- Iite are given in Table 3. The r-ray powder diffraction pattern of the syn- thetic getchelliteproduced in the synthesisof 4] days duration wassome- what more diffuse than that of natural getchellite,but no significant visible differences,other than degreeof exposure,were observedbetween the powder patterns of natural getchelliteand synthetic getchellitepro- duced in the synthesesof 14- and 50-daysduration. The powder pattern was indexedby graphically comparingthe measuredspacings of natural getchellitewith the interplanar spacingsobtained graphicallv from the cell dimensionsof synthetic getchellite.The interplanar spacingslisted GETCHELLITE t823

Talln 3. X-Rav PomrR DrFlRAcrroN ParrnnNs ol GETcHELLTTET eNo Svxtnntrc GntcllBr-r-rtnz

Getchellite Synthetic Getchellite d (olc.) A d (meas.)A I/It d (meas ) A

9.19 40 9!7 40 001 9.11 6.+6 50 6.46 40 111 6.44 011 640 5.91 5 594 5 20I 586

5.40 5 530 J 200 530 4.97 50 492 50 211 4.91 111 4.85 460 50 broad 4..58 50 202 462 2to 457 oo2 4.55 4.44 80 4.44 o20 4.50 112 4.43 5 212

40 broad 120 4.10 4.06 o12 4.06 121 404 021 401 311 361 220 3.43 312 3.42 112 3.37 122 3.37 203 336 333 20 310 3 31 1 1L 10 222 022 320 31+ 40 40 213 3.14 113 3 1,2 3.08 40 10 003 304 2.99 10 10 321 296 289 100broad 100broad 402 293 401 292 313 29r 202 288 130 288 013 288 a19 286 122 283 320 278 419 2.78 223 269 268 231 2 6t- 403 267 2.60 2.59 230 261 132 258 2.54 2.53 413 204 254

1 Getchellite from the Getchell Mine, Humboldt County, Nevada. 2 Synthetic getchellite hydrothermally crystallized from AsaSbSsglass in an aqueous sodium sulfide solu- tion held at 260' C. and 1,000 bars pressurefor 14 days Data from photographs taken with a 1146 mm diameter Debye-Scherrer camera, uing Cu radiation with Ni filter, powder suspendedin a rod of "Duco" cement, and no correction for film shrinkage 1824 B. G. WEISSBERG

Trern 3 (conti.nued.)

Synthetic Getchellite d (calc ) A d (meas.) A d (neas.) A

410 2.54 323 254 113 2.52 023 2.52 232 251 2.46 2.46 214 2.45 L1' 245 )L1 240 321 243 222 2+2 314 239 238 2.33 t l1 400 2.31 t32 2.31 423 230 2.25 312 226 040 2.25 203 22+ 414 2.2+ 2s3 224 2.23 224 2.22 411 222 513 2.21 o14 2.2r t4.0 2.20 123 2.18 324 2.17 522 2.r0 24! 2.ro 432 2.O9 431 209 510 206 424 2.O5 52I 2.0s 421 2 -01 024 2.O3 s23 203 203 2+2 202 205 202 420 1 986 r.978 133 | 975 21.5 1.972 602 1 971 612 1.925 342 1 922 s20 1.919 1.919 1.913 334 1 911 613 1 908 134 | 902 412 1 902 601 1 901 115 1.901 GETCHELLITE I 825

Tl.rl.E 3-(continued)

Synthetic Getchellite d (calc. ) A d (meas.) A I/10 d (meas) A

1.865 10 1 864 1 9)L 10 r 822

I 793 J 1.789 1 758 10 1.741, not resolved 1.733 10 1.699 20

1 658 30 broad 1.59.5 5 1 567 30 I 573 1.536 10 r 526 1 471 10 1.451 | 426 10 1.432 1 382 5 1.288 10 were calcuiatedfrom the lattice dimensionsof synthetic getchellitefor those planesthe spacingsof which, determinedgraphically, agreedwith the measuredvalues of natural getchellite. Inasmuch as all the getchellitecrystals found were badly deformed, synthetic cr)'stals of getchellite were used in the single crystal fi-ray in- vestigations.Weissenberg (equi-inclination) photographs of crystals of synthetic getchellite rotated about the o-crystallographicaxis and D- crystallographic axis were taken of the zero, first, and second layers' Cell dimensions of synthetic getchellite determined from the photo- graphsarei a:11.85 +0.02,6:8.99+0.02, c:10.1610.02 A, 0:1160 27' I l2t . The unit cell has a volume of 969 A3 and contains eight AsSbS3. The calculatedspecific gravity is 4.01 in comparisonwith 3.92 ior the natural material. The spacegroup is P21fa, reflections(hOl) and (0k0) being absentwhen h and k are odd.

AcxlqowLroGMENTS The author wishes to acknowledge the financial support of the Na- tional ScienceFoundation, Grant N. P862,to Prof. F. W. Dickson for ore genesis research, in sponsoring the initial field investigation in which getchellite lvas found and also for providing, through the grant, the pres- sure vessels and accessoryequipment necessary for the hydrothermal synthesis of the getchellite crystals used in this study. The General Manager and staff at the Getchell Mine were most courteous and helpful during the author's visits to the mine. Special thanks are extended to the staff of the Department of Geology at the University of , Riverside, for providing space,facilities and support during most of the 1826 B. G, WEISSBERG investigation while the author lvas there as a researchfeliow. Polished sectionswere expertly made b1' Mr. John deGross6of the Department of Geology, University of California, Los Angeles. Appreciation is ex- pressedto the New Zealand GeologicalSurvey for making available to the author reflectingand heating stagemicroscopes. Thanks are given to Dr. G. A. Challis and Prof. GeorgeTunell for critically reviewing and improving the manuscript.

RolBnrNcrs

Herov, R. A. (1941) Geology of the Getchell Mina Am. Inst. Min. Met. Eng., Trans.,l44, 147-t50. Hrrrnenaro,W. l-., C. E. F. LuNnnll, M. S.Bnrcnr.tNo J. E. Horr.ueNrl(1953) Applien Inorganic Analysis.2nd Ed., John Wiley& Sons,Inc., New York. Jonar.nuox, Pnrrn (1951) The occurrence of gold at the Getchell Mine, Nevada. Ecoz. Geol.46, 267-310. Sltrn, G. F. aNo R. L. Mav (1941) Use of bromate in volumetric analysis-Determination of arsenic and antimony using internal indicators at ordinary temperatures. In d,ustrial Engi.neering Chemical,Analysi;, Analyti.col, Ed., 13, 460-467.

Manu.script rueiaed.,Morck 11, 1965; acceptd Jor publication, March 29, 1965.