AmericanfrIineralogist, Volume 63 , pages832-839, I 978

On cooperite,braggite, and vysotskitel

Louts J. Cesnt, J. H. Gllus Le,rLemltn,JoHN M. SrBwnRr

Canada Centre for Mineral and Energy Technology 555Booth Street, Ottawa, CanadaKl A 0GI

KsNr TunNsn INP BRtnNJ' SrtNNrn

Department of Geology and Geophysics, Yale Uniuersity New Hauen, Connecticut06520

Abstract

phase Detailedmineralogical analyses ol cooperite,braggite, and vysotskite,together with of equilibrium studies,reveal that, though there is no uncertaintyregarding the identity cooperite(PtS;P4r/mmc), one may considerbraggite (Pt,Pd)S and vysotskitePdS to be nicfeloanmembers of an isomorphoussolid-solution series (Pd,Pt)S (P4z/ m)' Theresults also the suggestthat this solid-solutionseries (braggite series) may be subdividedby restricting namevysotskite to thosemembers with lessthan about l0 molepercent PtS. Cooperite and is only braggitecan both form at magmatictemperatures of 1000"Cor above,but vysotskite fo.ir"a at submagmatictempiratures, possibly by crystallizationfrom a residualimmiscible sulfide-richmelt or by solid-statereaction.

Introduction has since been reported, but not documentedin de- Montana (Cabri Cooperite,braggite, and vysotskiteare the only Pt- tail, from the Stillwater Complex of Isles deposit, Pd sulfide minerals with ideal compositions in the and Laflamme, 1974)and the Lac des system Pt-Pd-S. Cooperite is ideally PtS, braggite Ontario (Cabri and Laflamme,1976). (Pd,Pt)S,and vysotskitePdS. All analyses,however, In this note we report new data on all three miner- of report the presenceof significantamounts of Ni, so als from the Precambrian Stillwater Complex the variations mineralogicallythey must be consideredas being in Montana. Pageet al. (1976) discussed and ultramafic the system Pt-Pd-Ni-S. We aim to clarify the no- of PG-elementsin the different mafic identifiedby Page menclatureand to presentnew data on the composi- layersof the complex.In the layers Zones we have identi- tions of the minerals.It is a pleasureto be able to do et al. as the Banded and Upper minerals in the same so in the Frondel-Hurlbut issue of The American fied, for the first time, the three found in Mineralogist, becausethese two distinguished miner- deposit. The three minerals have not been doesnot alogists have clarified so many similarly confusing contact so it is probablethat the assemblage can issuesin the nomenclatureof minerals. representequilibrium, and its true significance Braggite and cooperite are both important plati- only be guessedat. num-group minerals (PG-minerals) and are known studies from many areasof'the world; in the great Merensky Previous mineralogical Reef deposits of the Bushveld Igneous Complex in Cooperite the Transvaalthey are major ore minerals.Vysotskite samples from the has not so far been found in sufficient quantities to be Cooperite was discovered in (1928) named by Wagner consideredan important ore mineral. First discov- Bushveld by Cooper and Mr. Wartenweiller ered in the Noril'sk deposits of the USSR and re- (lg2g) following a suggestionby a presentationof ported by Genkin andZvyagintsev(1962), vysotskite in a published discussionfollowing Cooper's paper. Bannister and Hey (1932) deter- with a : 4.91 and I MineralsResearch Program, Processing Contribution No. 88. mined the mineral to be tetragonal 832 CABRI ET AL: COOPERITE, BRAGGITE, AND VYS7TSKITE

: c 6.10A.Though they recognizedthat this wasa c spacegroup P4r/m, Z : 8, and a compositionap- face-centeredcell and that the true primitivecell was proximating". . . (Pt,Pd,Ni)Scontaining about 20 : : p4r/mmc, a 3.47, c 6.10,4.,space group they percentPd and 5 percentNi . . . ." Braggiteis even neverthelessused the centeredcell in order to empha- more abundantthan cooperitein somemines of the size the relationshipto the unit cell of platinum. BushveldComplex. It averages35.9 volume percent Berryand Thompson(1962) reported a unit cellwith of the PG-mineralsfrom four areasin the Western : a: 3.48and c 6.llA, Z:2, in closeagreement Transvaal(Vermaak and Hendriks,1976) and 60 vol- with the primitivecell of Bannisterand Hey. In their umepercent at the Atok Platinummine in theeastern powder pattern,however, they observedbut could Bushveld(Schwellnus et al., 1976). not index three lines at 2.81(2),1.765(6), and The crystalstructure of braggitewas determined by 1.397A(Vr)respectively. Genkin (1968), reporting on Childsand Hall (1973)on a samplefrom the Potgie- cooperitefrom two differentlocations in theNoril'sk tersrustdistrict, Transvaal. They reported cell param- area,presented two new powder patterns.Neither etersof a : 6.380(l),c : 6.570(l)A,Z: 8. Childs pattern included the two weak lines (2.81 and andHall alsoproposedthat2 Pd atomswere ". . . the 1.397A)of Berry and Thompson,suggesting they minimum requirementin the formationof the pdS were probably due to impurity admixturesin the (braggiteor vysotskite)structure rather than the ptS Berry and Thompsonsample. Both of the Noril'sk (cooperite)structure." However, four of the sixbrag- samplepatterns contained the strongestline reported gite analysesreported by Schwellnuset al. (1976\ by Berry and Thompson,observed at 1.763and haveless than two atomsof Pd per cell,thus casting 1.770Arespectively, thus stronglysuggesting that it considerabledoubt on the Childs and Hall sugges- belongsto the pattern.The solutionto the problem ' tion. becomesapparent from the work of Grdnvold et al. (1960),who studiedsynthetic PtS, for which they Vysotskite : obtaineda cellwith q 3.4700and c :6.1096,{. The In 1962,Genkin andZvyagintsevreported the min- Grdnvoldpowder pattern contains a strongreflection eral vysotskite,for which theyindicated the formula at 1.7544,which is indexedas (103). (Pd,Ni)S,though they did not considerNi to be an Cooperitemakes up a significantfraction of the essentialconstituent. They found the mineralto be PG-mineralspresent in areasof the BushveldCom- tetragonalwith a : 6.371and c : 6.5404,and the plex wherethe MerenskyReef is mined le.g. 13.36 powderpattern was very similar to that reportedfor volumepercent average from four areasin the West- braggite.The powder pattern was also very similar to ern Transvaal,Vermaak and Hendriks(1976); 10.3 that of syntheticPdS, for whichGaskell (1937) had volumepercent at the WesternPlatinum mine, Bry- reporteda : 6.43(2)and c : 6.63(2)A,space group nardet al. (1976);and 25 volumepercent at the Atok P4r/m, Z : 8. Thesedata suggestedstrongly that Platinummine, Schwellnus el al. (1976)1.Inspite of braggiteand vysotskitewere membersof an iso- the significanceof cooperitein the MerenskyReef, morphousseries of which PdSwas the end-member the only publishedmicroprobe analyses are thoseof composition. Schwellnuset al. (1976)and Brynardet al. (1976), togetherwith a singleanalysis by A. M. Clark re- Phasesin the systemPt-Pd-S ported by Cabri (1972).The fivecooperite analyses of The ternaryphase relations for Pt-Pd-Sat 1000"C Schwellnuset al. (1976)are from samplesfrom the were reportedby Skinneret al. (1976).Those at easternBushveld, and they reveal a rangeofcomposi- 800'C havenow beencompleted and arereported in tion for Pd (by weight)from 0.15to 5.78percent and Figure l. No solidphases more sulfur-richthan PtS for Ni from 0.68to l.l8 percent;the analysesalso or PdSappear, thus we canconfine discussion to the revealPb (1.14to 1.35percent) and Bi (0.41to 0.71 phase-volumePtS-PdS-Pd-Pt. Four phasesappear: percent).Six analysesof cooperitefrom the western a continuousalloy from Pt to Pd, a liquid field lo- Bushveldby Brynardet al. (1976)have a narrower catedon the Pd-PdSjoin, andtwo sulfides.The first rangefor Pd from 0.5to 1.5percent, with Ni ranging sulfideis a solid solutionidentical with cooperite from 0.6 to 1.6percent and tracesof Rh. extendingfrom PtS,along the compositionjoin to- ward PdS, to (Pt'.zoPdo.a0)Sat 1000.C Braggite and (Pto.b4Pdo.G)Sat 800oC. The secondis a phaseidenti- Braggitewas named by Bannisterand Hey (1932) cal in propertiesto braggite.At 1000'Cthe braggite- : : for a tetragonalmineral with a 6.37,c 6.584, like phaseranges in compositionfrom (Pdo.ooPto60)5 834 CABRI ET AL,, COOPERITE, BRAGGITE, AND VYSOTSKITE

S cumulate layer situated some 6 feet stratigraphically below the main olivine cumulate horizon (main marker) which contains large pyroxene oikocrysts; sample 2 came from the "dark anorthosite" which forms part of the ore horizon immediatelyabove the main marker; and sample4 camefrom an adjoining section of the same ore horizon, referred to as the "tri-colored anorthosite." The three samples pro- vided by the Corporation are located as follows: sample 5(1824-PP-20)is a sulfide concentrate milled and floated from a bulk sampletaken from the West Fork adit. Sample 6(DDH 77-73-13) is from the Camp zone, located within the lower part of the Banded zone and 4.7 miles WNW along strike from the West Fork adit. Sample 7(789-T-l-l) is from a pyroxenecumulate layer in a trench in the southeast corner of claim Coors 8, located 6.5 miles WNW along strike from the adit. The Lac des Islesvysotskite sample was discovered in a partly crusheddrill core sample,Pl4-7, and the Pt to il arroy r o ;-- Pd Rustenburg (M19398) and Potgietersrust(MI9388) from the Fig. l. Phase relations in the Pt-Pd-S system at 1000' and samples were PG-mineral concentrates 800'C. The braggite solid solution at 800"C includesvysotskite. Royal Ontario Museum collections' : PtS: PdS, in atomic percent, for specificpoints are: A : 70: 30; B The samplesthat neededcrushing and concentra- = : = 60:40; C 16:84; D 54:46:E 24:'76. tion were processedas outlined by Cabri and La- flamme (1974). X-ray powder diffraction data were to (Pdo.roPto.r.)S,but does not reachthe composition obtained by the film method using Gandolfi and PdS.PdS is stable below 912 + 5oC,and at 800oCthe Debye-Scherrercameras. Film-shrinkage corrections braggite-likesolid solution ranges from PdS to were applied and the unit-cell parameterswere re- (Pdo.?.Pto.ro)S. fined by a least-squarescomputer program Pnnllt Two conclusionscan immediatelybe drawn from (Stewart et al., 1972). the synthesisstudies. First, braggite and cooperite are The compositionswere determinedusing a Materi- not polymorphs,even though the Pd-rich limit of als Analysis Company Model400 electronprobe mi- cooperitemight slightlyoverlap the Pt-richlimit of croanalyzer,operated at 25 and 20 kV with a speci- braggite.Second, pure PdS, with powderpatterns men current of about 0.03 microamperes' The identicalto vysotskite,is compositionallycontinuous following lines and synthetic standards were used: with a phaseidentical in all X-ray propertieswith PtLa(Pto rPdo.BS, PtrFe, PtrFe, PtbFer); braggite.Clearly, therefore, the identityof braggite PdLa(Pto?Pdo.sS, PdS, metal); SKa(Pto.?Pdo.gS,PdS); and vysotskiteas minerals must rest on theircompo- CuKa(Pt, oFeoraCuo.l.); FeKa(PtrFe, PtuFer). Correc- sitionranges. tions to theseX-ray data were applied with a modi- fied version of the Eltpeon VII computer program of Analysesof PG-sulfideminerals Rucklidgeand GasParrini(1969). We had availableto us samplesof PG-minerals Results from the StillwaterComplex, the Rustenburgarea of the MerenskyReef, Potgietersrust, and Lac desIsles. Fifty grains of Pt-Pd sulfidesfrom the Stillwater The Stillwatersamples were collected in 1975from sampleswere analyzedand the resultsare plotted in the WestFork adit of the propertyworked by Johns- Figure 2. In addition, analysesof grains from the ManvilleCorporation (1,2,4) andsupplemented by Rustenburg,Potgietersrust, and Lac desIsles samples samples5, 6 and 7 providedby the Corporation. are also plotted in Figure 2, together with analyses Using local mine terminology,sample I camefrom reported by Laputina and Genkin (1975)' Brynard e/ the Bastardzone, which is a thin (2-6 in.) pyroxene al. (1976)and Schwellnuset ql' (1916).Some analyses CABRI ET AL.: COOPERITE, BRAGGITE, AND VYS}TSKITE 835

aregiven in TableI whileTable 22lists all the analy- ses.A considerablenumber grains of werenot ho- o STITLWATERANALYSES (HOM <3) mogeneous, a STILLWATERANALYSES (HOM >3) especiallygrains of vysotskiteand brag- LAC DES ISLES ANALYSIS K gite.These are identified as filled circleswith a HOM P index )3 tr in Figure 2, and the rangeof variability x betweenPt and Pd, in particular,is demonstratedby o the bars in Figure3. A few grainswere X-rayed to confirmidentifications and to provideX-ray data. Al "/" It is apparentthat cooperiteapproaches an ideal Ni-freecomposition more closelythan do braggite and vysotskite,and no difficulty or uncertainty cloudsits identity.From analysesto date,it appears that braggiteand vysotskitecontain a minimum of about l0 molepercent Ni replacingthe othermetals. The spreadshown by theseanalyses, when plotted in Pds the PdS-PtS-NiScomposition triangle (Fig. 3), high- lights the problemconcerning the nomenclatureof Fig. 2. Cooperite, braggite, and vysotskite analysesplotted in the vysotskite-braggite. PtS-PdS-NiS composition triangle. Both braggiteand vysotskitewere named without specificationof compositionalranges, prior to knowl- from rn situ analysesfor 19 homogeneousgrains is edgeof phaserelations in the Pt-Pd-S system,and veryclose to that givenby Bannisterand Hey (1932). prior to theextensive microprobe analyses now avail- The dataplotted in Figure2, despitethe uncertainties ablefrom a wide rangeof samples.It hasgenerally shownin Figure3, do suggestcompositional group- come to be acceptedthat braggiteis (pt,pd,Ni)S ingswhich might be usedto clarifythe nomenclature (P4"/mspace group), with a compositionbetween the of braggiteand vysotskite. compositionalend-members PdS and ptS. It is inter- Vysotskitesare, as definedby Genkin and Zvya- estingto note,however, that theaverage composition gintsev,(Pd,Ni)S compounds. All analyzedsamples containa smallamount of Pt aswell, but in naturally 'A occurringsamples a distinctbreak separatesvysot- copyof Table2 is availableas Document AM-7g-077 from the BusinessOffice, MineralogicalSociety of America, 1909K skitesfrom braggites(Fig. 2). Thus,vysotskites can StreetN.W., Washington,D. C. 20006 pleaseremir $1.00in be definedas all membersof the braggitesolid solu- advancefor the microfiche. tion seriescontaining less than l0 mole percentPtS.

Table l. Electron probe analysesofcooperite, braggite,and vysotskite

!'lei 9ht percent Atomic proportions Pt Pd Ni S TotaI Pt Pd Ni XM

Cooper i te High Pt 85.8 0.77 0.50 14.I l0t.t7 0.98 0.02 0 .02 1.02 0.98 Low Pt 71.8 4.0 7.3 t7.3 100.4 0.69 0.07 0.23 0.99 t.0t AVE(i4anal. ) 83.5 i.4 1.2 14.5 I 00.6 0.94 0.03 0.04 l.0t 0.99 Grain C* 83.5 1.4 0.64 14.2 99.74 0.96 0.03 0 .02 I .0I 0.99 Bragq i te High Pt 68.1 8.5 5.6 16.7 98.9 0.67 0. I 5 0. 18 1.00 1.00 Low Pt 40.9 33.9 5.1 19.6 oo R 0.34 0.52 0. l4 I .00 1.00 AVE(19anal.) 56.4 20.3 5.1 18.2 100.0 0.51 0. 34 0.15 1.00 I .00 Grain H* oq. o I J.5 3.9 l7.l qq I 0.63 0.24 0.12 0.99 .01 Vysotskite High Pd (cr. 6)* 2.0 69.5 3.9 23.7 00 I 0.01 0.89 0.09 0.99 .01 Low Pd 4.3 65.7 5.3 23.7 99.0 0.03 0.84 0.12 0.99 .01 AVE(3anal.) z.z b/.b 5.9 24.1 99.8 0.0? 0.85 0.t3 I.00 .00 Bi , Pb sought for but not detected. Analysesused in averagin had H0M<3. 836 CABRI ET AL.: COOPERITE, BRAGGITE, AND VYSOTSKITE

PG-minerals.One of the phasesthat will crystallize from the coolingsulfide melt is a braggite-seriesmin- eral.As the temperaturedrops, the braggitemineral will beincreasingly Pd-rich and will, in fact,fall in the vysotskitecomposition range. Thus, we suggestthat most braggitesare probablyformed at high, mag- matic temperatures,while all vysotskitesare formed at sub-magmatictemperatures. Unless an early- formedbraggite grain happens,by chance,to beadja- cent to a late-formedvysotskite grain and thereby able to becomehomogenized, we shouldexpect to observeseparate compositional populations of brag- giteand vysotskite, even if thetwo shouldbe found to coexistin the samerock. Crystal chemistry The cooperite X-ray powder diffraction pattern Fig. 3. Cooperite, braggite, and vysotskite analyseswith HOM (Table3) is freeof the extraneouslines observed by plotted PtS-PdS-NiS composition triangle to show ) 3 are in the workersand is also the first X-ray the rangeof inhomogeneity,mainly betweenPt and Pd. some previous patternreported from an analyzedgrain ofcooperite. The braggiteX-ray powderdiffraction pattern (Table Braggitewould then refer to all compoundsin the 4) hasbetter resolution than the pattern listed in PDF serieswith a PtScontent in excessof l0 molepercent. 9-421fe.g. the (102) and (210) reflectionsare re- The reasonfor two compositionalgroups in a con- solvedl.The vysotskiteX-ray powderdiffraction pat- tinuoussolid solutionseries is an interestingpoint, tern is comparedto that of Genkinand Zvyagintsev and a possibleanswer is suggestedby the phaserela- (1962) which has four bracketedunindexed reflec- tionsin Figurel. Braggitesformed at magmatictem- tions (Table 5), presumedto be beta.The consid- peraturesof 1000'C or abovemust all contain l6 erablyhigher intensityflor the (200)reflection for the molepercent PtS or more,because phases containing Noril'sk sampleis probablydue to its enhancement lowerPt contentsare not stable(Skinner et al.,1976). by thebeta for the(210). Unit cellsand other physical However,as temperaturesdecline, a sulfidemelt can data are listedin Table 6, the spacegroup assign- remainstable down to almost600oC, and, as pointed mentsfor cooperiteand vysotskitehaving been con- out by Skinneret al. (1976),this melt probablyplays firmed by single crystal examinationwith a pre- an importantrole in the crystallizationsequence of cessioncamera in this study' The cooperitecell dimensionsare very close to thosereported by Grdn- Table 3. X-rav nowder diffraction data of cooperite vold et at. (1960) for syntheticPtS. Braggite cell dimensionscompare favourably with thosereported r dmeas dcalc hkl r dmeas dcalc hkl by Childsand Hall (1973)for a grainwith a similar in in- 3 3.45 3.46 010 1.030'I I.031 13? composition. Except for some differences 3 3.051 3.052 002 .018 'lr.017 006 tensities,the powder pattern for vysotskiteis the l0 3.013 3.0]3 0lI'I I .004 .004 033 6 2.450 2.450 l0 0.9592 0.959r 125 that reportedfor Genkin and Zvyagintsev's 224 sameas 2 2.290'1 2.290 012 0.9557 0.9554 I .91|1 .910 112 0.9496 0.9494 231 (1962)in PDF l5-151,though the calculatedc-di- o 1.753 I.7s4 013 0.9397 0.9397 ll6 longerthan that given by 5 1.732 1.732 020 0.8901 0.8901 I34 mensionin Table6 is a little 3 l. f,zo | . JZo 004 0.8771 0.8774 026 Genkin andZvyagintsev.These data show that, while 7 1.507 r. s07 022 0.8691 0.8690 233 3 1.295 1.295 lt4 0.8664 0.8663 040 it is possibleto distinguishcooperite from braggite,it 1.231 1.232 123 0.8456 0.8457 017 betweenbraggite and I 1.224 1.225 220 0.8390 0.8391 035 is impossibleto differentiate 2 I.l50 r.15] 0ls 0.8327 0.8326 l4l vysotskiteby X-ray methods,because substitution of 3 1.144 1.145 024 0.7893 0.7890 332 'I 226 very smallshifts in cell edges .137 |.t37 'I222 0.7828 0.7827 Pt for Pd leadsonly to 2 I .096 1.096 30 0.7770 0.7768 143 0.7750 0.7749 240 in the braggite-vysotskitecell structure. 'I Childs and Hall (1973)inferred the structureof 14.6 mmDebye Scherrer camera,Ni -fi l tered Cu radiation, I = 1.5405. ROMM 19388,9r. C in Table l, braggitefrom Gaskell's(1937) determination of the from Potgietersrust. structureof PdS.They testedfive differentconfigura- CABRI ET AL.: COOPERITE, BRAGCITE. AND VYSOTSKITE

Table4. X-ray powderdiffraction data of braggite point out that this would also require additional dis- ordered replacementof the Pt positions by Pd. This dmeas dcalc hkl I dmeas dcalc hkL can only be confirmed by further crystal structure 't 'I J 4.55 4.56 0t .393 l 397 412 investigationof different ordering models. 5 3.71 3.71 llt 1.374 1.374 a5J I 3.?8 3.28 002 1.362 I .364 332 3 3.t8 3.18 200 1.270 \.273 500 Mineral associltions 'I 9 2.914 ?.916 102 1.259 1.261 4l3 0 2.848 ?.847 210 1.?49 L 250 501 Though the three minerals occur in the Stillwater 2.649 2.651 ll2 1.236 1.237 JJJ 2.610 a.btt ?11 'I1.226 1.226 151 samples,they werenot observedto coexist.Cooperite 2.148 2.t50 212 .210 1.212 025 2.066 2.068 103 I .200 1.202 234 Pto.eEPdo.olNioorSo.r, and braggite Pd'.doPto.a8Nio.roSr.o, 2.015 '| 2.019 301 3B 'rl . t91 1. 191 25 were found coexisting in only one sample (No. a). .921 1.924 3lI 3B .185 r .187 052 .854 '1 1.856 222 2B '1.165 1.167 152 Cooperite and also braggite occurred as contiguous 4 .799 I .802 203 3 .14l 1.142 044 3 .780 1.782 302 2 1.124 | .124 144 grains in the Rustenburgsamples, but microanalyses 4 .764 1.766 230 , 'l.085I .094 r .093 006 5 .731 revealed very little compositional variation in the I .735 213 2 'I 1.084 I tJ 6 I ,l .714 1.716 312 'I.079 I .078 106 coexistingmineral pairs, though the compositionsof .705 I .705 231 3B .061 I.063 il6 5 .638 I .640 004 1B 'l.0451.052 I .053 aJ3 .589 I .591 400 1 I.047 06r .542 1.544 410 'I I 'Ir.039 L040 253 Table 5. X-ray powder diffraction date of vysotskite .499 .500 330 4 .033 I .034 026 .479 ']1.48r 133 }B I .000 I .000 145 .460 463 331 I 0.9874 0.9879 335 LAC DES ISLES* NOR]L'SK** .420 1.421 124 I 0.9643 0.9649 245 dmeas, dcalc. hkz '1 14.6 mm Debye '1.5405;Scherrer camera, Ni-filtered Cu rad'iation, = = I B Broad. R0l414 19388, gr. H 5 13.22) potgietersrust. in,Table l, from Additional 23

Table 6. Physical data for the Pt-Pd sulfide minerals

Cell DjmensjonsA CelI vol. Dx Space 3 Compositi on Group a c AA u/z gcm

c o p |i I Pt Pd Ni S P4rlmnc 3.465(3) 6.104(3) 73.286 2 36.643 10.'l23 o e t e O. gO O. OI O. 0 2 0 . 99 g g 2 Pd Ni s P4r/m 6.367(3) 6.561(6) 265.974 I 33.246 9.383 b r a i t e Pt o . o g o . z + o . t z t . o l y i 3 Pt Ni S P4rlm 6.368(3) 6.562(3) 266.098 6 33.262 6.705 v s o r s k t e Pd o . a g o . o t o . 0 9 I . 0 1

l.Potgietersrust, R0MM.l9388, gr.C; 2.Potgjetersrust, ROMM'l9388, gr.H; 3.Lac des Isles, P14-7, 9r.6.

the braggites were significantly different to the Conclusions Stillwater sample. Cooperite ranged from The composition rangesand mineral assemblages Pto.r?Pdo.o2Nio.orSo.r"to Pto.esPd'.0rNio.02So eeand brag- of the PG-sulfideminerals agree reasonably well with gite from Pto.u.oPdo.3oNio.ruSr.o,to Pto.srPdo.z6Nie 1aS1.or. data obtained from equilibrium phase studies.The The significance of this cannot be assessedwithout data suggestthat in layered intrusive rocks, both further detailed sampling and mineralogicalstudies. cooperite and braggite can be formed at magmatic The occurrenceof Pt-Fe alloy with cooperite or temperaturesof 1000oCor above, while vysotskiteis braggite was not uncommon in the Stillwater sam- only formed at sub-magmatic temperatures and ples. These assemblageswere further examined be- probably arisesby crystallizationfrom a residualim- cause Skinner et al. (1976) found that cooperite * misciblesulfide-rich melt that separatedfrom the par- isoferroplatinum + pyrrhotite was a stable assem- ent magma. blage in the Pt-Fe-S ternary at 1000'C. The Pt-Fe We concludethat Ni is not an essentialelement in alloy grains coexisting with cooperite and braggite the formation of cooperite, braggite, or vysotskite, (Figs. 4, 5) were analyzed and, in all cases,were becauseeach of the phasescan be prepared,Ni-free, found to have compositionswithin the isoferroplati- in the laboratory. Presumably the high Ni-contents num field reported by Cabri and Feather (1975), in reflect the origin of the three minerals as phases good agreementwith the syntheticwork of Skinnerel formed in mafic or ultramafic igneous rocks rich in al. (1976). Typical compositions ranged from Ptr.r2 Ni. Should any of the minerals be formed in other (FerorNioorCuo.or)to Pt2.?6Pd'.0?(Fe'.seNi6.66Cu6.62).kinds of geologicalenvironments, where Ni was not The fifteen grains analyzedwere too small (from less readily availableor was alreadytied up in other min- than 5 X 5 pm to about 30 X 80 pm) to extractfor X- eralsand removed from reaction,we suggestthat Ni- ray diffraction in order to determinewhether the Pt- free or at least Ni-poor braggite, vysotskite, and Fe alloy was primitive cubic isoferroplatinumor face- cooperitecould be exPected. centeredcubic ferroan platinum. Finally, we conclude that braggite and vysotskite are simply compositionalvariants of the samephase,

Fig. 4. Pt-Fe alloy inclusion (white) in braggite (light grey)- silicateintergrowth. The l0 pm long inclusion in Pt-Fe is pent- Fig. 5. Pt-Fe alloy included in braggitecoexisting with pyrrho- landite.Samole 1824-PP-20.Stillwater Complex. tite (upper right, P). Sample2, StillwaterComplex. CABRI ET AL.: COOPERITE. BRAGGITE, AND VYSOTSKITE 839

(Pd,Pt,Ni)S,of whichthe end-membercomposition, Cooper, R. A. (1928)A new platinum mineral in the Rustenburg PdS,has not beenfound in nature.We suggestthat norites. ./ Chem. Metall. Mineral. Soc. South Africa, 28,281- the family 283. of mineralsbe calledthe braggiteseries Gaskell, T. F. (1937) The structure of braggite and palladium and that all compositionscontaining less than l0 suffide. Z. Kristallogr, 96, 203-213. molepercent PtS be calledvysotskite. Genkin, A. D. and O. E. Zvyagintsev (1962) Vysotskite, a new suffide of palladium and nickel. Zapiski Vses Mineralog. Acknowledgments Obshch..91. 718-725. - We would like to thank Mr. H. Keith Conn and Dr. E. L Mann (1968) Minerals of the Platinum Metals and their Associa- for their hospitality to one of us (L.J.C.) during a visit to the tions in the Copper-Nickel Ores of the Noril'sk Deposils Nauka, Johns-ManvilleCorporation operationsand for samplesprovided. Moscow (in Russian). The Lac desIsles samplewas obtained through Mr. J. P. Sheridan, Grdnvold, F., H. Haraldsen and A Kjekshus (1960)On the sul- Toronto, and the Royal Ontario Museum sampleswere obtained fides, selenidesand tellurides of platinum. Acta Chem Scand., with the help of Drs. R. I. Gait and J. A. Mandarino. We are also 14,1879-1893. grateful to Drs J. T. Szymanski and T. T. Chen for useful dis- Laputina, I. P. and A D. Genkin (1975)Minerals of the braggite- cussions.A portion of this work was supportedby NSF grant GA- vysotskite series. Izomorpizm Mineraly, Izdat. Nauka, 146-150 27523. (in Russian). Page,N. J, J. J. Rowe and J. Haffty (1976)Platinum metalsin the References Stilfwater Complex, Montana. Econ. Geol., 71, 1352-1363 Rucklidge,J. C. and E. L. Gasparrini (1969)Electron microprobe Bannister,F. A. and M. H. Hey (1932)Determination of minerals anafyticaf data reduction. Erraplnn YII. Department of Geology, in platinum concentratesfrom the Transvaalby X-ray methods. Uniuersity of Toronto. Mineral Mag., 28, 188-206. Schwellnus,J S. I., S. A. Hiemstra and E. Gasparrini (1976)The Berry, L. G. and R. M. Thompson (1962) X-ray Powder Datafor Merensky Reef at the Atok Platinum mine and its environs. Ore Minerals: the Peacock Atlas. Geol Soc. Am. Mem. 85. Econ Ceol . 7l. 249-260. Brynard, H J., J. P. R. De Villiers and E. A. Viljoen (1976) A Skinner, B J , F. D. Luce, J. A. Dill, D. E. Ellis, H. A. Hagan. mineralogicalinvestigation of the MerenskyReef attheWestern D. M. Lewis, D. A. Odell, D. A. Sverjenskyand N. Williams Platinum mine, near Marikana, South Africa. Econ. Geol ,71, (1976) Phaserelations in ternary portions of the systemPt-Pd- 1299-1307. Fe-As-S. Econ Geol., 71, 1469-14'75. Cabri, L. J. (1972) The mineralogy of the platinum-group ele- Stewart, J. M., G. J. Kruger, H. L. Ammon, C. Dickinson and ments.Minerals Sci.Eng,4, No.3,3-29. S R. Hall (1972) The X-ray system of crystallographicpro- - and C. E. Feather(1975) Platinum-iron alloys:a nomencla- grams Uniuersity of Maryland Compuler Science Report TR- ture based on a study of natural and synthetic alloys. Can. 192 Mineral., 13. l17-126. Vermaak, C. F. and L. P. Hendriks (1976)A reviewof the mineral- - and J. H G. Laflamme(1974) Rhodium, platinum and gold ogy of the Merensky Reef, with special referenceto new data on affoys from the Stillwater Complex. Can Mineral., I 2, 399-403. the precious metal mineralogy. Econ. Geol.,7l, 1244-1269 - and - (1976) Ore microscopyof some samplesfrom Wagner, P. A. (1929) The Platinum Deposils and Mines of South Lac des lsles, Ontario. G.A.C.-M A C. Program with Abstracts, Africa Oliver and Boyd, Edinburgh. 1, 56. Childs, J. D. and S. R. Hall (1973)The crystal structureof brag- Manuscript receiued, March 20, 1978; accepted gite, (Pt,Pd,Ni)5. Acta Crystallogr., 829, 1446-1451. for publication, April l l, 1978.