Characteristic Optical Data for Cooperite, Braggite and Vysotskite

Canadian Mineralogist Vol. 23, pp."149-162(1985)

CHARACTERISTICOPTICAL DATA FOR COOPERITE,BRAGGITE AND VYSOTSKITE

ALAN J. CRIDDLE AND CHRISTOPHER J. STANLEY Department of Mineraloglt, British Museum (Natural History), Cromwell Road, South Kensington, London, Great Britain SW7 sBD

ABSTRAcT monfent que cooperiteet pyrrhotine ne seressemblent pas. Les descriptionsoriginelles de Schneiderhcihn(1929a, b) Reflectance spectra for cooperite PtS and braggite portaient non pas sur la cooperite,mais probablementsur (Pt,Pd)S from the type locality at Potgietersrus, South la braggite, encore inconnue d 1'6poque.L'erreur s'est Africa, weremeasured between 2100 and 700nm. Thesespec- aggravdequand le "spectre" de rdflectance de la pynho- tra, and qualitative descriptionsof the grains, completethe tine. extraite de concentrescens6s contenir de lia cooperite, characterization of the minerals made in 1932by Bamister fut attribud d la cooperite. & Hey. The mineral association, optical properties and reflectancespectra of cooperite,braggite and vysotskitePdS (Traduit par la R€daction) from the Minneapolis Adit area of the Stillwater complex, Montana, are also described.The data for cooperiteand Mots-clds: cooperite, braggite-vysotskite, spectrede rdflec- braggite are compared with those for the type material; the tance, 6valuationdes couleurs, caract6ristiques optiques' data for vysotskite may, in the absenceof appropriate data VHN, intervalle de composition, Potgietersrus,com- for the type material from Noril'sk, U.S.S.R., be consi- plexe de Stillwater, min6raux du groupe du platine. dered characteristic for the mineral. Electron-microprobe data are provided for all ofthe grains measuredinthe reflectance study, and VHN for most of them. The extraordi- IurnoructroN nary confusion that has surrounded the quantitative data and qualitative descriptions of theseminerals is described. Cooperite,braggite and vysotskiteare important Reflectancespectra and color valuesdemonstrate the false- sourcesof platinum and palladium in many of the hood of the long-standing belief that cooperite resembles world's largest depositsof platinum-group minerals. pyrrhotine. The (Schneiderhcihn original descriptions It is thus surprising that their optical characteristics, l929a,b) were not probably of cooperitebut of the then. particularly those of cooperite and braggite, have unknown mineral braggite. This error was compounded present, when the reflectance'spectrum' of pyrrhotine, from the remainedill-defined sincetheir discovery.At supposedlycooperite-bearing concentrates, was mistaken theh identificationwith the microscopeis uncertain, for that of cooperite. owing to erroneousand conflicting descriptiveand quantitative data in the literature. Keywords: cooperite, braggite-vysotskite,reflectance spec- We first ericounteredthis problem in 1980when tra, color values,optical characteristics,YHN, composi- examiningsamples from the MinneapolisAdit area tional range, Potgietersrus, Stillwater complex, of the Howland Reef (Bow el ol. 1982), Stillwater platinum-group minerals. Valley, Montana. Here, unusually, all three minerals occur in close associationbut, during our initial SOMMAIRE investigation, we were unable to identify them without the aid of the electronmicroprobe. Indeed, On a mesur6, entre 400 et 700 nm, le spectre de r6flec- at first, we believed cooperite to be a new mineral tance de la cooperitePtS et de la braggite (Pt,Pd)S de la and braggiteto be cooperite.Closer examination of localit6 type, Potgietersrus(Afrique du Sud). Ces specfies, the literature revealedthat the earliestdescriptions ainsi que la description qualitative gxains, des complEtent of cooperite (Schneiderh6hn l929a,b) were on la caractdrisationdes deux esplces amorc€e en 1932par Ban- nister et Hey, On d€crit aussiI'association min6ralogique, material of dubious authenticity. Thesedescriptions, les propri6tds optiques et Iesspectres de rdflectancede coo- and in particular a suggestedsimilarity to pyrrho- perite, braggite et vysotskite PdS du secteur dit Minnea- tine, are completelymisleading. Unfortunately, when polis Adit du complexe de Stillwater (Montana). Les don- cooperite and braggite were characterized@annister n6essur la cooperite et la braggite sont compar6esi celles & Hey 1932), their optical characteristics were not des dchantillons types; vu le manque de donn€espertinen- determined.It remainedfor Edwards et al. (1942) tes sur le mat6riautype de Noril'sk (U.R.S.S.), les obser- briefly to describethe qualitative optical properties vations ici consign6espeuvent servir d caractdriserla vysot- of a type specimen of braggite. The erroneous skite. On pr€senteles resultatsd'analyses d la microsonde descriptions and reflectance data for cooperite 6lectronique pour tous les grains dont on a d6termin6 la r6flectance, et la durete VHN pour la plupart d'entre eux. (Schneiderhdhn1929a, b, Schnederh6hn& Ramdohr On ddcrit la confusion extraordinaire qui 6mane des des- 1931,Frick 1930)and the incompletedata for bragg- criptions, tant qualitatives que quantitatives, de ces espd- ite hinderedthe identificationof the mineralsby ore ces.Les specfiesde rdflectanceet l'6valuation des couleurs microscopy. Thus, although confirmation of their 149

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identity was certain by X-ray diffraction and Clearly, a thorough re-investigatioi of all three chemicalanalysis, the problem remainedof identify- minerals using procedures recornmendedby the ing the right mineral in the first instance.As a result, COM was overdue.In particular, the confusionsur- until quite recentlycooperite was regarded as a'rar- rounding the color and brightnessof the minerals ity' (Ramdohr 1980),and braggitewas reported to required measurementof the dispersion of their be a 'very rare mineral' (Picot & Johan 1982). reflectancethroughout the visible spectrum and cal- Attention was first drawn to the 'conflicting and culation from thesespectra of unambiguouscolor- overlapping data' for braggite and cooperite by values.More than this, we consideredit essentialto Leonard et ol. (1969),who also noted that adequate relate thesemeasurements to the composition of each quantitative data were lacking. The situation grain that was measured and to obtain X-ray data improved somewhat in the 1970s,when electron for selectedgrains. We were fortunate in having at microprobesand microscope-photometerscapable our disposal the type specimensof cooperite and of measurementsof spectralreflectance became more braggite of Bannister & Hey (1932), as well as the widely available. The first spectral-reflectancedata, intergrown grains of cooperite, braggite and vysot- though restricted to four wavelengthsthat cor- skite from the MinneapolisAdit. The latter, fortui- respondto the perceivedbrightness and color ofthese tously, occur in a matrix consisting predominantly minerals, were those of Cabri (1972)for braggiteand of pyrrhotine, pentlandite and pyrite; hencedirect Schwellnuset ol. (1976)for cooperiteand braggite. comparisonof cooperiteand pyrrhotine was possi- More completespectra and descriptiveinformation ble. What follows, therefore, may be regardedas the were reported by Picot & Johan (1977). Unfor- completion of the characterizationof braggiteand tunately, the spectraand descriptionsfor cooperite cooperite, with additional and more complete data aretransposed in their book and appearas for brag- for vysotskite. g$e (this mistake occurredat an editorial stage;Picot In addition to measuringthe reflectancespectra, & Johan, pers. comm.). Their description of calculating color values, and measuring micro- cooperiteis also in error; it correspondsneither to indentation hardness(VHN) values, we examinedin cooperitenor to braggite. In 1981,Cabri reviewed detail the apparentcolor, brightness,bireflectance the situation and concludedthat the reflectancespec- and reflectancepleochroism ofthe minerals in plane- tra of braggite neededconfirmation, that those of polarized light, and their rotation tints between cooperite neededredetermination, and that more crossedpolars. This was done not only to complete work was required on the qualitative optical proper- their characterization but to determine whether the ties of vysotskite.The latter, despitethe uncertain qualitative properties alone were sufficiently distinc- propertiesof braggite,had beencharacterized with tive to be used as a meansof identification. minimal optical data in l962by Genkin &ZWaejnt- sev. In 1982,Picot & Johan publishedcharacteris- EXPERIMEI.ITALPROCEDURE tic reflectance spectra and descriptive data for cooperite. Their R spectra for braggite are also All of the Pt-Pd-Ni-S-bearing samplesfrom the characteristic, but their description of this mineral Stillwater Yalley examined (fourteen polished does not correspondto thesespectra; in fact, it is mounts, ten polishedgrain-mounts, and five polished almost identical to the description of thin sections)came from an unusually coarse-grained Schneiderhdhn'scooperite rhat had previouslybeen pegmatiticgabbro from the first cross-cutof the Min- reiteratedin various forms in the diagnostictexts of neapolisAdit. Thesesamples were selectedfrom a Farnham (1931),Ramdohr (1950and all later edi: suiteof specimensBM 1981,134. The equipmentand tions), Schouten(1962)and Uytenbogaardt& Burke polishing procedure used for the mounts and grain (1971).Correct reflectancevalues determined at the mounts were describedin Criddle & Stanley(1979), four wavelengthsrecommended by the Commission Comminution, separationand mounting procedures on Ore Microscopy (COM) for braggite and for the grain mounts are those describedby Cabri cooperitefrom the Rustenburgmine, South Africa, & Laflamme (lW6), The samepolishing procedures werereported, together with brief descriptivenotes, wereapplied to the grain mounts of the type speci- by Kingston & El-Dosuky (1982).These authors go mensfrom Potgietersrusas were used on the Still- someway toward resolvingthe confusion concern- water samples. ing the two minerals; however,their assertionthat The equipmentand proceduresused for measur- the incorrect description'of the color of cooperite ing the reflectanceof all the samplesare as described (tlat had becomeentrenched in the literature) is more in the Appendix to Cabri et al. (1981), except that applicableto braggitehas introduced a new source x 16 air and oil objectives,adjusted to provide effec- of confusion. Further, their conclusionthat the origi- tive numerical aperturesof 0.2, were used. Micro- nal source of the mistaken indentity of cooperite indentation hardnessmeasurements were made at arose from a description of braggite-cooperiteinter- 100-gramforce with a Leilz Miniload 2 hardness growhs is one that we cannot support. tester equippedwith a Vickers indenter. The areas

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selectedfor both sets of measurementswere first tion and vocabularybetween the two observers,the checked for compositional homogeneity with the results were very consistent. electron microprobe, a Cambridge Instruments Initially, we were under the impressionthat the MicroscanIX, operatedwith an acceleratingvoltage rotation tints differed betweenthe two microscopes of 20 kV and a beamcurrent of 2.50 x 10-oAon but, on checkingour notes,it becameclear that the the Faradaycage. The pure elementsPt, Pd and Ni, difference was not in the color but in the relative as well as FeS, were usedas standards.The results intensity,or brightness.This phenomenonwas most were coriected using the Cambridge Instruments marked in the polymineralic Stillwater samples, ZAF programme V028. wherethe eyewas further confusedby the rotation For the qualitative examination of the optical tints of the associatedminerals. As a result, in the propertiesof the mineralswe used aLeitz Ortholux subjective terminology applied to the strength of Pol-BK microscope as well as the Zeiss MPM03 anisotropy and rotation tints (Galopin & Herry 1972, microscope-photometer.It was apparentthat even p. 243), the anisotropy of the samemineral could when using the samelightbulbs, adjustedto the same be described,depending on orientation and associ voltage, and hence the same nominal color- ation, as weak, distinct, or strong. We emphasizethis temperatureof ca. 3100K, the relative brightnessand point becauserotation tints have recently been the intensity of the rotation tints were greaterwith recommendedas one of the most important diagnos- the Leitz than with the Zeiss equipment. This we tic propertiesfor ore minerals@icot & Johan 1982). ascribeto the longer light-path of the microscope- photometer (it includesa modulator that extendsthe light-path by some 16 cm). We recordedseparately Oprtcal Pnopenrrss with both instrumentsour observationsconcerning the appearanceof the minerals in plane-polarized Qualitative opticol datafor the type specimensfrom light and the sequenceof rotation tints between Potgietersrus crossedpolars and with the analyzeruncrossed by 3'. Allowing for slight differencesin coloqpercep- Five discrete and monomineralic grains of

T]IBIT 1. AIJISOTROPY AID ROIAIIO]I TINIS

cooPmrn BRAGCITT CRND POUE:

PotgieteraruF li6h! peenish pey to Eid-grey to yc'llorlsh b.om purnlish grey 10 browniah grcl AlR S1 i I lvater Areenish gey to brtAht rhttish yollop to hrom- d. blniah./purpltsh grey to pinkish brom

ar€y to bright and pale brom

slightlJ Potgietrersros nrcentsh Sey to briAhl rhitish yello{ to b.om- subdued purplish grcl to Bid-grel to grey to bright and pale brcm broMleh dark grcy OIL

St il lsater greeniah groy to brlght vhitiah Jellor to brom- sllrducd purp)lsh EIc.t to pinkiGh brom

grey to bright and lale brcm

ATALISER TIX-CRGSBD BY 3C:

Potgleteraru6 pal. and n€talllc groentsh rhite to slaie or Itrle and briaht purplish/hluish grey ro palc nnd

blulFh hright bromiEh Arer IIR srel brosn St i I lrator pale Freoniah grey 10 brilliant, elighlly crenBy, slAic or bluish AruI to lale Iellorish rhile to ntd-gry

pey lo nld-gret to sltahllf Pol aietartsrus lalc JelloriFh or Frcnieh hrorn to hrllliant. slron8 purJrlish,/blnish to slighill cradn'. rhite to lishl tinkieh Frey to rcddish Bid-gral ro lellosish liFht Fr.I ol t, LromiFh did-qrcl

hlniFh 1o sliFlllI mddish nii- Sl i I lsnter nnlo {roonish groy 1o brilllonl. rhlrleh l.llot 1. rlnte or nid-Atrr

lighi pinkiFh Fr.I 10 Ft.al-Froy io rollorish arat lo liFhl !.llorish ar.t

WSOIT'KITT, PtknlloTl \[ (.RNSED NOIII6:

AIR {lark Lrom ta rlark bluo-Arfl liAhi Frcorish FrcI !. nid-{rfr tr) darh rrf\ Sl i I lrtrtor oll \/v. vcrt albdudrl dark hrorn 10 dnr.h hlu.-gri liqhi Arofnish sr.l,/rlritc lo nid-ar(I lo pnrPlish nid-arcl lo nnsro to roddish brorn

lxALlsER tN-ClossrD BI 30:

AIR rhoF trinnod, briAht .oddtevp[rplish {rey to rliFhlll ar.cnish liaht arlt to groeniFh bid-grcy

pale light bluo to fam. Nore soEbre rhcr. un- to brohiFh dark grer Stillrarer

the sre tints oa ln air but Flightly &rk.r hrtFhl liAht arey to puqliah pal€ grel lo nid-Frer

to reddiah broh to rich reddiah brorn

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cooperite from BM L932, L30L were polished in mount E.700. In planeseclion all are anhedral,with angular outlines and curved a16 angular embayments.They vary in sizefrom I to 1.5mm. In plane- polarized light in air they appear pleochroic from white or creamy white to bluish white. They do not appearto be bireflectant (c/ quantitative properties). In immersion oil, under otherwisethe samecondi- tions, they arc apparentlypleochroic and bireflectant from white (higherR) to bluish white. Between crossedpolars their anisotropy in both media is strong. Their rotation tints between crossedand slightly uncrossed polars are summarized in Table Il.[',t,n'nxxilnnn l. None of the grains is twinned. FIc. l. Photomacrographof a polishedsection from Still- Three roughly equant grains of braggite fromBM water. The braggite B prism is characteristically frac- 1932, l3M were polished in E.701. They are all tured. At both ends of the prism and to one side are slightly lessthan 1 mm acrossand have rounded out- compositionally homogeneousareas of Wsotskite V. A lines (one has small angular embayments).In plane- color Jilter wasused to exaggeratethe differencebetween polarized the groundmass (dark grey) of yellower pyrrhotine, light, in air and in oil, braggite is white pentlandite and chalcopyrite, and the whiter F!, Pd sul- and very sliehtly bireflectant; it lacks reflectance fides. As a result, pyrite dispersedthroughout the pleochroism. Its moderately high reflectanceis simi- groundmassalso appearswhite. The bar scaleis in rnil- lar to that of cpoperite.Between crossed polars, its limetres. anisotropy is distinct in air and is reduced, to weak to distinct, in oil (Table l). Like cooperite,all of the grains are untwinned. Thus, tHe featuresthat enablethe two minerals to be distinguished where isolated in separate grain- mounts, are 1) the absencein braggiteofthe apparent reflectance-pleochroismof cooperiteand 2) the much more strongly anisotropic character of cooperite. In our grain mounts we did not have any pyrrhotine for comparison but this was remedied in the Still- water samples.

Qualitative optical dota for the minerals from Stillwater

The Stillwater samplesoffer an ideal opportunity to compare the optical properties of the three Pt- Pd-Ni-S minerals with pyrrhotine and with many of the other minerals with which they are commonly associated. Details of the mineral association and paragenesisof the Minneapolis Adit samplesare to be reported separately;however, insofar as they have a bearing on the diagnosticproperties of the three PGM, the associationwill be outlined here. The three sulfides occur in a pegmatitic gabbro in associationwith: major pentlandite,pyrrhotine, chalcopyrite and pyrite, minor cubanite and nickelian mackinawite, and accessorygold and the PGM, e.9., moncheite,isoferroplatinum, kotulskite, keithcon- nite, palladian tulameenite and a number of other unnamed minerals. Unusually large (by PGM standards)prismatic crystalsof braggile, up to 8 mm long (Fig. l), are not uncornmon.All of them are frac- tured. They are compositionally homogeneous, braggite B. The section is oriented for R, cooperite. except that a distinctive rim of slightly more highly Plane-polarized light. reflecting vysotskiteinvariably occursat the 'ends'

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of the prisms. In addition, as is shown in Figure l, vysotskite may be separatedfrom, but is closely associatedwith, the braggiteprisms. Cooperitewas not found in associationwith theseeuhedra. It occurs as subhedralto anhedralinclusions, from 30 to 250 pm in size @igs. 2,3), within large (up to 15 mm) irregularareas ofbraggite (Fig. 4). Vysotskite,which was not found in direct contact with cooperite, is invariably present as a selvedgeto braggite at its boundarieswith the Fe, Ni and Cu sulfides(Fig. 5). Texturally, theie is abundant evidencethat vysotskite was the last of the three PG sulfidesto form and that it replaced braggite: it forms thin selvedgesto fractures in braggite, 'healed' fractures,and finger-like ffK,ff,ffilX-,nre,ffimn projections, or penetrationtextures (Fig. O. In this Ftc. 4. Photomacrograph of an irregular area of inter- latter form, the vysotskitechanges in composition, grown braggite, vysotskite and cooperite from Still- moving toward that of braggite the further it pene- water, The centml and smallergrain of cooperiteis illus- trated in Figure 2, The bar scaleis in millimetres. tratesinto the braggitefrom its boundaries$/ith the other sulfides. In someinstances the changeappears to have been sequential and is detectable optically as a subtly steppedreduction in reflectanceand intensity of rotation tints. Compositionally homogeneous vysotskite occurs as small and irregular areas near the larger areasof braggite. Here, and in the thicker selvedgesto braggite (Fig. 5), guided penetration and vermiform textures are common. The dominant Fff ,r + ;i;", t interpenetrant minerals are pyrrhotine, pentlandite .*,1j!,:.}1 and chalcopyrite,although gold and the Pt-Pd tel- lurides are not uncommon in theseveinlets, which are usually compound. Our examination of the qualitative optical behaviorof the three mineralsfrom Stillwater illus- trates how the assessmentof brightnessand the relative intensitiesof color and rotation tints of one mineral Frc. 5. Oil-immersion photomicrograph of vysotskite V could be influencedby the presenceof other ' minerals in the same field. selvedgeto braggite B with associatedpentlandite Pe, pyrrhotine Note tle guided In plane-polarizedhghq cooperitefrom Stillwater, Po and minor chalcopyrite. penetration of chalcopyrite, pyrrhotine and unlike that from Potgietersrus, textures is bireflectant and pentlanditein the vysotskite.Plane-polarized light. apparently pleochroicfrom white (higherR) to bluish white in both media. Braggite,which surroundsit, appears slightly lower rbflecting, slightly ereenish white against white cooperite, and slightly higher reflecting and pinkish white against the bluish white cooperite. The higher bireflectance and more pronouncedpleochroism of the Stillwater samples are, however, more apparent than real, as will be demonstrated in the description of quantitalive properties. In comparisonwith cooperite, pyrrhotine is significantly lessbright and pale brown to pale milky brown in color. Betweencrossed polars, the anisotropy of cooperiteis strong, apparentlystronger that that of the type specimens,and as strong, or strongerthan that ofpyrrhotine. The rotation tints of cooperitefrom both localitiesare virtually identical and differ from those of pyrrhotine, a differ- FIc. 6. Oil-immersion photomicrographof vysotskiteV ence that is even more marked with the analyzer selvedgeto twinned braggite B. Note the optical uncrossedby 3' (Table l). Simple twinning was behaviorof the vysotskiteas it penetratesthe braggite. observedin only one small graiu of cooperite@ig. Slightly uncrossedpolars.

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2), ard undulatory extinction, which may be due to It has beensuggested recently that in the isomor- deformation, in one of the larger grains. phous solid-solution seriesfrom braggite (Pt,Pd)S In plane-polarizedlight, the single crystals of to vysotskitePdS, the lJamevlsotskite be restricted braggite are white and neither bireflectant nor to mineral$ containing less than l0 mole q0 PtS pleochroic;none is twinned. A characteristicfeature (Cabri et ol. 1978).This followed from the observa- of the polycrystallineaggregates of grainsthat con- tion that, at the time, no membersof the seriescon- stitute the larger anhedralareas of braggiteis poly- taining betweenl0 and 30 mole VoPtS wereknown. synthetic and simple twinning and, to a lesserextent, In our investigation, whereasmost of the larger kinkbanding [c/. Schneiderhdhn (1929a,b) on grainsofvysotskite analyzedcontain about l0 mole cooperite,and Kingston& El-Dosuky(1982) on brag- 9o PtS, a significant number of homogeneousand gitel. In theseareas bireflectance is weakly percept- inhomogeneousareas and grainswere found in the ible. In fact, as will be demonstratedbelow, the 10-30 mole Vo PtS range. In this account, we will bireflectanceis minimal, I to 1.590in both media, limit ourselves to descriptions of those optical and lower than that of the Potgietersrusbraggite. propertiesof vysotskitethat are characteristicand Nevertheless,this small difference in reflectanceis to othersthat might causeconfusion with braggite. within the visual thresholdof the eye,and discrimi- The latter arise from a gradual change in these nation is helpeil by the small-scaletwin lamellae properties as the composition moves from that of wherebright and lessbright lamellaeare jucaposed. one end-membertoward that of the other. A more Betweencrossed polars, anisotropyis distinct in air detailed account of the compositionalvariation in for both the single crystals and the polycrystalline the series,together with textural relationships,will areas(Table l). In oil, it is reducedto "weak" for be reported separately. the singlecrystals but remainsdistinct (again,prob- In plane-polarizedlight, in air and in oil, vTsol- ably becauseof the juxtaposition of lamellaewith skile (containing l0 mole 9o or lessftS) has a slightly different huesand relative intensities)in the twinned higher reflectancethan braggite and is a slightly areas.It shouldbe clearfrom the above,from Table creamierwhite. Neither bireflectancenor reflectance l, and from our comparisonof the appearanceof pleochroism is detectable visually. In isolation, cooperiteand braggite,that thereis little likelihood homogeneousvysotskite could be mistaken for of confusing the two, and it is equally improbable braggiteand vice verso.Similarly in intergro\rr'nareas, that braggite could be confusedwith pyrrhotine. where the constituents are inhomogeneous,it is usually impossible to distinguish them by eye. Betweencrossed polars, vysotskite is appreciablyless TABLE2. CHEM1CALCO||IPOSITION OF COOPERI'TE,BRAGGITE AND VYSOTSKITE* anisotropicthan braggite, and its sombrerotation- r-t.X E""*-=-g- tints in air are evenmore subduedin oil (Table l). pr Pd lli s rmat Pt Pd t{i s gootrp.ite Even more distinctive are the tints that are seenwhen F(IICIEENSTUS the analyzer is uncrossedby 3o (Table l). In un- 8.7m,1 46.1 0.6 0.? 13.9 1m.3 .98 .o1 .03 .94 twinned areasthese tints are lessbright than those E.7@,3 u.2 o.5 0.? 74.7 99.1 .99 .01 .oit .94 8.700,4 u.r 1.5 0.8 13.0 [email protected] .9? .0:t .o3 .w of braggite but, when twinned, as is usually the case, E.7@r6 44.5 0.6 0.4 13.8 99.5 .99 .o1 .03 .97 8.7@,6 they are much brighter. Other, more unusual 45.2 o.8 0.7 13.6 100.3 .99 .o2 .03 .96 'bleaching' STIIITASR phenomenawere observed: a gradual or E.6A9rl 78.6 6.5 0.2 74.9 [email protected] .a6 .13 .OOA 1.m desaturation of the tints away from a saturated 8.349,2 ?s.9 0.o o.7 74.7 70/J.7 .AA .12 .006 .99 8.692,1 79.O 0.9 o.2 14.4 1m.5 .aa .14.0G.97 centre, and a play of color resemblingiridescence, 8.591r1 ?9.7 6.6 0.1 14.5 9S.7 .90 .12.O4.94 8.39!,2 n.g 6.7 0.2 74.A Sa.8 .8a .12 .Ooa 1.m or undulose extinction of randomly oriented 8.391r3 80.6 5.5 0.2 15.9 100.2 .92 .12.o04.96 aggregatesof minute grains (no grain boundaries Eeagito were detectable,however, in theseareas). Possible FOICIEISBNF' phenomena 8.7O7,7 92.7 14.4 4.4 17.3 99.6 .60.4.16 1.OO explanationsfor these are compositional E.?O7,2 63.2 76.4 4.4 r7.4 100.4 .60 .2? .1,1 1.OO inhomogeneityand deformation. E.m1r5 01.2 7A.A 4.2 77.6 99.5 .58 .20 .13 1.00 ATILITATEn To summarize,the qualitative optical properties E.589 62.9 1a.3 1.7 16.9 99.a .61 .33 .06 1.OO of cooperiteare sufficiently distinctive for it not to 8.6S6,6 62.7 19,O 2.O 16.9 lm.o .60 .34 .06 1.OO 4.391 62.O 75.2 1.S 16.9 1m.o .60 .34 .06 1.OO be confusedwith other minerals.Those of braggite Vy@t€klto and vysotskitesimilarly are distinctive,but they are Sf,IIITAEB difficult to distinguish from eachother exceptwhere 8.d02 13.5 60.4 4.1 22.1 100.5 .10 .82 .10 .98 E.3gA, it 15.5 59.2 4.O 21.a [email protected] .11 .80 .10 .S near end-membercompositions occur together. E.395,s 14.6 49.4 4.9 n.! 100.o .11 .80 .10 .99 8.398,c-f 66.9 3.4 8.4 707.4 .o4 .45 .13 .94 Quantitative optical dato

hmhottoe Fo Co l{i S Fe Co t{t S SIIUIADB Reflectancespectra are reported for five grains of !.392 60.4 0.r. 0.6 39.4 [email protected] .844 .m2 .mA 1.@ cooperitefrom Potgietersrusand six from Stillwater, r electron-mlcrcDrobedata. three grains of braggitefrom both localities,and four

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grains of vysotskitefrom Stillwater; thesewere meas- TABLE3. REPRESENTATIVEN AND @F SPECTMFOR COOPERITE, ured at an interval of 10 nm from 400 to 700 nm. PYRRHOTINE,BMGGITE AND VYSOTSKIIE Measurementswere made in air and in Zeiss oil Np 1.515against a ZeissWTiC standardno. 314. In addition, severalgrains of plrrhotine from Stillwater were measured for comparison, principally with cooperite.Every grain measuredwas also analyzed by electronmicroprobe (Iable 2), and as many grains as practicablewere indented to detetmine their VHN (Table 5). The identity of grains representativeof each mineral was confirmed by X-ray diffraction. The identification numbers quoted in all of the Tables refer to individual grains in the different polishedmounts. It would be impracticalto publish the very large.amount of data that we obtained; therefore, we have selectedfrom the completeset reflectance $pectra that are representative of each mineral (Table 3). Similarly, although the complete data-setswere used to calculatethe color values,we a.t 8r have omitted the chromaticity co-ordinatesfrom 6A 6.r 6.? s.r ,.6 a.3 n.t a.6 Table 4. The completeset are availablefor inspec- 5.o a.€ &.2 U.6 fr.A

tion at the British Museum (Natural History). @ 6.1 &.a The reflectance spectra of. cooperite from Still-

water are similar to those of the type specimensfrom &.6 e.t $,c Potgietersrus,but are lessdispersed (Fie. 7) and less &'5 3l'o

bireflectant (cl, qualitative observations). Through- a.r 9.r out the visible spectrumthe bireflectancesin air and in oil of the type specimensare 5-8 90, and those

of the Stillwatercooperite 4-690 (Tables3,4). These o.a ,!.! e.. lla differencescan be attributed to differencesin com- TABLE4. COLORVALUES FOR COOPERITE, PYRRHOTINE, BRAGGITE AND VYSOTSKITE

LLWIW C [4ryIW I

AIR

nnldld PaZ ?a' n n ld ^a n % la la ..! ,ut t^, (& MGIMNB tt", .t R.) E.mrl i9.0 45.6 4&3 4a7 5.1 3.O 5.3 g).1 141 1S 3.5 s.5 45.3 493 49t 1.4 2.' 4.4 9.7 402 4$ 4.7 2.2 E.70r3 39.5 44.3 4S 4S 4.9 3.2 5.5 S.4 4A2 4& 8.3 s.9 4i.8 49 4S 2.8 2.O 5.o 2?.0 403 4S 4.7 3.2 f,.m,4 s.9 46.7 481 4S 5. t 31.2 441 {S i.a $.4 46.i 492 49t 3.O 1.5 ::.5 m.a 492 4S 4.4 2.4 E.?q) 5 $.4 4C.1 442 4S 5.0 1.4 5.0 r).6 aal {S 3.3 37.a 45.A 4r3 4S X.l 1.3 u.4 m.3 402 4$ {.4 2.1 E.ru,6 39.6 - 4S {.{ - 5.4-4S- 7.7 30,1-495-2.1 gl.I-{q-4.5 sTt uamn E.Sr.l 3r.3 44.4 49 4at 3.3 1.4 5.7 gt.5 {a3 4a3 5.8 3.4 s.9 44.6 495 45 2'O 1.O 8.4 9.2 4v 4,U 3.4 1.7 8.S0.2 30.6 {5.t 4A3 4& 3.6 1.4 5.a S.6 4S {g 2.6 39.1 45.1 {95 4S 2.1 1.O s.4 9.3 4q 405 3.? 1.6 8.392.' 3!.0 43.5 {45 4S ?.{ 1.5 A.t S.{ 4& 4& 3!.6 43.2 49i 41F 1.6 1.1 24.4 4.1 4r[ 407 2.6 1.4 8.f,!1.1 40.O 45.{ 4S 4S 2.4 t.O A.O &.1 4& 4V 4.3 1.6 39.6 45.2 {S 4S 1.4 o.7 24.2 S.9 495 {S 2.6 1.1 8.3!1.2 39.O 45.2 {a1 la2 3.3 0.9 24.3 N.; 4A1 4A2 39.5 45.! 403 4S 1.8 o.0 24.0 4.5 4!3 4S 2.4 1.O f.391.3 40.1 45.4 4& 405 2.3 0.6 24,6 D.9 4g 48 4.3 1.4 ao.a {5.3 4!? a$2 o.0 ?4.3 p.a 405 {s 2.6 1.O

t'ar) sttmam (R) ^/ lt'*r (Rt *2t ' -l -2' 8.392 6.5 S.1 5?O 579 0.9 9.9 8.7 X.3 S79 5ro 14.5 1{.2 37.3 S.a tS S7 14.5 14.4 24.4 5.O m m g),7 a).1 Englte-. JEo MTffiRtS (P R.) tldo , IR R.) E.to1.1 49.5 44.1 nS $7 o.3 1.1 t.3 9.O m ru o,2 1.3 42.4 44.1 514 nn O.3 1.3 :7.2 9.O S 5?5 0.3 1.6 E.7O1rg 42,7 q.i 572 ffi 0.2 1.6 t.5 S.? 4S S2 42.6 44.? 510 5n O.3 1.4 n.4 8.7 &t W9 0.6 1.0 0.Q !.?o1 ri {2.4 {5.0 5& S 0.6 2.i :7.3 &.5 4$ S2 o.5 2.O 42.7 45.6 il1 5?6 0.5 2.4 27.2 l).4 W S9 2.2 saluam f,.so 43.O 44.1 4S K o.2 0,a P.a *.o 491 ffi o.5 1.0 42.9 {.t S ffi O.3 0.9 n.? 8.o' fi2 am 0.6 1.1 8.3S,5 42.8 44.1 S7 571 0.8 1.5 n.4 *.O 69 6?1 1.0 1.4 42.A 4.7 571 ffi O.9 1.S n.4 A.O 5?5 Are 1.2 2.3 8.f,g1 43.0 43.9 S2 32 o.5 1.? t.6 4.5 5g 5A 0.6 2.2 43.0 43.9 S? S 0.5 2.2 z.o 8.6 r{3 s1 o.5 2.9

slru!@ 8.302 45.S 45.7 574 sft 2.6 2.5 S.9 m.4 375 574 2.5 2.9 45.4 6.9 ffi g 3.5 3.5 m.0 &.6 s2 s 3.4 4.2 8.3tr.d 4.7 45.6 5Z 56 3.0 3.1 m.O &.a 5Z 5Z 2,9 2.4 {4.9 45.? S g 4.3 4.3 s.1 31.O S g 4.1 4.O E.35.e 4{.? 45.5 5? 5-.0 2.6 2.4 il.2 m.9 52 576 0.2 3.1 44.9 €.6 S g 3.4 3.5 &.6 31.O S S {.6 {.{ 8.393,c-f 44.3 46.0 5?6 ffi 2.6 2.4 m.4 31.2 576 36 2.6 2.3 {5.5 €.1 g S 3.6 3.3 s.6 31.r g s t.a 3.2

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position: the Stillwater sampleshave consistently explanationfor the perceptionof blue and white hues higher Pd and lower Pt and Ni contentsthan those is that the higher reflecting component Rr, is closer from Potgietersrus(Table 2). to the illuminant point and at most half as saturated One of the grains from the type locality is isotropic (in quantitative terms, the excitation purity, P"9o) (E.700,O.The reflectancesof this grain confirm the as the Ro component. Thus, what is perceivedas assignmentof Ro to the spectral curves of lower reflectance pleochroism is nothing more than a reflectance.Hence, the sign of the bireflectanceis difference in brightness (quantitatively, the positive.Earlier, we pointed out that cooperitefrom luminance, Yslo)ard is properly a function of the Potgietersrus is apporently pleochroic when mineral's bireflectance. immersedand cooperitefrom Stillwater is apparently Severalgrains of monoclinicpyrrhotine from the pleochroic in both media. The reflectancecurves and samepolished mounts weremeasured by N.S. Old- color values (Table 4) show that the dominant field. From thesemeasurements we selectedrepresen- wavelengths)r7 (in subjective terms, the hues) of tative spectra(Table 3) and plotted them with those both Ro and Re, are nearly constant,and hencethe of cooperite(Fie. 7). Inspectionof thesedata should mineral is, strictly speaking, not pleochroic. The prove that there are no similarities betweencooperite

"*::i:::::f;:::;:';;;;;;;;:1"1"""

LO 488 458 5AA qqr, 6ez 6,5A 7Zg Lombdo nm Frc. 7. R and ,z.i?spectra for cooperitefrom Potgietersrus(P, dotted) and Stillwater (S, continuous)compared with pyrrhotine (py, dashed)from Stillwater,

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and pyrrhotine. That point is made evenmore con- ing (by about l39o) than in air. In contrast, cooperite vincing in Table 4; here the values are referred to hasdominant wavelengthsin the 490nm range,and the two illuminants recommendedby the COM, C the reasonthat it doesnot appear more than slightly and A, with correlatedcolor-temperatures ol 6774 blue is due to its low excitation-puritiesof l-4.8t/0. and 2856K, respectively.It will be recalledthat our In terms of relative brightnessor luminance, the qualitative observationswere made with respectto bireflectanceof cooperiteimmediately distinguishes a lamp run at about 3100 K. It follows that our it from pyrrhotine and, although Wo Ry for pyr- descriptionswill correspondmost closelyto the quan- rhotine is about the sameas Wo Ro for cooperite, titative values for illuminant A. The brownish to the two could not be confusedbecause of the differ- brownish white color of pyrrhotine is explainedby encesin hue and saturation. its fairly high saturation or excilation purity of about The reflectancespectra of three grains of braggite l4v/o in air, and its hue, or dominant wavelength, from both localities (Fig. 8), plotted to the samescale of about 588 nm. The fact that darker browns are as those of cooperite (Fig. 7), illustrate that the seenin oil is a consequenceof it being more satu- mineral is much lessbireflectant than cooperite.They rated, with a P"of 20c/0,and lessstrongly reflect- also show that, Iike cooperite, the Potgietersrus

la 4@A 454 5ZA 55Z' 6AA 65A 7ZZ Lombdo nm FIc. 8. R and,zR spectrafor 3 grains of braggitefrom Potgietersrus(dashed lines) and 3 grains from Stillwater (continuous lines).

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braggite is slightly more bireflectant than its Still- levels of excitation purity for R", are a little higher, water counterpart. Theseslight differencescan be with the result that more consistent and reliable attributed, in the sameway as for cooperite, to com- dominant wavelengthsof about 570-580 nm are positional variation (Table 2): the Stillwater grains obtained. The difference in the dispersionof the are more Pd- and less Ni-rich than the type reflectanceof the two vibration directionsaccounts specimens. for the detection by eye of the small measured In terms of color, the undispersedRo spectra bireflectance.One factor that is not readily explained indicate that the mineral will be white, lacking any in terms of color scienceis the presenceof pinkish noticeablehue, but that it will take on the color and greenish tints to the white braggite where it is attributes of the light soluce, that is, it should appear in contact with cooperite. We can only speculatethat white to bluish white with a Csource and a creamier as a result of differencesin the polishing hardness white with the14 source.This fact is emphasizedby of the two minerals, we were observingKalb light- the very low levelsof excitation purity ln Table 4. lines. The scatter of the dominant wavelengths is in- Vysotskiteposed problems for reflectancemeas- significant sincethe chromaticity co-ordinatesplot urementssince, unlike the other two minerals, selec- within lessthan l9o of thoseof the illuminants. The tion of compositionally homogeneousareas large

wyaolek I t€

.b.qggi!€ I I I

____*___*___; __;_-_::a

2A l--+ +-+- + |.++-* 4ZA 454 5AA 552 6ZZ 652 7ZZ Lombdo nm

FIc. 9. RepresentativeR and i-R spectrafor vysotskiteand braggitefrom Stillwater, Note the lower bireflectanceof vysotskite.

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enough for photometry required that a large num- tinctly lower than that of braggite. The dispersion ber of compositebraggite-vysotskite grains be ana- of their reflectancealso differs from that of bragg- lyzed by electron microprobe. As we have men- ite, as can be seenin Figure 9. It was not possible, tioned, the serieswas discoveredto be continuous; becauseof the low bireflectance,to determinewhich however,grains suitable for photometry are, with of the two vibration directionsmeasured corresponds the exceptionof a near end-membervysotskite, all to Ro. This was assigned,by analogywith the Ro of of similar composition, that is, they contain abour braggite,to the spectraof lower luminance.It will l0 mole 9o PtS (Table 2). Also, owing to the ex- be apparent from Figure 9 why vysotskite, when tremely subduedrotation-tints and weak anisotropy, adjacentto braggite, appearsa creamierwhite and it wasnot possibleto be certain of the extinction posi- slightly brighter; the reflectancesof both vibration tions of any of thesegrains. For this reason,it was directionsof all of the grains are higher than those necessaryto usethe photometerto selectreflectance of braggiteabove 520 nm. This is confirmed by the maxima and minima for each grain. This was done color valuesin Table 4; the luminancesof vysotskite at 560 nm. arc l-Zvlo higher in air and l-390 higher in oil than The four grains measuredhave bireflectancesof thoseof braggite;the dominant wavelengthsfor the lessthan 190throughout the visible spectrum,dis- .4 illuminant are nearly constant between 580 and 5A

4Zn 45A 5AA 55U 6AA 652 7ZA Lombdo nm Ftc. 10. RepresentativeR and tmRspectra for cooperite,braggite and vysotskite.The ornament is the samefor each pair of curvesand for both media. S and P for Stillwater and Potgietersrus,respectively.

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590 nm and, perhapsmore importantly, the excita- skite. Pyrrhotine, by comparison,is softer than all tion purities are 34Vo higher than those of braggite. three. Thus there should be no difficulty in identifying Of the publisheddata, only thoseof Cabri (1981) the end membersof the braggite-lysotskite series, agreewith ours for braggite (his data for cooperite but it seemslikely that if grains intermediate in com- were made at a different loading and so are not composition were measured,their reflectancespectra parablewith ours). Comparedwith our results,King- would fall betweenthe extremesshown in Figure 9. ston & El-Dosuky (1982)obtained lower valuesfor This conclusionis supportedby the qualitative obser- braggite (meanYHN1g9 864) and higher valuesfor vations ofgradational and steppedreduction in the cooperite (mean VHN1* 939). In fact, their braggrte reflectanceof vysotskitereported earlier. valuesmatch ours for cooperite exceptthat the hard- To concludethis section,we haveplotted represen- nessanisotropy of their mineral is much lower and tative spectrafor the three minerals @ig. l0), includ- similar to our braggite. The only other data avail- ing examplesfrom the type specimensas well as for able are those of Rozhkov et al. (1962)for identa- material from Stillwater. Thesespectra are charac- tions at an unspecifiedload. For this reasontheir teristicfor the minerals.In our opinion, whereasthe data cannot be regardedwith any confidence. qualitative properties that we have described could We have no reasonto doubt that our data are as suggestto the trained observerthe possibleidentity accurateas the techniquepermits; whereaswe do not of one or other of theseminerals, reflectance spec- considerthe VHN to be useful as a 'primary' means tra provide the only reliable meansof confirming this of identification, we believethat it has somevalue identifi cation optically. as a support to identificationsmade on the basisof refl ectancedeterminations. Mic ro- inden t at io n hardness

The published VHN values for cooperite and DtscussloN AND CoNCLUSIoNS braggiteare few and contradictory; there are none for vysotskite.All of our measurementswere mace The main objectivesof this study were l) to com- with a Vickers indenterat a 100-gramforce. Grains plete the characterizationof braggiteand cooperite wereselected for their compositionalhomogeneity, made by Bannister& Hey (1932),our predecessors and the number of indentationsmade in eachgrain at the BM(NH),2)to provide characteristicoptical wasgoverned by its size.Wherever possible, at least data for Wsotskite (acceptingthat the material at our 10 indentationswere made, but fewer in the caseof disposalis not from the type locality at Noril'sk), the smallergrains of vysotskiteand cooperitefrom 3) to dispel the extraordinary confusion that has sur- Stillwater (the actual number is shown as a subscript rounded braggite and cooperite and that was to the mean VHN in Table 5). Our results are developing around vysotskite and, finally, 4) to reproducible,similar for the samemineral from the attempt to explain how this occurred. two localities,and their precisionis good (Table 5). We haveshown that the qualitative propertiesof In terms of micro-indentation hardness,cooperite cooperite are parti\ularly distinctive and those of is softer than braggite but in the samerange as vysot- braggite and vysotskiteare sufficiently distinctive for end membersof the seriesto be identifiedwith some confidence.Reflectance spectra for the three minerals provide evengreater assuranceof a positive identifi- TABLE5. VALUESOF MICRO.INDENTATIONHARDNESS cation, and havethe addedadvantage of supplying b- wloo 1oo Ceperl.te the data necessaryfor deriving unambiguouscolor- polqrgEnsnns removethe subjectivityof color 8.700,4 76 9n 835t.^t f-Bcc values.In this, they E.7Oo,5 7.tlL1O18 878ii;i f-scc descriptionand the assessmentof relativebrightness sTutllm. 8.391,1 ?az-N? S39(i) f-scc which, evenfor the experiencedmicroscopist, cause Beggr.to confusion through the eye'sability to modify impres- PMGIEEBSUF sions of color and brightness depending on the E.?O1,1 9?4-708,X '331[i3] 8.707,3 94S-1026 i:::: appearanceof the associatedminerals. gTil,I'AER for us to try and explain why the E.tag 94{'-1m3 It only remains 8.396,5 981-1064 original descriptionsof cooperitewere so mislead- E.391 95t-1044 #ifiiil:: ing. When Schneiderhdhndescribed cooperite in VJrstskito platinum STIIffAEN 1929,itwas consideredto be a sulfarsenide E.396rd m &4 &(o) Bf-cc (Cooper 1928).This was revisedby Cooper (1929) E.3e5,o 473-a4? 83o;;i sf-cc E.3e6,ef ?75-7s2 ? ei;i f-cc to PtS2,and it was not until 1932that Bannister& Pyrhott@ Hey, using X-ray diffraction to identify individual STIII'AM grains in the concentratesfor wet-chemicalanaly- 8.892 2€2-331 318(aO) ef-cc sis, proved it to be PtS. Schneiderhdhn's(1929a,b)

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descriptions of his mineral as white with a distinct minerals.As we have shown, the optical properties greyish yellow tint, with high reflectivity in air and of both minerals are distinctive. in oil, distinct anisotropy with grey-pink and grey- The explanationfor the continuedmisidentifica- greenrotation tints, and characteristicpolysynthetic tion of the minerals and the mistaken belief that they twinning, would not fit our description of cooperite. are rare followed naturally from the acceptanceof This descriptionof the appearanceof his prismatic the original mistake. It would, after all, be impossi- grains does,however, correspond to a mineral which, ble to identify cooperite from the 1929 or l93L at that time, was unknown: braggite. In the same descriptions.What is more difficult to understand papers,Schneiderhdhn recorded the resultsof Frick,s is how the mistakewas perpetuated when combined determinationl with the reflection-photometer ocu- optical and electron-microprobestudies were made 'absolute lar of the reflectivities' of cooperite tn of the PGM in the 1970s. green, orangeand red light. Thesevalues, 37, 37 and The qualitative and quantitative data in the liter- 3690, respectively,are exactly the same as those ature for vysotskite are few and contradictory. Near determined at the same time'for pyrrhotine (and end-membervysotskite, we have shown, can be iden- within 290 of those for chalcopyrite). In l93l the tified with some confidence. Its reflectanceis not description was changed by Schneiderhdhn & lower than that of chalcopyrite(Genkin 1968),it is Ramdohr; the reflectanceof cooperitewas described slightly higher, but nowherenear as high as reported as similar to that of chalcopyriteand pyrrhotine, and by Vyalsov (1973). The dispersionof his reflectance its anisotropy as weak. The reflectance values were spectrumfor vysotskite is similar to that of our spec- also revised to 41,34 and 34r/o (thesewere later tra but is inexplicably7-8Vo higher. Until suchtime attributed to Frick by Ramdohr 1969).Frick, him- as type material from Noril'sk is thoroughly re- self, reported (1930)slightly different valuesagain examined,our data for vysotskitecan be taken as of 37.8, 37.8 and 36.40/o.Subsequently, Ramdohr characteristic,as they are for cooperiteand braggite. (1950),describing Schneiderh6hn's observations in 1929,stated that they were incorrect because2 or 3 different mineralswere consideredas one. Obvi- ously, fifty yearsafter the event,we cannot be cer- ACKNoWLEDGEMENTS tain of what happened,but suggestthat the most plausible explanation is: A.J.C. is grateful to the Anaconda Copper Com- l. Schneiderhiihnoriginally describedbraggite as pany for providing accessto the MinneapolisAdit cooperite(braggite, although found in concentrates and, in particular, to Alistair Turner, Marvin Rat- from the samelocality by Bannister& Hey in 1932, cliff and Mike Zdepski for organizingthe samplecol- was unknown in 1929). lection and for guidancein the field. We are grate- 2, Theconcentratesexamined by Schneiderhdhn ful to Nick Oldfield, formerly a vacation studentin contained pyrrhotine, and it was pyrrhotine and not this Department, for measuringthe reflectancespec- braggite or cooperitethat was measuredby Frick tra of pyrrhotine and collating the color valuesand (1930),leading to the apparentidentity of cooperite VHN. We thank Drs. A.C. Bishopand A.M. Clark and pyrrhotine. Frick's (1930)tables of reflectance for their constructivecomments on this work. We for the opaque minerals, except for a consistent are particularly grateful tri Dr. R.F. Martin for underestimateof the reflectance in green light, con- improvementsto the text. Drs. G.A. Kingston and form well with reflectances obtained with modern J.F.W. Bowles reviewedthe manuscript. microscope-photometers.So the fact that he repeat- edly. obtained reflectances that match pyrrhotine more closelythan cooperite-braggitetends to confirm this error. REFERENCES 3. Schneiderhdhnaccepted these values, and he and Ramdohr in l93l noted the similarity between BarNrsrEn, F.A. & Hrv, M.H. (1932): Determination cooperite and pyrrhotine. This was to remain firmly of minerals in platinum concentrates from the Trans- entrenchedin the literature into the 1980s. vaal by X-ray methods. Mineral. Mag. 2, 188-206. Kingston & El-Dosuky (1982)suggested that the original mistake made by Schneiderhithn(1929) and Bow, C., Worrcneu, D., Tumren, A., BenNEs,S., (1982): repeatedby Schneiderhdhn& Ramdohr (1931),Frick EvaNs, J., Znspsrr, M. & Boupnseu, A. Investigations of the Howland Reef of the Stillwater (1930)and Ramdohr (1969),was that of confusing 'braggite-cooperite Complex, Minneapqlis Adit area: stratigraphy, braggitewith cooperiteand inter- structure, and mineralization. Econ. Geol. 77, €rowths' as braggiteshowing'pleochroism'. Whereas t48t-1492. we considerthat Schneiderhdhn'soriginal description was that of braggite, not cooperite, this was not CasRr, L.J. (1912): The mineralogy of the platinum- a questionof mistakenidentity betweentwo known group elements, Minerals Sci. Eng. 4(3),3-29.

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(1981): The platinum-group minerals. 1n MerenskyReef at the Rustenburgplatinum mine. Platinum-Group Elements: Mineralogy, Geology, Econ. Geol. 77. 1367-1384. Recovery (L.J. Cabri, ed.). Con. Inst. Mining Metall,, Spec. Vol.23, 83-150. LsoNano,B.F., Dsssonoucn,G.A. & Pacr, N.J (1969):Ore microscopyand chemicalcomposition CnrnprB,A.J., Lanarr,rME,J.H.G., BsARNp, of somelaurites. Amer. Mineral. 54, 1330-1346. G.S. & Hannrs, D.C. (1981): Mineralogical study of complex Pt-Fe nuggets from Ethtopia. Bull. Prcor, P. & Jonar, Z. (1977):Atlas des mindraux Mindral. 104. 508-525. mdtalliques.Bur. Rech.G6ol. Minidres, Mdm.90.

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KrNcsroN,G.A. & Er--Dosury,B.T. (1982):A contri- Received July 16, 1984, revised manuscript accepted bution on the platinum-groupmineralogy of the November 19,1984.

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