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Characteristic Optical Data for Cooperite, Braggite and Vysotskite

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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 reflec- tance 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 149 150 THE CANADIAN MINERALOGIST 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 apparent color, 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
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