FORMATION OF OPAQUE MINERALS IN CK CHONDRITES

The opaque minerals of 19 CK chondrites from Antarctica and the Nullarbor Region, Australia were studied mineralogically by optical and analytical methods. The most striking feature of the CK chondrite-indicating the oxidized nature of these meteorities-is the presence of magnetite as the most abundant opaque phase. It occurs in modal abundances between 1.2 and 8.1 vol. %, averaging about 4 vol %. Magnetite occurs in two different populations: as tiny micron-sized grains dispersed throughout the meteorite; and as rounded aggregates up to 1 mm in size. In the latter type exsolution lamellae of ilmenite and spinel were observed. Chemically, the magnetites contain small amounts of MgO, Al2O3 and TiO2 (usually below 1 wt. %). and an average of about 4. % Cr2O3. Sulphide phases occur in abundances below 1 vol. % (except Karoonda 1.3 vol. %). However, six different phases were found: pentlandite ((Fe,Ni)(9)S-8), pyrite (FeS2), pyrrhotite (Fe1-xS), millerite (NiS), and chalco-pyrite (CuFeS2). Usually, the parageneses pentlandite-pyrrhotite/MSS-pyrite and pentlandite-pyrite were found. The chemical compositions of the observed pentlandites vary substantially regarding the Fe/Ni ratio. In all phases various amounts of Co and, in some cases Cr were detected. Associated with magnetite and/or suplhides, micron-sized noble metal-rich sulphides, tellurides and arsenides were found. Among the sulphides the most frequently occuring phase is (Os,Ru,Ir)S-2. In addition, PtS, (Au,Fe,Ag)(2)S, and (Fe,Au,Co)(2)S-3 were discovered. For the formation of the observed opaque phases and assemblages a multistage process under increased oxygen fugacities is proposed. Magnetite is formed from Fe,Ni-metal during a metamorphic event under oxygen fugacities on the order of the Ni-NiO oxygen buffer. Ni became disengaged, oxidized and diffused into olivine. The observed exsolution of ilmenite and spinel was established during cooling. Based on stability conditions the observed sulphide parageneses must have been formed by subsolidus reactions under low temperatures from MSS. The noble metal elements which were either dissolved in the Fe, Ni-metal or in the MSS, formed noble metal-rich phases by exsolution during the oxidation of the mental phases or during the subsolidus breakdown of MSS.