Oberthürite, Rh3(Ni,Fe)32S32 and torryweiserite, Rh5Ni10S16, two new platinum-group minerals from the Marathon deposit, Coldwell Complex, Ontario, Canada: Descriptions, crystal-chemical considerations, and comments on the geochemistry of rhodium

Abstract

ABSTRACT Oberthürite, Rh3(Ni,Fe)32S32, and torryweiserite, Rh5Ni10S16, are two new platinum-group minerals discovered in a heavy-mineral concentrate from the Marathon deposit, Coldwell Complex, Ontario, Canada. Oberthürite is cubic, space group , with a 10.066(5) Å, V 1019.9(1) Å3, Z = 1. The six strongest lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 3.06(100)(311), 2.929(18)(222), 1.9518(39)(115,333), 1.7921(74)(440), 1.3184(15)(137,355) and 1.0312(30)(448). Associated minerals include: vysotskite, Au-Ag alloy, isoferroplatinum, Ge-bearing keithconnite, majakite, coldwellite, ferhodsite-series minerals (cuprorhodsite–ferhodsite), kotulskite, and mertieite-II, and the base-metal sulfides, chalcopyrite, bornite, millerite, and Rh-bearing pentlandite. Grains of oberthürite are up to 100 × 100 μm and the mineral commonly develops in larger composites with coldwellite, isoferroplatinum, zvyagintsevite, Rh-bearing pentlandite, and torryweiserite. The mineral is creamy brown compared to coldwellite and bornite, white when compared to torryweiserite, and gray when compared chalcopyrite and millerite. No streak or microhardness could be measured. The mineral shows no discernible pleochroism, bireflectance, or anisotropy. The reflectance values (%) in air for the standard COM wavelengths are: 36.2 (470 nm), 39.1 (546 nm), 40.5 (589 nm), and 42.3 (650 nm). The calculated density is 5.195 g/cm3, determined using the empirical formula and the unit-cell parameter from the refined crystal structure. The average result (n = 11) using energy-dispersive spectrometry is: Rh 10.22, Ni 38.83, Fe 16.54, Co 4.12, Cu 0.23 S 32.36, total 100.30 wt.%, which corresponds to (Rh2Ni0.67Fe0.33)Σ3.00(Ni19.30Fe9.09Co2.22Rh1.16Cu0.12)∑31.89S32.11, based on 67 apfu and crystallochemical considerations, or ideally, Rh3Ni32S32. The name is for Dr. Thomas Oberthür, a well-known researcher on alluvial platinum-group minerals, notably those found in deposits related to the Great Dyke (Zimbabwe) and the Bushveld complex (Republic of South Africa). Torryweiserite is rhombohedral, space group , with a 7.060(1), c 34.271(7) Å, V 1479.3(1), Z = 3. The six strongest lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 3.080(33)(021), 3.029(58)(116,0110), 1.9329(30)(036,1115,1210), 1.7797(100)(220,0216), 1.2512(49)(0416), and 1.0226(35)(060,2416,0232). Associated minerals are the same as for oberthürite. The mineral is slightly bluish compared to oberthürite, gray when compared to chalcopyrite, zvyagintsevite, and keithconnite, and pale creamy brown when compared to bornite and coldwellite. No streak or microhardness could be measured. The mineral shows no discernible pleochroism, bireflectance, or anisotropy. The reflectance values (%) in air for the standard COM wavelengths are: 34.7 (470 nm), 34.4 (546 nm), 33.8 (589 nm), and 33.8 (650 nm). The calculated density is 5.555 g/cm3, determined using the empirical formula and the unit-cell parameters from the refined crystal structure. The average result (n = 10) using wavelength-dispersive spectrometry is: Rh 28.02, Pt 2.56, Ir 1.98, Ru 0.10, Os 0.10, Ni 17.09, Fe 9.76, Cu 7.38, Co 1.77 S 30.97, total 99.73 wt.%, which corresponds to (Rh4.50Pt0.22Ir0.17Ni0.08Ru0.02Os0.01)∑5.00(Ni4.73Fe2.89Cu1.92Co0.50)Σ10.04S15.96, based on 31 apfu and crystallochemical considerations, or ideally Rh5Ni10S16. The name is for Dr. Thorolf (‘Torry') W. Weiser, a well-known researcher on platinum-group minerals, notably those found in deposits related to the Great Dyke (Zimbabwe) and the Bushveld complex (Republic of South Africa). Both minerals have crystal structures similar to those of pentlandite and related minerals: oberthürite has two metal sites that are split relative to that in pentlandite, and torryweiserite has a layered structure, comparable, but distinct, to that developed along [111] in pentlandite. Oberthürite and torryweiserite are thought to develop at ∼ 500 °C under conditions of moderate fS2, through ordering of Rh-Ni-S nanoparticles in precursor Rh-bearing pentlandite during cooling. The paragenetic sequence of the associated Rh-bearing minerals is: Rh-bearing pentlandite → oberthürite → torryweiserite → ferhodsite-series minerals, reflecting a relative increase in Rh concentration with time. The final step, involving the formation of rhodsite-series minerals, was driven via by the oxidation of Fe2+ → Fe3+ and subsequent preferential removal of Fe3+, similar to the process involved in the conversion of pentlandite to violarite. Summary comments are made on the occurrence and distribution of Rh, minerals known to have Rh-dominant chemistries, the potential existence of both Rh3+ and Rh2+, and the crystallochemical factors influencing accommodation of Rh in minerals.