Abstract
Tamuraite, ideally Ir5Fe10S16, occurs as discrete phases (≤20 μm) in composite inclusions hosted by grains of osmium (≤0.5 mm across) rich in Ir, in association with other platinum-group minerals in the River Ko deposit of the Sisim Placer Zone, southern Krasnoyarskiy Kray, Russia. In droplet-like inclusions, tamuraite is typically intergrown with Rh-rich pentlandite and Ir-bearing members of the laurite–erlichmanite series (up to ~20 mol.% “IrS2”). Tamuraite is gray to brownish gray in reflected light. It is opaque, with a metallic luster. Its bireflectance is very weak to absent. It is nonpleochroic to slightly pleochroic (grayish to light brown tints). It appears to be very weakly anisotropic. The calculated density is 6.30 g·cm−3. The results of six WDS analyses are Ir 29.30 (27.75–30.68), Rh 9.57 (8.46–10.71), Pt 1.85 (1.43–2.10), Ru 0.05 (0.02–0.07), Os 0.06 (0.03–0.13), Fe 13.09 (12.38–13.74), Ni 12.18 (11.78–13.12), Cu 6.30 (6.06–6.56), Co 0.06 (0.04–0.07), S 27.23 (26.14–27.89), for a total of 99.69 wt %. This composition corresponds to (Ir2.87Rh1.75Pt0.18Ru0.01Os0.01)Σ4.82(Fe4.41Ni3.90Cu1.87Co0.02)Σ10.20S15.98, calculated based on a total of 31 atoms per formula unit. The general formula is (Ir,Rh)5(Fe,Ni,Cu)10S16. Results of synchrotron micro-Laue diffraction studies indicate that tamuraite is trigonal. Its probable space group is R3m (#166), and the unit-cell parameters are a = 7.073(1) Å, c = 34.277(8) Å, V = 1485(1) Å3, and Z = 3. The c:a ratio is 4.8462. The strongest eight peaks in the X-ray diffraction pattern [d in Å(hkl)(I)] are: 3.0106(216)(100), 1.7699(420)(71), 1.7583(2016)(65), 2.7994(205)(56), 2.9963(1010)(50), 5.7740(102)(45), 3.0534(201)(43) and 2.4948(208)(38). The crystal structure is derivative of pentlandite and related to that of oberthürite and torryweiserite. Tamuraite crystallized from a residual melt enriched in S, Fe, Ni, Cu, and Rh; these elements were incompatible in the Os–Ir alloy that nucleated in lode zones of chromitites in the Lysanskiy layered complex, Eastern Sayans, Russia. The name honors Nobumichi Tamura, senior scientist at the Advanced Light Source of the Lawrence Berkeley National Laboratory, Berkeley, California.
Highlights
The heavy-mineral fraction recovered from the River Ko deposit and the placer in the neighboring River Seyba is mainly derived from ultramafic units of the Lysanskiy layered complex, Eastern Sayans, Russia [1,2,3]
The grains of tamuraite (≤20 μm; Figure 2a–d, Figure 3a,b) occur with Rh-rich pentlandite and cavernous grains of laurite–erlichmanite solid solution in droplet-like polymineralic inclusions (≤50 μm). These platinum-group minerals (PGM) are hosted by placer grains of the Ir-bearing native Os (Figures 2a–d and 3a,b; Table 1), which corresponds to the mineral osmium [7]
The new mineral is gray to brownish gray in reflected light
Summary
A new mineral species of ideal composition Ir5 Fe10 S16 , was found in a placer along the River Ko, south of Krasnoyarsk, Russia The heavy-mineral fraction recovered from the River Ko deposit and the placer in the neighboring River Seyba is mainly derived from ultramafic units of the Lysanskiy layered complex, Eastern Sayans, Russia [1,2,3]. The development of placers bearing platinum-group minerals (PGM) corresponds to the Sisim river system in the southern portion of Krasnoyarskiy Kray in South Central Siberia (Figure 1a,b). Our aims are to describe the mode of occurrence, associations and properties of tamuraite. The mineral and its name were approved (IMA 2020-098) by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association
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