Nickeltyrrellite, CuNi₂Se₄, a new member of the spinel supergroup from El DragÓn, Bolivia

Abstract

Nickeltyrrellite, ideally CuNi₂Se₄, is a new selenide species from the El Dragón mine, Department of Potosí, Bolivia. It most frequently occurs as anhedral to subhedral grains (up to 20 μm in size) in association with cerromojonite, klockmannite, clausthalite, and penroseite, forming fracture fillings in pre-existing krut'aite−penroseite solid solutions. Nickeltyrrellite is non-fluorescent, black, and opaque with a metallic luster and black streak. It is brittle, with an irregular fracture and no obvious cleavage and parting. In plane-polarized incident light, nickeltyrrellite is cream to pale pinkish, displaying no internal reflections. Between crossed polarizers, it is isotropic. The reflectance values in air for the COM standard wavelengths are 45.9 (470 nm), 47.6 (546 nm), 48.1 (589 nm), and 49.8 (650 nm). Electron-microprobe spot analyses (n = 28) of the grain populations used for the EBSD study yielded a mean composition of Cu 13.01, Fe 0.27, Co 6.66, Ni 16.98, S 1.04, Se 61.91, total 99.87 wt.%. The empirical formula, normalized to 7 apfu, is Cu1.00(Ni_(1.42)Co_(0.56)Fe_(0.02))_(Σ2.00)(Se_(3.84)S_(0.16))_(Σ4.00). The ideal formula is CuNi₂Se₄, which requires (in wt.%) Cu 12.79, Ni 23.63, Se 63.58, total 100. EBSD patterns reveal nickeltyrrellite as cubic, space group Fd3m , with a = 9.99 Å, V = 997.0 Å3, Z = 8. The calculated density is 7.36 g cm⁻³. It formed in response to alteration of quijarroite + hansblockite + watkinsonite + clausthalite + penroseite aggregates by oxidizing, low-T, Cu-Ni-Co-Pb-bearing fluids at a fSe₂/fS₂ ratio greater than unity. Nickeltyrrellite is a new selenide belonging to the tyrrellite subgroup of the spinel supergroup. The new species has been approved by the IMA-CNMNC (2018-110) and is named for constituting the Ni-analogue of tyrrellite.

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