Tetrahedrite-(Ni), Cu6(Cu4Ni2)Sb4S13, the first nickel member of tetrahedrite group mineral from Luobusa chromite deposits, Tibet, China

Abstract

Abstract Tetrahedrite-(Ni) (IMA2021-031), ideally Cu6(Cu4Ni2)Sb4S13, is the first natural Ni-member of tetrahedrite group mineral found in Luobusa chromite deposit, Tibet, China. The new species occurs as anhedral grains 2 to 20 μm in size, associated with gersdorffite, vaesite, and chalcostibite, which are disseminated in a matrix of dolomite, magnesite, quartz, Cr-rich mica, and Cr-bearing clinochlore. Tetrahedrite-(Ni) is black in color with a reddish-black streak and metallic luster. It is brittle with uneven fractures and has a calculated density of 5.073 g·cm–3. The mean values of 9 electron microprobe analyses (wt%) are Cu 39.83, Ni 5.67, Fe 1.45, Sb 21.69, As 5.45, S 25.39, total 99.48, and the empirical formula calculated on the basis of cation = 16 apfu is M(2)Cu6.00M(1)[Cu4.03(Ni1.55Fe0.42)Σ1.97]Σ6.00X(3)(Sb2.85As1.16)Σ4.01S12.67. Tetrahedrite-(Ni) is cubic, with space group I43m, a = 10.3478(4) Å, V = 1108.00(14) Å3, and Z = 2. Its crystal structure has been solved by X-ray single-crystal diffraction on the basis of 188 independent reflections, with a final R1 = 0.0327. Tetrahedrite-(Ni) is isostructural with tetrahedrite group minerals. It represents the first natural tetrahedrite-group mineral with a Ni-dominated charge-compensating constituent. Tetrahedrite-(Ni) may be the product of late-serpentinization at moderately high-temperature conditions around 350 °C. In this case, tetrahedrite-(Ni) and its mineral paragenesis record an entire geological process of nickel enrichment, migration, activation, precipitation, and alteration from deep mantle to shallow crust.