Tennantite-(Ni), Cu₆(Cu₄Ni₂)As₄S₁₃, from Luobusa ophiolite, Tibet, China: a new Ni-member of the tetrahedrite group

The new mineral tennantite-(Ni), Cu 6 (Cu 4 Ni 2 )As 4 S 13 , has been discovered from the Luobusa Chromitite, Tibet, southwestern China. Tennantite-(Ni) occurs as anhedral grains ranging from 2 to 20 μm in size. In reflected light microscopy, tennantite-(Ni) is isotropic and appears yellow-greenish grey. Reflectance data for Commission on Ore Mineralogy wavelengths in air for tennantite-(Ni) are: 31.0 (470 nm), 29.6 (546 nm), 29.6 (589 nm) and 29.3 (650 nm). Electron microprobe analysis for holotype material gave the empirical formula (on basis of total cations = 16 apfu): M (2) Cu 6 M (1) [Cu 4.00 (Ni 0.97 Cu 0.53 Fe 0.50 ) Σ2.00 ] Σ6.00 X (3) (As 2.94 Sb 1.06 ) Σ4 S 12.77 . Tennantite-(Ni) is cubic, with space group I $\bar{ 4}$ 3 m (#217), a =10.2957(9) Å, V = 1091.4(3) Å 3 and Z = 2. By using single-crystal X-ray diffraction, the crystal structure has been determined and refined to a final R 1 = 0.0423 on the basis of 163 independent reflections [ F o > 4σ ( F o )]. The calculated seven strongest powder X-ray diffraction lines [ d in Å ( I ) ( hkl )] are: 2.972 (100) (222), 1.820 (83) (440), 2.574 (28) (400), 1.552 (18) (622), 3.640 (10) (220), 1.880 (10) (521) and 1.287 (7) (800). Tennantite-(Ni) is isostructural with other tetrahedrite-group minerals, and nickel is hosted at the tetrahedrally coordinated M (1) site, along with Cu and minor Fe. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2021-018).