Tetrahedrite-(Cu), Cu₁₂Sb₄S₁₃, from Bankov near Košice, Slovak Republic: a new member of the tetrahedrite group

Tetrahedrite-(Cu), Cu 12 Sb 4 S 13 , has been approved as a new mineral species (IMA2022–078) from the Bankov magnesite deposit near Košice, Slovak Republic where it occurs as anhedral grains, up to 0.4 mm across, associated with skinnerite, chalcostibite, famatinite, tetrahedrite-(Fe) and zoned aggregates of tennantite-(Cu) to tennantite-(Fe). Tetrahedrite-(Cu) is steel-grey, with a metallic lustre. Mohs hardness is ca . 3½–4 and calculated density is 5.029 g.cm –3 . In reflected light, tetrahedrite-(Cu) is isotropic and grey with a bluish shade. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 31.1; 546: 30.1; 589: 29.9; and 650: 28.1. The empirical formula, based on electron-microprobe data (mean of 17 spot analyses), is Cu 11.42 Zn 0.26 Fe 0.19 (Sb 4.06 As 0.08 ) Σ4.14 S 12.99 . The ideal formula is Cu 6 (Cu 4 Cu 2 )Sb 4 S 13 , which requires (in wt.%) Cu 45.76, Sb 29.23 and S 25.01, total 100.00. Tetrahedrite-(Cu) is cubic, I 4̄ 3 m , with unit-cell parameters a = 10.3296(15) Å, V = 1102.2(5) Å 3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R 1 = 0.0347 on the basis of 261 unique reflections with F o > 4σ( F o ) and 22 refined parameters. Tetrahedrite-(Cu) is isotypic with other tetrahedrite-group minerals. Previous findings of tetrahedrite-(Cu) are reported and some nomenclature issues, related to the Fe and Cu oxidation states, are discussed. At the Bankov deposit, tetrahedrite-(Cu) is related to hydrothermal, most probably Alpine, solutions strongly enriched in Cu, Sb and S.