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

Tetrahedrite-(Cu), Cu12Sb4S13, was approved as a new mineral species from the Bankov magnesite deposit near Koxˇice, 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 metallic luster. Mohs hardness is ca. 31/2 - 4, calculated density is 5.029 g.cm-3. In reflected light, tetrahedrite-(Cu) is isotropic, grey with bluish shade. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 31.1; 546: 30.1; 589: 29.9; 650: 28.1. The empirical formula, based on electron-microprobe data (mean of 17 spot analyses), is Cu11.42Zn0.26Fe0.19(Sb4.06As0.08)Σ4.14S12.99. The ideal formula is Cu6(Cu4Cu2)Sb4S13, which requires (in wt.%) Cu 45.76, Sb 29.23 and S 25.01, total 100.00. Tetrahedrite-(Cu) is cubic, I 3m, with unit-cell parameters a = 10.3296(15) Å, V = 1102.2(5) å3, Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1= 0.0347 on the basis of 261 unique reflections with Fo> 4σ(Fo) 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 4 discussed. At the Bankov deposit, tetrahedrite-(Cu) is related to hydrothermal, most probably Alpine, solutions strongly enriched in Cu, Sb and S.