Stibiogoldfieldite, Cu-12(Sb2Te2)S-13, a new tetrahedrite-group mineral

Stibiogoldfieldite, Cu-12(Sb2Te2)S-13, was approved as a new mineral species from the Mohawk mine, Goldfield mining district, Esmeralda County, Nevada, USA. It occurs as metallic anhedral grains, dark grey in colour. It is associated with quartz, pyrite and an Ag-Bi-(S,Se) phase (holotype material) and with quartz, pyrite, calaverite, bismuthinite, bohdanowiczite, and the Ag-Bi-(S,Se) phase (cotype material). In reflected light, stibiogoldfieldite is isotropic, grey in colour, with indistinct brownish shade. Reflectance data in air [R (%)] are: 31.1 at 470 nm, 30.9 at 546 nm, 30.8 at 589 nm and 31.0 at 650 nm. Electron microprobe analysis for holotype material gave (in wt.% - average of 60 spot analyses): Cu 45.03(60), Ag 0.26(7), Fe 0.02(3), Zn 0.13(15), Sn 0.02(4), Pb 0.05(6), Sb 8.02(62), As 2.80(65), Bi 2.77(87), Te 15.15(1.24), S 24.50(32), Se 0.52(11), total 99.27(69). On the basis of (As + Sb + Te + Bi) = 4 atoms per formula unit (apfu), the empirical formula of stibiogoldfieldite is (Cu12.05Ag0.04Zn0.03Fe0.01)(sigma 12.13)(Sb1.12As0.63Bi0.23Te2.02)(sigma 4.00)(S12.99Se0.11)(sigma 13.10). Chemical data on an additional sample from the same locality (cotype material) gave the following results (in wt.% - average of 181 spot analyses): Cu 43.84(63), Ag 0.21(7), Sb 5.92(78), As 2.63(45), Te 20.07(1.19), S 25.13(53), Se 0.97(35), total 99.47(66). On the basis of (As + Sb + Te + Bi) = 4 apfu, the empirical formula of cotype material is (Cu11.30Ag0.03)(sigma 11.33)(Sb0.80As0.57Bi0.06Te2.57)(sigma 4.00)(S12.83Se0.20)(sigma 13.03). Stibiogoldfieldite is cubic, I (4) over bar 3m, with unit-cell parameters a = 10.3466(17) angstrom, V = 1107.6(5) angstrom(3) and Z = 2 (holotype). Unit-cell parameters for the cotype sample are a = 10.3035(2) angstrom and V = 1093.83(7) angstrom(3). The crystal structure of holotype stibiogoldfieldite was refined by single-crystal X-ray diffraction data to a final R-1 = 0.032 on the basis of 285 reflections with F-o > 4 sigma(F-o) and 20 refined parameters. Stibiogoldfieldite is isotypic with other members of the tetrahedrite group.