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  (Formerly Journal of the Czech Geological Society)

Original paper

Jiří Sejkora, Jakub Plášil, Emil Makovicky

Stibioústalečite, Cu6Cu6(Sb2Te2)Se13, the first Te-Se member of tetrahedrite group, from the Ústaleč, Czech Republic

Journal of Geosciences, volume 67 (2022), issue 4, 289 - 297


Stibioústalečite, Cu12(Sb2Te2)Se13, was approved as a new mineral species from the Ústaleč mine, 15 km west of Horažďovice, SW Bohemia, Czech Republic. It occurs as metallic anhedral grains up to 0.1-0.3 mm in size, dark grey in color, in a calcite gangue. It is directly associated with hakite-(Hg), berzelianite, the not-yet approved phase Cu12(As2Te2)Se13 and uraninite. Stibioústalečite is brittle, with an indistinct cleavage and a conchoidal fracture; the calculated density is 5.676 g/cm3. In reflected light, stibioústalečite is isotropic, and grey in color; internal reflections were not observed. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 33.3; 546: 33.2; 589: 33.1; 650: 33.0. Electron microprobe analysis for holotype material (grain used for single-crystal X-ray study) gave (in wt. % - average of 7 spot analyses): Cu 34.10, Ag 1.22, Fe 0.04, Zn 0.09, Hg 0.33, Sb 9.39, As 0.70, Te 12.41, S 3.76, Se 37.59, total 99.63. On the basis of (As + Sb + Te) = 4 atoms per formula unit (apfu), the empirical formula of stibioústalečite is M(2)(Cu5.75Ag0.25)Σ6M(1)(Cu5.93Hg0.04Zn0.03Fe0.02)Σ6.02X(3)(Te2.12Sb1.68As0.20)Σ4(Se10.36S2.55)Σ12.91. The ideal formula is Cu6Cu6(Sb2Te2)Se13, which requires Cu 33.33, Sb 10.64 Te 11.16 Se 44.87, total 100 wt. %. Stibioústalečite is cubic, I 3m, with unit-cell parameters a = 10.828(4) Å, V = 1269.6(9) Å3, Z = 2. The crystal structure of stibioústalečite was studied by single-crystal X-ray diffraction data and it is isotypic with other members of the tetrahedrite group. The mineral is named after its type locality Ústaleč and its chemical composition, being the (Sb/Te) end-member in the possible ústalečite series.

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