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

Stibioústalečite, Cu 12 (Sb 2 Te 2 )Se 13 , 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 0.1-0.3mm in size, dark grey in color, in a calcite gangue.It is directly associated with hakite-(Hg), berzelianite, the not-yet approved phase Cu 12 (As 2 Te 2 ) Se 13 and uraninite.Stibioústalečite is brittle, with an indistinct cleavage and a conchoidal fracture; the calculated density is 5.676 g/cm 3 .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) (Cu 5.75 Ag 0.25 ) Σ6 M(1) (Cu 5.93 Hg 0.04 Zn 0.03 Fe 0.02 ) Σ6.02 X(3) (Te 2.12 Sb 1.68 As 0.20 ) Σ4 (Se 10.36 S 2.55 ) Σ12.91 .The ideal formula is Cu 6 Cu 6 (Sb 2 Te 2 ) Se 13 , which requires Cu 33.33, Sb 10.64 Te 11.16 Se 44.87, total 100 wt.%.Stibioústalečite is cubic, I4̄ 3m, with unit-cell parameters a = 10.828( 4) Å, V = 1269.6(9)Å 3 , 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.