Arsenoústalečite, Cu₁₂(As₂Te₂)Se₁₃, a new mineral, and crystal structures of arsenoústalečite and stibioústalečite

Abstract Arsenoústalečite is a new mineral discovered in a sample collected from the abandoned Ústaleč deposit near Horažďovice, SW Bohemia, Czech Republic. It occurs as rare anhedral grains, up to 40 μm in size, in a calcite gangue, associated with stibioústalečite, hakite-(Hg), berzelianite and uraninite. Arsenoústalečite is dark grey, with a metallic lustre. Mohs hardness is ca . 3½–4; calculated density is 5.730 g/cm 3 . In reflected light, arsenoústalečite is pale grey with a yellowish shade; it is isotropic. Internal reflections were not observed. Reflectance values for the four COM wavelengths in air [ R (%) λ (nm)] are: 33.3 (470); 33.1 (546); 33.0 (589); and 32.9 (650). The empirical formula of arsenoústalečite is (Cu 5.81 Ag 0.17 ) Σ5.98 (Cu 5.95 Fe 0.02 Zn 0.02 Hg 0.01 ) Σ6.00 (As 1.40 Sb 0.87 Te 1.73 ) Σ4.00 (Se 10.30 S 2.32 ) Σ12.61 . The ideal formula is Cu 12 (As 2 Te 2 )Se 13 , which requires (in wt.%) Cu 34.76, As 6.83, Te 11.63, Se 46.78, total 100.00. Arsenoústalečite is cubic, I $\bar{4}$ 3 m , with unit-cell parameters a = 10.6580(19) Å, V = 1210.7(6) Å 3 and Z = 2. The strongest reflections of the calculated powder X-ray diffraction pattern [ d , Å ( I , %) hkl ] are: 3.077 (100) 222, 2.848 (10) 321, 1.946 (12) 521, 1.884(52) 440 and 1.608(21) 622. According to the single-crystal X-ray diffraction data ( R 1 = 0.0285 on the basis of 334 unique reflections with F o > 4σ F o and 24 refined parameters), arsenoústalečite is isotypic with other tetrahedrite-group minerals. The crystal structure of co-existing stibioústalečite, with an empirical formula of (Cu 5.69 Ag 0.07 ) Σ5.76 (Cu 5.80 Zn 0.13 Fe 0.06 Hg 0.01 ) Σ6.00 (Sb 1.82 As 0.42 Te 1.76 ) Σ4.00 (Se 9.52 S 3.10 ) Σ12.62 and unit-cell parameters a = 10.6975(16) Å, V = 1224.2(5) Å 3 and Z = 2, was refined to R 1 = 0.0191 on the basis of 267 unique reflections with F o > 4σ F o and 24 refined parameters. Structural relationships and crystal-chemistry of both members of the ústalečite series are discussed. Arsenoústalečite is named after its type locality, the Ústaleč deposit and its chemical composition. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2022-116).