Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Pavel Škácha, Jiří Sejkora, Jakub Plášil, Emil Makovicky

Pošepnýite, a new Hg-rich member of the tetrahedrite group from Příbram, Czech Republic

Journal of Geosciences, volume 65 (2020), issue 3, 173 - 186

DOI: http://doi.org/10.3190/jgeosci.308

The new mineral pošepnýite was found in the mine dump of shaft No. 16 Háje, one of the mines in the Příbram uranium and base-metal district, central Bohemia, Czech Republic. Pošepnýite is associated with příbramite, dzharkenite, ferroselite, hakite-(Hg), tetrahedrite-(Zn), antimonselite and uraninite in a calcite-dominant gangue. The new mineral occurs as idiomorphic to hypidiomorphic grains up to 100 μm in size. Pošepnýite is steel-grey and has a metallic lustre. Mohs hardness is ca. 3.5-4; the calculated density is 6.23 g cm^-3. In reflected light, pošepnýite is light brown. No bireflectance, pleochroism, anisotropy or internal reflections were observed. The average empirical formula, based on electron-microprobe analyses (62 spot analyses), is (Cu_3.33Ag_0.28)_Σ3.61 (Hg_3.43Cu_2.50Zn_0.03Cd_0.03Fe_0.01)_Σ6.00 (Sb_3.88As_0.12)_Σ4.00 (Se_10.83S_1.29)_Σ12.12. The ideal formula is (Cu⁺_3+x☐_3-x)_Σ6 (Hg²⁺_4-xCu⁺_2+x)_Σ6Sb₄ (Se_12.5☐_0.5)_Σ13, 0 ≥ x << 2, which requires Cu 12.25, Hg 30.93, Sb 18.77 and Se 38.05, total 100.00 wt. % (for x = 0) or Cu 17.65, Hg 23.87, Sb 19.32, Se 39.16, total 100.00 wt. % (for x = 1). Pošepnýite is cubic, I-43m, a = 10.964(1) Å, with V = 574.82(15) ų and Z = 2. The strongest reflections of the calculated powder X-ray diffraction pattern are [d, Å (I) (hkl)] are: 3.165(100) (222), 2.930(24) (321), 2.0017(16) (521), 1.9381(65) (440), 1.6528(31) (622)]. According to the single-crystal X-ray diffraction data (R_obs = 0.051), pošepnýite is isostructural with minerals of the tetrahedrite group. The cubic structure of pošepnýite is a tetrahedral framework with cavities and a part of tetrahedra replaced by SbSe₃ coordination pyramids. It has one type of pyramidal Sb position, one type of tetrahedrally coordinated cation position M1 occupied by (Hg, Cu), and one type of partly vacant cation site M2 with triangular coordination. The main anion site Se1 participates in all of these cation coordination polyhedra. Minority anion ’Se2’ ties the six coordination triangles of Cu2 in the cavity of the tetrahedral framework into one cluster. Pošepnýite is named after the František Pošepný, a famous 19^th-century geologist, often regarded as the father of the modern economic geology.