Posepnyite, a new Hg-rich member of the tetrahedrite group from Pribram, Czech Republic

The new mineral posepnyite was found in the mine dump of the shaft No. 16 Haje, one of the mines in the Pribram uranium and base-metal district, central Bohemia, Czech Republic. posepnyite is associated with pribramite, 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 mu m in size. posepnyite 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, posepnyite 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 (Cu3.33Ag0.28)(Sigma 3.61)(Hg3.43Cu2.50Zn0.03Cd0.03Fe0.01)(Sigma 6.00)(Sb3.88As0.12)(Sigma 4.00 (Se)10.83(S)1.29())Sigma 12.12 . The ideal formula is (Cu-3+x(+),square(3-x) ),(Hg-4-x(2)+ Cu-2+x(+))(Sigma 6)Sb-4(Se-12.5 square(0.5) ), 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). posepnyite is cubic, I-43m, a = 10.964(1) angstrom, with V = 574.82(15) angstrom(3) and Z = 2. The strongest reflections of the calculated powder X-ray diffraction pattern are [d, angstrom (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), posepnyite is isostructural with minerals of the tetrahedrite group. The cubic structure of posepnyite 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 An 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. posepnyite is named after the Frantisek Pokpny, a famous 19th-century geologist, often regarded as the father of the modern economic geology.