Pribramite, CuSbSe2, the Se-analogue of chalcostibite, a new mineral from Pribram, Czech Republic

The new mineral pribramite was found on the dump of shaft No. 16, one of the mines in the Pribram uranium and basemetal district, central Bohemia, Czech Republic. Pribramite is associated with antimonselite, permingeatite, dzharkenite, ferroselite, hakite, tetrahedrite, chameanite, giraudite, a new Hg-, Sb-, Cu selenide, a new Sb-, Cu selenide and uraninite in a calcite-dominant gangue. The new mineral occurs as columnar crystals up to 60 x 12 mu m (in the section), growing together in aggregates up to 150 mu m across in the calcite gangue. Pribramite crystals form intergrowths and replace older permingeatite; antimonselite crystals were observed in close association as well. Pribramite is brittle, is lead grey in colour, and has a metallic lustre. Mohs hardness is ca. 3-4; the calculated density is 5.884 g cm(-3). In reflected light pribramite is grey with a yellowish hue, bireflectance is medium, and pleochroism is weak with grey tints. Anisotropy is strong with grey to brownish rotation tints. Internal reflections were not observed. The empirical formula, based on electron-microprobe analyses, is Cu-1.00(Sb1.02As0.01)(1.03)(Se1.81S0.15)(1.96). The ideal formula is CuSbSe2, which requires Cu 18.52, Sb 35.47 and Se 46.01, total 100.00 wt%. Additional elements as Fe, Pb, Tl and Hg were found in small concentrations above detection limits. Pribramite is orthorhombic, Pnma, a = 6.3042(15), b = 3.980(1), c = 14.989(4) angstrom, with V = 376.09(2) angstrom(3) and Z = 4. The strongest reflections of the calculated powder X-ray diffraction pattern [d, angstrom (I)(h k l)] are: 3.152(40) (2 0 0), 3.113(100)(0 1 3), 3.085(40)(2 0 1), 3.395(63)(0 1 5), 1.9900(38)(0 2 0), 1.8442(42)(3 1 1) and 1.8329(33)(3 0 4). According to single-crystal X-ray diffraction data (R-obs = 0.0480), pribramite is unequivocally isostructural with chalcostibite and emplectite. The structure of pribramite contains one Cu, one Sb and two Se sites (last mentioned is occupied both by Se and S atoms). It is built up from square Sb pyramids linked to form chains of SbSe2 along b and CuSe4 tetrahedra forming chains of CuSe3 parallel to b. These two types of infinite chains are then linked to form sheets that are stacked perpendicular to c. The effect of the larger Se atom, compared with S, is well reflected by the increased unit-cell volume of pribramite (V-pribramite similar to 380 angstrom(3)) compared with that of chalcostibite (V-chalcostibite similar to 330 angstrom(3)). Pribramite is named after the type locality, the Pribram uranium and base-metal district.