Příbramite, CuSbSe2, the Se-analogue of chalcostibite, a new mineral from Příbram, Czech Republic

Abstract

The new mineral přibramite was found on the dump of shaft No. 16, one of the mines in the Přibram uranium and base-metal district, central Bohemia, Czech Republic. Přibramite 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 × 12 μm (in the section), growing together in aggregates up to 150 μm across in the calcite gangue. Přibramite crystals form intergrowths and replace older permingeatite; antimonselite crystals were observed in close association as well. Přibramite 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 přibramite 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 (Sb 1.02 As 0.01 ) 1.03 (Se 1.81 S 0.15 ) 1.96 . The ideal formula is CuSbSe 2 , 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. Přibramite is orthorhombic, Pnma , a = 6.3042(15), b = 3.980(1), c = 14.989(4) A, with V = 376.09(2) A 3 and Z = 4. The strongest reflections of the calculated powder X-ray diffraction pattern [ d , A (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), přibramite is unequivocally isostructural with chalcostibite and emplectite. The structure of přibramite 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 SbSe 2 along b and CuSe 4 tetrahedra forming chains of CuSe 3 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 přibramite ( V přibramite ~ 380 A 3 ) compared with that of chalcostibite ( V chalcostibite ~ 330 A 3 ). Přibramite is named after the type locality, the Přibram uranium and base-metal district.