Roterbärite, PdCuBiSe3, a new mineral species from the Roter Bär mine, Harz Mountains, Germany

Abstract

Roterbarite, PdCuBiSe3, is a new mineral species from the Roter Bar mine, Harz Mountains, Germany. It forms euhedral to subhedral grains, up to 50 μm across, embedded in clausthalite, which is spatially associated with gold, mertieite-II, bohdanowiczite, hematite, chalcopyrite, baryte, ankerite and dolomite. Roterbarite is brittle and has a metallic luster. In plane-polarized light, roterbarite is slightly pleochroic in shades of dark cream to slightly greenish cream; it has weak anisotropy with rotation tints in shades of pale orange brown to grey and exhibits no internal reflections. Reflectance values of roterbarite in air (R2, R1 in %) are: 42.4/43.0 at 470 nm, 45.4/44.4 at 546 nm, 46.8/44.6 at 589 nm and 47.7/44.6 at 650 nm. Eighteen electron-microprobe analyses of roterbarite gave an average composition (in wt%): Pd 18.1 wt%, Bi 35.2 wt%, Cu 10.5 wt%, Se 33.5 wt% and S 2.6 wt%, totalling 99.8 wt% and corresponding to the empirical formula Pd1.01Cu0.98Bi1.00(Se2.53S0.48)3.01, based on 6 atoms; the average of ten microanalyses on its synthetic analogue is: Pd 17.62 wt%, Cu 10.74 wt%, Bi 33.59 wt% and Se 38.70 wt%, the total of which is 100.65 wt% and corresponds to Pd1.01Cu1.03Bi0.98Se2.98. The ideal formula is PdCuBiSe3. The density, calculated on the basis of the empirical formula, is 7.23 g/cm3. The mineral is orthorhombic, space group P212121, with a 5.00520(10), b 7.9921(2) A, c 13.5969(2) A, V 543.90(2) A3 and Z = 4. The crystal structure was solved and refined from the powder X-ray diffraction data of synthetic PdCuBiSe3. Roterbarite crystallizes in the (Bi,Sb)CuNiS3 structure-type, being isostructural with minerals of the lapieite group – i.e., lapieite (CuNiSbS3), muckeite (CuNiBiS3) and lisiguangite (CuPtBiS3). Roterbarite belongs to the vast group of sulfosalts and related compounds (a selenio-salt). The strongest lines in the powder X-ray diffraction pattern of the synthetic PdCuBiSe3 [d in A (I) (hkl)] are: 3.3593 (97) (103), 3.1226 (100) (120), 3.0434 (75) (121), 2.3894 (39) (105), 1.9210 (70)(223).