Litochlebite, Ag2PbBi4Se8, a new selenide mineral species from Zalesi, Czech Republic: description and crystal structure

Moravia, Czech Republic. It occurs as irregular grains up to 200 mm, which form aggregates up to 1–2 mm in size in a quartz gangue. These aggregates are replaced along the margins and fractures by a heterogeneous supergene Bi–Se–O phase. Other associated minerals included uraninite, hematite, and uranophane. Litochlebite is opaque, dark grey to black, has a dark grey streak and a metallic luster. No cleavage was observed; the mineral is brittle with an irregular fracture. The VHN10g microhardness 230 (227–234) kg/mm2 corresponds to a Mohs hardness of about 3; the calculated density is 7.90 g/cm3. In reflected light, litochlebite is white, with weak bireflectance (only in oil) and pleochroism from white with a very faint yellowish tint to white with a very faint bluish tint. Between crossed polars, the anisotropy is moderate both in air and in oil, with dark grey to brown polarization-colors. Reflectance values in air (Rmin, Rmax in %, l nm) are: 44.5–49.9 (470), 45.1–50.5 (546), 45.6–51.3 (589), 45.4–51.8 (650). Litochlebite is monoclinic, space group P21/m, with a 13.182(2), b 4.1840(8), c 15.299(2) Å, b 109.11(1)°, V 797.3(2) Å3, and a:b:c 3.1506:1:3.6565. The main lines of the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 3.684(53)(301), 3.201(76)(104), 3.028(100)(311), 2.980(88)(312) and 2.892(95)(005). Its average composition (electron-microprobe data) is Cu 0.10, Ag 10.27, Cd 0.05, Pb 11.73, Bi 43.27, Se 32.93, S 0.01, total 98.36 wt.%. The resulting empirical formula, written on the basis 15 apfu, is (Ag1.84Cu0.03)S1.87 (Pb1.09Cd0.01)S1.10Bi3.99Se8.04. The ideal formula, Ag2PbBi4Se8, requires Ag 11.41, Pb 10.96, Bi 44.22, Se 33.41, total 100.00 wt.%. The crystal structure of litochlebite has been solved by direct methods and refined to R1 = 3.89% on the basis of 938 unique reflections [Fo > 4s(Fo)] collected on a Bruker AXS diffractometer with a CCD detector and MoKa radiation. The crystal structure contains one lead site, four independent Bi site