• journal_title：The Canadian Mineralogist
• Contributor：Jiří Sejkora ; Emil Makovicky ; Dan Topa ; Hubert Putz ; Georg Zagler ; Jakub Plášil
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.2.639
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：2
• firstpage：639
• section：Articles

Litochlebite, Ag₂PbBi₄Se₈, is a new selenide mineral from the Zálesí uranium deposit, Rychlebské hory Mountains, northern Moravia, Czech Republic. It occurs as irregular grains up to 200 μm, 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 VHN_10g 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 (R_min, R_max in %, λ 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, β 109.11(1)°, V 797.3(2) ų, 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)(311̅), 2.980(88)(312̅) 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 (Ag_1.84Cu_0.03)_∑1.87 (Pb_1.09Cd_0.01)_∑1.10Bi_3.99Se_8.04. 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 [F_o > 4σ(F_o)] collected on a Bruker AXS diffractometer with a CCD detector and MoKα radiation. The crystal structure contains one lead site, four independent Bi sites, four silver sites and eight independent Se sites. One Ag site is an octahedrally coordinated (2 + 4) site in the pseudotetragonal layer, the other Ag site has a distorted tetrahedral coordination. The remaining Ag sites have low occupancies. Litochlebite is an Ag-dominant isotype of watkinsonite, Cu₂PbBi₄Se₈, and structurally related to berryite, Cu₃Ag₂Pb₃Bi₇S₁₆.