TY - JOUR
T1 - Litochlebite, Ag2PbBi4Se8, a new selenide mineral species from Zalesi, Czech Republic: description and crystal structure
AU - Makovicky, Emil
AU - Topa, Dan
AU - Sejkora, Jiri
AU - Putz, Hubert
AU - Zagler, Georg
AU - Plasil, Jakub
PY - 2011/4
Y1 - 2011/4
N2 - 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 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 (R min, Rmax in %, λ nm) are: 44.5-19.9 (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) 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)Σ1.87Pb 1.09Cd0.01)Σ1.10 Bi3.99Se8.04. 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 > 4σ(F 0)] 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, Cu3Ag2Pb3Bi 7S16.
AB - 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 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 (R min, Rmax in %, λ nm) are: 44.5-19.9 (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) 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)Σ1.87Pb 1.09Cd0.01)Σ1.10 Bi3.99Se8.04. 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 > 4σ(F 0)] 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, Cu3Ag2Pb3Bi 7S16.
U2 - 10.3749/canmin.49.2.639
DO - 10.3749/canmin.49.2.639
M3 - Journal article
SN - 0008-4476
VL - 49
SP - 639
EP - 650
JO - Canadian Mineralogist
JF - Canadian Mineralogist
IS - 2
ER -