LEAD-ANTIMONY SULFOSALTS FROM TUSCANY (ITALY). VIII. ROUXELITE, Cu2HgPb22Sb28S64(O,S)2, A NEW SULFOSALT FROM BUCA DELLA VENA MINE, APUAN ALPS: DEFINITION AND CRYSTAL STRUCTURE

摘要

Rouxelite, Cu₂HgPb₂₂Sb₂₈S₆₄(O,S)₂, is a new species of sulfosalt discovered in the Buca della Vena Fe–Ba deposit, Apuan Alps, Italy. It occurs as acicular metallic crystals in calcite veins; reflectance values in air are (R%, in nm): 38.2 (470), 36.2 (546), 35.4 (589), 32.8 (650). Rouxelite is monoclinic, space group C2/m, with a 43.113, b 4.059, c 37.874 Å, ß 117.35°, V 5887 Å3, Z = 2, D_calc = 5.86 g/cm3. The eight most intense lines of the X-ray powder-diffraction pattern [d (in Å)(I)(hkl)] are: 3.84(31)(.0.8), 3.402(100)(.0.2, .0.8, 115), 3.369(74)( 14, 0.0.10), 2.815(70)(317, .1.4), 2.756(36)(516, 12.0.3), 2.251(31)(1.1.12, 3.1.11), 2.116(31)( .1.15), and 1.955(30)( .1.5, 11.1.8). Electron-microprobe analyses gave (wt.%; mean of six spot analyses): Cu 1.34(5), Hg 1.76(9), Pb 45.08(15), Sb 31.50(14), S 20.07(11), O (struct.) 0.20, sum 99.95. The chemical formula, calculated on the basis of a cation total of 53 atoms, is Cu_2.20Hg_0.92Pb_22.78Sb_27.10S_65.53O_1.31 (Z = 2), giving the idealized formula Cu₂HgPb₂₂Sb₂₈S₆₄(O,S)₂, which is charge-balanced. An X-ray single-crystal study, despite a poor R value (0.169), revealed a quite original structure, with Cu and Hg in two specific sites, a tetrahedral one for Cu, and a flattened octahedral one for Hg. There is also a split O,S position, bound to two Sb atoms. For the main part, Pb, Sb and S atoms are organized in columns with a pseudoternary axis, derived from hexagonal Ba₁₂Bi₂₄S₄₈. The structural formula is Cu₂HgPb_22.6Sb_27.4S_64.67O_1.33. The structure is built up of two types of ribbons parallel to b and alternating along a, rimmed by a ribbon-layer parallel to (010); the result is a three-component structure of the boxwork type, like for neyite. Very large topologically equivalent columns are also recognizable in rouxelite and kobellite, which have about 90% of their structure motif in common. The formation of rouxelite is associated with the hydrothermal remobilization and deposition of mercury in the Apuan Alps during the Apenninic tectonometamorphic event, as illustrated in the nearby Hg deposits of Ripa and Levigliani. The name honors Jean Rouxel (1935–1998), solid state chemist, professor and founder of the Institut des Matériaux, University of Nantes (France).