Tillmannsite, (Ag_3Hg)(V,As)O_4, a new mineral: its description and crystal structure

摘要

Tillmannsite, (Ag_3Hg)(V,As)O_4, was found in the old copper mines of Roua (Alpes-Maritimes, France), associated with pecoraite, vesignieite, olivenite, kolfanite, janggunite, chlorargyrite, cuprite, native copper, native silver, native silver containing 2 of mercury, domeykite, djurleite and algodonite. It forms aggregates (0.2 mm diameter) consisting of pseudooctahedral crystals (50 μm maximum dimension). The crystals are red, brownish red. The mineral is tetragonal, I(4-bar), a = 7.727(7) A, c = 4.648(5) A, V = 277.5(5)A~3, Z = 2 and D_(calc) = 7.733(3) g/cm~3. The strongest lines in the X-ray powder diffraction pattern (d_(obs) in A, (hkl), I_(vis)) are: 5.45, (110), 25; 2.772, (211), 100; 2.324, (002), 30; 2.254, (301), 20. Luster is adamantine translucent, streak is brownish red; crystals are uniaxial (+) with ω～2.3, ε～2.5 at 589 nm. Pleochroism is intense with ε = red orange intense, ω = orange brown. The crystal structure was solved from data collected using synchrotron radiation by traditional direct methods and refined using 350 observed unique reflections to R(F) = 0.037, Rw(F~2) = 0.075. The structure of tillmannsite containes isolated tetrahedra (V,As)O_4 and tetrahedral clusters (Ag_3Hg) formed by metallic atoms. Each (Ag,Hg) metallic atom is coordinated by 3 metallic neighbors and by 3 oxygens.