Tillmannsite, (Ag₃Hg)(V, As)O₄, a new mineral : its description and crystal structure

Tillmannsite, (Ag³Hg)(V, As)O⁴, was found in the old copper mines of Roua (Alpes-Maritimes, France), associated with pecoraite, vésigniéite, 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, I4̄, a = 7.727(7) Å, c = 4.648(5) Å, V= 277.5(5) ų, Z = 2 and D_calc = 7.733(3) g/cm³. The strongest lines in the X-ray powder diffraction pattern (d_obs in Å, (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(F²) = 0.075. The structure of tillmannsite containes isolated tetrahedra (V, As)O₄ and tetrahedral clusters (Ag₃Hg) formed by metallic atoms. Each (Ag, Hg) metallic atom is coordinated by 3 metallic neighbors and by 3 oxygens.