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Tillmannsite, (Ag₃Hg)(V, As)O₄, a new mineral

¹⁾ Département de Minéralogie du Museum d'Histoire naturelle de Genève, 1, route de Malagnou, CH-1208 Geneve, Switzerland
²⁾ Geology Department, Moscow State University, 119899 Moscow Russia
³⁾ CCLRC Daresbury Laboratory, Daresbury, Warrington. Cheshire WA4 4AD, UK

^* e-mail: dmitp{at}geol.msu.ru

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, I, 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 2.3, 2.5 at 589 nm. Pleochroism is intense with = red orange intense, = 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.

Key-words: tillmannsite, crystal structure, vanadate, silver, mercury, Roua (France).

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