Theoparacelsite, Cu-3(OH)(2)As2O7, a new mineral: Its description and crystal structure

Theoparacelsite, Cu-3(OH)(2)As2O7, was found in the old copper mines of Rou a (Alpes-Maritimes, France), associated with olivenice, cornubite, clinotyr olite, connellite:, brochantite, malachite, trippkeite, pharmacosiderite, s trashimirite, gilmarite, native copper, cuprite, algodonite and domeykite. It forms aggregates in geodes of cuprite (1 mm diameter) formed by xenomorp h crystals (0.2 x 0.1 x 0.05 mm maximum dimension), rectangular elongated c rystals (90 x 10 x 5 mu rn), perfect little rectangular crystals (10 x 7 x 2 mum), equidimentional crystals blunted at their extremities (20 mum), pse udomorphs after thin acicular crystals of olivenite, or finally in very fin e grained aggregates. The dark pistachio green rectangular crystals are elo ngated along b, flattened on (001), and have a perfect cleavage on {001}. T hey are not twinned. The mineral is orthorhombic, Pmma, a = 8.3212(8), b = 2.9377(3), c = 4.6644(5) Angstrom, V = 114.02(2) Angstrom (3), Z = 2/3, and D-calc = 4.72 (1) g/cm(3). The strongest lines in the X-ray powder diffrac tion pattern (d(obs) in Angstrom, (hkl), I-vis) are: 3.104, (201), 100;2.48 6, (011), 70; 2.400, (210), 25; 1.672, (212), 30; 1.596, (411), 25; 1.330, (601)(221), 25. The Mohs' hardness could not be measured. Luster is vitreou s to adamantin, streak is yellowish green; crystals are biaxial (+) with al pha = 1.81(1), beta = ;1.82(1), gamma = 1.86(1) at 590 nm; 2V(obs) = 57(3)d egrees, 2V(calc) = 54(1)degrees. Pleochronism is moderate with gamma = dade green pistachio, beta = olive green, ct = light olive green. The orlisral structure was solved by direct methods (MoK alpha radiation) and refined us ing 207 observed unique reflections to R = 0.063, R-W = 0.064. The structur e of theoparacelsite is disordered and formed by (001) sheets of CuO6 octah edra with the composition CuO3. The sheets are linked in the c direction by chains of AsO4 tetrahedra parallel to b and occupied from 2/3 giving so th e composition As2/3O3.