Rollandite, Cu3 (AsO4)2•4H2O, a new mineral:: its description and crystal structure

Rollandite, Cu-3 (ASO(4))(2). 4H(2)O, was found in the old copper mines of Roua (Alpes-Maritimes, France), associated with olivenite, conichalcite, clinotyrolite, cornubite, kolfanite, pharmacosiderite, gerhardtite, atacamite, gilmarite, wallkilldellite-Fe, cuprite, domeykite, algodonite and native copper. Tt forms aggregates (1mm diameter) consisting of perfect crystals (0.5 x 0.15 x 0.1 mm maximum dimension). The bottle-green crystals are elongated along a, slightly flattened on {010}, and have a good cleavage on {001}. They are not twinned. The mineral is orthorhombic, Pnma, a = 5.6906(4), b = 17.061(1), c = 9.732(1) Angstrom, V = 944.9(1) Angstrom(3), Z = 4, D-meas = 3.9(1) and D-calc = 3.84 g/cm(3). The strongest lines in the X-ray powder diffraction pattern (d(obs) in Angstrom, (hkl), I-vis) are: 8.52, (020), 100; 3.721, (131), 60; 3.221, (141) (051), 90; 3.102, (132), 40; 2.817, (103)(033), 35; 2.795, (142), 35; 2.350, (143) (053), 25; 2.133, (080) (251), 25. Mohs' hardness is 4-4.5. Luster is vitreous transparent, streak is very light green; crystals are biaxial (-) with alpha = 1.745(5), beta = 1.755(5), gamma = 1.760(5) at 590 nm; 2V(obs) = 71(2)degrees, 2V(calc) = 70.1(4)degrees. Non pleochroic. The crystal structure was solved by direct methods (MoK alpha radiation) and refined using 1407 observed unique reflections to R = 0.086, R-w = 0.046. The structure of rollandite is formed by (020) sheets of CulO(6) octahedra and [100] chains of Cu2O(5) square pyramids linked by AsO4 tetrahedra. The water molecule O6H(2) is bonded by hydrogen bonds in the channels running along [100].