Rollandite, Cu (sub 3) (AsO (sub 4) ) (sub 2) .H (sub 2) O, a new mineral; its description and crystal structure

Rollandite, Cu (sub 3) (AsO (sub 4) ) (sub 2) .4H (sub 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. It forms aggregates (1mm diameter) consisting of perfect crystals (0.5X0.15X0.1 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) Aa, V = 994.9(1) Aa (super 3) , Z = 4, D (sub meas) = 3.9(1) and D (sub calc) = 3.84 g/cm (super 3) . The strongest lines in the X-ray powder diffraction pattern (d (sub obs) in Aa, (hkl), I (sub vis) ) are: 8.52, (020), 100; 3.721, (131), 60; 3.221, (141) (051), 90; 3.102, 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 (sub obs) = 71(2) degrees , 2V (sub calc) = 70.1(4) degrees . Non pleochroic. The crystal structure was solved by direct methods (MoKalpha radiation) and refined using 1407 observed unique reflections to R = 0.086, R (sub w) = 0.046. The structure of rollandite is formed by (020) sheets of CulO (sub 6) octahedra and [100] [chains of Cu (sub 2) O (sub 5) square pyramids linked by AsO (sub 4) tetrahedra. The water molecule O (sub 6) H (sub 2) is bonded by hydrogen bonds in the channels running along [100].