Calcio-andyrobertsite-2O, KCaCu5(AsO4)4[AsO2(OH)2] • 2H2O:: its description, crystal structure and relation with calcio-andyrobertsite-1M

Calcio-andyrobertsite-2O, ideally KCaCu5(AsO4)(4)[AsO2(OH)(2)](.)2H(2)O, is a polytype of calcio-andyrobertsite-1M and occurs in a specimen of quartz gangue found at the Tsumeb mine, Namibia, together with cuprian adamite. It forms platy crystals with a maximum dimension of 7 x 3 x 0.5 mm. The intense blue crystals are elongated along [011] or [001], platy on {100}, and have a good cleavage on {100} and conchoidal fracture. They are not twinned. The mineral is orthorhombic, Pnma, a = 19.647(2), b = 10.087(l), c = 9.963(l) Angstrom, V= 1974.4(4) Angstrom(3), Z = 4, D-meas = 3.9(1) and D-calc = 3.80(1) g/cm(3). The strongest lines in the X-ray powder diffraction pattern (d(obs) in Angstrom, (hkl), Ivis) are: 9.80, (200), 50; 7.084, (011), 70; 6.665, (111), 60; 4.830, (102), 70; 4.487, (220), 90; 3.965, (302), 60; 3.116, (322)(610), 100; 2.760, (132), 90; 2.674, (431), 40; 1.758, (840)(11.01), 40. Mohs' hardness is 3 to 4. Lustre is vitreous, streak is pale blue; crystals are transparent and biaxial (-) with alpha = 1.715(5), beta = 1.730(5), gamma = 1.735(5) at 589 nm; 2V(obs) = 55(5)degrees, 2V(calc) = 60(3)degrees. Pleochroism is moderate with gamma = intense blue, beta = blue, alpha = pale blue. The crystal structure was solved by direct methods (single crystal, MoKalpha radiation) and refined using all 1911 observed unique reflections to R(F) = 0.039, wR(F-2) = 0.105.