Calcio-andyrobertsite-2O, KCaCu (sub 5) (AsO (sub 4) ) (sub 4) [AsO (sub 2) (OH) (sub 2) ] . 2H (sub 2) O; its description, crystal structure and relation with calcio-andyrobertsite-1M

Calcio-andyrobertsite-2O, ideally KCaCu (sub 5) (AsO (sub 4) ) (sub 4) [AsO (sub 2) (OH) (sub 2) ] . 2H (sub 2) O, is a polytype of calcio-andyrobertsite-1M and occurs is a specimen of quartz gangue found at the Tsumeb mine, Namibia, together with cuprian adamite. It forms platy crystals with a maximum dimension of 7X3X0.5 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(1), c = 9.963(1) Aa, V = 1974.4(4) Aa (super 3) , Z = 4, D (sub meas) = 3.9(1) and D (sub calc) = 3.80(1) g/cm (super 3) . The strongest lines in the X-ray powder diffraction pattern (d (sub obs) in Aa, (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 = 1.73(5), gamma = 1.735(5) at 589 nm; 2V (sub obs) = 55(5) degrees , 2V (sub 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 (super 2) ) = 0.105.