Original paper

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

Sarp, Halil; Černý, Radovan

Abstract

Calcio-andyrobertsite-2O, ideally KCaCu5(AsO4)4[AsO2(OH)2]·2H2O, 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(1), c = 9.963(1) Å, V = 1974.4(4) Å3, Z = 4, Dmeas= 3.9(1) and Dcalc= 3.80(1) g/cm3. The strongest lines in the X-ray powder diffraction pattern (dobs in Å, (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 α = 1.715(5), β = 1.730(5) γ = 1.735(5) at 589nm; 2Vobs = 55(5)°, 2Vcalc = 60(3)° Pleochroism is moderate with γ = intense blue, β = blue, α= pale blue. The crystal structure was solved by direct methods (single crystal, MoKα radiation) and refined using all 1911 observed unique reflections to R(F) = 0.039, wR(F2) = 0.105.

Keywords

calcio-andyrobertsite-2opolytypecrystal structurearsenatehydrogen bondingtsumebnamibia