Calcio-andyrobertsite-2<I>O</I>, KCaCu<SUB>5</SUB>(AsO<SUB>4</SUB>)<SUB>4</SUB>[AsO<SUB>2</SUB>(OH)<SUB>2</SUB>]·2H<SUB>2</SUB>O: its description, crystal structure and relation with calcio-andyrobertsite-1<I>M</I>

Calcio-andyrobertsite-2O, ideally, CaCu5AsO4[AsO4(AsO2] • 2H2O, is a polytype of calcio-andyrobertsite-1 M, 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 × 3 × 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 589 nm; 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(F)=0.105.