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Calcio-andyrobertsite-2O, KCaCu₅(AsO₄)₄[AsO₂(OH)₂] · 2H₂O

Halil SARP¹ and Radovan KERN^2,*

¹ Departement de Minéralogie du Muséum d'histoire naturelle de la Ville de Genève, 1 route de Malagnou, CP 6434, CH-1211 Genève 6, Switzerland
² Laboratoire de Cristallographie, Université de Genève, 24 quai Ernest-Ansermet, CH-1211 Genève 4, Switzerland

^* Corresponding author, e-mail: Radovan.Cerny{at}cryst.unige.ch

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 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) ų, Z = 4, D_meas= 3.9(1) and D_calc= 3.80(1) g/cm³. The strongest lines in the X-ray powder diffraction pattern (d_obs 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 = 1.715(5), β = 1.730(5), = 1.735(5) at 589 nm; 2V_obs= 55(5)°, 2V_calc = 60(3)°. Pleochroism is moderate with = intense blue, β = blue, = pale blue. The crystal structure was solved by direct methods (single crystal, MoK radiation) and refined using all 1911 observed unique reflections to R(F) = 0.039, wR(F)=0.105.

Key-words: calcio-andyrobertsite-2O, polytype, crystal structure, arsenate, hydrogen bonding, Tsumeb, Namibia.

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