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

Authors: Sarp, Halil; Černy, Radovan

Source: European Journal of Mineralogy, Volume 16, Number 1, 1 January 2004 , pp. 163-169(7)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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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, 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 α = 1.715(5), β = 1.730(5) γ = 1.735(5) at 589nm; 2V_obs = 55(5)°, 2V_calc = 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-2O; POLYTYPE; CRYSTAL STRUCTURE; ARSENATE; HYDROGEN BONDING; TSUMEB; NAMIBIA

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2004/0016-0163

Publication date: 2004-01-01

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