Cornubite, Cu₅(AsO₄)₂(OH)₄, first occurrence of single crystals, mineralogical description and crystal structure

Abstract

The first single crystals of the rare copper arsenate hydroxide cornubite, Cu5(AsO4)2(OH)4, have been found from a silicified barite vein at Reichenbach, Odenwald near Bensheim, Hessen, W.-Germany. Cornubite occurs as apple-green, translucent and mostly tabular crystals, up to 0.3 X 0.3 mm in size and up to 0.05 mm thick. The lustre is vitreous, streak light-green and hardness 4. The indices of refraction are between 1.8 and 1.9 and the optical angle is very large. The results of electron microprobe analysis are As2O5 32.01, CuO 58.60, SiO2 0.34, P2O5 0.25, Al2O3 0.15, Fe2O3 0.32, H2O (calculated) 5.27 weight total 96.94 weight %. The empirical formula is Cu5.03 (As1.903 P0.024 Fe0.027 Al0.020 Si0.038)2.013O8 (OH)4. X-ray single crystal diffraction studies gave triclinic symmetry, space group P1 with a = 6.121(1), b = 6.251(1), c = 6.790(1) Å, α = 92.93(1)°, β = 111.30(1)° and γ = 107.47(1)°, V = 227.09 Å3, Z = 1, Dx = 4.85 Mg/m3. The crystal structure contains sheets of edge-sharing CuO6-octahedra // (011) connected by AsO4 tetrahedra via common corners and by hydrogen bonds.

Keywords

• cornubite
• copper arsenate hydroxide