Description and crystal structure of Yvonite, Cu(AsO3OH)2H2O

Yvonite, Cu(AsO3OH)2H(2)O, was found in the Salsigne mine near Carcassone (Aude, France). It forms aggregates or radiating spherules consisting of individual crystals (maximal size 0.3 x 0.15 x 0.06 mm) of turquoise blue color. They are elongated in c, flattened on (010), and have a perfect cleavage on (100). The mineral is triclinic, P (1) over bar, a = 7.632(3), b = 11.168(3), c = 6.020(3) Angstrom, alpha = 89.32(3), beta = 86.55(5), gamma = 74.43(3)degrees, V = 493.4(3) Angstrom(3), Z = 4, D-meas = 3.20(2) g/cm(3), and D-calc = 3.22(1) g/cm(3). Mohs hardness 3.5-4. Luster vitreous transparent, streak blue; optically biaxial (-) with alpha = 1.615(2), beta = 1.660(2), and gamma = 1.700(2) at 589 nm; 2 V-obs = 82(2)degrees, 2 V-calc = 84(1)degrees. Pleochroism weak with Z = blue, Y = light blue, and X = light blue to colorless. Associated minerals: geminite, lindackerite, arsenopyrite, native bismuth, chalcopyrite, and pushcharovskite. The crystal structure was solved by direct methods (MoK alpha radiation) and refined using 1429 observed unique reflections to R = 0.069, R-w = 0.043. There are two symmetrically independent distorted CuO5(H2O) octahedra in the structure. They share edges and form cis [CuO3(H2O)] chains parallel to [001]. Two symmetrically independent distorted AsO3(OH) tetrahedra cross-link these chains to form sheets parallel to (100). Two symmetrically independent H2O molecules are located between the sheets, which are linked by a network of hydrogen bonds, accounting for the perfect cleavage of yvonite. The mineral is structurally related to geminite, Cu(AsO3OH)(H2O), and fluckite, CaMn[(AsO3OH)(H2O)](2).