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Yvonite, Cu(AsO 3 OH)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 × 0.15 × 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, P1̅, a = 7.632(3), b = 11.168(3), c = 6.020(3) Å , α = 89.32(3), β = 86.55(5), γ = 74.43(3)°, V = 493.4(3) Å 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 α = 1.615(2), β = 1.660(2), and γ = 1.700(2) at 589 nm; 2 V obs  = 82(2)°, 2 V calc  = 84(1)°. 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 (MoKα radiation) and refined using 1429 observed unique reflections to R = 0.069, R w  = 0.043. There are two symmetrically independent distorted CuO 5 (H 2 O) octahedra in the structure. They share edges and form cis [CuO 3 (H 2 O)] chains parallel to [001]. Two symmetrically independent distorted AsO 3 (OH) tetrahedra cross-link these chains to form sheets parallel to (100). Two symmetrically independent H 2 O 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(AsO 3 OH)(H 2 O), and fluckite, CaMn[(AsO 3 OH)(H 2 O)] 2 .

Description and crystal structure of Yvonite, Cu(AsO 3 OH)2H 2 O