Crystal structure and chemistry of conichalcite, CaCu(AsO4)(OH)

The crystal structures of conichalcite [Ca(Cu, Mg)(AsO4)(OH)] samples obtained from Gozaisho mine, Fukushima, Japan, and Higgins mine, Arizona, USA, were refined by single-crystal X-ray diffraction (XRD) using an imaging plate detector. The results revealed that conichalcite is orthorhombic and belongs to the P2(1)2(1)2(1) space group. The positions of the hydrogen atom and the donor and accepter atoms in the structure were determined by difference Fourier and bond-valence sum methods. The crystal comprised three highly distorted coordination polyhedra: an AsO4 tetrahedron, a CuO4(OH)(2) octahedron, and a CaO7(OH) square antiprism. The CuO4(OH)(2) octahedron was distorted by the Jahn-Teller effect. The observed As-O distances were shorter than those predicted by ionic bonding. The electronic structure of the atoms affected the distortion of the polyhedra that did not have a symmetry center. The CuO4(OH)(2) octahedron shared its edges to form linear chains, which were further linked by the vertices of the AsO4 tetrahedron and the CaO7(OH) square antiprism to form a three-dimensional network. The arrangement of these linear CuO4(OH)(2) chains was very similar to that of CuO6 chains in CuGeO3, a spin-Peierls material. Comparisons with several isomorphous minerals revealed that the Jahn-Teller distortion effect caused by Cu atoms differed significantly between the minerals in the Ca series and those in the Pb series. Because of hydrogen bonding, the Cu-O(5) and Cu-O(5)* distances tended to become shorter than the other Cu-O distances in all isomorphous minerals.