Acta Cryst. (2008). E64, i53-i54  [ doi:10.1107/S1600536808024173 ]

Redetermination of conichalcite, CaCu(AsO₄)(OH)

R. R. Henderson, H. Yang, R. T. Downs and R. A. Jenkins

Abstract: The crystal structure of conichalcite [calcium copper(II) arsenate(V) hydroxide], with ideal formula CaCu(AsO₄)(OH), was redetermined from a natural twinned specimen found in the Maria Catalina mine (Chile). In contrast to the previous refinement from photographic data [Qurashi & Barnes (1963). Can. Mineral. 7, 561-577], all atoms were refined with anisotropic displacement parameters and with the H atom located. Conichalcite belongs to the adelite mineral group. The Jahn-Teller-distorted [CuO₆] octahedra share edges, forming chains running parallel to [010]. These chains are cross-linked by eight-coordinate Ca atoms and by sharing vertices with isolated AsO₄ tetrahedra. Of five calcium arsenate minerals in the adelite group, the [MO₆] (M = Cu, Zn, Co, Ni and Mg) octahedron in conichalcite is the most distorted, and the donor-acceptor O-HO distance is the shortest.

Online 6 August 2008

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