The crystal structures of conichalcite [Ca(Cu,Mg)(AsO
4)(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
12
12
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 AsO
4 tetrahedron, a CuO
4(OH)
2 octahedron, and a CaO
7(OH) square antiprism. The CuO
4(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 CuO
4(OH)
2 octahedron shared its edges to form linear chains, which were further linked by the vertices of the AsO
4 tetrahedron and the CaO
7(OH) square antiprism to form a three-dimensional network. The arrangement of these linear CuO
4(OH)
2 chains was very similar to that of CuO
6 chains in CuGeO
3, 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.
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