Cobaltaustinite, CaCo(AsO₄)(OH)

This study presents the first structural report of a natural cobaltaustinite sample (calcium cobalt arsenate hydroxide) based on single-crystal X-ray diffraction data. Cobaltaustinite, with the ideal formula CaCo(AsO₄)(OH), belongs to the adelite mineral group. The CoO ₆ octahedra share edges to form chains extending parallel to the c axis, which are cross-linked by Ca²⁺ ions and by sharing vertices with isolated AsO₄ tetrahedra. The Ca²⁺ ions are situated in square antiprisms formed by eight O atoms. The major structural difference between the five calcium arsenates in the adelite group is shown in the bonding environments around the octahedrally coordinated M²⁺ cations (M = Cu, Zn, Co, Ni and Mg), with the average M-O distance decreasing from Cu-O in conichalcite, CaCu(AsO₄)(OH) to Zn-O in austinite, CaZn(AsO ₄)(OH), Co-O in cobaltaustinite, Ni-O in nickelaustinite, CaNi(AsO₄)(OH), and Mg-O in adelite, CaMg(AsO₄)(OH). The donor-acceptor O-H⋯O distance [2.721 (7) Å] in cobaltaustinite is similar to those in austinite and nickelaustinite, but different from those in adelite and conichalcite.