Robertsite, (Ca2Mn3O2)-O-III(PO4)(3)center dot 3H(2)O

Robertsite, ideally Ca2Mn3O2(PO4)(3)center dot 3H2O [calcium manganese((III)) tris(orthophosphate) trihydrate], can be associated with the arseniosiderite structural group characterized by the general formula Ca(2)A(3)O(2)(TO4)(3)center dot nH(2)O, with A = Fe, Mn; T = As, P; and n = 2 or 3. In this study, single-crystal X-ray diffraction data were used to determine the robertsite structure from a twinned crystal from the type locality, the Tip Top mine, Custer County, South Dakota, USA, and to refine anisotropic displacement parameters for all atoms. The general structural feature of robertsite resembles that of the other two members of the arseniosiderite group, the structures of which have previously been reported. It is characterized by sheets of [MnO6] octahedra in the form of nine-membered pseudo-trigonal rings. Located at the center of each ninemembered ring is a PO4 tetrahedron, and the other eight PO4 tetrahedra sandwich the Mn-oxide sheets. The six different Ca2+ ions are seven-coordinated in form of distorted pentagonal bipyramids, [CaO5(H2O)(2)], if Ca-O distances less than 2.85 angstrom are considered. Along with hydrogen bonding involving the water molecules, they hold the manganese-phosphate sheets together. All nine [MnO6] octahedra are distorted by the Jahn-Teller effect.