Robertsite, ideally Ca2Mn3O2(PO 4)3·3H2O [calcium manganese(III) tris(orthophosphate) trihydrate], can be associated with the arseniosiderite structural group characterized by the general formula Ca2A 3O2(TO4)3·H2O, 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 PO 4 tetrahedra sandwich the Mn-oxide sheets. The six different Ca 2+ ions are seven-coordinated in form of distorted pentagonal bipyramids, [CaO5(H2O)2], if Ca-O distances less than 2.85 A 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.
Andrade, MB; Morrison, SM; Di Domizio, AJ; Feinglos, MN; Downs, RT
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