Robertsite, Ca₂Mn^III₃O₂(PO₄)₃·3H₂O

Robertsite, ideally Ca₂Mn₃O₂(PO₄)₃·3H₂O [calcium manganese(III) tris­(orthophosphate) trihydrate], can be associated with the arseniosiderite structural group characterized by the general formula Ca₂A₃O₂(TO₄)₃·nH₂O, 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 nine-membered 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 Å 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.

Redetermination of durangite, NaAl(AsO₄)F

The crystal structure of durangite, ideally NaAl(AsO₄)F (chemical name sodium aluminium arsenate fluoride), has been determined previously [Kokkoros (1938). Z. Kristallogr. 99, 38-49] using Weissenberg film data without reporting displacement parameters of atoms or a reliability factor. This study reports the redetermination of the structure of durangite using single-crystal X-ray diffraction data from a natural sample with composition (Na_0.95Li_0.05)(Al_0.91Fe³⁺_0.07Mn³⁺_0.02)(AsO₄)(F_0.73(OH)_0.27) from the type locality, the Barranca mine, Coneto de Comonfort, Durango, Mexico. Durangite is isostructural with minerals of the titanite group in the space group C2/c. Its structure is characterized by kinked chains of corner-sharing AlO₄F₂ octa­hedra parallel to the c axis. These chains are cross-linked by isolated AsO₄ tetra­hedra, forming a three-dimensional framework. The Na⁺ cation (site symmetry 2) occupies the inter­stitial sites and is coordinated by one F^- and six O^2- anions. The AlO₄F₂ octa­hedron has symmetry -1; it is flattened, with the Al-F bond length [1.8457 (4) Å] shorter than the Al-O bond lengths [1.8913 (8) and 1.9002 (9) Å]. Examination of the Raman spectra for arsenate minerals in the titanite group reveals that the position of the band originating from the As-O symmetric stretching vibrations shifts to lower wavenumbers from durangite, maxwellite [ideally NaFe(AsO₄)F], to tilasite [CaMg(AsO₄)F].