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Crystal structure of rimkorolgite, Ba[Mg₅(H₂O)₇(PO₄)₄](H₂O), and its comparison with bakhchisaraitsevite

¹ Department of Crystallography, Faculty of Geology, St. Petersburg State University, University Emb. 7/9, St. Petersburg 199034, Russia
² Department of Mineral Deposits, Faculty of Geology, St. Petersburg State University, University Emb. 7/9, St. Petersburg 199034, Russia
³ Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame IN 46556–0767, USA
⁴ Geological Institute, Kola Science Centre, Russian Academy of Sciences, Fersmana 14, 184200-RU Apatity, Russia

^* corresponding author, e-mail: sergey{at}cryst.geol.pu.ru

The crystal structure of rimkorolgite, ideally Ba[Mg₅(H₂O)₇(PO₄)₄](H₂O), (monoclinic, P2₁/c, a = 8.3354(9), b = 12.8304(13), c = 18.313(2) Å, β = 90.025(2)°, V = 1958.5(4) ų, Z = 4) has been solved by direct methods and refined to R₁ = 0.052 using X-ray diffraction data collected from a crystal twinned on (001). There are five symmetrically independent Mg²⁺ cations that are each octahedrally coordinated by four O atoms and two H₂O groups. One symmetrically independent Ba²⁺ cation is coordinated by eight O atoms and two H₂O groups. The Mg₆ octahedra ( = O, H₂O) and PO₄ tetrahedra form sheets parallel to (001). Their main elements are zigzag chains of the Mg₆ edge-sharing octahedra. The chains are linked via common vertices to form an octahedral sheet in which Mg atoms are located at the vertices of the 6³ hexagonal net. The PO₄ tetrahedra are above and below hexagonal rings of Mg octahedra and are linked to them by sharing common O vertices. The Ba atoms and H₂O(1) and H₂O(22) groups are located between the sheets providing their linkage into three-dimensional structure. The structure of rimkorolgite is closely related to that of bakhchisaraitsevite, Na₂Mg₅(PO₄)₄7H₂O. Both structures are based on the octahedral-tetrahedral sheets of the same type. In bakhchisaraitsevite, the sheets are linked into three-dimensional framework by edge-sharing between the Mg₆ octahedra from two adjacent sheets, whereas in rimkorolgite, there is no linkage between adjacent sheets. The structure of rimkorolgite can be considered as bakhchisaraitsevite-like framework interrupted by the presence of large Ba²⁺ cations.

Key-words: rimkorolgite, crystal structure, bakhchisaraitsevite.

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