SYNCHROTRON X-RAY DIFFRACTION STUDY OF THE STRUCTURE OF SHAFRANOVSKITE, K2NA3(MN,FE,NA)4[SI9(O,OH) 27](OH)2·NH2O, A RARE MANGANESE PHYLLOSILICATE FROM THE KOLA PENINSULA, RUSSIA

Abstract

The structure of shafranovskite, ideally K2Na3(Mn,Fe,Na)4[Si9(O,OH) 27](OH)2·nH2O (n ∼ 2.33), a K-Na-manganese hydrous silicate from Kola peninsula, Russia, was studied using synchrotron X-ray radiation and a MAR345 image-plate detector at the Swiss-Norwegian beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The structure [trigonal, space group P31c, a = 14.519(3), c = 21.062(6) Å, V = 3844.9(14) Å3] was solved by direct methods and partially refined to R1 = 0.085 (wR2 = 0.238) on the basis of 2243 unique observed reflections ( Fo ≥4σF). Shafranovskite is a 2:1 hydrous phyllosilicate. Sheets of Mn and Na octahedra (O sheets) are sandwiched between two silicate tetrahedral sheets (T1 and T2). The 2:1 layers are parallel to (001). The upper tetrahedral sheet T1 consists of isolated [Si13(O,OH)37] islands composed of three six-membered rings. The octahedral sheet O consists of Mnφ 6, Na1φ6, and Na2φ6 octahedra (φ = O, OH, H2O). This unit can be considered as a trioctahedral sheet with each 20th octahedron vacant. The lower tetrahedral sheet T2 consists of [Si13(O,OH) 37] islands linked into a sheet through an additional SiO3OH tetrahedron. The Na3, K1, K2 atoms, and H 2O32 groups are between the 2:1 layers and provide their linkage along c.