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

The structure of shafranovskite, ideally K2Na3(Mn,Fe,Na)(4)[Si-9(O,OH)(27)](OH)(2).nH(2)O (n similar to 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) Angstrom, V = 3844.9(14) Angstrom(3)] was solved by direct methods and partially refined to R-1 = 0.085 (wR(2) = 0.238) on the basis of 2243 unique observed reflections (\F-o\ greater than or equal to 4sigma(F)). Shafranovskite is a 2:1 hydrous phyllosilicate. Sheets of Mn and Na octahedra (O sheets) are sandwiched between two silicate tetrahedral sheets (T-1 and T-2). The 2:1 layers are parallel to (001). The upper tetrahedral sheet T-1 consists of isolated [Si-13(O,OH)(37)] islands composed of three six-membered rings. The octahedral sheet O consists of Mnphi(6), Na1phi(6), and Na2phi(6) octahedra (phi = O OH, HO). This unit can be considered as a trioctahedral sheet with each 20(th) octahedron vacant. The lower tetrahedral sheet T, consists of [Si-13(O,OH)(37)] islands linked into a sheet through an additional SiO3OH tetrahedron. The Na3, K1, K2 atoms, and H(2)O32 groups are between the 2:1 layers and provide their linkage along c.