Tiettaite K4Na12Fe3+Si16O41(OH)4 ⋅ 2H2O: A Mineral with a Novel Type of Microporous Heteropolyhedral Framework

Crystallography Reports - Microporous silicate tiettaite from the Khibiny alkaline massif (Kola Peninsula, Russia) has been studied using X-ray diffraction analysis, electron probe microanalysis,...     

Ordering of cations in the voids of the anionic framework of the crystal structure of catapleiite

The pseudohexagonal crystal structure of the mineral catapleiite Na_1.5Ca_0.2[ZrSi₃(O,OH)₉] · 2(H₂O,F) from the Zhil’naya Valley in the central part of the Khibiny alkaline massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0346): a = 20.100(4), b = 25.673(5), and c = 14.822(3) Å; space group Fdd2, Z = 32, and ρ_calcd = 2.76 g/cm³. Fluorine atoms substituting part of H₂O molecules in open channels of the crystal structure have been found for the first time in the catapleiite composition by microprobe analysis. The pattern of distribution of Na and Ca atoms over the voids of the mixed anionic framework consisting of Zr-octahedra and three-membered rings of Si-tetrahedra accounts for the pronounced pseudoperiodicity along the a and c axes of the pseudohexagonal unit cell and for the lowering of crystal symmetry to the orthorhombic one. It is shown that part of the hydrogen atoms of water molecules is statistically disordered; their distribution correlates with the pattern of the population of large eight-vertex polyhedra by Na and Ca atoms.     

Crystal structure of [Rb0.24(H2O)0.76]VO(H2O)(PO4), a new monoclinic variety in the series of layered vanadyl phosphates

The crystal structure of a new monoclinic variety of hydrous rubidium vanadyl phosphate [Rb_0.24(H₂O)_0.76]VO(H₂O)(PO₄) doped with Al³⁺ ions is studied by X-ray (R = 0.054) diffraction: a = 6.2655(4) Å, b = 6.2712(3) Å, c = 6.8569(5) Å, β = 107.805(7)°, space group P2₁/m, Z = 2, and D _x = 2.792 g/cm³. The new phase obtained by the hydrothermal synthesis in the V₂O₅-Rb₂CO₃-AlPO₄-H₂O system has a layer-type structure in which Rb atoms and water molecules are located between layers of vertexsharing [VO₅(H₂O)] octahedra and [PO₄] tetrahedra. Rb intercalates based on VOPO₄ · 2H₂O are described by general formula [Rb _x (H₂O)_{1 ? x} ]V _1?x ^V V _x ^IV O(H₂O)(PO₄), where x ≤ 0.5, and the amount of reduced vanadium and interlayer water molecules is determined by the amount of introduced rubidium atoms.