THE CRYSTAL STRUCTURE OF SCOMIOKITE-(Y), Na3Y(CO3)33H2O. Pushcharovsky, Dmitry Yu.1, Rastsvetneva, Ramisa K2, Pekov, I.V.1, 1Department of Geology, Moscow State University, 119899 Moscow, Russia; 2Institute of Crystallography RAN, 117333 Moscow, Russia
Among 11 carbonate minerals, which contain Y atoms in the individual positions, schomiokite-(Y), Na3Y(CO3)33H2O, is chemically related to orthorhombic tengerite Y2(CO3)32-3H2O, kimuraite CaY2(CO3)46H2O, lokkaite CaY4(CO3)79H2O and tetragonal kamphaugite-(Y) CaY(CO3)2(OH)H2O. The aim of the present study is to determine the position of schomiokite inside of this crystal chemical family.
The parameters of the orthorhombic cel1 are: a=17.358, b=10.034, c=5.946Å, sp. gr. Pna21. The structure was refined up to Rhkl 0.059 for 1973 observed reflections. The structure contains isolated mixed complexes, formed by YO9 polyhedra, which share their three edges with CO3 groups. YO9 polyhedra can be considered as 3-caped pseudotrigonal antiprisms, where one base is formed by three H2O molecules. Three CO3 groups are characterized by pyramidal distortion with the deviations of C atoms from the planes, formed by O atoms, 0.006, 0.003 and 0.01Å respectively.
The mixed complexes are considered as relicts of the corrugated polyhedral layers, described in tengerite, kimuraite and lokkaite, and are connected by Na octahedra and 5-fold polyhedra as well as by a system of H-bonds. The interpretation of the structure in terms of the interrupted layers is confirmed by the values of its lattice parameters ~6 and 10Å, which are parallel to the plane of the sheet and are close to the corresponding values of tengerite, kimuraite and lokkaite.