A NOVEL [SI18O45]18- SHEET IN THE CRYSTAL STRUCTURE OF ZERAVSHANITE, CS4 NA2 ZR3 [SI18O45] (H2O )2

Abstract

The crystal structure of zeravshanite, ideally Cs4 Na2Zr3 (Si18O45 (H2O)2, monoclinic, a 26.3511(8), b 7.5464(3), c 22.9769(8) Å, β 107.237(1)°, V 4363.9(4) Å3, space group C2/c, Z=4, D(calc.) = 3.170 g.cm-3, was solved by direct methods and refined to an R1 index of 2.8% based on 4508 observed [F0 > 4σF] unique reflections measured with MoKα X-radiation on a Bruker P4 diffractometer equipped with it CCD detector. Electron-microprobe analysis gave the composition (Cs3.80 Na0.18 K0.02)∑4.00 Na2.00 (Zr2.76 Ti0.19 Sn0.05 Fe3+0.05)∑3.05 [Si18O45] (H2O)2 based on 47 O apfu. In the crystal structure, there are nine tetrahedrally coordinated Si sites with = 1.614 Å. There are two [6]-coordinated M sites, mainly occupied by Zr (with minor Ti, Fe3+ and Sn4+), with = 2.067 Å. There is one [5]-coordinated Na site with <Na-O, H2O> = 2.400 Å. There are two A sites primarily occupied by Cs (with minor Na and K), a [12]-coordinated A(1) site and an [11]-coordinated A(2) site with <A(1)-O, H2O> = 3.369 and <A(2)-O> = 3.396 Å. In the crystal structure of zeravshanite, (SiO4) tetrahedra link together to form an [Si18O45]19- sheet parallel to (101), with five- and eight-membered silicate rings. The topology of the sheet was not previously known for any silicate mineral. It can be described as a linkage of wollastonite-like [Si3O9]6- chains. The silicate sheets, (MO6) and (NaO0) polyhedra share common vertices to form a mixed [Na2 [Zr3 (Si18O45)] (H2O )2]- framework with cages that occlude A atoms. The (NaO5) square pyramids share common vertices to form [NaO4] zig-zag chains along [010]. Every square pyramid shares one edge with an M(2) octahedron, and M(2) octahedra decorate the [NaO4] chain in a cis fashion. One (H2O) site that is fully occupied by (H2O) is a ligand of Na and A(1), with one strong (1.94 Å) and one very weak birfurcated hydrogen bond (∼ 3.10 Å) to oxygen atoms of the framework.