Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Résumé Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Uvarova, Y. A. Articles by Agakhanov, A. A. Search for Related Content GeoRef GeoRef Citation

A NOVEL [Si₁₈O₄₅]^18– SHEET IN THE CRYSTAL STRUCTURE OF ZERAVSHANITE, Cs₄ Na₂ Zr₃ [Si₁₈O₄₅] (H₂O)₂

Yulia A. Uvarova^1,, Elena Sokolova¹, Frank C. Hawthorne¹, Leonid A. Pautov² and Atali A. Agakhanov²

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Fersman Mineralogical Museum, Russian Academy of Sciences, RU–117071 Moscow, Russia

E-mail address: umuvarov{at}cc.umanitoba.ca

The crystal structure of zeravshanite, ideally Cs₄ Na₂ Zr₃ (Si₁₈O₄₅) (H₂O)₂, monoclinic, a 26.3511(8), b 7.5464(3), c 22.9769(8) Å, ß 107.237(1)°, V 4363.9(4) ų, 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 [F_o > 4F] unique reflections measured with MoK X-radiation on a Bruker P4 diffractometer equipped with a CCD detector. Electron-microprobe analysis gave the composition (Cs_3.80 Na_0.18 K_0.02)_4.00 Na_2.00 (Zr_2.76 Ti_0.19 Sn_0.05 Fe_0.05 ³⁺)_3.05 [Si₁₈O₄₅] (H₂O)₂ based on 47 O apfu. In the crystal structure, there are nine tetrahedrally coordinated Si sites with <Si–O> = 1.614 Å. There are two [6]-coordinated M sites, mainly occupied by Zr (with minor Ti, Fe³⁺ and Sn⁴⁺), with <M–O> = 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]^18– 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 (NaO_n) 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.

Keywords: zeravshanite, Cs–Zr silicate, crystal structure, [Si₁₈O₄₅] sheet, Dara-i-Pioz, Tajikistan.