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VLASOVITE, Na₂ Zr (Si₄O₁₁), FROM THE KIPAWA ALKALINE COMPLEX, QUEBEC, CANADA: CRYSTAL-STRUCTURE REFINEMENT AND INFRARED SPECTROSCOPY

Elena Sokolova^1,, Frank C. Hawthorne¹, Neil A. Ball¹, Roger H. Mitchell² and Giancarlo Della Ventura³

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Department of Geological Sciences, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada
³ Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy

E-mail address: elena_sokolova{at}umanitoba.ca

The crystal structure of vlasovite, Na₂ Zr (Si₄O₁₁), from the Kipawa alkaline complex, Quebec, Canada, monoclinic, a 11.0390(5), b 10.0980(3), c 8.5677(4) Å, ß 100.313(1)°, V 939.6(1) ų, space group C2/c, Z = 4, D (calc.) = 3.008 g.cm^–3, was refined to an R₁ index of 1.9% based on 1352 observed [|F_o| > 4F] unique reflections measured with MoK X-radiation on a Bruker P4 diffractometer equipped with a 4K CCD detector. An electron-microprobe analysis gave the composition Na_1.98 Zr_1.00 (Si₄O₁₁) based on 11 O apfu. In the crystal structure, there are two tetrahedrally coordinated Si sites with a grand <Si–O> distance of 1.612 Å, and one octahedrally coordinated Zr site with <Zr–O> = 2.085 Å. There are two Na sites: the Na(1) site is [7]-coordinated with <Na(1)–O> = 2.620 Å. The Na(2) site is split into two subsites, Na(2A) and Na(2B), with a separation of 0.829 Å, and the symmetry-related Na(2B) sites are separated by 1.525 Å. Na(2A) is octahedrally coordinated, and Na(2B) is [5]-coordinated, and their occupancies are 0.826(8) and 0.092(4) Na, respectively. Thus the Na(1) and aggregate Na(2) sites are each fully occupied. In the crystal structure of vlasovite, four-membered rings of (SiO₄) tetrahedra link together to form an [Si₄O₁₁]^6– chain. Chains of tetrahedra and Zr octahedra link together to form a heteropolyhedral framework with channels extending along [001]. Three channels contain the Na atoms, which are bonded to O atoms of the octahedron–tetrahedron framework. Vlasovite is not significantly damaged by exposure to X-rays. Infrared spectroscopy shows that vlasovite can be free of H₂O or can contain variable amounts of H₂O; single crystals can be anhydrous in some regions and hydrous in other regions. Moreover, the H₂O content and its spectral response are unaffected by exposure of crystals to X-rays.

Keywords: vlasovite, Na–Zr silicate, crystal-structure refinement, infrared spectroscopy, H₂O.