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

Abstract

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) A, β 100.313(1)°, V 939.6(1) A3, space group C 2/ c , Z = 4, D (calc.) = 3.008 g.cm−3, was refined to an R 1 index of 1.9% based on 1352 observed [| F o| > 4σ F ] unique reflections measured with Mo K α X-radiation on a Bruker P 4 diffractometer equipped with a 4K CCD detector. An electron-microprobe analysis gave the composition Na1.98 Zr1.00 (Si4O11) based on 11 O apfu . In the crystal structure, there are two tetrahedrally coordinated Si sites with a grand distance of 1.612 A, and one octahedrally coordinated Zr site with = 2.085 A. There are two Na sites: the Na (1) site is [7]-coordinated with = 2.620 A. The Na (2) site is split into two subsites, Na (2A) and Na (2B), with a separation of 0.829 A, and the symmetry-related Na (2B) sites are separated by 1.525 A. 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.