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) Å, β 100.313(1)°, V 939.6(1) Å3, 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 [IFol > 4σF] unique reflections measured with MoKα X-radiation on a Bmker P4 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 <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(l) 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(l) 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 ah [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.
