CRISTOBALITE-RELATED PHASES IN THE NAALO2-NAALSIO4 SYSTEM - II - A COMMENSURATELY MODULATED CUBIC STRUCTURE

The crystal structure of a new cubic cristobalite-related sodium alumi nosilicate Na1.45Al1.45Si0.55O4 [P2(1)3, a = 14.553 (1) Angstrom] has been modelled using a modulation wave approach and the model tested ag ainst X-ray powder diffraction data using the Rietveld method. Owing t o there being 64 independent positional parameters and eight independe nt Na sites, refinement of the tetrahedral framework atom positions an d Na occupancies was not possible. The framework was modelled successf ully in terms of q(1) = 1/4[020](p)-type (p = parent) modulation wave s with the requirement that the MO4 (M = Al0.725Si0.275) tetrahedra be as close to regular as possible. Na/vacancy ordering was modelled suc cessfully in terms of q(2) = 1/4[220](p) modulation waves. Only the N a-atom positions were refined. The significance of this unique modulat ed cubic cristobalite-related structure and the possible insight it pr ovides to understanding beta-cristobalite are discussed.