Acta Crystallographica Section B

Structural Science

Volume 53, Part 2 (April 1997)

research papers

Acta Cryst. (1997). B53, 203-220 [ doi:10.1107/S0108768196012335 ]

A Modulation Wave Approach to the Structural Characterization of Three New Cristobalite-Related Sodium Magnesiosilicates

R. L. Withers, C. Lobo, J. G. Thompson, S. Schmid and R. Stranger

Abstract: The crystal structures of three new cristobalite-related sodium magnesiosilicates [Na₂MgSiO₄, M_r = 162.37, orthorhombic, Pna2₁, a = 10.835 (5), b = 5.279 (12), c = 7.067 (8) Å, D_x = 2.668 g cm^-3, Z = 4, Cu K [alpha] , [lambda] = 1.5418 Å, [mu] = 75.96 cm^-l, F(000) = 319.87; Na_1.74Mg_0.79Al_0.15Si_1.06O₄, M_r = 157.02, orthorhombic, Pbca, a = 10.487 (7), b = 14.351 (4), c = 5.243 (6) Å, D_x = 2.643 g cm^-3, Z = 8, Cu K [alpha] , = 1.5418 Å, [mu] = 76.70 cm^-l, F(000) = 619.04; Na_1.8Mg_0.9Si_1.1O₄, M_r = 158.15, tetragonal, P4₁2₁2, a = 5.330 (6), c = 7.086 (5) Å, D_x = 2.609 g cm^-3, Z = 2, Cu K [alpha] , [lambda] = 1.5418 Å, [mu] = 75.44 cm^-l, F(000) = 155.94] are determined by Rietveld refinement from X-ray powder diffraction data. Plausible starting models were derived from a modulation wave approach based on the ideal C9 structure type and assuming regular SiO4 and MgO₄ tetrahedra.