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

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 Kα, λ = 1.5418 Å, μ = 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 Kα, λ = 1.5418 Å, μ = 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 Kα, λ= 1.5418 Å, μ = 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.

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