Structure-composition relations and Raman spectroscopy of high-pressure sodium silicates |
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| Authors: | Michael E Fleet Grant S Henderson |
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| Institution: | Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada, CA
Department of Geology, University of Toronto, Toronto,Ontario M5S 3B1, Canada, CA
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| Abstract: | ?-Na2Si2O5, ζ-Na2Si2O5, Na2SiSi2O7], and Na6Si3Si9O27] have been synthesized using an MA6/8 superpress. Densification in high-pressure sodium silicates is effected largely by changes in packing. In the relaxed (1 bar) structures, cation polyhedra and thermal/displacement parameters are similar to those of low-pressure silicates, but the extra-framework cation positions are oversized. The two mixed 4]Si and 6]Si framework silicates of known structure (Na2SiSi2O7] and Na6Si3Si9O27]) belong to the limited homologous series Na2mSimSin–mO2n+m], with m<n. The structure-composition relationships of wadeite-type, A2Ge4O9-type, and Na6Si3Si9O27] silicates and germanates depend on T-O distance and size of the large extra-framework cation. Characteristic features of the SiO4 tetrahedral units are present in micro-Raman spectra of mixed 4]Si and 6]Si framework silicates, but bands uniquely attributable to SiO6 octahedra are weak or obscured. However, 6]Si has a profound indirect influence on the Raman spectra, resulting in intense and complex low-frequency bands, assigned to symmetric bending modes with coupled displacements at both bridging oxygens and nonbridging oxygens bonded to 6]Si, and a shift to higher frequency and reduction in intensity of the high-frequency bands assigned to symmetric 4]Si-Onbr stretching vibrations. Raman spectroscopy does not appear to be a useful structural probe for small amounts of 6]Si in silicate glasses and melts. |
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