STRUCTURE-COMPOSITION RELATIONS AND RAMAN-SPECTROSCOPY OF HIGH-PRESSURE SODIUM SILICATES

epsilon-Na2Si2O5, zeta-Na2Si2O5, Na2Si[Si2O7], and Na6Si3[Si9O27] have been synthesized using an MA6/8 superpress. Densification in high-pre ssure sodium silicates is effected largely by changes in packing. In t he relaxed(1 bar) structures, cation polyhedra and thermal/displacemen t parameters are similar to those of low-pressure silicates, but the e xtra-framework cation positions are oversized. The two mixed Si-[4] an d Si-[6] framework silicates of known structure (Na2Si[Si2O7] and Na6S i3[Si9O27]) belong to the limited homologous series Na2mSim[Sin-mO2n+m ], with m<n. The structure-composition relationships of wadeite-type, A(2)Ge(4)O(9)-type, and Na6Si3[Si9O27] silicates and germanates depend on T-O distance and size of the large extra-framework cation. Charact eristic features of the SiO4 tetrahedral units are present in micro-Ra man spectra of mixed Si-[4] and Si-[6] framework silicates, but bands uniquely attributable to SiO6 octahedra are weak or obscured. However, Si-[6] has a profound indirect influence on the Raman spectra, result ing in intense and complex low-frequency bands, assigned to symmetric bending modes with coupled displacements at both bridging oxygens and nonbridging oxygens bonded to Si-[6], and a shift to higher frequency and reduction in intensity of the high-frequency bands assigned to sym metric Si-[4]-O-nbr stretching vibrations. Raman spectroscopy does not appear to be a useful structural probe for small amounts of Si-[6] in silicate glasses and melts.