Changes of structure and physical properties of silicate melts with pressure

Abstract

The structures of quenched melts in the system CaO-MgO-SiO₂ have been studied using the Raman spectroscopic technique. Quenched melts along the joins Ca₂SiO₄-SiO₂and Ca MgSiO₄-SiO₂consist principally of three different structural units; with increasing SiO₂ along these joins the structural units change from monomer (SiO₄^4-) +dimer (Si₂ O₇^6-) +chain (SiO₃^2-) through monomer+chain+sheet (Si₂O₅^2-) to chain+sheet + 3-dimentional network (SiO₂) . The ratios of these structural units change little with pressure in the compositional ranges studied. However, the structure of 3-dimentional network appears to change with pressure. The viscosity of melts enriched in the 3-dimentional network decreases with increasing pressure at constant temperature. Particularly NaAlSi₂O₆ (jadeite) melt shows a large decrease in viscosity. Glass quenched from the same melt shows an increase of density, a shift of AlKα radiation, and changes of both infrared absorption and Raman spectra with increasing preassure of quenching. These changes are due to the structural changes of melts probably caused by decrease in size of rings consisting of (Si, Al) O₄tetrahedra. At higher pressures (>50 kbar) Al in the melts may shift from 4-fold to 6-fold coordination, resulting in further decrease in viscosity and increase in density.