Silicon coordination and speciation changes in a silicate liquid at high pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with ²⁹Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na₂Si₂O₅ glass quenched from 8 gigapascals and 1500°C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.