Silicon Coordination and Speciation Changes in a Silicate Liquid at High Pressures

Science  01 Sep 1989:
Vol. 245, Issue 4921, pp. 962-964
DOI: 10.1126/science.245.4921.962


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.