Effects of High Temperature on Silicate Liquid Structure: A Multinuclear NMR Study

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Abstract
The structure of a silicate liquid changes with temperature, and this substantially affects its thermodynamic and transport properties. Models used by geochemists, geophysicists, and glass scientists need to include such effects. In situ, high-temperature nuclear magnetic resonance (NMR) spectroscopy on 23 Na, 27 Al, and 29 Si was used to help determine the time-averaged structure of a series of alkali aluminosilicate liquids at temperatures to 1320°C. Isotropic chemical shifts for 29 Si increase (to higher frequencies) with increasing temperature, probably in response to intermediate-range structural changes such as the expansion of bonds between nonbridging oxygens and alkali cations. In contrast, isotropic chemical shifts for 27 Al decrease with increasing temperature, indicating that more significant short-range structural changes take place for aluminum, such as an increase in mean coordination number. The spectrum of a sodium aluminosilicate glass clearly indicates that at least a few percent of six-coordinated aluminum was present in the liquid at high temperature.