EFFECTS OF HIGH-TEMPERATURE ON SILICATE LIQUID STRUCTURE - A MULTINUCLEAR NMR-STUDY

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 Na-23, Al-27, and Si-29 was used to help determine the time-averaged structure of a series of alkali aluminosilicate liquids at temperatures to 1320-degrees-C. Isotropic chemical shifts for Si-29 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 Al-27 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.