Temperature Independent Thermal Expansivities of Silicate Melts in the System Anorthite-Wollastonite-Gehlenite (CAS) system

Calcium aluminosilicate melts are, in addition to their model role in geochemistry, also important for both the glass and stone wool industry. Contributions to the PVT equation of state of such melts are needed for geochemical and geophysical modelling, as well as for providing tests of structure-property relationships for magma. The temperature-independent thermal expansivities of ten melts included in the anorthite-wollastonite-gehlenite (An-Wo-Geh) compatibility triangle were determined on glassy and liquid samples using a combination of calorimetry and dilatometry. The melts have either 0.5 or 1 non-bridging oxygens per tetrahedraly coordinated cations. The volumes at room temperature were derived from density measurements using the Archimedean buoyancy method. Each sample had a cooling-heating history of 10-10 K/min at 298K and a precise dimension of the samples allowed calculation of the density from the sample geometry at room temperature. The thermal expansion coefficient of the glass from 298K to the glass transition interval was measured by a dilatometer and the heat capacity was measured using a differential scanning calorimeter over a 298-1135K temperature range The thermal expansion coefficient and the heat flow was determined at a heating rate of 10 K/min on glasses which was previously cooled at 10 K/min. Supercooled liquid molar thermal expansivities were indirectly determined by combining differential scanning calorimetric and dilatometric measurements assuming that kinetics of enthalpy and shear relaxation are equivalent. This low-temperature combined determination of supercooled liquid density, molar volume and molar expansivities was tested against high-temperature data obtained by using the Lange-Carmichael (1987), Lange (1997) and Courtial-Dingwell (1999) model. The best linear fit provides a combination of data presented in this study and high temperature data calculated using the Courtial-Dingwell (CAS) model. This dilatometric/calorimetric method of liquid molar expansivity determination greatly increases the temperature range accessible for thermal expansion measurements. These results contrast strongly with those obtained for geologic multicomponent melts, as well as anorthite-diopside eutectic and diopside melts, which exhibit a clear temperature-dependence of expansivity. The temperature-dependence of the thermal expansivity of melts is herewith confirmed to be a sensitive function of composition. This leads us to speculate that its origins may indeed lie in the temperature-dependence of the coordination number of specific cations with temperature.