Nonlinear composition dependence of molar volume of melts in the CaOAl₂O₃₃SiO₂ system

Density measurements have been performed on nine liquids in the system CaOAl₂O₃SiO₂, in the temperature range of the melting point up to 1800°C, using both Pt- and Ir-based double-bob Archimedean method. In addition, the molar volume has been determined at 1800°C for one liquid along the SiO₂Al₂O₃ join. The data of this study show that, within the temperature and the composition range investigated, the molar volume of Ca-aluminosilicate melts at low SiO₂ content does not behave linearly as function of the composition. This nonideality results clearly from interaction between SiO₂ and CaO, whereas our data in addition with literature data suggest that SiO₂Al₂O₃ and CaOAl₂O₃ interactions could also occur in these Ca-aluminosilicate melts at high Al₂O₃ content. These different results were analysed using a regression equation from which the partial molar volume of each component was obtained by the method of least squares. A nonideal model (SC) for Ca-aluminosilicate melts is thus proposed, involving an excess volume term between SiO₂ and CaO and leading the calculation of the liquids molar volume within 0.5% of uncertainties except for Ca- and Al-rich compositions. A better constrain of this model in this extreme composition range requires new data of pure alumina liquid. Some structural implications of this nonideal behaviour with respect to the molar volume are also discussed.