Modelling Configurational Entropy of Silicate Melts

The Adam-Gibbs theory provides a robust connection between the transport or relaxation properties of melts and their thermochemical properties. In its expanded form: log η = A + B/(T [S_c (Tg) + 〖Cp〗_c ln (T⁄Tg)) the equation has adjustable unknown parameters A, B and Sc(Tg) which can be estimated from experimental estimates of configurational heat capacity (Cpc), glass transition temperature (Tg) and viscosity (). Here, we use recently published datasets for anhydrous and hydrous silicate melts and glasses (N~50) for which there are measurements of log and calorimetric measurements of Cpc and Tg. Our fitting strategy follows the approach developed and modified by previous workers with the sole exception that we assume all silicate melts converge to a common, but unknown, viscosity value at high temperature (e.g., A = log ∞). Our optimal value for A is -3.51 0.25. A consequence of a common, high-temperature limit to silicate melt viscosity is that the corresponding model values of glass transition temperature (Tg12), melt fragility (m), and the ratio Cpc /Sc are constrained to lie on a single plane approximated as: 〖Cp〗_c/S_c = -〖Tg〗^12/243399 - m/(15.518 )+0.996 . therebywhich establishinges a quantitative connection between calorimetric and rheological measurements. Lastly, we compare values of Tg12 and fragility (m) from this Adam-Gibbs based model of melt viscosity to corresponding values predicted by the GRD viscosity model for multicomponent silicate melts (cf. Giordano et al., 2008a).