ENERGY BARRIERS TO VISCOUS-FLOW AND THE PREDICTION OF GLASS-TRANSITION TEMPERATURES OF MOLTEN SILICATES

Within the framework of the Adam-Gibbs (configurational entropy) theor y of viscosity, it is shown that for a given composition, the ratio of parameters B-e (a temperature independent constant) to S-c(T-g) (the configurational entropy at the glass transition) is proportional to th e height of the average potential energy barrier to viscous flow (Delt a mu) and the size of the rearranging domains at the glass transition [z(T-g)]. The ratio B-e/S-c (T-g) is evaluated for several silicate a nd aluminosilicate compositions of variable polymerization. It is foun d that the ratio B-e/S-c(T-g) shows simple compositional variations th at correspond closely to those that may be expected qualitatively for the height of the potential energy barrier to viscous flow. Assuming t hat z(T-g) is a constant for all compositions, the available data for B-e/S-c(T-g) are parameterized as a function of Delta mu. The physica l basis of this parameterization will therefore allow extension to mor e complex systems as additional data become available. The A(e) term i n the Adam-Gibbs equation (another temperature independent constant) s hows only minor compositional variation (-2.6 +/- 1), but the variatio n that does exist is found to be a linear function of B-e/tetrahedron. The proposed parameterizations of B-e/S-c(T-g) and A(e) are shown to be sufficient for estimating the glass transition temperature to withi n 15-20 K. Calculated glass transition temperatures may be combined wi th existing models for viscosities in the range 10-10(5) Pa.s. Interpo lation provides the whole viscosity curve and thus also an estimate of the departure from Arrhenian behavior. Although further work is neces sary to verify and extend the parameterizations to compositions of dir ect geological relevance, this work represents a step toward a fully g eneralizable predictive model of silicate melt viscosity based within a physical framework.