VISCOSITY, FRAGILITY, AND CONFIGURATIONAL ENTROPY OF MELTS ALONG THE JOIN SIO2-NAALSIO4

Viscosities of fourteen melts close to the join SiO2-NaAlO2 were measu red in the range 1-10(12) Pa.s (700-1650 degrees C) using a combinatio n of concentric cylinder and micropenetration techniques. These compos itions cover five isopleths in silica content from 50 to 82 mol% and v ary from mildly peralkaline to mildly peraluminous. Greatly improved c onstraints on the temperature dependence of viscosity in the system Si O2-NaAlO2 result because exactly the same compositions were used for b oth high-and low-temperature measurements, viscosities over an extende d range of silica contents were measured at temperatures close the gla ss transition, and several compositions at constant silica content and variable alkali/Al ratio were measured, allowing interpolation of dat a to compositions exactly along the join SiO2-NaAlO2. At high temperat ure (1600 degrees C) viscosity and activation energy are shown to be a pproximately a linear function of silica content, but large nonlineari ties occur at temperatures close to the glass transition range. Defini ng fragility as the gradient of the viscosity curve at the glass trans ition temperature (T-g taken to be the 10(12) Pa.s isokom) on a reduce d temperature scale (T-g/T), it is found that the fragility increases in a nonlinear fashion as NaAlO2 is substituted for SiO,, with fragili ty increasing more rapidly at lower SiO, contents. The viscosity data are combined with heat capacity data available in the literature to es timate configurational entropies of albite, jadeite, and nepheline gla sses using the Adam-Gibbs theory. Fragility, when defined in terms of the Adam-Gibbs parameters, is shown to increase with configurational h eat capacity (difference in heat capacity between the liquid and the g lassy states) but to decrease with increasing configurational entropy at the glass transition. In the light of independent phase equilibria and spectroscopic and calorimetric evidence that suggests the Al-Si or dering increases as silica content decreases from SiO2 to nepheline, t he modeling of configurational entropy in terms of Al-Si mixing sugges ts the following: (1) Melt configurational entropy has contributions f rom both cation mixing (chemical contribution), as well as variations in the topology of the O network (topological contribution), of which the latter dominates. (2) The chemical contribution is due to mixing o f tetrahedral rather than O sites. (3) At the glass transition (10(12) Pas isokom) the topological contribution shows little, if any, variat ion.