Abstract

As a step toward structural characterization of magmatic liquids and to correlate their structure with melt properties, in-situ, high-temperature structural data have been obtained along the joins Na2Si3O7 - Na2(NaAl)3O7, and Na2Si4O9 - Na2(NaAl)4O9. A total of 211 data points containing anionic speciation information have been determined and combined with published structure data from other alkali aluminosilicate melt systems. Numerical description of the relationship between abundance of structural units, temperature, and bulk composition was derived by stepwise regression of expressions such as: XQi= a + b(NBO/T) + cT + d/T + e[Al/(Al+Si)] + f[Al/(Al+Si)]2.

The equilibrium constants, K, for the two relevant anionic equilibria, 2Q3¤Q2+Q4 (1), and 2Q2¤Q1+Q3 (2), were determined at temperatures above the glass transition. The K-values were fitted via stepwise regression to expressions of the form; ln K = a + b/T + c[Al/(Al+Si)] + d[Al/(Al+Si)]2 + e(NBO/T).

The enthalpy of reaction for the anionic equilibria was extracted from the linear relationships, ln K = a + b/T. The enthalpy values for reaction (1) are in the range -30-70 kJ/mol. The values are systematic functions of bulk composition generally increasing with increasing Al/(Al+Si) and with increasing ionization potential of the metal cation. The enthalpy is not sensitive to bulk melt polymerization.

The configurational heat capacity of individual anionic species were derived by combining structure data for metal oxide silicate melts with published thermodynamic data. The configurational entropy and heat capacity of other aluminosilicate melts were then estimated from those data combined with the data on abundance of structural units. The topological and mixing contributions to those parameters were also evaluated. The topological contribution to both heat capacity and entropy represents more than 90% of the total value.