High temperature heat contents and heat capacities of liquids and glasses in the system NaAlSi3O8-CaAl2Si2O8

Abstract

Enthalpies and heat capacities of glasses and of stable liquids in the system NaAlSi3O8-CaAl2Si2O8 have been measured by drop and differential scanning calorimetry. Within experimental error, values of C p and of H T 300 of three intermediate compositions fall on straight line interpolations between the end members for both liquids and glasses, indicating that excesses in true and in mean heat capacities [(H T −H 300)/(T−300)] are small or absent. A value for the heat capacity of the An100 liquid component can therefore be derived, and is probably a better estimate than that based on measurements on the pure substance alone. On the assumption of zero excess heat capacity in this system, heats of mixing in the stable liquids are equal to those measured in the glasses by solution calorimetry, and can be as negative as -2 kcal mol−1. Heat capacities of solids and glasses in the Ab-An system are similar and do not vary greatly with composition. The C P's of the liquids, however, increase strongly with An content, suggesting major structural changes take place across the binary.