Entropy and structure of silicate glasses and melts

Low-temperature adiabatic C_p measurements have been made on NaAlSi₂O₆, MgSiO₃, Ca₃Al₂Si₃O₁₂ and Ca_1.5Mg_1.5Al₂Si₃O₁₂ glasses. Above about 50 K, these and previous data show that the heat capacity is an additive function of composition to within ±1% throughout the investigated glassforming part of the system CaO-MgO-Al₂O₃-SiO₂. In view of the determining role of oxygen coordination polyhedra on the low-temperature entropy, this is interpreted as indicating that Si and Al are tetrahedrally coordinated in all these glasses, in agreement with structural data; whereas Ca and Mg remain octahedrally coordinated. In contrast, heat capacities and entropies are not additive functions of composition for alkali aluminosilicates, indicating increases in the coordination numbers of alkali elements from about six to nine when alumina is introduced. A thermochemical consequence of additivity of vibrational entropies of glasses is that entropies of mixing are essentially configurational for calcium and magnesium aluminosilicate melts. For alkali-bearing liquids, it is probable that vibrational entropies contribute significantly to entropies of mixing. At very low temperatures, the additive nature of the heat capacity with composition is less well followed, likely as a result of specific differences in medium-range order.