Entropy of calcium and magnesium aluminosilicate glasses

The heat capacity of a magnesium and two calcium aluminosilicate glasses has been measured adiabatically at low temperatures. The residual entropy of quenched Mg2Al4Si5O18 glass is 94 ± 13 J mol−1 K−1. Above ∼ 150 K, available data show that Cp is an additive function of composition to within ±0.5% throughout the investigated glass-forming part of the system CaOMgOAl2O3SiO2. The breakdown of these linear variations observed at the lowest temperatures does not affect significantly the additive nature of room-temperature vibrational entropies. Excess heat capacities with respect to Debyeʹs model are not a special feature of glasses. Also observed for crystals, they are the strongest for framework silicate minerals and determine the entropy difference between silicate glasses and crystals. Comparisons between entropy data for glasses and crystals confirm that Ca and Mg depart little from octahedral coordination in alkaline-earth aluminosilicate glasses.