The heat capacity of a magnesium and two calcium aluminosilicate glasses has been measured adiabatically at low temperatures. The residual entropy of quenched Mg 2 Al 4 Si 5 O 1 8 glass is 94 ± 13 J mol - 1 K - 1 . Above 150 K, available data show that C p is an additive function of composition to within ± 0.5% throughout the investigated glass-forming part of the system CaO-MgO-Al 2 O 3 -SiO 2 . 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 Debye's 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.