Heat capacity of aluminum-free liquid silicates

Drop calorimetry measurements made between 900 and 1800 K are reported for six MO-SiO₂ liquids (M = Li₂, K₂, SrandBa) and two titanium alkalisilicate melts. These results, together with data from the literature, are used to derive a model for calculating the heat capacity of Al-free silicate melts as a function of temperature and chemical composition. Twenty-one major or minor oxides have been considered and, except for K₂O-bearing melts, the available data do not indicate deviations of the heat capacities from an additive function of composition. Simple energy calculations show that large variations of the temperature of the liquids result in structural changes of a magnitude similar to those of crystal-liquid transitions. It is suggested that network-modifier cations play an important role in changing the configuration of the liquid in response to temperature variations. The specificity of the behavior of the cations is shown by the lack of a simple relationship between the heat capacities of the liquids and characteristics of the alkali and alkaline-earth cations such as ionic potential or field strength.