HEAT CAPACITIES OF HAPLOGRANITIC GLASSES AND LIQUIDS

Abstract

The heat capacities of 27 glasses have been determined from room temperature to temperatures corresponding to supercooled liquid behavior. The investigated compositions are based on a haplogranite (near the 2-kbar pH2O minimum melt composition in the system NaAlSi3O8-KAlSi3O8-SiO2) to which alkali and alkaline earth oxides (Li2O, Na2O, K2O, Rb2O, Cs2O, MgO, CaO, SrO, and BaO) have been added individually. Where comparison is possible, our data below the glass transition are consistent with predictive models previously proposed in the literature. In addition, the partial molar heat capacity of Li2O in silicate glasses is determined from our data. Extrapolated glassy and relaxed liquid enthalpies intersect at a temperature defined as the limiting fictive temperature (Tf'). At this temperature, the glassy heat capacity of all the studied compositions is close to the theoretical upper limit of 3R per gram-atom, where R is the gas constant. For samples cooled at 5 K/min, liquid viscosity of all samples is 1012.56+/-0.43 Pa s at Tf'. For other cooling rates, this result implies that log η(Tf') = 11.5 - log Q, where η(Tf') is the viscosity at the limiting fictive temperature and Q is the cooling rate (K/s). Liquid heat capacity is found to generally increase with addition of all oxides, although the details of the variations are obscured by the fact that experimental uncertainties are of a similar magnitude to variations in heat capacity caused by compositional change. On the other hand, the ''configurational heat capacity'' (Cpc°nf(Tf')), defined as the difference between the fully relaxed liquid heat capacity and the glassy heat capacity at the limiting fictive temperature, shows much less dispersion as a function of composition. Its variation is a nonlinear function of composition, with little, if any, change for additions of oxide less than 10 mol%, but increasing values for greater additions of oxides. By use of previously determined liquid expansivities, we calculate that volume changes account for ~15% of the configurational heat capacity. We conclude that liquid heat capacity should be considered as the sum of a vibrational contribution, of value close to 3R per gram-atom, and a configurational contribution related to liquid structure, rather than trying to define a single partial molar heat capacity for each liquid oxide component.