Heat capacities of haplogranitic glasses and liquids

The heat capacities of 27 glasses have been determined from room temperatur e to temperatures corresponding to supercooled liquid behavior. The investi gated compositions are based on a haplogranite (near the 2-kbar pH(2)O mini mum 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 o f Li2O in silicate glasses is determined from our data. Extrapolated glassy and relaxed liquid enthalpies intersect at a temperature defined as the li miting fictive temperature (T-f'). At this temperature, the glassy heat cap acity of all the studied compositions is close to the theoretical upper lim it of 3R per gram-atom, where R is the gas constant. For samples cooled at 5 K/min, liquid viscosity of all samples is 10(12.56 +/-0.43) Pa s at T-f'. For other cooling rates, this result implies that log eta (T-f') = 11.5 - log Q, where eta (T-f') is the viscosity at the limiting fictive temperatur e and Q is the cooling rate (K/s). Liquid heat capacity is found to general ly increase with addition of all oxides, although the details of the variat ions are obscured by the fact that experimental uncertainties are of a simi lar magnitude to variations in heat capacity caused by compositional change . On the other hand, the "configurational heat capacity" (C-p(conf)(T-f')), defined as the difference between the fully relaxed liquid heat capacity a nd 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 oxid e less than 10 mol%, but increasing values for greater additions of oxides. By use of previously determined liquid expansivities, we calculate that vo lume changes account for similar to 15% of the configurational heat capacit y. 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 conf igurational contribution related to liquid structure, rather than trying to define a single partial molar heat capacity for each liquid oxide componen t. Copyright (C) 2001 Elsevier Science Ltd.