Previous page     Contents     Next page

Isobaric heat capacities (C_p) of 28 haplogranite glasses have been determined at 1 bar from room temperature through the glass transition and into the supercooled liquid. The investigated compositions are based on the 2 kbar pH₂O minimum melt composition in the system NaAlSi₃O₈-KAlSi₃O₈-SiO₂ (HPG8) to which alkali and alkaline-earth oxides (Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, MgO, CaO, SrO, BaO) have been added individually. Measured values of the glassy heat capacities are in excellent agreement with the predictions of the ideal mixing model of Richet (1987). Furthermore, the onset of the glass transition (rapid increase in C_p as a function of temperature) always occurs when the heat capacity of the glass is close to the Dulong and Petit limit of 3R J/atom K. However, liquid heat capacity data cannot be adequately described by ideal mixing of partial molar heat capacities of oxide components. To some extent this may be explained by the fact that absolute variations in measured liquid C_p as a function of composition are on the same order of magnitude as the accuracy of the liquid measurements (±3%). On the other hand, configurational heat capacities (defined as the difference between the liquid and glassy heat capacity at the glass transition temperature) have an uncertainty close to the precision of the measurements (<0.5%) and show a well-defined, but non-linear variation as a function of added cation. It is suggested that the observed non-linearities may be linked to some feature of the medium-range order of the liquids such as Al-Si order-disorder and/or the presence of triclusters. Using previously determined liquid expansivities and calculated values of ultrasonic wave velocities it is estimated that C_v (constant-volume heat capacity) represents between 90 and 99.5% of measured C_p values, and the difference between constant pressure and constant volume heat capacities is greatest for the most depolymerised compositions.

The heat capacity data obtained in this study have been combined with viscosity data for these samples in order to extract parameters of the Adam-Gibbs equation which links shear viscosity and configurational entropy. Calculated values of the configurational entropy of glasses show that the addition of alkali and alkaline-earth oxides to the stoichiometric tectosilicate base melt results in an increase in the configurational entropy. This increase becomes greater in the order Li₂O < Na₂O < K₂O < Rb₂O < Cs₂O per mole of added alkali oxide, whereas the increase per mole of added alkaline-earth oxide is similar to that caused by Li₂O. These variations are interpreted to reflect the varying extent to which the added alkali and alkaline-earth oxides may exchange with Na and K present in network-stabilising, charge-balancing roles for aluminate tetrahedra in the haplogranitic base.