HEAT-CAPACITY OF WADEITE-TYPE K2SI4O9 AND THE PRESSURE-INDUCED STABLEDECOMPOSITION OF K-FELDSPAR

The heat capacity of synthetic, stoichiometric wadeite-type K2Si4O9 ha s been measured by DSC in the 195 less than or equal to(K)less than or equal to 598 range. Near the upper temperature limit of our data, the heat capacity observed by DSC agrees with that reported by Geisinger et al. (1987) based on a vibrational model of their infrared and Raman spectroscopic data. However, with decreasing temperature, the Cp obse rved by DSC is progressively higher than that predicted from the vibra tional model, suggesting that the standard entropy of K2Si4O9 is likel y to be larger than 198.9 +/- 4.0 J/K.mol computed from the spectrosco pic data. A fit to the DSC data gave: Cp(T) = 499.13 (+/-1.87) - 4.350 14.10(3)(+/-3.489.10(1)).T-0.5, with T in K and average absolute perce nt deviation of 0.37%. The room-temperature compressibilities of kalsi lite and leucite, hitherto unknown, have been measured as well. The da ta, fitted to the Murnaghan equation of state, gave K-o = 58.6 GPa, K- o' = 0.1 for kalsilite and K-o = 45 GPa, K-o' = 5.7 for alpha-leucite. Apart from the above mentioned data on the properties of the individu al phases, we have also obtained reaction-reversals on four equilibria in the system K2O-Al2O3-SiO2. The Bayesian method has been used simul taneously to process the properties of 13 phases and 15 reactions betw een them to derive an internally consistent thermodynamic dataset for the K2O-Al2O3-SiO2 ternary. The enthalpy of formation of K2Si4O9 wadei te is in perfect agreement with its revised calorimetric value, the st andard entropy is 232.1 +/- 10.4 J/K mol, similar to 15% higher than t hat implied by vibrational modeling. The phase diagram, generated from our internally consistent thermodynamic dataset, shows that for all p robable P-T trajectories in the subduction regime, the stable pressure -induced decomposition of K-feldspar will produce coesite + kalsilite rather than coesite + kyanite + K2Si4O9 (cf Urakawa et al. 1994).