Enthalpies of mixing for disordered alkali feldspars at high temperature: a test of regular solution thermodynamic models and a comparison of hydrofluoric acid and lead borate solution calorimetric techniques

Enthalpies of solution have been measured for an 11-member analbite-sanidine series at 704 °C in molten lead borate. Enthalpies of K-Na mixing determined from the data are statistically equivalent to those determined by HF solution calorimetry at 50 °C (Hovis, 1988). This indicates that regular solution thermodynamic models, which assume enthalpies of mixing to be independent of temperature, are valid for the disordered alkali feldspars. The experiments also demonstrate the consistency of the HF and lead borate solution calorimetric techniques. Volumes of K-Na mixing at room temperature also are reported.