Enthalpies of solution in 2PbO · B2O3 at 981 K have been measured for glasses in the system albite-orthoclase-silica and along the join Na1.6Al1.6Si2.4O8-K1.6Al1.6Si2.4O8. The join KAlSi3O8-Si4O8 shows zero heat of mixing similar to that found previously for NaAlSi3O8-Si4O8 glasses. Albite-orthoclase glasses show negative heats of mixing symmetric about Ab50Or50 (Wn = - 2.4 ± 0.8 kcal). Negative heats of (Na, K) mixing are also found at Si (Si + Al) = 0.6. Ternary excess enthalpies of mixing in the glassy system Ab-Or-4Q are positive but rarely exceed 1 kcal mol-1. Using earlier studies of the thermodynamic properties of the crystals, the present calorimetric data and the "two-lattice" entropy model, the albite-orthoclase phase diagram is calculated in good agreement with experimental data. Attempts to calculate albite-silica and orthoclase-silica phase diagrams reveal complexities probably related to significant (but unknown) mutual solid solubility between cristobalite and alkali feldspar and to the very small heat and entropy of fusion of SiO2.
ASJC Scopus subject areas
- Geochemistry and Petrology