Calorimetric investigation of Na-K mixing and polymorphism in the alkali feldspars

Heat of solution measurements of the enthalpies of Na-K mixing, Al-Si ordering, and fusion for crystalline solutions and end-member phases in the system Na(AlSi₃O₈)–K(AlSi₃O₈) were obtained with a vacuum-jacketed, isoperibolic calorimeter using 20.1 percent hydrofluoric acid at temperatures ranging from 40 to 60°C. Heat of solution data for artificially prepared microcline (–low-albite) crystalline solutions at 49.7°C are represented by

ΔH_sol = −149.408 + 5.230 N₂ − 5.928 N₁N₂² − 8.457 N₂N₁² kcal/mole

based on 39 measurements of 14 different bulk compositions, where N₁ and N₂ denote the mole fractions of NaAlSi₃O₈ and KAlSi₃O₈ respectively. The thermochemical uncertainty of the above least-squares formulation is ± 0.032 kcal/mole.

Unit-cell parameters determined by least-squares refinement of diffraction data indicate that the excess volumes of Na-K mixing of these microcline solutions are positive over the entire composition range. Combining the calorimetric and crystallographic data leads to the following expression for the variation of molar enthalpy of mixing with pressure:

ΔH_ex = (5928 + 0.110 P) N₁N₂² + (8457 + 0.110 P) N₂N₁² cal/mole.

The sign and magnitude of ΔH_ex determined by calorimetry agree closely with ΔH_ex calculated from a third-order Margules formulation of high-temperature two-phase equilibrium data in the critical region.