THERMODYNAMIC MIXING PROPERTIES OF NEPHELINE-KALSILITE CRYSTALLINE SOLUTIONS

A solution calorimetric investigation has been carried out on two nepheline-kalsilite ion-exchange series, each consisting of thirteen members. Dissolutions were performed at 50 degrees C in 20.1 wt percent HF under isoperibolic conditions. Heats of solution varied linearly with K/Na ratio for both sodium-rich nephelines and all kalsilites, interpreted as ideal thermodynamic behavior. Potassic nephelines, however, exhibited positive enthalpies of K-Na mixing associated with the entry of K+1 into relatively small Na sites; this also correlated well with the miscibility gap in the middle of the series. []Si substitution was found to have very significant effects on the heats of solution of these minerals, and vacancies associated with both the latter substitution and with the substitution of Ca+2 in the alkali sites were found to increase substantially enthalpies of K-Na mixing and also to increase the critical temperature of the solvus. Enthalpies were simultaneously modelled for both series as a function of nepheline, kalsilite, and vacancy contents both as a modified Taylor series and also in Extrapolation of the formulation suggested that enthalpies of K-Na mixing might be near-zero in the absence of vacancies.