The Substantial Effect of Mineral Structure on Phase Equilibria and Thermodynamic Behavior In the Nepheline - Kalsilite System

Nepheline - kalsilite crystalline solutions vary chemically both in K:Na and Al:Si ratio: (K,Na)_1-x[]_xAl_1-xSi_1+xO₄, where [] represents alkali-site vacancies. We have investigated three ion-exchange series in this system, one with nearly stoichiometric Al:Si (0.983:1.017), another having 0.125 excess Si (Al:Si = 0.875:1.125), and a third with 0.052 excess Si plus K:Ca content that varied across the series via []Ca = 2K ion exchange. Reversed phase equilibria from solvus experiments produced critical temperatures for the three series that range from 961 °C to 1265 °C, despite very limited variation of []Si and []Ca. The details of structural variation, nepheline with its 3:1 ratio of smaller oval to larger hexagonal alkali sites and kalsilite with its single ditrigonal site, are critical to phase behavior. Potassium occupancy of the smaller oval alkali site in nepheline produces strain; the compositionally displaced sodic limbs of the three solvi therefore involve compositional pairs where one of the quenched phases is nepheline in which K has been forced into the smaller site. The potassic limbs of the three solvi, which compositionally are superimposed upon one another below 900 °C, depend solely on Na content of the one and only ditrigonal alkali site of kalsilite. Structural details also explain thermodynamic behavior. Enthalpies of solution (20.1 wt % HF, 50 °C, isoperibolic conditions) display linear trends with composition so long as K is replacing Na in the larger hexagonal site of nepheline. Once K initiates occupancy of the smaller alkali site, these trends are replaced by curved relationships that reflect positive enthalpies of mixing, which indeed one would associate with the presence of a solvus. The most silicic series displays the lowest critical temperature, which is associated with the greatest number of vacancies, diminished K=Na substitution, and positive enthalpies of mixing that are spread over a shorter compositional range.