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Thermodynamic, phase equilibrium, and crystal chemical behavior in the nepheline–kalsilite system

^* Department of Geology and Environmental Geosciences, Lafayette College, Easton, Pennsylvania 18042, USA
^** Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA
^*** Physique des Minéraux et des Magmas, IPGP, 4 place Jussieu, 75252 Paris cedex 05, France

hovisguy{at}lafayette.edu

Reversed phase equilibria have been determined from 500 to 1050 °C along the nepheline–kalsilite solvus for crystalline solutions having 12.5 mole percent excess Si relative to stoichiometric Al:Si = 1:1. The resulting 961 °C critical temperature (T_c) for this high-Si series is substantially lower than those (1108 and 1265 °C) of two previously studied series containing less Si. Hydrofluoric acid solution calorimetric data collected at 50 °C for mid-compositional members of this high-Si series show positive enthalpies of mixing (H_ex) that are lowest in magnitude among the three series, correlating well with the lower T_c. The thermodynamic and phase behavior of minerals in the nepheline–kalsilite system are related to structural strain associated with the entrance of K⁺ into the smaller of nepheline's two alkali crystallographic sites and also by equilibrium between minerals with related, yet distinct, structures. Positions of the sodic limbs of the solvi are governed in part by the preference of nepheline's large hexagonal alkali site for vacancies (instead of sodium) near room temperature, with an apparent switch to preference for K⁺ at T 500 °C. Positions of the potassic limbs of the solvi are governed only by the substitution of relatively small Na⁺ into the ditrigonal site of kalsilite, which is accompanied by ideal thermodynamic behavior of the solution calorimetric data. The collective structural and crystal chemical distinctions among the various members of these crystalline solutions produce differences in thermodynamic behavior and miscibility that are substantial for the modest observed differences in system chemistry.

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