Statistical mechanical models for aluminum–silicon disorder in plagioclases Available to Purchase

The equilibrium aluminum–silicon disorder which occurs in plagioclase solid solutions is computed using an extension of the statistical mechanical model of Mazo. The model includes aluminum avoidance and a composition-dependent T₁₀ site preference energy. The configurational entropy and order parameter are calculated as functions of the temperature and composition, and reasonable qualitative agreement is obtained with the known composition dependence of the order parameter. A two-dimensional analogue model of albite is introduced which includes local charge balance. A limiting high-temperature configurational entropy of 1.12R per mole of Al is obtained in the two-dimensional model when local charge balance is rigorously obeyed.