Immiscibility in binary silicates: chemical and phase equilibria

Abstract

The driving force for immiscibility in binary silicates was examined using a statistical theory which combines a Flory-type theory, based on the compatibility of structural units, and a chemical equilibrium model, that describes the preferred associated species distribution of silicates. Restrictions imposed by a preferred distribution of associated species lead to a limitation of the number of ways to form a complete network structure, or equivalently to a decrease in the configurational entropy. An increase in configurational entropy is achieved by inducing a phase separation. The role of the enthalpies of species formation is examined relative to the configurational entropic contribution to phase coexistence. The theory leads to a rich set of phase coexistence behavior that includes three-phase equilibrium and a higher order critical point.