Activity/composition relations among silicates and aqueous solutions. I. Thermodynamics of intrasite mixing and substitutional order/disorder in minerals

Abstract The activities of thermodynamic components of clay minerals corresponding i composition to pyrophyllite, muscovite, paragonite, and margarite were computed from chemical nalyses reported inthe literature assuming ideal mixing of atoms on homological sites in the minerals. These activities were then used to generate stability fields for smectites, illites, and mixed-layer clays on logarithmic a tivity diagrams representing equilibrium among minerals and aqueous solutions at 25 ~ and 1 bar. Comparative analysis indicates that he approach affords close approximation f both mineral and water compositions i geologic systems