MECHANISMS AND GEOCHEMICAL SIGNIFICANCE OF SI-AL SUBSTITUTION IN ZEOLITE SOLID SOLUTIONS

Abstract

Rock-forming zeolites often exhibit complex solid solutions reflecting isomorphous substitutions between Si and Al in tetrahedral framework sites, between charge-balancing extraframework cations, and between water molecules and vacancies. Although the number of moles of charge on extraframework cations in a zeolite must equal the moles of Al in order to maintain charge balance, the relationships between Si-Al and extraframework substitutions vary considerably across this mineral group. Review of available compositional data suggests that there are three main modes of Si-Al substitution in zeolites: 1) coupled CaAl-NaSi substitution; 2) coupled substitution of a single extraframework cation plus Al for Si; and 3) completely uncoupled substitution among extraframework cations and Si and Al on tetrahedral sites. Among zeolites that exhibit the latter two modes of solid solution, Si-Al substitution can be described by an SiO2 (± H2O) compositional exchange vector from a hypothetical, pure-silica endmember composition. Recent calorimetric, structural, and theoretical investigations suggest that Si-Al substitution follows a non-ideal, athermal solution model characterized by no excess enthalpies of mixing and negative excess entropies of mixing. Because Si-Al exchange in these minerals can be explicitly or implicitly described by exchange of an SiO2 component, the Si/Al ratio in their framework can be predicted solely as a function of temperature, pressure, and the chemical potential of SiO2. Application of this model leads to calculated Si/Al ratios in stilbite (coexisting with albite), analcime, and chabazite consistent with observed mineral compositions and parageneses in very low-grade metamorphic environments. Coexistence of silica polymorphs with zeolites containing SiO2 · nH2O exchange vectors potentially provides a means of performing thermobarometric calculations in very low-grade metamorphic and diagenetic environments. © 2005 Elsevier B.V. All rights reserved.