It has been proposed that smectite clays, the predominant minerals in bentonite, are metastable solid solutions whose compositional heterogeneity prevents strict adherence to fixed ion activities and activity ratios characterizing invariant equilibria among stoichiometric phases [1,2]. This is qualitatively confirmed in the present analysis of exceptionally well-constrained experimental data [3] using a solid-solution model defined by the phase rule and estimated ideal site-mixing parameters. The results suggest that equilibria are approached among smectite solid solutions and kaolinite or halloysite, and that these minerals coexist metastably with respect to asiO2(aq)· Irreducible uncertainties are generated in the model by analytical and conceptual deficiencies in understanding compositional variability in smectite. However, their estimated effects on smectite’s stability are relatively small, and are comparable to the effects of experimental uncertainty in standard Gibbs energies on the stabilities of stoichiometric minerals.

An alternate analysis of the data further confirms that smectite does not behave like a stoichiometric phase [3]. Ion-exchange models for this clay mineral may therefore be thermodynamically ill defined because stoichiometric behavior is assumed implicitly under limiting conditions of fixed proportions of cations on exchange sites. Ion exchange is a pragmatic simplification enabling empirical analysis of some experimental data. However, its empirical, rather than thermodynamic, basis should not be overextended to conditions that are beyond an experimentally calibrated range.