Experimental Evaluation of Two Operational Standard States for Montmorillonite in Metastable Hydrolysis Reactions

Abstract

AbstractTwo conventions have been used to represent smectite formulas in mineral hydrolysis reactions: exchangeable cations are either included in (monophase structure) or excluded from (charged structure) the Standard State phase designation. Replicate mixtures of K‐saturated Belle Fourche montmorillonite, kaolinite, goethite, and magnesite were taken through a series of consecutive equilibrations with 0.01, 0.1 or 1.0 M KCl solutions using low solution/solid ratios. Apparent chemical equilibrium with respect to magnesite was used as an indication of system‐wide meta‐equilibrium. Following equilibration, the aqueous phases were recovered by immiscible displacement. Total soluble K, Mg, Si, pH, and ambient CO2 were measured. The solutions were chemically speciated using an ionassociation computer model. Hydrolysis reactions were developed based on the two smectite standard state phase designations. Reaction equilibrium constants and Gibbs free energies of formation (ΔG°) for montmorillonite were calculated. In the case of the monophase structure model, these parameters were functions of exchangeable cation composition. The mean ΔG° (montmorillonite) values for the charged structure and monophase structure models differed in absolute value but were equally precise. These results suggested that a characteristic metastable equilibrium was obtained in the experimental system. Justification for the selection of one convention over the other must be based on criteria other than precision.