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This review updates and expands an earlier study (Glynn 1990), that is presently out of print. The principal objectives of this chapter are (1) to review the thermodynamic theory of solid-solution aqueous-solution interactions, particularly as it pertains to low-temperature systems (between 0 and 100°C), and (2) to summarize available data on the effects of ionic substitutions on the thermodynamic properties of binary sulfate solid-solutions. Selected carbonate and halide solid-solutions are also considered.
Studies of solid-solution aqueous-solution (SSAS) systems commonly focus on measuring the partitioning of trace components between solid and aqueous phases. The effect of solid-solution formation on mineral solubilities in aqueous media is rarely investigated. Several studies, however, have examined the thermodynamics of SSAS systems, describing theoretical and experimental aspects of solid-solution dissolution and component-distribution reactions (Lippmann 1977, 1980; Thorstenson and Plummer 1977, Plummer and Busenberg 1987, Glynn and Reardon 1990, 1992; Glynn et al. 1990, 1992; Glynn 1991, 1992; Glynn and Parkhurst 1992, Königsberger and Gamsjäger 1991, 1992; Gamjäger et al. 2000). The present chapter describes and compares some of the concepts presented by the above authors, and presents methods that can be used to estimate the effect of SSAS reactions on the chemical evolution of natural waters. Sulfate minerals are particularly well suited for the investigation of the thermodynamics of SSAS systems, because their generally high solubilities facilitate the attainment of thermodynamic equilibrium states and their commonly large crystal structures tend to allow considerable ionic substitution.
Field or laboratory observations of miscibility gaps, spinodal gaps, critical mixing points, or distribution coefficients can be used to estimate solid-solution excess-free-energies, which are needed for the calculation of solid-solution solubilities and of potential component-partitioning behavior. Experimental measurements of the solubility and thermodynamic properties of solid solutions are …