In the ternary system CsCl – NaCl – H2O, at a temperature of 298.15 K, a double salt with the stoichiometric formula CsCl∙2NaCl∙2H2O(cr) is known to be formed. This double salt and the anhydrous CsCl(cr) are the end-members of a solid solution. For the pure double salt, the solubility constant was determined. The obtained value was applied to calculate the solubility diagram also of the quaternary system CsCl – NaCl – KCl – H2O and the quaternary-reciprocal system Cs+, Na+ || Cl−, SO42− – H2O. The solubility constant together with a solid solution between CsCl·2NaCl·2H2O(cr) and CsCl(cr) were implemented in THEREDA, which extends the applicability of the existing cesium dataset.

The THEREDA model has been extended by phases and species formed within the title system. A temperature range of 0 °C to 100 °C is considered. Most of the solubility products, Pitzer parameters and association constants were adapted from the model of Königsberger et al. (Königsberger, E., Königsberger, L.-C., Gamsjäger, H.: Low-temperature thermodynamic model for the system Na2CO3−MgCO3−CaCO3−H2O. Geochim. Cosmochim. Acta, 63, (1999), pp. 3105–3119, DOI 10.1016/S0016-7037(99)00238-0). Literature on solubility data of carbonates of magnesium and calcium is critically evaluated. As far as experimental solubility data were available, they were compared with model calculations. The data comprise the minerals magnesite (MgCO3), nesquehonite (MgCO3∙3H2O), lansfordite (MgCO3∙5H2O), hydromagnesite (4MgCO3∙Mg(OH)2∙4H2O), dypingite (4MgCO3∙Mg(OH)2∙5H2O), calcite (CaCO3), aragonite (CaCO3), vaterite (CaCO3), ikaite (CaCO3∙6H2O), dolomite (MgCO3∙CaCO3), huntite (3MgCO3∙CaCO3), magnesian calcite (solid solution between CaCO3 and MgCO3), eitelite (Na2CO3·MgCO3), pirssonite (Na2CO3·CaCO3·2H2O), gaylussite (Na2CO3·CaCO3·5H2O), natrite (Na2CO3), thermonatrite (Na2CO3·H2O), Na2CO3·7H2O, natron (Na2CO3·10H2O), nahcolite (NaHCO3), wegscheiderite (Na2CO3·3NaHCO3) and trona (Na2CO3·NaHCO3·2H2O). There are minerals known as artinite (MgCO3∙Mg(OH)2∙3H2O) or barringtonite (MgCO3·2H2O), for which no solubility data exist. The literature survey also covers geochemical observations on the existence of carbonate minerals, laboratory investigations on the decomposition, formation and conversion as well as spectral and XRD characterization of carbonate phases. For important minerals, such as magnesite or hydromagnesite, the uncertainty of the solubility constants is large. The added Pitzer parameters, concern the binary cation - carbonate, cation - hydrogen carbonate and the ternary interactions, including mixing parameters of the anions HCO3-, CO32- and OH-. Mixing parameters of these anions with chloride or sulfate are not considered in this work. From the data situation and the modelling results, conclusions are drawn with respect to future experimental work to obtain more reliable equilibrium data.