Solid–liquid equilibria of Sorel phases and Mg(OH)2 in the system Na-Mg-Cl-OH-H2O. Part II: Pitzer modeling

For geochemical calculations of solubility equilibria between Sorel phases, Mg(OH) 2 , and oceanic salt solutions, the polythermal THEREDA dataset (based on the HMW model at 25°C) was extended. With both models, H + solution concentrations in equilibrium with Mg(OH) 2 (s) and the 3-1-8 Sorel phase at 25°C can be calculated in good agreement. In contrast, calculated OH − solution concentrations do not agree. Using the solubility constants (lg K s ) determined up to 60°C in Part I of this work, together with available solubility isotherms up to 120°C, temperature functions for the 3-1-8 phase (25°C–100°C), 2-1-4 phase (60°C–120°C), and 9-1-4 phase (100°C–120°C) were derived. In order to accurately model the OH − solution concentrations, it was necessary to implement the solution species Mg 3 (OH) 4 2+ (∆ R G m ° temperature function) in addition to the MgOH + already contained in the previous model. Finally, fitting Pitzer mixing coefficients for both species now allow the calculation of the solubility equilibria of Mg(OH) 2 (s) and the Sorel phases in agreement with the experimental data in the Mg-Cl-OH-H 2 O and Na-Mg-Cl-OH-H 2 O systems.