Low-temperature thermodynamic model for the system Na2CO3-MgCO3-CaCO3-H2O

A comprehensive low-temperature thermodynamic model for the geochemically i mportant Na2CO3 -MgCO3 -CaCO3-H2O system is presented. The model is based o n calorimetrically determined Delta(f)H degrees(298) values, S degrees(298) values and C degrees(p)(T) functions taken from the Literature as well as on mu degrees(298), values of solids derived in this work from solubility m easurements obtained in our laboratories or by others. When these thermodyn amic quantities were combined with temperature-dependent Pitzer parameters taken from the literature, solubilities calculated for a wide range of cond itions agree well with experimental data. The results for several subsystem s were summarized by depicting the respective phase diagrams. For the MgO-C O2-H2O subsystem, it was found that the commonly believed stability relatio ns must be revised, i.e., in the temperature range covered, nesquehonite ne ver becomes more stable than hydromagnesite at p(CO2) less than or equal to 1 arm. Although the recommended set of thermodynamic data on sparingly sol uble solids was derived from experimental results on mainly NaClO4 systems, it can be incorporated in databanks containing additional Pitzer parameter s for modeling more complex fresh- or seawater systems. Copyright (C) 1999 Elsevier Science Ltd.