Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars

We report new measurements of equilibrium relative humidities for stable and metastable hydration-dehydration equilibria involving several magnesium sulfates in the MgSO4-nH(2)O series. We also report a comprehensive thermodynamic treatment of the system including solution properties and experimental data from the published literature, i.e. solubilities, heat capacities and additional decomposition humidities. While for some magnesium sulfate hydrates solubility data in the binary system MgSO4-H2O are sparse, there is a reasonable database of solubility measurements of these hydrates in the ternary MgCl2-MgSO4-H2O and the quaternary reciprocal Na+-Mg2+-Cl--SO42--H2O systems. To make these data suitable for the determination of solubility products, we parameterized a Pitzer ion interaction model for the calculation of activity coefficients and water activities in mixed solutions of these systems and report the ion interaction parameters for the Na+-Mg2+-Cl-SO42--H2O system. The model predicted solubilities in the reciprocal system are in very good agreement with experimental data. Using all available experimental data and the solution model an updated phase diagram of the MgSO4-H2O system covering the whole temperature range from about 170 to 473 K is established. This treatment includes MgSO4 center dot H2O (kieserite), MgSO4 center dot 4H(2)O (starkeyite), MgSO4 center dot 5H(2)O (pentahydrite), MgSO4 center dot 6H(2)O (hexahydrite), MgSO4 center dot 7H(2)O (epsomite) and MgSO4 center dot 11H(2)O (meridianiite). It is shown that only kieserite, hexahydrite, epsomite and meridianiite show fields of stable existence while starkeyite and pentahydrite are always metastable. Due to sluggish kinetics of kieserite formation, however, there is a rather extended field of metastable existence of starkeyite which makes this solid a major product in dehydration reactions. The model predicted behavior of the magnesium sulfates is in excellent agreement with observations reported in the literature under terrestrial temperature and relative humidity conditions. We also discuss the implications of the new phase diagram for sulfates on Mars. (C) 2011 Elsevier Ltd. All rights reserved.