Sulfate salts, particularly those of magnesium, are important components of the near-surface environment on Mars. They are thought to play significant roles in the planetary H₂O cycle in equatorial regions where they release to or remove from the atmosphere water vapor through dehydration and hydration reactions, respectively, driven by diurnal variations in temperature and relative humidity. Realistic evaluation and modeling of the role of Mg-sulfate salts in the planetary H₂O cycle require an accurate and precise understanding of the phase equilibria and thermodynamic properties of these phases.

     Thermodynamic properties of Mg-sulfate salts reported in the literature are in poor agreement, which hinders our ability to estimate their stabilities under various conditions. To address this problem, the humidity-buffer technique was used previously to determine equilibrium constants and derive thermodynamic properties for the epsomite-hexahydrite reaction (Chou and Seal, 2003, Astrobiology, 3, 619). The same method was used in this study for the reaction hexahydrite (MgSO₄•6H₂O) = starkeyite (MgSO₄•4H₂O) + 2 H₂O. Reversals along four humidity-buffer curves between 37 and 61 °C at 0.1 MPa yield ln K (±0.06) = 35.68 + 13231.6/T, where K is the equilibrium constant, and T is temperature in Kelvin. The derived standard Gibbs free energy of reaction at 298.15 K is 21.57± 0.15 kJ/mol, which is 3.30 kJ/mol lower than the value calculated from the data compiled by DeKock (1986, Bur. Mines Inf. Cir., 9081). The intersection of our hexahydrite-starkeyite boundary extrapolated to the boundary of hexahydrite-saturated solutions, which was based on the vapor-pressure measurements of Carpenter and Jette (1923, J. Am. Chem. Soc., v. 45, 578), defines the invariant point for the assemblage hexahydrite-starkeyite-aqueous solution-vapor at 73.9 °C and 79.8% RH. This invariant point needs to be verified. Our results indicate that starkeyite could be a significant mineral on Mars, as suggested recently by Chipera et al. (2005, Lunar and Planetary Sci., XXXVI, 1497). Assuming rapid and equilibrium reactions, a kilogram of MgSO₄ can release to or remove from the Martian atmosphere between a minimum of 0.45 kg of H₂O to a maximum of 1.65 kg H₂O through hydration or dehydration reactions, respectively, during a diurnal cycle.