EXPERIMENTAL STABILITY OF MAGNESIUM SULFATE HYDRATES THAT MAY BE PRESENT ON MARS

The dominant naturally occurring members of the MgSO4·nH2O series on Earth are epsomite (MgSO4·7H2O), hexahydrite MgSO4·6H2O), and kieserite (MgSO4·H2O). In addition, other magnesium sulfates hydrates (sanderite - MgSO4·2H2O, Starkeyite - MgSO4·4H2O, pentahydrite - MgSO4·5H2O) have been identified as efflorescent salts in limited localities in the United States, Chile, and Germany. Significant attention has been recently given to Mg-sulfate salts as they are believed to be a widespread cementing agent in martian soils. Most studies examine only the possible occurrences of epsomite, hexahydrite, and kieserite at the martian surface, on the assumption that the other MgSO4 hydrates are unstable or metastable.

To investigate the possibility that the other hydrates are stable at conditions not common on the Earth’s surface, various hydrate forms of MgSO4 were placed into ovens at 50° and 75°C over saturated salt solutions to control humidity. Samples chosen for this study include anhydrous MgSO4, reagent monohydrate MgSO4 (non-kieserite structure), synthesized keiserite, synthesized starkeyite, and hexahydrite. Equilibration times varied from 10 to 30 days. Although hexahydrite is a common hydrate on the Earth’s surface, the largest stability fields were for kieserite and starkeyite at elevated temperatures. Although sanderite formed in several experiments, its stability field, if thermodynamically stable, appears to be rather limited. To date, we were only able to form pentahydrite as a metastable phase during rehydration of amorphous MgSO4 hydrate. In addition, we were able to form MgSO4-hydrates whose identities in X-ray diffraction analysis are uncertain, as there were no matching phases in the International Centre for Diffraction Data database.

Results of this study show that MgSO4 hydrates other than epsomite, hexahydrite, and kieserite can be stable (especially starkeyite) but not at conditions normally encountered on the Earth’s surface. Unlike the sharp hexahydrite/epsomite transition, the stability fields for the other MgSO4 hydrates show considerable overlap, which is probably indicative of both metastability and kinetic limitations. As would be expected, the stable form of the MgSO4·nH2O series is significantly dependent on both RH and temperature.