EXPERIMENTAL STABILITY OF MAGNESIUM SULFATE HYDRATES THAT MAY BE PRESENT ON MARS
To investigate the possibility that the other hydrates are stable at conditions not common on the Earths 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 Earths 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 Earths 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.