Phase transition pathways of the hydrates of magnesium sulfate in the temperature range 50°C to 5°C: Implication for sulfates on Mars

Dehydration/rehydration experiments were conducted on pure Mg-sulfates and mixtures of epsomite with Ca-sulfates, Fe-sulfates, Fe-oxide, and Fe-hydroxide. The goal was to investigate the stabilities and phase transition pathways of Mg-sulfate hydrates, under temperature and relative humidity conditions relevant to Mars, as a function of starting structure and coexisting species. Two pathways were found to form Mg-sulfate monohydrates between 5 degrees C and 50 degrees C through dehydration of epsomite or hexahydrite. Two polymorphs of Mg-sulfate monohydrates were characterized in this study. It is important to distinguish among these phases on Mars because they have different formation conditions that have the potential to provide additional information on surface and subsurface geologic processes. We found that Mg-sulfates with moderate hydration states (especially starkeyite and amorphous Mg-sulfates) can be very stable under current Martian surface conditions. On the basis of NIR spectral features, these phases are good candidates for polyhydrated sulfates identified on Mars by OMEGA and CRISM; thus, they may contribute to the high hydrogen concentrations found by the neutron spectrometer on the orbiting Odyssey spacecraft. Our experiments indicate that the maximum number of water molecules per SO4 held by the amorphous Mg-sulfate structure is three. In addition, the amorphization rate of Mg-sulfates is strongly dependent on temperature. The low temperature (approximately -80 degrees C) in the early morning hours during the Martian diurnal cycle would slow the dehydration rate, which would favor the stability of starkeyite over amorphous Mg-sulfates and would lead to a low abundance of the latter.