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Dehydration and Metastable States of Mg-Sulfate Hydrates that May be Present on Mars

Introduction: 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 sulfate hydrates (sanderite 2H2O, starkeyite 4H2O, pentahydrite 5H2O) have been identified as efflorescent salts in limited localities in the United States, Chile, and Germany. Chipera et al., [1,2] conducted an extensive study of the relative stability among the various Mg-sulfate hydrates. They, observed Mg-sulfate hydrates outside of their predicted stability field, and for some phases found in nature, were not able to establish that there was a stability field at all. One of the conclusions from the earlier studies was that the Mg-sulfate hydrate system is dominated by kinetics, metastability, and what was believed to be reaction path dependence. They initially conducted short experimental runs (2-3 weeks) before the significant control that kinetics exerted on the Mg-sulfate hydrate system became apparent. Subsequently, longer experiments were conducted (2-4 months) in an attempt to attain equilibrium conditions. Longer runs at the same conditions produced more complete reactions although even some of the longer experiments produced polyphase mixtures, indicating that they also may not have gone to completion. This raised the question of how the Mg-sulfates dehydrate and whether they dehydrated in a stepwise fashion with intermediate phases as suggested by the limited available data. The present study was carried out to examine the possibility that Mg-sulfate hydrates dehydrate along established reaction paths. We also explored the behavior of sanderite which appears to violate the expected dehydration trend, forming after a 1-H2O stage with decreasing RH and increasing temperature (i.e., 7 6 4 1, then the sanderite 2hydrate). It is currently believed that sanderite is a metastable intermediate form.