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Abstract: Martian layered deposits and regolith at Gale Crater may contain multiple hydrated mineral phases, tentatively identified as hydrated Mg-sulfate minerals and smectites. We have used humidity buffer experiments to assess the stability of hydrated Mg-sulfate minerals in the presence of smectites in order to improve our understanding of the probable behavior of Mg-sulfate minerals within multiphase geological materials on Mars. A series of long-term experiments employed temperature (−25 to +23°C) and relative humidity (RH) conditions (7–100%) that emulate near-equatorial martian surface conditions. Our results indicate that the hydration state of Mg-sulfate minerals is affected by the presence of RH-sensitive clay minerals (i.e., smectites). The formation of gypsum and bassanite in dry mineral mixtures via cation exchange between Ca-bearing smectite and Mg-sulfate minerals indicates that Ca-sulfate minerals may be useful indicators of H2O and metal mobility at Mars-relevant temperatures (−25 to +23°C). The presence of smectites also suppresses deliquescence of Mg-sulfate minerals at 100% RH and low, but non-freezing, temperatures. Co-existence of smectites and Mg-sulfate minerals appears to buffer RH within mixtures of these minerals, which can result in production or preservation of Mg-sulfate phases that are inconsistent with measured values of atmospheric RH. Consequently, hydrated Mg-sulfate minerals may persist beyond their expected T–RH equilibrium fields on longer timescales within smectite–MgSO4 mixtures than in the pure MgSO4–H2O system. Dehydration of highly hydrated Mg-sulfate minerals appears to slow in the presence of smectite, which may have important implications for long-term preservation of organic biosignatures within Mg-sulfate crystals on Mars. Together, these observations suggest that Mg-sulfate mineral behavior (and thus cycling and bioavailability of H2O and metals) may be impacted by the presence of smectites within mineralogically complex martian layered deposits and regolith. [Copyright &y& Elsevier]