Low-temperature stability of Mg-sulfate minerals in the presence of smectites: Implications for tracing the water cycle of Mars

Hydrated Mg-sulfate minerals are common near the surface of Mars, where they may be found in association with other hydrated mineral phases such as smectites. The hydration states of Mg-sulfate minerals are strongly dependent on temperature and relative humidity (RH). Thus, detection of particular species of hydrated Mg-sulfate minerals (either remotely, using vibrational spectroscopy, or directly, using the X-ray diffraction capabilities of the upcoming Mars Science Laboratory mission) could potentially be used to understand the effects of RH, temperature, and exchange of H₂O between sulfate minerals and their local environment. Exchange of structural H₂O and cations occurs between Mg-sulfate minerals and Ca-rich smectites under conditions of varying RH similar to those at the arid surface of Mars. This exchange of Mg for Ca results in the production of a more Mg-rich smectite and precipitation of Ca-sulfate minerals such as gypsum (CaSO₄. 2H₂O) and bassanite (CaSO₄.~0.5H₂O). Our experiments show that cation-exchange reactions can take place in the absence of free, liquid H₂O and are likely mediated by the presence of thin films of H₂O at smectite-sulfate grain boundaries. The ability of Mg-sulfate minerals and smectites to exchange H₂O suggests that these minerals may have the capacity to impact one another's hydration state. In order to improve our understanding of the probable behavior of Mg-sulfate minerals within multiphase geological materials, such as the martian regolith and layered deposits of clays and sulfate salts, we have used humidity buffer experiments to assess the stability of hydrated Mg-sulfate minerals in the presence of smectites. A series of long-term microcosm experiments employed ranges of temperature conditions (-25°C to +23°C) and RH conditions (7 to 100%) that more closely emulate martian surface conditions than have been used previously. Our results indicate that Mg-sulfate mineral equilibria, although still sluggish and path-dependent, are significantly affected by the presence of other RH-sensitive minerals (i.e., smectites). The formation of gypsum (even at -25°C) indicates that Ca-sulfate minerals may be useful indicators of cation-exchange and mobility of H₂O at Mars-relevant temperatures. The presence of smectites suppresses deliquescence of Mg-sulfate minerals at higher temperatures and RH and appears to buffer humidity within mineral mixtures. In these mixtures hydrous Mg-sulfate minerals may occur beyond their expected T-RH equilibrium fields on timescales of months to a year. Our experiments also show that formation of meridianiite (MgSO₄.11H₂O) from epsomite (MgSO₄.7H₂O) does occur at sub-freezing temperatures, but this transformation may take upwards of 10 months even at very high RH. Together, these observations strongly suggest that the results of previous studies of mineral stability in the MgSO₄-H₂O system may be inadequate predictors of phase stability (and thus cycling and bioavailability of H₂O) within mineralogically complex martian regolith.