The Role of Sulfate in Cation Exchange Reactions: Applications to Clay–Brine Interactions on Mars

Phyllosilicates on Mars record a complex history of aqueous activity, including at Gale crater and Meridiani Planum, where stratigraphic differences in clay mineralogy have been recorded in outcrops that also contain calcium sulfate minerals. Thus, characterizing associations between phyllosilicates and calcium sulfates may provide constraints that are useful for constraining the geochemical environments that formed these outcrops. Previous studies have documented calcium sulfate precipitation as a result of clay–salt–atmospheric H 2 O interactions, but the compositions of brines throughout Mars’ history would have depended on the volume of water available on the Martian surface. Variations in brine composition influence the type and extent of reactions between the brines and the minerals that they come in contact with. To better understand how clay–brine interactions affected near-surface mineral assemblages on Mars, we performed two sets of experiments. The first set of experiments examined the effect of differing total brine concentrations and the second set explored variations in Na + and SO 4 2− concentrations independently. The results of this study show that gypsum readily forms due to cation exchange between montmorillonite and Na 2 SO 4 brines of any concentration, but only near-saturated MgSO 4 brines produced gypsum, and these also produced higher quantities of epsomite. Additionally, we found that the amount of gypsum produced from clay–Na 2 SO 4 brine reactions is more strongly influenced by SO 4 2− than Na + or Cl − concentrations. Understanding how rapidly gypsum forms as a product of clay–brine interactions, as well as the influence of SO 4 2− on cation exchange, will aid interpretations of sediments and environments that are observed on Mars.