Spectral Properties of Chloride Salt-Bearing Assemblages: Implications for Detection Limits of Minor Phases in Chloride-Bearing Deposits on Mars

Chloride salt-bearing deposits have been identified throughout the southern highlands of Mars and have been suggested to be two component mixtures of anhydrous chloride salt and regional basaltic regolith. On Earth, chlorides typically occur as evaporite salts associated with other evaporite minerals such as carbonate and sulfate, and chemical weathering products like clays. In this laboratory study, we document the visible/near-infrared reflectance and midinfrared emissivity spectral characteristics of a series of chloride salt-bearing mineral assemblages. Our data show that the spectral features of various powder mixtures change systematically with variable additional mineral concentrations and particle sizes. Specifically, small amounts of calcite, gypsum, or nontronite have a relatively large effect on the bulk spectral properties of halite/labradorite mixtures making them easily distinguishable from two-component halite/labradorite mixtures. This fundamental laboratory work suggests that additional minor phases could only be present at no more than 1–5 wt% in Martian chloride salt-bearing deposits.