CHLORIDE AND SULFATE EXCHANGE IN SHORT-TERM, LOW TEMPERATURE BRINE + JAROSITE EXPERIMENTS

Chlorine and other halogens in high salinity brines may readily exchange with sulfate and other ionically bonded anions available in other secondary phases observed on the surface of Mars, leading to diverse mineral assemblages. We conducted short-term, low-temperature flow-through and batch reactor experiments investigating jarosite (KFe³⁺₃(OH)₆(SO₄)₂) dissolution and reaction products in 50, 20 and 5 weight % CaCl₂ brines. Akaganeite (Fe³⁺OOH,Cl)) and antarcticite (CaCl₂·6H₂O) were observed via powder X-ray diffraction (XRD) in all experiments alongside Ca-sulfate minerals. Antarcticite is likely present due to excess CaCl₂ brine in the samples prior to analysis. However, the presence of akaganeite and Ca sulfate minerals indicate that Cl^- and SO₄^2- exchange readily in both flow-through dissolution and batch reactor experiments despite short durations and low temperature.

Akaganeite has been observed in association with sulfate and sulfide minerals (Peretyazhko et al., 2016) by the CheMin-XRD at Yellowknife Bay, Gale Crater, Mars (Vaniman et al., 2014) and at other locations on Mars via orbital imaging spectroscopy (CRISM) (Carter et al., 2015). Our results suggest that low temperature diagenesis in chloride brines may have produced these mineral assemblages observed on Mars. Ongoing research aims to further characterize these chloride- and sulfate-bearing reaction products using Visible and Near Infared and Raman Spectroscopy to better understand the spectral signatures of mixed sulfate-chloride assemblages, as well as Transmission Electron Microscopy to investigate the textural relationships between the reaction products.