Partitioning behavior of Br and Cl during jarosite precipitation and its implications for sedimentary rock on Mars

Abstract

National Aeronautics and Space Administration (NASA)′s Opportunity rover landed at Meridiani Planum on Mars in 2004, and discovered the first sedimentary record on Mars called Burns formation. The detection of magnesium- and iron- sulfates, jarosite (KFe3(SO4)2(OH)6), and hematite (Fe2O3) in the outcrop of Burns formation indicates that aqueous activities have been once present in this region. The Alpha Particle X-ray Spectrometer (APXS) onboard the rover detected enrichment of elemental Br in rock and soil samples which varied by three orders of magnitude, and primarily controlled the variations of Br/Cl ratios in these samples. Although aqueous related processes have been suggested to explain the enrichment of Br, the speciation of Br and the mechanisms for Br variations are poorly constrained. Jarosite has been reported to be able to preferentially incorporate Br− over Cl− during precipitation, and result in Br enrichment and Br/Cl fractionation. However, the partitioning behavior of Br− and Cl− during jarosite formation and how the incorporation of halogens would influence properties of jarosite are not well known. In this work, we synthesized a series of halogen bearing jarosite with Br− and Cl− concentration gradients by oxidation of ferrous sulfate at room temperature. After synthesis, we used X-ray diffraction (XRD), Raman spectrometer, infrared spectrometer (IR), scanning electron microscope (SEM) to analyze structure information and surface morphology of jarosite solids. Anion and cation concentrations in solution and solid were analyzed by ion chromatography (IC), atomic absorption spectrophotometer (AAS) and X-ray fluorescence (XRF). Our work shows that Br and Cl contents in the synthesized jarosite are positively correlating with its initial concentrations in solutions. Starting at the same concentrations in solution, incorporation of Br− in jarosite is about two orders of magnitude higher than that of Cl−. Coexisting Br− can significantly interfere with Cl− partitioning into jarosite. The distribution coefficients of Br− and Cl− are negatively correlated with their initial concentrations in solutions, and in the Br− and Cl− coexisting setting, the distribution coefficients of Cl− decrease as the initial Br− concentration increase in the solution. Therefore, during precipitation of jarosite, Br− prefers to participate into jarosite while Cl− prefers to remain in solution. Incorporation of halide anions into jarosite directly affected the range of hydroxyl and water in Raman spectra, indicating that Br− and Cl− substitute for OH position in jarosite without changing its fundamental structure. We calculated that in our experiment settings, Br/Cl ratios in jarosite are about two orders of magnitude higher than that of initial solutions. Therefore, if jarosite precipitates from a brine containing Br− and Cl−, it can enrich Br over Cl and bearing significant higher Br/Cl ratio signature comparing to the initial brine. For sedimentary outcrop composed of substantial amount of jarosite, Br/Cl ratios may not be controlled solely by evaporation and precipitation of halide evaporites, but might also be influenced by precipitation and dissolution of halogen bearing jarosite. The aqueous stability of halogen bearing jarosite and the possible release of halide anions during jarosite dissolution require further evaluation.