291-4 Clay Mineral Alteration by Natural Acid Sulfate Waters under Mars-Analogue Conditions: Implications for Clay–Sulfate Assemblages on Early Mars

Session: Mineralogy in the Solar System

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Abstract:

Orbital and rover observations have revealed widespread occurrences of phyllosilicates (clays), sulfates, iron oxides, and silica across Martian surface, indicating complex aqueous alteration histories that evolved from clay-forming to increasingly acidic, sulfate-rich environments. To evaluate mineralogical pathways behind this shift, we exposed Mars-relevant clays (kaolinite, illite, chlorite, Al-montmorillonite, Mg-saponite, and two Fe-rich nontronite: NAu-1 & NAu-2) plus an amorphous silica control to natural acid rock drainage (ARD) in field flow-through experiments for up to 5 months. Natural ARD is rich in sulfate and dissolved cations (Fe, Al, Mg, Ca, K), providing a realistic analogue for Martian fluids and enabling complex alteration reactions that are unrealized in simple sulfuric acid solutions at equivalent pH. Upon retrieval, reacted solids were extracted from intact dialysis membranes for characterization. We used X-ray diffraction (XRD) to monitor layer stacking and crystallinity, Fourier transform infrared spectroscopy (FTIR) to detect tetrahedral (Si–O) and octahedral (OH) structural changes, and X-ray fluorescence (XRF) to track cation leaching from reacted clays.

ARD exposure induced structural and/or chemical alterations across all samples. Kaolinite remained essentially unaltered, while illite underwent progressive structural alteration initiated by interlayer K release. Chlorite showed limited octahedral alteration, though Fe oxidation and partial Mg release occurred while preserving the crystalline structure. Swelling clays (smectites) exhibited variable structural responses alongside secondary Fe-Al precipitation. Among smectites, nontronites underwent initial tetrahedral distortion followed by structural reordering; montmorillonite exhibited moderate alteration alongside amorphous silica precipitation; and trioctahedral saponite experienced heavy leaching of octahedral Mg and tetrahedral Si. Consequently, clay stability under sulfate and Fe-rich acidic conditions follows the order: Kaolinite > Chlorite > Nontronite 2 > Illite >  Nontronite 1 > Montmorillonite > Saponite. Unlike pure-acid solutions, natural ARD promotes adsorption, secondary metal precipitation, and structural reorganisation due to high ambient cation activities, leading to rapid surface passivation and demonstrating that complex fluids preserve clay frameworks for far longer during Martian acid alteration.

These results imply that clay persistence in Hesperian and Amazonian acid-sulfate units does not require brief or mild fluid exposure, as cation-rich acidic fluids naturally buffer clay destruction. The established stability ranking provides a predictive framework for rover and orbital observations, explaining how localized silica and iron sulfate deposits can form via selective leaching while preserving resilient clay frameworks across Martian sedimentary basins.

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