Mineral assemblages with smectites in simulated basalt alteration: implications for Martian aqueous environments

This study addresses the limitations of individual mineral phases for paleoenvironmental reconstruction on Mars, by investigating smectite-associated mineral assemblages formed across a broad pH range (pH = 4–13). Hydrothermal experiments reveal that aluminum content controls iron crystallization pathways under acidic conditions. Low aluminum content favors direct nontronite formation, while high aluminum content leads to iron (hydr)oxide precipitation. Under oxidizing environments, moderately acidic conditions enhance Fe/Mg enrichment, explaining the elevated Fe/Mg ratios observed in Martian smectites. With increasing pH toward alkaline conditions, zeolite species shift from gobbinsite to phillipsite, and analcime crystal morphology changes from purely cubic to a combination of cubic and tetragonal trisoctahedra between pH 10 and 13. These trends make smectite-zeolite assemblages promising diagnostic indicators of alkalinity. Raman spectroscopy reliably distinguishes zeolite species, overcoming visible and near-infrared reflectance (VNIR) technology limitations. Furthermore, evaporite minerals exhibit exquisite sensitivity to variations in temperature and humidity, undergoing structural transitions (e.g., Na 2 SO 4 -III → Na 2 SO 4 - V ) and ionic substitution (e.g., hexahydrite → blödite; thermonatrite and sodium sulfate → burkeite). These pose challenges for reliable paleoenvironmental reconstruction and Mars sample return curation. These results provide a mineralogical framework for deciphering smectite-related aqueous environments on Mars.