eochemical Transformations of Gypsum under Multiple Environmental Settings and Implications for Ca-Sulfate Detection on Mars

Calcium sulfate minerals are found in multiple environments on Earth and Mars, with chloride (Cl) salts widely distributed on both planets. Low-temperature studies have explored geochemical processes including formation of transient liquid water and ion migration on Mars. Some Cl-salts (e.g., NaCl and CaCl2) can dissolve gypsum (CaSO4·2H2O) in certain environments, making gypsum-Cl salt interactions significant. Additionally, gypsum's geochemical transformation at high temperatures reveals dehydration pathways crucial for understanding Mars' aqueous history and potential for life. This study examines gypsum dehydration through (i) thermal analyses and (ii) interactions with Cl-salts over a temperature range of −90 to 400 °C. We applied three spectroscopic techniques (Raman, visible/near-infrared (VNIR), and mid-IR) plus X-Ray Diffraction (XRD) to analyze these samples under variable conditions. This study also provides a low-temperature spectral dataset for gypsum and gypsum-Cl salt mixtures, beneficial for orbital analyses. Our findings reveal that experimental (i) heating rates, (ii) temperature ranges, (iii) relative masses of gypsum and Cl-salts, and (iv) dehydration environments (e.g., in situ, in vacuum) influence Ca-sulfate phase formation. Although we find different results in some cases, this study demonstrates that changing experimental conditions affect the detectability and transformation of gypsum. Further, these results indicate that the geochemical environmental condition on Mars plays a role in gypsum's geochemical transformation to dehydrated components. This study also provides structural and chemical data for Ca sulfate assemblages from vibrational spectroscopy and XRD, which extends our knowledge of gypsum and related materials under variable conditions, thus aiding orbital and surface planetary analyses which may help to advance our understanding of planetary geochemistry on Mars.