Gamma-CaSO4 With Abnormally High Stability From a Hyperarid Region on Earth and From Mars

Ordinary gamma-CaSO4 is a metastable calcium sulfate, while gamma-CaSO4 from the hyperarid region on Earth and from Mars has been found with abnormally high stability. In this study, we used multiple microanalyses to characterize the chemical and structural properties of two such gamma-CaSO4: one from Atacama soil (#10-d30) and the other from Martian meteorite MIL03346,168. Silicon was determined to be quasi-homogeneously distributed in Atacama gamma-CaSO4, while both silicon and phosphorus were detected in Martian gamma-CaSO4. We found the abnormally high stability of those gamma-CaSO4 from hyperarid environments was due to the chemical impurities which filled their structural tunnels and blocked the entrance of atmospheric H2O, with non-detectable structural distortion. We propose that the gamma-CaSO4 with Si or Si and P impurities could have igneous origin or evaporative origin. Due to the extreme similarity in the structures of bassanite and gamma-CaSO4, their XRD patterns are almost non-distinguishable; thus some martian bassanite minerals identified by Curiosity's CheMin instrument at Gale crater can actually be gamma-CaSO4. The structural tunnels in gamma-CaSO4 would allow ions and ionic groups to fill, thus providing meaningful insights about the geological and geochemical processes experienced by it during the formation and transformation. The Raman spectrometer carried by the Perseverance and by ExoMars rovers will help the selection of samples enriched in gamma-CaSO4 at Jezero Crater and Oxia Planum, which should be sampled for in-depth analysis on Mars and back to Earth.

Automated summary

This study characterized the chemical and structural properties of gamma-CaSO4 from the hyperarid regions on Earth and Mars, finding that the abnormally high stability of these minerals was due to chemical impurities filling their tunnels and blocking the entrance of water molecules. The findings suggest that gamma-CaSO4 with impurities may have igneous or evaporative origins.