AbstractOrdinary γ‐CaSO4 is a metastable calcium sulfate, while γ‐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 γ‐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 γ‐CaSO4, while both silicon and phosphorus were detected in Martian γ‐CaSO4. We found the abnormally high stability of those γ‐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 γ‐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 γ‐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 γ‐CaSO4. The structural tunnels in γ‐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 γ‐CaSO4 at Jezero Crater and Oxia Planum, which should be sampled for in‐depth analysis on Mars and back to Earth.