Mineralogical approach on laboratory weathering of uncontaminated Ryugu particles: Comparison with Orgueil and perspective for storage and analysis

Although CI chondrites are susceptible to terrestrial weathering on Earth, the specific processes are unknown. To elucidate the weathering mechanism, we conduct a laboratory experiment using pristine particles from asteroid Ryugu. Air-exposed particles predominantly develop small-sized euhedral Ca-S-rich grains (0.5–1 μm) on the particle surface and along open cracks. Both transmission electron microscopy and synchrotron-based computed tomography combined with XRD reveal that the grains are hydrous Ca-sulfate. Notably, this phase does not form in vacuum- or nitrogen-stored particles, suggesting this result is due to laboratory weathering. We also compare the Orgueil CI chondrite with the altered Ryugu particles. Due to the weathering of pyrrhotite and dolomite, Orgueil contains a significant amount of gypsum and ferrihydrite. We suggest that mineralogical changes due to terrestrial weathering of particles returned directly from asteroid occur even after a short-time air exposure. Consequently, conducting prompt analyses and ensuring proper storage conditions are crucial, especially to preserve the primordial features of organics and volatiles.