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Nonequilibrium spherulitic magnetite in the Ryugu samples
We have investigated several particles collected during each of two touchdowns of the Hayabusa2 spacecraft at the surface of the C-type asteroid 162173 Ryugu using various electron microscope techniques. Our detailed transmission electron microscopy study shows the presence of magnetite with various morphologies coexisting in close proximity. This is characteristic of CI chondrite-like materials and consistent with the mineral assemblages and compositions in the Ryugu parent body. We describe the microstructural characteristics of magnetite with different morphologies, which could have resulted from the chemical conditions (growth vs. diffusion rate) during their formation. Furthermore, we describe the presence of magnetites with a spherulitic structure composed of individual radiating fibers that are characterized by pervasive, homogeneously distributed euhedral to subhedral pores that have not been described in previous chondrite studies. This particular spherulitic structure is consistent with crystallization under nonequilibrium conditions. Additionally, the presence of a high density of defects within the magnetite fibers, the high surface/volume ratio of this morphology, and the presence of amorphous materials in several pores and at the edges of the acicular fibers further support their formation under nonequilibrium conditions. We suggest that the growth processes that lead to this structure result from the solution reaching a supersaturated state, resulting in an adjustment to a lower free energy condition via nucleation and rapid growth.
The Hayabusa2 project has been developed and led by JAXA in collaboration with Deutsches Zentrum für Luft- und Raumfahrt (DLR) and Centre national d'études spatiales (CNES), and supported by NASA and Australian Space Agency (ASA). We thank all of the members of the Hayabusa2 project for their technical and scientific contributions. TM is supported by JSPS grants (No. 21K13981 and 21H05431). AJB is supported by NASA Emerging Worlds grant 80NSSC18K0731 and the JEOL NEOARM at the University of New Mexico by NSF MRI grant 1828731. HAI is supported by NASA Laboratory Analysis of Returned Samples grant 80NSSC19K0936. Analytical work at the University of Hawai‘i at Mānoa was supported by the Hawai‘i Institute of Geophysics & Planetology. Work at the Molecular Foundry was supported by the Office of Science, Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Author affiliationSchool of Physics & Astronomy, University of Leicester
- AM (Accepted Manuscript)