Mineralogy and crystallography of some Itokawa particles returned by the Hayabusa asteroidal sample return mission

Abstract

We studied seven Itokawa particles provided by the Japan Aerospace Exploration Agency (JAXA) as first International Announcement of Opportunity (AO) study mainly using electron and synchrotron radiation X-ray beam techniques. All the analyzed particles were collected from the first-touchdown site and composed of olivine and plagioclase with traces of Ca phosphate and chromite, and do not contain pyroxenes. Optical microscopy of these particles shows minor undulatory extinction of olivine and plagioclase, suggesting minor shock metamorphism (shock stage: S2). The electron microprobe analysis shows that olivine is Fo70-73 and plagioclase is An13-10Or5-7. The synchrotron radiation X-ray diffraction (SR-XRD) analysis of olivine crystals gives cell dimensions of a = 4.708 to 4.779 A, b = 10.271 to 10.289 A, c = 6.017 to 6.024 A, corresponding to the Fo content of Fo~70 by Vegard's law. This composition matches the result obtained by the electron microprobe analysis. The olivine compositions of the analyzed particles are consistent with those of LL chondrites. The cell dimensions of two plagioclase crystals (a = 8.180 to 8.194 A, b = 12.53 to 12.893 A, c = 7.125 to 7.23 A, α = 92.6° to 93.00°, β = 116.36° to 116.75°, γ = 90.03° to 90.17°) indicate that their equilibration temperatures are 800°C ± 10°C. This temperature is near the peak metamorphic temperature recorded by equilibrated ordinary chondrites. The size of plagioclase crystals and the homogeneity of olivine compositions indicate that their petrologic type is ≥5. We also analyzed plagioclase by SR iron X-ray absorption near-edge structure (SR-XANES) and found that its Fe3+/(Fe2+ + Fe3+) ratio is approximately 0.5. Such high Fe3+ abundance indicates the formation under a relatively oxidizing environment. Thus, all these analyses have reconfirmed that the Itokawa particles returned by the Hayabusa spacecraft are very weakly shocked equilibrated LL chondrites, which matches the results of the preliminary examination team.