MINERALOGYAND
CRYSTALLOGRAPHY
OF
ITOKAWA PARTICLES BY ELECTRON BEAM AND
SYNCHROTRON RADIATION X-RAY ANALYSES.
T. Mikouchi1, M. Komatsu2, K. Hagiya3, K. Ohsumi4, V. Hoffmann5,6, M. Zolensky7, J. Martinez7, Y. Terada4, N. Yagi4, M.
Takata4, W. Satake1, Y. Aoyagi1, A. Takenouchi1, R. Hochleitner8, M. Kaliwoda8, Y. Karouji9, M. Uesugi9, T. Yada9, M.
Miyamoto1. 1University of Tokyo, 2Waseda University,
3
University of Hyogo, 4JASRI, 5University of München,
6
University of Tübingen, 7NASA-JSC, 8Mineral. State Collection,München, 9JAXA-ISAS. E-mail: mikouchi@eps.s.u-tokyo.ac.jp.
Introduction: Itokawa particles returned by the JAXA
Hayabusa spacecraft are providing significant information about
the formation and evolution of meteorite parent bodies [e.g., 1].
The preliminary examination (PE) of these particles revealed that
their mineral compositions are close to those of equilibrated LL
chondrites [e.g., 2]. In this abstract we report mineralogical andcrystallographic studies on 4 new and 3 PE particles that wereallocated by JAXA as 1st international AO study.
Samples and Methods: At first 3 PE samples were allocated.
They are RA-QD02-0036, 0041 and 0049-2. We additionally received 4 new particles (RA-QD02-0179, 0138, 0100, and 013301) and made polished sections after being embedded in epoxy.
These particles were analyzed by optical microscope, FEG-SEM(Hitachi S-4500) and electron microprobe (JEOL JXA8900L).
They were also analyzed by synchrotron radiation (SR) XRD(SPring-8, BL37XU) and Fe-XANES (KEK, PF, BL-4A).
Results: The optical microscopic observation showed that allparticles were mostly composed of single crystal except for RAQD02-0179 (polycrystalline plagioclase). The shock stage wasestimated to be S2 for all samples because of the presence of minor undulatory extinction. The SEM-EDS analysis revealed thatthey are mainly composed of olivine with small amounts of plagioclase. Submicron-sized chromite and Ca phosphate were alsofound. No pyroxene was present in our samples. Olivine shows ahomogeneous composition in each particle, and is Fa30 with 0.4
wt% MnO. Olivine in RA-QD02-0179 is slightly Mg-rich (Fa26).
Plagioclase is also homogeneous (An12Or6, ~0.1 wt% FeO). TheSR-XRD analysis gave sharp diffraction spots, suggesting minor
shock metamorphism. Olivine gives the following cell dimensions: a=4.77-4.78, b=10.27-10.29, c=6.02 (Å), corresponding to
the chemical composition of Fa31-32 [3]. This matches the composition obtained by electron microprobe analysis. The cell dimensions obtained for plagioclase are a=8.18, b=12.53, c=7.13 (Å)
with α=93.0, β=116.4, γ=90.2 (o). The Fe-XANES analysis ofplagioclase suggests the presence of small amounts of Fe3+ although longer counting time is required in future analysis.
Discussion and Conclusion: The newly-analyzed particles
are all compositionally close to equilibrated LL chondrites. The
presence of Fe3+ in plagioclase is consistent with formation undera slightly oxidizing environment which is also consistent with LL
chondrites. Because the plagioclase crystallite size is larger than
2 µm, the petrologic type is ≥5. We did not find any unequilibrated particles as reported in [2]. Thus, this study further confirmsthat the Itokawa particles belong to equilibrated LL chondrites
with minor shock metamorphism.
References: [1] Nagao K. et al. 2011. Science 333:1128–1131. [2] Nakamura T. et al. 2011. Science 333:1113–1116.
[3] Akimoto S. and Fujisawa H. 1968. Journal of GeophysicalResearch 73:1467–1479.