The objectives of the proposal are to study olivine and pyroxene microstructure in the Itokawa particles as well as Fe-Mg cation ordering in orthopyroxene. For these two objectives, we will use transmission electron microscopy (TEM). The requested samples are thus microtomed slices.
(1) The microstructure is a marker of thermal and mechanical events which may have affected the Itokawa grains. Typically the microstructure is revealed through crystal defects (dislocations, twins, intergrowths, precipitates, staking faults, subgrain boundaries). With our experience on microstructural studies of silicates in meteorites, we propose to study the dislocation microstructure in olivine and Ca-rich pyroxene. We will characterise the dislocations character (Burgers vector, line orientation and slip systems). We will then relate our observations with literature data to give an estimate of the intensity of shock event(s). We would like to study several samples to track down a possible heterogeneous distribution of dislocations in the regolith grains.
(2) Preliminary studies by the Japanese team showed that the Itokawa grains are compatible with an equilibrated LL chondrite type, suggesting that Itokawa is a collection of fragments from at once larger asteroid. On a local scale (submicrometer level) we wish to study the magnitude of Fe-Mg ordering in enstatite, which is related to the thermal history. This part of study is based on the use ofprecession electron diffraction. We wish to estimate the Fe-Mg ordering rates in the Itokawa grains and determine if their thermal histories are similar or not. Two orthopyroxene rich samples are asked for this part of study.