Pyroxenes microstructure in comet 81P/Wild 2 terminal Stardust particles

We report the examination by transmission electron microscopy (TEM) of four Stardust terminal particles extracted from two neighboring tracks (32 an 69). The particles are made of well-preserved crystalline grains dominated by low-Ca pyroxene ranging from nearly pure enstatite to pigeonite. Some olivine grains are also found, in chemical equilibrium with the Surrounding pyroxenes. Various microstructures are observed, as a function of the composition of the grains. They include (100)-twinned pigeonite, clino/ortho domains in enstatite and exsolution in a Ca-rich grain. The microstructures are mostly consistent with a formation by cooling from high-temperature phases, which could be associated to igneous processes. Some dislocations in glide configuration are also present, probably attesting for small intensity shocks. Possible effects of the rapid heating/cooling stage and thermal shock associated to the collect are discussed. It appears that most of the microstructural features reported here are plausibly pristine.