Microstructural indications for protoenstatite precursor of cometary MgSiO3 pyroxene:: A further high-temperature component of comet Wild 2

Microstructural studies by transmission electron microscope (TEM) techniques of a microtomed cometary enstatite (Mg/Si 0.858; Fe/Si 0.027; Ca/Si 0.01; Al/Si 0.009; Cr/Si 0.01) from Wild 2 sampled during NASA's Stardust mission were conducted. The enstatite is characterized by high stacking disorder parallel (100) which includes alternating clinoenstatite (CLEN) and orthoenstatite (OREN) lamellae and (100) twins. In addition a widespread occurrence of 4.5 angstrom wide half-planes parallel (100) are detected, which leads to 13.5 and 22.5 angstrom polytypes of the structure. These microstructural features are indicative of the direct transformation from a protoenstatite (PEN) precursor, which requires temperatures of more than 1275 K and rapid cooling (>10 K hr(-1)). Our finding represents a further high-temperature component originally present in the cold icy region where comet Wild 2 was formed.