Energy-filtered transmission electron microscopy (EFTEM) of intergrown pyroxenes

It is demonstrated that energy-filtered transmission electron microscope (E FTEM) imaging in a conventional TEM (CTEM), equipped with a field-emission gun (FEG) electron source, can be used to characterize the local chemical d istribution in exsolved pyroxenes. EFTEM imaging, which can be performed in one to tens of minutes, yields two-dimensional compositional maps that can have nanometer-scale resolution. The combination of electron energy-loss s pectroscopy (EELS) and EFTEM imaging with techniques such as bright-field a nd dark-field imaging, high-resolution TEM (HRTEM) imaging, energy-dispersi ve X-ray spectroscopy (EDS), and electron diffraction allows for the chemic al and structural characterization of any sample able to withstand the elec tron beam. EFTEM imaging, HRTEM, and EDS data suggest that the augite --> orthopyroxen e reaction in the samples examined occurs in two-steps; augite --> pigeonit e --> orthopyroxene. In this two-step process, the chemical and structural components are accomplished separately, suggesting that it is energetically or kinetically favorable to dissociate the two components rather than have them occur simultaneously. This two-step transformation is supported by th e pigeonite --> orthopyroxene transformation, which appears to be an isothe rmal martensitic transformation since the pigeonite and orthopyroxene compo sitions are identical within analytical error.