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Abstract:
Coexistence in natural samples of zircon (ZrSiO4) and reidite (high-pressure polymorph of ZrSiO4) is attributed to the effect of hypervelocity impact events. The grains and intergrowths in those minerals can be merely a few nanometers in size, which makes phase identification by standard methods of structure analysis difficult. However, analytical scanning transmission electron microscopy (STEM) utilizing electron energy-loss spectroscopy (EELS) can provide important information on phase transition mechanisms and pressure-temperature conditions of the associated shock event at the nanoscale. Here we demonstrate that the valence as well as oxygen core-loss EELS can be employed for nanoscale mapping of zircon-reidite distributions in zircon-reidite aggregates. Moreover, other accompanying phases, e.g., baddeleyite, could also be identified and mapped by this method. We further compare the EELS maps with a 4D-STEM nanobeam precession electron diffraction data, and demonstrate the advantages of the EELS mapping, which provides spatial resolution down to the nanometer scale and is independent on crystal orientation.
