Abstract
The behavior of natural monazite $(\mathrm{La},\phantom{\rule{0.16em}{0ex}}\mathrm{Ce},\phantom{\rule{0.16em}{0ex}}\mathrm{Nd})\mathrm{P}{\mathrm{O}}_{4}$ at high pressure is investigated by Raman spectroscopy. Changes in the Raman spectra indicate a phase transition from monazite to a post-monazite phase. Single-crystal x-ray diffraction measurements of the decompressed sample reveal that the post-monazite phase has a distorted barite-type structure with space group $P{2}_{1}/n$. This phase transition is irreversible under quasihydrostatic conditions, and is reversible under nonhydrostatic conditions. Therefore, it is expected that the post-monazite phase cannot be found as a metastable phase at ambient conditions in impact craters due to nonhydrostatic conditions during shock metamorphism, supporting recent mineralogical observations in impact craters.
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