Polarised infrared spectra of synthetic single crystals and radiation-damaged natural samples were collected to examine hydroxyl incorporation in monazite. The IR spectra of pure synthetic monazite contain two OH stretching bands at 3163 and ∼3335 cm−1 with contrasting bandwidths of ∼40 and ∼90 cm−1, respectively. The two OH bands show strong pleochroism and dominant infrared absorption in the Y direction. The IR spectra of natural monazite contain a weak pleochroic OH band centred around 3400 cm−1 with a bandwidth of more than 200 cm−1. During step-heating experiments, this broad OH band split into several bands, and these bands differ from those observed in the spectra of synthetic samples. The OH stretching signals in the spectra of both natural and synthetic samples disappeared after heating at 1000 °C. The H2O concentration inferred from the infrared absorbance was 22(2) ppm in the synthetic monazite and 130–520 ppm in the natural samples. These results suggest that OH defects in natural monazite arise because of secondary hydration facilitated by radiation damage, as in the case of natural zircon and xenotime.

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