Raman microspectroscopy of monazite-(Ce) and xenotime-(Y): examples from experiments and nature

Abstract

This work presents an investigation of naturally and experimentally unaltered and altered monazite-(Ce) and xenotime-(Y), as well as secondary phases and synthetic REE phosphates (YPO4 and LaPO4–LuPO4) with Raman microspectroscopy, using 488 nm, 532 nm, 633 nm and 780 nm excitation lasers, supplemented by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Monazite-(Ce) spectra of the 532 nm laser from experimental products display a wide range of band positions of the v1(PO4) symmetric stretching band from 962 to 981 cm-1. Secondary fluorcalciobritholite spectra display a prominent band at 962–965 cm-1, a broader band at ca. 860 cm-1 and broad distinct luminescence effects at mid-range. Xenotime-(Y) spectra from experimental products display only minor spectral changes, except for a few spectra near secondary Y-rich fluorcalciobritholite. Spectra of Y-rich fluorcalciobritholite from 532 nm laser display characteristic broad luminescence effects at the range of 1500–3000 cm-1, including a strong luminescence band at ca. 2600 cm-1 (Sm3+ electronic transition), and are accompanied by a broad band at 974 cm-1 (488 nm laser) and 964 cm-1 (633 nm laser). The characteristic changes in monazite-(Ce) spectra near secondary phases are emphasised with hyperspectral maps and EPMA-WDS X-ray compositional maps, which provide spatial context and reveal underlying interferences, important to be noted, when evaluating altered minerals. In all cases, experimental and natural, high values of the full width at half maximum (FWHM) of the v1(PO4) band of unaltered monazite-(Ce) domains represent moderate radiation damage. In contrast, lower FWHM values were observed in altered domains. This work expands the Raman database onto examples of naturally and experimentally altered monazite-(Ce) and xenotime-(Y) enabling the identification of internal structural changes in unaltered and altered domains and differentiation of monazite-(Ce) or xenotime-(Y) from fluorcalciobritholite, Y-rich fluorcalciobritholite and other secondary phases.