Infrared spectroscopic study of vibrational modes in Th-bearing, multi-REE natural monazites

The characterisation of rare earth elements (REE) in geological samples is of significant interest in critical mineral resource exploration and development. Hyperspectral technologies involving mid-infrared (MIR) and thermal (TIR) wavelength regions have a high potential for detecting REE-hosted minerals, such as phosphates and carbonates. Despite some technological advancements, the infrared spectral signatures are complicated by various scattering effects making their interpretation a significant challenge. In this study, the infrared spectral properties of single crystals of natural monazite samples were investigated using microscope-based Fourier transform infrared (micro-FTIR) spectroscopy. Two different micro-FTIR spectroscopy methods (i.e., Reflectance and Attenuated Total Reflectance, ATR) were used to evaluate the effect of crystal orientation, crystal face and surface/mineral impurities on the infrared spectral signatures. We found that the infrared spectrum of monazite is influenced by crystal faces hosting other accessory phases (e.g., muscovite, apatite), and this was confirmed by scanning electron microscopy (SEM) analysis. A correlation between monazite chemistry and peak positions has been established, with band shifts observed due to the sample's varying chemistry, particularly for the peak centered at ∼947 cm^-1. Trends were observed with Ca, Th, and Si substituting for P and REE in the crystal structure. We report for the first time infrared reflectance spectra of natural monazites and this is complemented by microanalytical characterisation. Our findings reveal no intra-sample compositional variation in monazite, and the FTIR spectroscopy results are interpreted in the context of accessory phases that contribute to the overall spectral signature. Furthermore, the spectra obtained from 'relatively pure' monazite regions are highly reproducible, reinforcing the reliability of the monazite infrared spectrum as a potential reference for proximal sensing and TIR spectroscopy during remote sensing applications.