Quantification of the REE3+ aqua ions and chloride species in aqueous fluids by in situ Raman spectroscopy using perturbations of the water band

Abstract

Acidic NaCl-rich aqueous fluids play a crucial role in forming hydrothermal rare earth elements (REE) mineral deposits. Aqueous REE mobility is mostly controlled by the stability of REE³⁺ and REE chloride species. Our current knowledge of REE speciation is based on solubility data, thermodynamic models and in situ spectroscopic measurements, sometimes coupled with molecular simulations. Here we investigate Nd and Yb speciation in pH 2 chloride-bearing solutions at 25 °C and 0.1 MPa with variable Cl/REE ratios using Raman spectroscopy in solutions with 0.1 to 0.6 mol/kg NdCl₃ or YbCl₃ and 0.2 to 3.2 mol/kg NaCl. Due to the challenges in resolving the REE-Cl band, we develop a new method using the water vibrational mode and multivariate curve resolution (MCR) analysis. The Raman spectra for the vibrational band of water (2700 to 3900 cm⁻¹) were collected at 25 °C and fitted by three Gaussian sub-peaks, then quantified using MCR analysis to de-convolute the water band into bulk H₂O and the perturbations caused by of Cl⁻, REE³⁺, and REE chloride species. REE speciation based on the perturbations of the water band indicates that REE³⁺ aqua ions dominate in acidic solutions at 25 °C, but up to ∼20 mol% YbCl²⁺ forms at high YbCl₃ concentrations. The new method is promising for quantifying in situ speciation of the REE³⁺ aqua ions and REE chloride species in aqueous fluids while providing information on the hydration of ions. This method improves our molecular level understanding of REE aqueous species and their role in REE mobilization during fluid-rock interaction.