Mechanistic Insights into the Early-Stage Crystallization and Nanophase Formation of Metastable Light Rare-Earth...

Rare-earth element (REE) carbonates play a crucial role in geochemistry due to their prevalence in carbonatite ore deposits, which are extensively mined globally for REE extraction. Two primary mineral groups of interest are lanthanites (REE₂(CO₃)₃·8H₂O) and bastnäsites (REECO₃(OH,F)), typically enriched in light REE. This study aims to elucidate the mechanisms and kinetics of La-, Ce-, Pr-, and Nd-carbonate crystallization reactions at the earliest stages. REE-carbonates were synthesized through homogeneous crystallization by combining CO₃ ^2- and REE-bearing solutions at temperatures ranging from near-ambient to low hydrothermal conditions (5-80 °C). The crystallization processes were monitored in situ and in real-time using UV-vis spectrophotometry and synchrotron-based wide-angle X-ray scattering (WAXS). The characterization and quantification of the newly formed phases were conducted using a combination of conventional powder X-ray diffraction, high-resolution scanning electron microscopy with energy-dispersive spectroscopy and Fourier transform infrared spectroscopy. Our findings reveal a complex, multistep crystallization pathway specific to each REE, influenced by factors such as temperature, solution concentration and ratio, phase stability, and REE ionic potential. Additionally, the REE-carbonate crystallization pathways align with a progressive dehydration sequence involving multiple intermediate nanophases and reversible reactions. Notably, a reversible reaction between lanthanite and nanotengerite was observed at ambient temperature, involving structural rearrangements and hydration-dehydration processes. Our findings emphasize the importance of nanophase formation during the initial stages of REE-carbonate crystallization, with implications for the development of more efficient REE extraction methods.