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

Abstract

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.

This study reveals complex, multistep pathways for light rare earth carbonate crystallization, emphasizing the role of nanophases and reversible reactions at early stages.