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

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 (REE(CO)·8HO) and bastnäsites (REECO(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.

Chemical Textures on Rare Earth Carbonates: An Experimental Approach to Mimic the Formation of Bastnäsite.

The interaction between multi-component rare earth element (REE) aqueous solutions and carbonate grains (dolomite, aragonite, and calcite) are studied at hydrothermal conditions (21-210 °C). The effect of ionic radii of five REEs (La, Ce, Pr, Nd, Dy) on solid formation are analyzed using two solution types: equal REE concentrations and concentrations normalized to Post Archean Australian Shale Standard (PAAS). The interaction replaces the host Ca-Mg carbonate grains with a series of REE minerals (lanthanite → kozoite → bastnäsite → cerianite).

Impact of Rare Earth Elements on CaCO Crystallization: Insights into Kinetics, Mechanisms, and Crystal Morphology.

This study investigated the crystallization kinetics and mechanisms of calcium carbonate (CaCO) in the presence of rare earth elements (REEs) including lanthanum (La), neodymium (Nd), and dysprosium (Dy). Through a comprehensive approach utilizing UV-vis spectrophotometry, powder X-ray diffraction, and high-resolution electron microscopy, we examined the effects of REEs on CaCO growth from solution at varying concentrations and combinations of REEs. Our findings highlight that even trace amounts of REEs significantly decelerate the rate of CaCO crystallization, also leading to alterations in crystal morphology and mechanisms of growth.

The role of nanocerianite (CeO) in the stability of Ce carbonates at low-hydrothermal conditions.

The formation of cerianite (CeO) was investigated at low hydrothermal conditions (35-205 °C) two experimental settings: (1) crystallisation from solution experiments, and (2) replacement of Ca-Mg carbonates (calcite, dolomite, aragonite) mediated by Ce-bearing aqueous solutions. The solid samples were studied with a combination of powder X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The results revealed a multi-step crystallisation pathway: amorphous Ce carbonate → Ce-lanthanite [Ce(CO)·8HO] → Ce-kozoite [orthorhombic CeCO(OH)] → Ce-hydroxylbastnasite [hexagonal CeCO(OH)] → cerianite [CeO].

Targeted Crystallization of Rare Earth Carbonate Polymorphs at Hydrothermal Conditions via Mineral Replacement Reactions.

The interaction between rare earth element (REE)-rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO)), and aragonite (CaCO) at low temperature hydrothermal conditions (25-220 °C) is studied. The experiments result in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE-carbonates. The host grains are replaced from periphery inward.