2025-09-12 パシフィック・ノースウェスト国立研究所（PNNL）
Goethite is found in laterite soils and hosts rare earth elements with heavy rare earth elements preferentially incorporated.
(Image by Derek Munson | Pacific Northwest National Laboratory)
＜関連情報＞
- https://www.pnnl.gov/publications/iron-minerals-can-fractionate-critical-rare-earth-elements
- https://pubs.rsc.org/en/content/articlelanding/2025/ra/d5ra04022a
構造への組み込みがゴエタイト中の希土類元素を分画する Structural incorporation into goethite fractionates rare earth elements
Sebastian T. Mergelsberg,Alex J. Kugler,Elaine D. Flynn,Eric J. Bylaska,Duo Song,Jeffrey G. Catalano and Eugene S. Ilton
RSC Advances Published:14 Aug 2025
DOI:https://doi.org/10.1039/D5RA04022A
Abstract
Up to 20% of rare earth elements (REEs) in ion adsorption deposits (IADs) are associated with iron oxide minerals, primarily goethite. Often termed “non-extractable”, goethite-hosted REEs are thought to be structurally incorporated into the mineral lattice. The large mismatch in size and charge density between REEs (ionic radius, r = 0.86–1.03 Å) and Fe3+ (r = 0.65 Å), however, makes direct substitution energetically unfavorable. To determine REE compatibility with and incorporation into goethite on the atomic level, we used X-ray pair distribution function analysis (PDF) and LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The compatibility of REEs with goethite and the precursor ferrihydrite (FH) is Lu ≥ Yb ≫ Dy > Nd for both phases. Nd and Dy primarily form secondary amorphous phases, with <30% Nd and Dy incorporated into goethite. In the FH precursor at pH 6.8, Yb and Lu assumed a local REE-OOH like structure with next nearest neighbor Fe. The Yb, Lu, and Nd-FH samples were also matured at ambient conditions for 100 days; despite the presence of only ∼5% goethite, Lu and Yb were 42% and 100% in goethite-like structural environments, respectively, whereas the PDF and EXAFS of Nd showed little evidence of any incorporation. Using ab initio molecular dynamics (AIMD) to model the EXAFS, we determined the presence of protonated Fe vacancies, edge-sharing with structural Lu and Yb, likely helped accommodate these REEs in the goethite structure. Incorporation into Fe oxyhydroxides thus potentially fractionates the REEs during weathering associated with formation of lateritic and ion adsorption deposits.
