PNNL

The Science

Iron oxide minerals, such as goethite, are known hosts of rare earth elements (REEs), with significant deposits that could potentially be used to bolster domestic supply chains. Despite the size difference between iron and REE ions, the REEs are directly incorporated into the mineral. Researchers combined experimental structural characterization and theoretical modeling to i) demonstrate incorporation and ii) identify the REE incorporation mechanism into goethite. They found that the presence of iron vacancies and nearby REE ion pairs enables the substitution of a REE atom for iron within the mineral structure. Incorporation favors smaller heavy REEs over larger light and medium REEs.

The Impact

REEs present a significant challenge for critical mineral extraction, as various REEs have similar properties and cannot be easily separated from one another. One potential avenue to help ease the separations process is to target materials that are naturally enriched with specific REEs. These results validate and explain the observed enrichment of heavy REEs in iron minerals at environmentally relevant conditions. This constitutes a natural mechanism that fractionates the REEs—a necessary step toward separating the closely related elements for industrial use. The new mechanistic understanding can be used to develop guidance for mining and processing these critical resources.

Summary

Up to 20% of REEs in weathering-based clay deposits are associated with iron oxide minerals, primarily goethite. Goethite-hosted REEs are structurally incorporated into the mineral lattice despite large mismatches in physical and chemical properties. To determine REE compatibility with and incorporation into goethite on the atomic level, researchers used high-resolution X-ray techniques to measure the structure of the minerals. They found that heavy REEs were more compatible and more easily incorporated than medium or light REEs. Using ab initio molecular dynamics to model the X-ray spectroscopy data, the team determined that the presence of protonated Fe vacancies, edge-sharing with structural Lu and Yb, likely helps accommodate the REEs in the goethite structure. Heavy REEs appear to form dimers inside the iron oxide, favoring paramagnetic ytterbium over non-magnetic lutetium. These discoveries provide mechanisms that explain the mystery of how iron minerals can accommodate such large cations. Beyond the fundamental mechanistic insight, these results highlight how incorporation into Fe oxyhydroxides acts as a method of fractionating for REEs during weathering processes associated with the formation of ore deposits.

Contact

Sebastian Mergelsberg, Pacific Northwest National Laboratory, sebastian.mergelsberg@pnnl.gov 

Eugene Ilton, Pacific Northwest National Laboratory, Eugene.Ilton@pnnl.gov

Funding

The work of J. G. C. and E. D. F. were supported by the Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences (BES), Critical Minerals and Materials program under Award Number DE-SC0022213. S. T. M, A. J. K., E. J. B., D. S., and E. S. I. were supported on the same award under FWP 78843. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the DOE, SC, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. This research used beamline 6-BM BMM of the National Synchrotron Light Source II, a DOE SC user facility operated for the DOE SC by Brookhaven National Laboratory under Contract No. DE-SC0012704. This research used resources of the Advanced Photon Source; a DOE SC user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract no. DE-AC02-06CH11357. A portion of this research was performed using the Environmental and Molecular Sciences Laboratory, a national user facility at PNNL sponsored by DOE's Biological and Environmental Research program. PNNL is a multiprogram national laboratory operated by Battelle Memorial Institute under contract no. DE-AC05-76RL01830 for the DOE. Simulations were performed using PNNL Institutional Computing resources and the National Energy Research Scientific Computing Center, a user facility supported by the SC of the DOE under Contract No. DE-AC02-05CH11231.