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Structural state of rare earth elements in eudialyte-group minerals

DOI: 10.1180/mgm.2019.50 DOI Help

Authors: A. M. Borst (University of St Andrews) , A. A. Finch (University of St Andrews) , H. Friis (University of Oslo) , N. J. Horsburgh (University of St Andrews) , P. N. Gamaletsos (KU Leuven; Technical University of Denmark) , J. Goettlicher (Karlsruhe Institute of Technology) , R. Steininger (Karlsruhe Institute of Technology) , K. Geraki (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Mineralogical Magazine

State: Published (Approved)
Published: August 2019
Diamond Proposal Number(s): 14793 , 15903

Abstract: Eudialyte group minerals (EGM) attract global interest as potential resources for High Field Strength Elements (HFSE, e.g. Zr, Nb, Ta, and Rare Earth Elements, REE), i.e. critical materials for modern technologies. They are particularly valued for their relative enrichment in the most critical lanthanides, i.e. Nd and heavy REE (Gd-Lu). However, REE substitution mechanisms into the EGM structure are still poorly understood. Light and heavy REE may occupy different sites and there may be ordering and/or defect clustering in the structure. This study uses X-ray Absorption Spectroscopy (XAS) to determine the structural state of REE in EGM from prospective eudialyte-bearing complexes. Yttrium K-edge and Nd L3-edge spectra were collected as proxies for heavy and light REE, respectively, and compared to natural and synthetic REE-bearing standards. Extended X-ray Absorption Fine Structure (EXAFS) data yield best fits for Y in six-fold coordination with Y-O distances of 2.24-2.32 Å, and a second coordination sphere comprising Fe, Na, Ca, Si and O at radial distances of 3.6-3.8 Å. These findings are consistent with dominant Y3+ substitution for Ca2+ on the octahedral M1 site in all samples studied, and exclude preferential substitution of Y3+ onto the smaller octahedral Z site or the large low-symmetry N4 site. Using lattice strain theory, we constructed relative partitioning models to predict site preferences of lanthanides we have not directly measured. The models predict that all REE are favoured on the Ca-dominant M1 site and that preferential partitioning of heavy over light REE increases in EGM containing significant Mn in the M1-octahedral rings (oneillite 3 subgroup). Thus, the flat REE profiles that make EGM such attractive exploration targets are not due to preferential partitioning of light and heavy REE onto different sites. Instead, local ordering of Mn- and Ca-occupied M1 sites may influence the capacity of EGM to accommodate heavy REE.

Journal Keywords: Critical Metals; Eudialyte Group Minerals; Peralkaline Igneous Rocks; Rare Earth Elements; Partitioning Models; Lattice Strain Theory; XANES; EXAFS; X-Ray Absorption Spectroscopy; High Field Strength Elements

Subject Areas: Earth Science

Instruments: I18-Microfocus Spectroscopy