Structural state of rare earth elements in eudialyte-group minerals

Abstract

Eudialyte-group minerals (EGM) attract global interest as potential resources for high-field-strength elements (e.g. Zr, Nb, Ta, and rare-earth elements), 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 rare earth elements (Gd–Lu). However, rare earth element (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 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 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 the 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 measured directly. 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 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.

Adsorption of rare earth elements in regolith-hosted clay deposits

“…Analytical standards and blanks measured during the XAS studies were prepared as powders from natural and synthetic REE minerals in which the REEs occupy different coordination states ( Supplementary Data 1). Microcrystalline Ydoped NdPO 4 was synthesised at the University of St. Andrews following procedures described in Borst et al 40 . Synthetic crystals of Nd-doped YPO 4 were provided by Lynn Boatner, ORNL 66 .…”

“…Spectra for Y 2 O 3 (CN = 6) show a second peak at 17,064 eV (Feature B) that is as prominent as the main peak at 17,056 eV, comparing well to published Y K-edge XANES spectra for cubic Y 2 O 3 39 . EGMs are characterised by a double-peaked XANES with a minor peak at 17,053 eV (Feature A) and a broad maximum between A and B at 17,061 eV 40 . The spectra show a systematic shift in the position and shape of the broad peak around 17,104 eV (Feature C), shifting to higher energy with decreasing coordination numbers (CNs) (from 17,104 eV for Y in low symmetry 11-to 8-fold sites, to 17,110 eV for phases with Y in six-coordinated sites 40 ).…”

“…EGMs are characterised by a double-peaked XANES with a minor peak at 17,053 eV (Feature A) and a broad maximum between A and B at 17,061 eV 40 . The spectra show a systematic shift in the position and shape of the broad peak around 17,104 eV (Feature C), shifting to higher energy with decreasing coordination numbers (CNs) (from 17,104 eV for Y in low symmetry 11-to 8-fold sites, to 17,110 eV for phases with Y in six-coordinated sites 40 ). Figure 5b, c shows Y XANES for relict minerals and supergene phases in the laterite samples.…”

“…Standards in which LREEs are inferred to occupy higher symmetry sites (yttrofluorite, xenotime (YPO 4 -Nd), Nd 2 O 3 ) show more pronounced Nd XANES features (several or higher peaks at Feature A, or a stronger Feature B, Fig. 6a) and higher white lines than XANES of minerals in which LREEs occupy lower symmetry, or multiple sites 40 . XANES for Nd in aqueous solution (in weak HNO 3 ) demonstrates the highest white line among the standards, and additional features that are not observed in the other spectra (Fig.…”

Critical metal mineralogy and ore genesis revisited: thematic set arising from the Third International Critical Metals Meeting, Edinburgh

“…Understanding the geological and mineralogical processes which fractionate the REE to the right 'balance' is essential in order to ensure metal demand is appropriately met. In the second paper of this thematic set, Borst et al (2020) investigate the structural state of the REE in eudialyte-group minerals in order to determine why this phase has a well-balanced proportion of REE. Borst et al (2020) use X-ray spectroscopy to demonstrate that, in eudialyte, REE substitute for Ca in a single crystallographic site.…”