Niobium speciation in minerals revealed byL2,3-edges XANES spectroscopy

Abstract

The complexity of niobium (Nb) mineralogy, dominated by more than one hundred oxides of similar crystal chemistry, makes it particularly challenging to determine Nb speciation in minerals.Here, we describe the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) data on Nb-bearing minerals and synthetic oxides relevant to geological contexts. The interpretation of Nb L2,3-edges XANES spectra in light of crystal-field theory mirrors the sensitivity of spectra to the local site symmetry and the electronic configuration around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter of Nb 5+ , ranging from 2.8 to 3.9 eV depending on Nb coordination and Nb-O distances. In particular, the 10Dq vs. R -5 relationship (where R represents the average Nb-O bond distance) is observed for octahedrally-coordinated Nb, as expected in a point-charge model. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The Nb-O bonds show an equivalent mixing between ionic and covalent character unlike more ionic 3d metal-O bonds. These interpretations provide information on the mechanisms by which Nb 5+ substitutes for Fe 3+ , Ti 4+ or Ce 4+ in oxides common in environmental contexts. Whereas the substitution of Nb 5+ for Ce 4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb 5+ for Ti 4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site.Conversely, the substitution of Nb 5+ for Fe 3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of Nb L 2,3 -edges XANES spectroscopy to constrain Nb speciation in minerals with direct relevance for understanding the processes that can concentrate this critical metal at economic levels.

Multiscale processes controlling niobium mobility during supergene weathering

“…A large set of natural and synthetic Nb-bearing reference materials were considered as potential compounds for X-ray absorption near-edge structure (XANES) analyses (Bollaert et al, 2023). Apart from columbite-(Mn) and Nb-bearing ilmenite which were provided by École Nationale Supérieure des Mines de Paris (MINES ParisTech), Nb reference materials were synthesized in order to incorporate Nb concentrations representative of natural enrichment.…”

“…The synthesis of Nb-substituted anatase and rutile has been carried out using a sol-gel method at moderate temperature. Detailed mineral synthesis procedures are described in Bollaert et al (2023). The synthesis of Nb-sorbed phases at concentrations representative of natural lateritic media could not be carried out because Nb oxides reprecipitation instantly before adsorption.…”

“…The atomic environment of Nb in type-1 and type-2 Ti oxides, Fe oxides, ilmenite and columbite has been characterized using Nb L 3 -edge micro-XANES spectroscopy and compared with spectra of mineral references (Bollaert et al, 2023).…”

“…Goethite is a major host of Nb in the weathering profile (Table 5). Due to similar ionic radius ( [6] Fe 3+ = 0.645 Å and [6] Nb 5+ = 0.64 Å, -Shannon, 1976) substitution between Nb 5+ and Fe 3+ is made possible by the formation of Fe vacancies (5Fe 3+ for 3Nb 5+ ) which maintain charge balance (Bollaert et al, 2023). In goethite, the Nb 5+ −Fe 3+ substitution process could be accompanied by the loss of two H + occupying the tunnels between the double chains which would lead to the formation of NbO 5 (OH) octahedra.…”

“…By contrast, the similarity between the spectra of type-2 Ti oxide and Nb-bearing rutile reference indicates similar Nb environment and therefore similar mechanisms of substitution. In absence of Fe, the incorporation of Nb results in the increased distortion of the Nb site and Nb−O distances (Bollaert et al, 2023). In this case, the incorporation of Nb is made possible by the formation of one Ti 4+ vacancy for four Nb 5+ which leads to the increase of Nb−O distances (Bollaert et al, 2023).…”