Niobium speciation in minerals revealed by <i>L</i>2,3-edges XANES spectroscopy

Abstract The systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides similar chemistry. determination Nb speciation in solids (i.e., the distribution between phases and crystal-chemical environment Nb) thus a challenge geological contexts. Here, we present first L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various minerals synthetic relevance. interpretation XANES spectra light crystal-field theory shows sensitivity to local site symmetry electronic around atoms. Crystal-field multiplet simulations give estimates 10Dq parameter values for Nb5+, which range from 2.8 3.9 eV depending coordination Nb—O distances. Rather vs. R–5 relationship (where R represents average Nb-O bond distance) expected point-charge model, find R–3 dependence splitting reference materials octahedrally coordinated Nb. Complementary ligand-field provide evidence charge transfer O. contribution ionic covalent characters bonds equivalent, unlike 3d metal–O bonds. This characterization spectral properties provides information mechanisms Nb5+ substitutes Fe3+, Ti4+, or Ce4+ common Whereas substitution does not modify cation cerianite, Ti4+ rutile anatase results an increase cation-ligand distance decrease site. Conversely, Fe3+ hematite goethite smaller distortion. Our study demonstrates usefulness spectroscopy determine understand processes enrichment this critical metal.