Atomic-scale environment of niobium in ore minerals as revealed by XANES and EXAFS at the Nb K-edge

<p>The mineralogy of niobium (Nb) is characterized by multicomponent oxides such as AB<span class="inline-formula">₂</span>O<span class="inline-formula">₆</span>, A<span class="inline-formula">₂</span>B<span class="inline-formula">₂</span>O<span class="inline-formula">₇</span>, ABO<span class="inline-formula">₄</span>, and ABO<span class="inline-formula">₃</span> in which Nb is incorporated in the B site. Such complex crystal-chemistry prevents their unambiguous identification in ore deposits such as hydrothermal rocks and laterites which exhibit complex and fine-grained textures. The understanding of the processes controlling Nb ore deposit formation in various geological settings is therefore limited, although Nb is a critical element. In this study, we use X-ray absorption spectroscopy (XAS) at the Nb K-edge to investigate the local atomic-scale structure around Nb in a large set of natural and synthetic minerals of geological and technological importance. Our X-ray absorption near-edge structure (XANES) data at the Nb K-edge show three major features of variable position and intensity and then can be related to the local distortion and coordination number of the Nb site. Shell-by-shell fits of the extended X-ray absorption fine structure (EXAFS) data reveal that the NbO<span class="inline-formula">₆</span> octahedra are distorted in a variety of pyrochlore species. At least two distinct first shells of O atoms are present while reported crystallographic data yield regular octahedra in the same minerals. Next-nearest Nb–Nb distances in pyrochlore and Nb-bearing perovskite mirror a corner-sharing NbO<span class="inline-formula">₆</span> network, whereas the two Nb–Nb distances in columbite are typical of edge- and corner-sharing NbO<span class="inline-formula">₆</span> octahedra. Such a resolution on the Nb site geometry and the intersite relationships between the next-nearest NbO<span class="inline-formula">₆</span> octahedra is made possible by collecting EXAFS data under optimal conditions at 20 K and up to 16 Å<span class="inline-formula">⁻¹</span>. The local structure around substituted Nb<span class="inline-formula">⁵⁺</span> in Fe<span class="inline-formula">³⁺</span>, Ti<span class="inline-formula">⁴⁺</span>, and Ce<span class="inline-formula">⁴⁺</span> oxides suffers major changes relative to the unsubstituted structures. The substitution of Nb<span class="inline-formula">⁵⁺</span> for Ti<span class="inline-formula">⁴⁺</span> in anatase leads to the increase in the interatomic distances between Nb and its first and second Ti<span class="inline-formula">⁴⁺</span> neighbors. The substitution of Nb<span class="inline-formula">⁵⁺</span> for Ce<span class="inline-formula">⁴⁺</span> in cerianite reduces the coordination number of the cation from eight to four, and the Nb–O bonds are shortened compared to Ce–O ones. In hematite, Nb<span class="inline-formula">⁵⁺</span> occupies a regular site, whereas the Fe<span class="inline-formula">³⁺</span> site is strongly distorted suggesting major site relaxation due to charge mismatch. The sensitivity of XANES and EXAFS spectroscopies at the Nb K-edge to the local site geometry and next-nearest neighbors demonstrated in this study would help decipher Nb speciation and investigate mineralogical reactions of Nb minerals in deposit-related contexts such as hydrothermal and lateritic deposits.</p>