Structural environment of Nb~(5+) in dry and fluid-rich (H_2O, F) silicate glasses: a combined XANES and EXAFS study

摘要

We performed Nb K-edge XAFS spectroscopy experiments on a series of model compounds, natural glasses and anhydrous and fluid-bearing (H_2O, F) synthetic glasses doped with 0.1 to 5 wt. percent Nb_2O_5. In addition, we decribe a new ICP-AES method to provide chemical information on micro quantities of glass samples. In dry, H_2O- and F-bearing peraluminous and peralkaline glasses, Nb~(5+) is present as NbO_6 moeties. These share corners with SiO_4 and AlO_4 tetrahedra through non-bridging oxygen (NBO) atoms. No evidence for 4-, 5- or 7-coordinated Nb was found. Slight changes in site centrosymmetry were detected among the compositions (ordered in NBO-rich compositions, more disordered and distorted in peraluminous ones). In agreement with Nb_2O_5 solubility experiments, bond-valence theory suggests that network-modifying cations such as Na are present in the vicinity of Nb. This environment most closely resembles those observed in alkali niobosilicates such as vuonnemite and labuntsovite-group minerals. In dry, peraluminous glasses, the sites containing Nb are disordered, as in columbite. Similar information on speciation was obtained for a set of natural glasses. The local environment around the Nb atoms is therefore highly sensitive to melt depolymerization, alkalinity and H_2O, F contents, all of which will increase the solubility of Nb in the melt and explains a number of its geochemical properties in evolved magmatic systems.