| Popis: |
We performed Nb K -edge XAFS spectroscopy experiments on a series of model compounds, natural glasses and anhydrous and fluid-bearing (H 2 O, F) synthetic glasses doped with 0.1 to 5 wt.% Nb 2 O 5 . In addition, we decribe a new ICP–AES method to provide chemical information on micro quantities of glass samples. In dry, H 2 O- 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 2 O 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 2 O, 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. |
