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3.6 b. Niobium in silicate glasses: Structural and geochemical role by XAS (D.B. Dingwell and E. Paris, in collaboration with C. Romano/Rome, G. Giuli/Florence and I. Davoli/Tor Vergata)

In spite of their low concentrations in natural rocks, the high-field strength elements (Nb, Ta, W, Zr...) can play an important role in affecting the behavior of a magma. Distinctive trace element concentrations are associated with different magma generation environments and structural characterization can provide important information on the petrogenesis of the magma as well as on all the processes that occur during the uprising of the magma, from the source region to the external surface of the Earth, which are responsible for the generation of the wide compositional spectrum of terrestrial igneous rocks. Although considered as network-modifiers in the melt structure, elements like Nb can be present in minerals (i.e. in crystalline structures) both in tetrahedral and octahedral coordination. This suggests that the structural role of Nb in glasses could be present in a variety of different polyhedral geometries, affecting the polymerization state of the melt and its structure-related properties. X-ray absorption spectroscopy (XAS) provides information on the coordination number of these elements as well as the cation-anion (oxygen) bond-length. We have examined Nb-bearing glasses by collecting Nb K-edge XANES and EXAFS spectra of a series of aluminosilicate glasses. The glasses were synthesized in the system SiO2-Al2O3-Na2O-K2O to which Nb was added in different concentrations, ranging from 0.1 to 10wt% Nb2O5. The spectra were collected at ESRF (GILDA) in fluorescence mode using a multidetector of high-purity Ge. A Nb metal foil was used for energy calibration and Nb2O5 as crystalline reference material. Comparison of the experimental data with multiple scattering calculations, using clusters with Nb having different coordination numbers and geometries, suggests the presence of different and mixed coordination environments around the Nb atom in these glasses. Major differences were observed particularly when comparing glasses of different base composition (e.g. peralkaline vs. peraluminous). For example, the XANES spectra indicate the presence of prevalent tetrahedral Nb in the HPG8 glass but octahedral Nb in the HPG8 + 10% Na2O glass. EXAFS data, although preliminary, agree well with the XANES observations, giving a Nb-O distance of 1.93 Å for the former and Nb-O=2.03 Å for the latter, values consistent with those obtained for Nb in tetrahedral and octahedral coordination in mineral structures, respectively. These findings indicate that the environment of high-field strength cations is sensitive to both Al/Si and Al/(Na+K) ratios. On the contrary, a comparison of glasses having the same base composition but increasing Nb2O5 content shows that the local environment around Nb does not change as a function of Nb concentration within the compositional range that was studied. The results obtained in this study will help rationalize the behavior of high-field strength elements in silicate melts and to shed some light in the interpretation of the composition dependence of macroscopic properties affected by the presence of these elements.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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