François FARGES1,* and Stéphanie ROSSANO2
1 Laboratoire des Géomatériaux, Université de Marne-la-Vallée, (and Stanford University and Institut Universitaire de France) 5 Boulevard Descartes – Champs S/Marne 77454 – Marne la Vallée Cedex, France
2 Laboratoire de Minéralogie-Cristallographie, Universités de Paris 6, 7 et IPGP, UMR CNRS 7590 – 2 place Jussieu, 75252 – Paris cedex 05, France (now at the Laboratoire des Géomatériaux, Université de Marne-la-Vallée)
This paper was presented at «Fluid/minerals Interactions in the Crust», a symposium in honour of Martine Lagache, Paris, October 1999
The local environment of 1400 ppm Zr in hydrous synthetic glasses (sodium trisilicate and albitic) has been probed at the Zr K-edge by X-ray Absorption Fine Structure (XAFS) spectroscopy and compared to that for natural glasses (volcanic and tektites). In most glasses (anhydrous and hydrous), Zr is found to be 6-coordinated by oxygen, with Zr-O distances of 2.08 Å. However, in anhydrous albite glass, the Zr-O distance increases to 2.12 Å, related to the presence of highly-coordinated Zr-environments (7,8) in the glass. Network-forming, next-nearest neighbour T (T = Si/A1) contributions are measured near 3.6 Å, suggesting corner-sharing ZrO6-TO4 polyhedra. We observed no large effect due to the presence of water in these glasses, suggesting that molecular water and hydroxyls are not likely to complex Zr in these compositions. However, a slight structural relaxation of the tetrahedral framework (increased network fragility with a small production of NBO’s) is observed around Zr in albitic compositions with more than 1 wt.% water. Then, no strong evidence for depolymerization induced by water is observed around Zr (Zr being used in that study as a probe for trace amounts of NBO’s). By comparison with transition elements (Fe and Ni), Zr appears to be much less affected by the introduction of water. The conclusions for Zr are likely to be extrapolated to natural magmas, as the natural glasses probed show similar environments around Zr. Then, oxygen appears the most efficient anion to «complex» Zr, as compared to water, hydroxyls, halogens, CO2 or S.
Key-words: Zr, silicate glasses, natural glasses, XANES, EXAFS..