DOES ZR-F COMPLEXATION OCCUR IN MAGMAS

The local environment around Zr (0.8-3 wt%) was investigated in quench ed glasses (albitic, basaltic, dacitic and rhyolitic) using: XAFS (X-r ay absorption fine structure) spectroscopy at the Zr K-edge. The local environment around Zr in F-free glasses is characterized by two contr ibutions, arising from O nearest neighbors (similar to 6 at 2.07-2.08 +/- 0.01 Angstrom) and next-nearest Si neighbors at similar to 3.68 +/ - 0.05 Angstrom. In the most highly polymerized glass (rhyolitic compo sition), the significantly larger mean Zr-O distance (2.10 +/- 0.01 An gstrom) and the appearance of Zr-Zr contributions at similar to 3.67 /- 0.05 Angstrom are interpreted as evidence for the appearance of min or amounts of 8-coordinated Zr. In F-bearing albitic glass, no clear e vidence was found for F first neighbors around Zr. Nonetheless, F is l ikely to affect the medium-range environment around Zr in this silicat e glass because of the depolymerization induced by this halogen. The l ocal structure around Zr in this set of glasses is very similar to tha t found in synthetic melts up to 1500 K and in natural glasses from va rious origins throughout the world. The possibility for REE-F complexe s is also discussed.