Water in Zr-bearing synthetic and natural glasses
Water in Zr-bearing synthetic and natural glasses (in Fluid/mineral interactions in the crust, Anonymous)
European Journal of Mineralogy (December 2000) 12 (6): 1093-1107
The local environment of approximately 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 approximately 2.08 Aa. However, in anhydrous albite glass, the Zr-O distance increases to 2.12 Aa, related to the presence of highly-coordinated Zr-environments (7,8) in the glass. Network-forming, next-nearest neighbour T (T = Si/Al) contributions are measured near 3.6 Aa, suggesting corner-sharing ZrO (sub 6) -TO (sub 4) 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 deploymerization 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, CO (sub 2) or S.