Structural environments of incompatible elements in silicate glass/melt systems: I. Zirconium at trace levels

The structural environments of trace levels (2̃000 ppm) of Zr4+ in several silicate glasses were examined as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Glass compositions investigated were albite (NaAlSi3O8: AB) and a peralkaline composition (Na3.3AlSi7O17: PR)- Zirconium was added to the oxide-carbonate mix prior to melting in the form of ZrO2 (baddeleyite). A second set of Zr-silicate glasses containing 2000 ppm Zr and 1.0 to 2.4 wt% halogens (F as NaF and Cl as NaCl) was also synthesized. These included the Zr-AB and Zr-PR base-glass compositions as well as Zr-sodium trisilicate composition (Na2Si3O7: TS). In all glasses studied, Zr is mainly 6-coordinated by oxygen atoms (d[Zr-O] $̃2.07 ± 0.01 Å). In the most polymerized glass (AB), a small but significant amount of Zr was also found to occur in 8-coordinated sites (d[Zr-O] $̃2.22 Å). No clear evidence for F or Cl complexes of Zr was observed in any of the halogen-containing glasses. The regularity of the Zr site increases in the series AB < TS $̃PR. We attribute this change to an increase in the number of non-bridging oxygens in the first-coordination sphere of Zr related to the depolymerizing effects of halogens and/or sodium. Minor but significant interactions of Zr with the tetrahedral network were observed (d[Zr-{Si, Al}] $̃3.65–3.71 Å ± 0.03 Å), which are consistent with Zr-O-{Si, Al} angles close to 160–170°, as in catapleiite (Na2ZrSi3O9 · 2H2O). Intermediaterange order, as reflected by the presence and number of second-neighbor {Si, Al} around Zr, increases significantly with increasing melt polymerization. The local environment around Zr is more strongly influenced by bonding requirements than by the network topology of the melt. Stabilization of zirconium in 6-coordinated sites in relatively depolymerized melts should act to decrease the crystal-melt partition coefficients of Zr and may explain the normally incompatible character of Zr during magmatic differentiation. The presence of Zr in sites of higher coordination (ZrO8) in highly polymerized melts could be a precursor to the crystallization of zircon from such melts and thus may explain why Zr becomes a more compatible element, especially in the latest stages of magmatic differentiation.