STRUCTURAL ENVIRONMENTS OF INCOMPATIBLE ELEMENTS IN SILICATE GLASS MELT SYSTEMS .2. UIV, UV, AND UVI

The structural environments of trace to minor levels (almost-equal-to 2000 ppm to almost-equal-to 3.0 wt%) of U in several silicate glasses were examined as a function of oxygen fugacity, melt composition, and melt polymerization using X-ray (XANES and EXAFS) and optical absorption spectroscopies. Glass compositions were diopside (CaMgSi2O6:DI), anorthite (CaAlSi2O8:AN), albite (NaAlSi3O8:AB), sodium trisilicate (Na2Si3O7:TS), a peralkaline composition (Na3.3AlSi7O17:PR, approximately halfway between AB and TS), and a calc-alkaline rhyolite composition (RH). A second set of silicate glasses of the same base compositions containing almost-equal-to 2000 ppm to almost-equal-to 3.0 wt% U and almost-equal-to 0.6 to 2.5 wt% F or Cl was also synthesized. In the glasses synthesized under oxidizing conditions (in air), U(IV) occurs as uranyl groups with two axial oxygens at almost-equal-to 1.77-1.85 +/- 0.02 angstrom and four to five equatorial oxygens at almost-equal-to 2.21-2.25 +/- 0.03 angstrom. In glasses synthesized under more reducing conditions (f(O2), almost-equal-to 10(-3)-10(-7) atm), U(V) occurs in moderately distorted 6-coordinated polyhedra [d(U(V)-O) . 2.19-2.24 +/- 0.03 angstrom], which may co-exist with smaller numbers of U(VI) species and/or U(IV) species. Under the most reducing conditions used (f(O2), almost-equal-to 10(-8)-10(-12) atm), U(IV) occurs in less distorted octahedra [d(U(IV)-O) almost-equal-to 2.26-2.29 +/- 0.02 angstrom]. No clear evidence for U-F or U-Cl bonds was found for any of the halogen-containing glasses, suggesting that U-halogen ''complexes'' are not present. In addition, no U-U (second-neighbor) interactions were detected, indicating that no significant clustering of U atoms is present in any of the glasses studied. Bond strength-bond length calculations and constraints placed on local bonding by Pauling's second rule suggest that U(IV) and U(V) in 6-coordinated sites in silicate melts will preferentially bond to nonbridging oxygens (NBO's) rather than bridging oxygens (BO's). The unusually low 6-fold coordination of U(IV) and U(V) in relatively depolymerized silicate melts (e.g., peralkaline and halogen-rich melts) results in a high U-O bond strength in the melt that is not observed in crystalline U-bearing minerals. This difference in bond strength is partially responsible for the small crystal-melt partition coefficients of U(IV). In addition, the common silicate minerals comprising igneous rocks lack appropriate crystallographic sites which can stably accommodate this large and highly charged cation. These factors help explain the normally incompatible character of U(IV) during magmatic differentiation. In contrast, the low solubility of U(IV) and U(V) in more polymerized silicate melts, such as those produced during the late stages of magmatic differentiation, can be explained by a shortage of NBO's. Increasing amounts of 8-fold coordinated U should favor the incorporation of both U(IV) and U(V) in accessory minerals like zircon, thorite, titanite, apatite, uranium oxides, etc., thus its more compatible behavior in the latest stages of magmatic differentiation.