Search

High-uranium natural zircon with signs of hydrothermal (up to 300°C) treatment is studied via transmission electron microscopy, Raman spectroscopy, electron probe microanalysis, and local energy-dispersive analysis performed in situ with a transmission electron microscope. Uranium-based phases are not found in the regions with uranium concentrations of up to 10 wt % that exceed considerably the limit of solubility in synthetic zirconium silicate. Uranium is dissolved in a heterogeneous matrix that is formed as a result of the decomposition of radiation-damaged (a dose of 1017 α-decays/mg) zircon under hydrothermal influence. The matrix incorporates amorphous partially hydrated silicon oxide and nanocrystalline cubic (Zr,Hf)1-y MeyO2 stabilized with metal Me impurities (Y, Ca, Al, Fe; y ≈ 0.04). Cubic zirconium oxide is regarded as the preferred phase for uranium localization, while the total impurity concentration is ~0.13.

Structure and thermodynamic properties of zircon, coffinite, and zircon-coffinite solid solutions were subject to semiempirical atomistic simulation on the assumption of compositional disordering of mixed crystals: (Zr1 − x , U x )SiO4, where x = 0.02, 0.05, 0.08, 0.11, 0.14, 0.28, 0.50, 0.72, and 0.86. The solid solutions significantly depart from the Vegard and Retgers laws. The (Zr1 − x , U x )SiO4 mixed crystal structure is characterized by anisotropic expansion (largely in the a and b directions) under growth in x related to anisotropy of structural relaxation degree of the cation-oxygen interatomic spacing in the (Zr, U)O8 polyhedron. Increase in x parameters is accompanied by increase in average size of cation-oxygen polyhedrons and silica-oxygen tetrahedrons and by growth of interatomic spacing dispersion; maximum dispersion values are observed at x = 0.5–0.6. The distortions in local structures of ZrO8 and UO8 dodecahedrons and silica-oxygen tetrahedrons in the solid solution were analyzed on the basis of calculated functions of interatomic distance distribution. The obtained results demonstrate the possibility to assess numerically the structural (geometrical) disordering degree of the compositionally disordered solid solution depending on its composition. The calculated thermodynamic characteristics of solid solutions forecast the following solubility range limits: 2 mol % USiO4 in zircon and 5 mol % ZrSiO4 in coffinite under ∼1750 K.