Microstructure evolution and weathering reactions of Synroc samples crystallized form CaZrTi_2O_7 melts: TEM/AEM investigation and geochemical modeling

摘要

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and analytical electron microscopy (AEM) studies have been conducted on samples crystallized from a melt with a composition of zirconolite {Ca_(0.9)Gd_(0.1))Zr(Ti_(1.9)Al_(0.1))_2O_7}. The formation of a whole suite of Synroc minerals (zirconia, ZrTiO_4, zirconolite, perovskite, and rutile) has been observed. The formation of these minerals follows the crystallization sequence of Ti-bearing zirconia -> ZrTiO_4 phase -> Zr-rich zirconolite -> Zr-poor zirconolite -> rutile/perovskite. This sequence is induced by a fractional crystallization process, in which Zr-rich mineral phases tend to crystallize first, resulting in continuous depletion of Zr in melt. Consistent with this melt compositional evolution, Zr content in the zirconolite decreases from the area next to ZrTiO_4 phase to the area next to rutile or perovskite. High-resolution TEM images show that there are no glassy phases at the grain boundary between zirconolite and perovskite. The fractional crystallization-induced textural heterogeneity may have a significant impact on the incorporation of radionuclides into crystalline phases and the resistance of radioniclides to leaching proesses. Exsolution lamellae and multiple twining result from the phase transition from tetragonal zirconia to monoclinic zirconia may decrease durability of the Synroc. Fast cooling of melt may produce more zirconolite phase and relatively uniform texture. In general, however, a Synroc prepared by a through-melt method is less uniform in texture than that prepared by a through-sol-gel method. The reaction path calculation for the alteration of U-bearing zirconolite in al oxidizing fluid shows that zirconolite is first altered into a perovskite-like phase (CaZrO_3), followed by rutile (TiO_2), and U~(6+) -bearing phases of soddyite [UO_2)_2SiO_42H_2O] and haiweeite [Ca(UO_2)_2Si_6O_(15)5H_2O].