Microstructure evolution and weathering reactions of Synroc samples crystallized from CaZrTi{sub 2}O{sub 7} melts: TEM/AEM investigation and geochemical modeling
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 {l{underscore}brace}(Ca{sub 0.9}Gd{sub 0.1})Zr(Ti{sub 1.9}Al{sub 0.1}){sub 2}O{sub 7}{r{underscore}brace}. The formation of a whole suite of Synroc minerals (zirconia, ZrTiO{sub 4}, zirconolite, perovskite, and rutile) has been observed. The formation of these minerals follows the crystallization sequence of Ti-bearing zirconia {r{underscore}arrow} ZrTiO{sub 4} phase {r{underscore}arrow} Zr-rich zirconolite {r{underscore}arrow} Zr-poor zirconolite {r{underscore}arrow} rutile/perovskite. This sequence is induced by a fractional recrystallization 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{sub 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 radionuclides to leaching processes. Exsolution lamellae and multiple twinning 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 an oxidizing fluid shows that zirconolite is first altered into a perovskite-like phase (CaZrO{sub 3}), followed by rutile (TiO{sub 2}), and U{sup 6+}-bearing phases of soddyite [(UO{sub 2}){sub 2}SiO{sub 4}{center{underscore}dot}2H{sub 2}O] and haiweeite [Ca(UO{sub 2}){sub 2}Si{sub 6}O{sub 15}{center{underscore}dot}5H{sub 2}O].
- Research Organization:
- Univ. of New Mexico, Albuquerque, NM (US)
- Sponsoring Organization:
- National Science Foundation (NSF); National Aeronautics and Space Administration; State of New Mexico
- OSTI ID:
- 20015740
- Resource Relation:
- Conference: 1998 Materials Research Society Fall Meeting, Boston, MA (US), 11/30/1998--12/04/1998; Other Information: Single article reprints are available from University Microfilms Inc., 300 North Zeeb Road, Ann Arbor, Michigan 48106; PBD: 1999; Related Information: In: Scientific basis for nuclear waste management XXII. Materials Research Society symposium proceedings: Volume 556, by Wronkiewicz, D.J.; Lee, J.H. [eds.], 1355 pages.
- Country of Publication:
- United States
- Language:
- English
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