Surface features and alteration products of natural zirconolite leached in silica-saturated solutions
Zirconolite, CaZrTi{sub 2}P{sub 7}, has been proposed as an immobilization phase for the disposition of excess weapons Pu and other actinides (e.g., {sup 235}U). Due to actinide incorporation, zirconolite is expected to sustain {alpha}-decay event damage and become aperiodic (=metamict) over time. The leaching behavior of metamict zirconolite is, therefore, of interest. Because groundwater in a variety of geologic settings contains up to saturation concentrations of silicic acid, H{sub 4}SiO{sub 4}, silica-saturated solutions were used. Natural, metamict (>10{sup 26} {alpha}-decay events per m{sup 3}) zirconolite grains, nominally (Ca,Th)ZrTi{sub 2}O{sub 7} (US National Museum sample No. B20392, Walawada, Sri Lanka) were leached in two separate silica-saturated solutions at 150 C for 60 days. Surface features and alteration products were examined using scanning electron microscopy (SEM) and quantitative energy dispersive X-ray spectroscopy (EDS). Secondary electron (SEI) images of the surfaces of the leached grains from both experiments revealed pores, probably due to the accumulation of He-bubbles from {alpha}-decay events, of approximately 4% as estimated by contrast enhanced gray-scale analysis of digital images. SEI of the zirconolite surface before leaching showed a smooth surface. The pores not only increase the surface area of the metamict zirconolite, but also act as nucleation sites for alteration phase growth. One experiment was conducted in a silica-saturated solution containing approximately 100 ppm P as measured by atomic absorption spectroscopy (AAS). The main alteration phase was euhedral, monoclinic cheralite, (Th,Ca,Ce)(P,Si)O{sub 4} (monazite group). The second experiment was conducted in the absence of P. The main alteration phase was subhedral cubic thorianite, ThO{sub 2}.
- Research Organization:
- Univ. of New Mexico, Albuquerque, NM (US)
- OSTI ID:
- 20015755
- 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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