Coffinitization of Uraninite: SEM/AEM Investigation and Geochemical Modeling
Coffinite (USiO{sub 4}) has been found in numerous sedimentary and hydrothermal environments including those considered as natural analogues of nuclear waste repositories. Scanning electron microscopy (SEM) and analytical electron microscopy (AEM) studies have been conducted on a uraninite sample from a U-deposit in Canada. It is observed that the uraninite (UO{sub 2+x}) is replaced by coffinite (U[SiO{sub 4}].nH{sub 2}O) and the replacing coffinite coexists with quartz. The TEM study shows {alpha}-recoil damage, lattice distortion, and low-angle boundaries among neighboring uraninite domains. Coffinitization seems more closely associated with {alpha}-recoil-damaged uraninite areas. Electron energy-loss spectroscopy (EELS) spectrum indicates that the ratio of U(+6)U(+4) in the uraninite is about 2/3, while the coffinite is dominated by U(+4). A thermodynamic calculation indicates that coffinitization can take place most likely at temperatures below 130 C if dissolved silica concentrations are limited by amorphous silica mineral phase. In a sufficiently high silica concentration environment, coffinite can form under the oxygen fugacity of 10{sup -65}-10{sup -55} atm. The equilibrium model, however, is not able to explain the coexistence of coffinite with quartz. A kinetic model that takes account of Ostwald processes is thus proposed. The kinetic model indicates that the presence of U(+6) in uraninite and the enhanced uraninite dissolution rate may be an important factor controlling uraninite coffinitization.
- Research Organization:
- Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Sandia National Lab. (SNL-CA), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC04-94AL85000
- OSTI ID:
- 14096
- Report Number(s):
- SAND99-2626C; TRN: US0110965
- Resource Relation:
- Conference: ICEM '99, Nagoya (JP), 09/26/1999--09/30/1999; Other Information: PBD: 7 Oct 1999
- Country of Publication:
- United States
- Language:
- English
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