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| Titel |
Alteration of coffinite under reducing and oxidizing conditions |
| VerfasserIn |
Artur P. Deditius, Satoshi Utsunomiya, Veronique Pointeau, Rodney C. Ewing |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045266
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| Zusammenfassung |
| Coffinite, USiO4, is a one of the two naturally occurring actinide silicates (second is thorite,
ThSiO4) studied to elucidate the alteration of spent nuclear fuel (SNF) under reducing
conditions in a Si-rich environment. In order to understand the stability of coffinite under
different redox condition in natural systems, we have investigated coffinite the Grants
Uranium Belt, New Mexico, USA (reducing and oxidizing conditions) utilizing a variety of
electron microbeam techniques.
Fine-grained coffinite (-¤10 μm) from Woodrow Mine coexists with carbonate-fluorapatite
(CFAp) and (Ca,Sr)-(meta)autunite (M-Aut). It precipitated under reducing conditions
replacing CFAp, pyrite and aluminosilicates. Electron-microprobe analyses (EMPA) of
coffinite indicate limited incorporation of P2O5 and CaO, below 2.7 and 3.0 wt.%,
respectively, into the coffinite structure during replacement of CFAp. The chemical formula
of coffinite is (U0.95±0.09Ca0.15±0.02)Σ1.10±0.1(Si0.84±0.08P0.06±0.02)Σ0.90±0.08. Analysis by
high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
revealed that coffinite initially formed as crystals as large as 100 nm at the edges of altered
CFAp. Subsequently, infiltration of (Na,Ba,Sr)-rich oxidizing fluids into fractures resulted in
precipitation of Sr-rich M-Aut (up to 4 wt.% of SrO) at the expense of coffinite and CFAp.
High-resolution TEM reveals that Na-rich fluids caused a distortion of the ideal coffinite
structure and stabilized amorphous domains that formed due to alpha-decay event radiation
damage. Subsequently, the Na-enriched amorphous areas of coffinite were preferentially
altered, and secondary porosity formed at the scale of ~1 μm. Porosity also was
formed during alteration of CFAp to M-Aut, which facilitated the migration of
oxidizing fluids over distances of ~150 μm into CFAp, as evidenced by precipitation of
M-Aut.
These results show that micro-scale dissolution of apatite can create conditions conducive
to the precipitation of U(IV)- and U(VI)-minerals, leading to the reduced mobility of
U-species under both reducing and oxidizing conditions. It is concluded that coffinite
typically precipitates as micron- to nano-sized crystals within an amorphous matrix and is
highly stable under reducing conditions. The infiltration of Na-rich fluids into the amorphous
domains in coffinite prevented recrystallization by thermal annealing. It can be considered
that the amorphous domains were stabilized by the presence of Na. In contrast, coffinite is
altered extensively under oxidizing conditions and releases U into the ambient system,
however, the released U is immediately sequestered by the precipitation of secondary uranyl
minerals. |
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