Journal article
Precipitation and alteration of coffinite (USiO4·nH 2O) in the presence of apatite
European Journal of Mineralogy, Vol.22(1), pp.75-88
2010
Abstract
A mineral assemblage of coffinite, USiO4-TiH2O, n= 0-2, carbonate-fluorapatite (CFAp) and (Ca, Sr)-(meta)autunite (MAut) from the Woodrow Mine, Grants uranium region, New Mexico, has been investigated in order to understand the influence of a Prich micro-geochemical environment on precipitation of coffinite and its subsequent alteration under oxidizing conditions. Finegrained coffinite (< 10 μm) precipitated under reducing conditions replacing CFAp, pyrite and aluminosilicates. Electron-microprobe analyses (EMPA) of coffinite indicate limited incorporation of P 2O5 and CaO, <2.7 and <3.0 wt%, respectively, into the coffinite structure during replacement of CFAp. The chemical formula of coffinite is (U0.95±0.09Ca 0.15±0.02)Σ1.10± 0.1(Si0.84±0.08P 0.06±0.02)<0.90 ± 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. Highresolution 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 urn. Porosity also was formed during alteration of CFAp to M-Aut, which facilitated the migration of oxidizing fluids over distances of ∼ 150 μm; in to CFAp, as evidenced by precipitation of M-Aut. We report, for the first time, the precipitation of coffinite at the expense of apatite and the subsequent alteration of coffinite under P-rich, oxidizing conditions. These results show that micro-scale dissolution of apatite can create conditions conducive to the precipitation of U(IV)- and U(VT)-minerals, leading to the reduced mobility of U-species under both reducing and oxidizing conditions.
Details
- Title
- Precipitation and alteration of coffinite (USiO4·nH 2O) in the presence of apatite
- Authors/Creators
- A.P. Deditius (Author/Creator)S. Utsunomiya (Author/Creator)V. Pointeau (Author/Creator)R.C. Ewing (Author/Creator)
- Publication Details
- European Journal of Mineralogy, Vol.22(1), pp.75-88
- Publisher
- E. Schweizerbart'sche Verlagsbuchhandlung
- Identifiers
- 991005542479307891
- Copyright
- © 2009 E. Schweizerbart'sche Verlagsbuchhandlung.
- Murdoch Affiliation
- Murdoch University
- Language
- English
- Resource Type
- Journal article
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- 8 Earth Sciences
- 8.242 Nuclear Geology
- 8.242.1362 Uranium Chemistry
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