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Precipitation and alteration of coffinite (USiO4·nH 2O) in the presence of apatite

Deditius, A.P., Utsunomiya, S., Pointeau, V. and Ewing, R.C. (2010) Precipitation and alteration of coffinite (USiO4·nH 2O) in the presence of apatite. European Journal of Mineralogy, 22 (1). pp. 75-88.

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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.

Item Type: Journal Article
Publisher: E. Schweizerbart'sche Verlagsbuchhandlung
Copyright: © 2009 E. Schweizerbart'sche Verlagsbuchhandlung.
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