• journal_title：European Journal of Mineralogy
• Contributor：Artur P. Deditius ; Satoshi Utsunomiya ; Véronique Pointeau ; Rodney C. Ewing
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2010/0022-1990
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：22
• issue：1
• firstpage：75
• section：Articles

A mineral assemblage of coffinite, USiO₄·nH₂O, n = 0–2, carbonate-fluorapatite (CFAp) and (Ca, Sr)-(meta)autunite (M-Aut) from the Woodrow Mine, Grants uranium region, New Mexico, has been investigated in order to understand the influence of a P-rich micro-geochemical environment on precipitation of coffinite and its subsequent alteration under oxidizing conditions. Fine-grained coffinite (≤10 μm) precipitated under reducing conditions replacing CFAp, pyrite and aluminosilicates. Electron-microprobe analyses (EMPA) of coffinite indicate limited incorporation of P₂O₅ and CaO, <2.7 and <3.0 wt%, respectively, into the coffinite structure during replacement of CFAp. The chemical formula of coffinite is (U_0.95±0.09Ca_0.15±0.02)_∑1.10±0.1(Si_0.84±0.08P_0.06±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. 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(VI)-minerals, leading to the reduced mobility of U-species under both reducing and oxidizing conditions.