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

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 (U_0.95±_0.09Ca _0.15±_0.02)Σ_1.10± _0.1(Si_0.84±_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.