Precipitation and Alteration of Coffinite in the Presence of Apatite - 2009

Uranium (Nuclear Minerals) / Recovery, Remediation and other Mining-Related Technology

Deditius and three co-authors reported late 2009 that a mineral assemblage of coffinite, USiO_4^.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 mm) 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)_{∂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.

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 mm.

Porosity also was formed during alteration of CFAp to M-Aut, which facilitated the migration of oxidizing fluids over distances of ~150 mm into CFAp, as evidenced by precipitation of M-Aut.

The authors reported, 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.