Coffinite and ningyoite from the natural nuclear reactor at Bangombe, Gabon

Complete text of publication follows: P-rich coffinite, U(Si,P)O4.H2O, from the natural nuclear reactors in Bangombe, Gabon, is an important phase that incorporates percent levels of actinides and fission products. We have examined sample BAX03 (depth 12.2-12.3m) from Bangombe in order to understand micro- and nano-scale crystallo-chemical properties of P-coffinite. Electron microprobe analysis (EMPA) was completed only on coffinite inclusions (∼ 100 μm in size) in quartz to minimize the effect of alteration to U(VI)-phosphates and - sulfates. Based on the Si/P ratios three different chemical compositions; i) coffinite (≤1.45 wt.% of P2O5) without uraninite inclusions, ii) P-coffinite, and iii) Si-ningyoite, (U,Ca,Ce)2(PO4)2.1-2H2O. Phases ii) and iii) have inclusions of uraninite. The composition of coffinite i) is expressed to be (U0.79Ca0.05REE+Y0.04)0.88(Si1.03P0.05)1.08O4. The amount of (Y+REE)2O3 is ≤1.9 wt.%. Phosphorous substitutes for Si as evidenced by its negative correlation with Si (R2=0.87) and positive correlation with Y+REE (R2=0.6). The formula of P-coffinite (ii) is (U0.71-0.88 Ca0.06-0.11REE + Y0.07-0.18)0.92-1.08 (Si0.39-0.59P0.28-0.4 S0.03-0.12)0.8-0.96O4, and the P2O5 and (REE+Y)2O3 are as high as 9.3 wt.% and 8.64 wt.%, respectively. There is a positive correlation between Ca and P (R2=0.62), but no correlation among P, Y+REE and Si. The chemical formula of Si-rich ningyoite is (U1.43-1.73REE + Y0.2-0.3 Ca0.18-0.23)1.88-2.3 (P0.7-0.8 Si0.6-0.8 S0.1-0.3 As0-0.1)1.32-1.64O4. Silicon is positively correlated with P (R2=0.7). The EMPA elemental maps reveal homogeneous distribution of P, Si, Nd, Y and U in P-coffinite and Si-ningyoite. These results suggest that coffinite-(i) in Bangombe is not the alteration product of uraninite. But, P-coffinite and Si-ningyoite precipitated at the expense of uraninite under reducing conditions. The lack of correlation between P and Y+REE in P-coffinite and Si-ningyoite implies that these minerals may consist of nano-scale inter-growths of coffinite and homogeneously distributed (Ca,U,Y,REE)-phosphates. This is supported by the elemental maps, as well as TEM results