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

Utsunomiya, S., Deditius, A.P., Pointeau, V. and Ewing, R.C. (2008) Coffinite and ningyoite from the natural, nuclear reactor at Bangombé, Gabon. In: Goldschmidt Conference 2008, 13 - 18 July 2008, Vancouver, Canada.


P-rich coffinite, U(Si,P)04•H20 , 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 crystalo-chemical properties of P-coffinite.

Electron microprobe analysis (EMPA) was completed only on coffinite inclusions (-100 fliil 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 P20 5) without uraninite inclusions, ii) P-coffinite, and iii) Si-ningyoite, (U,Ca,Ceh(P04)2•1-2H20. Phases ii) and iii) have inclusions of uraninite. The composition of coffinite i) is expressed to be CUo.79Cao.osREE+ Yo.04)o. s7(Si,.03Po.os)J.0704. The amount of (Y+REE)203 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 (Uo.7J-o. ss Cao.o6-o.uREE+ Yo.o7-o.Js)o.92- J.os(Sio. 39-o.s9Po.2s-o.4So.03-0.J2)o.s-o.9604, and the P20 s and (REE+Y)203 m·e 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 (U~.43_u 3REE+ Y0.2_ o.3C<lo.J s-o.23)J.88-2.3(Po.7-o.sSio.6-o.sSo.J-o.3Aso-o.J)J. 32-1.6404. 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 Siningyoite precipitated at the expense of uraninite under reducing conditons. The lack of con·elation between P and Y +REE in P-coffinite and Si-ningyoite implies that these minerals may consist of nano-scale intergowths of coffinite and homogeneously disttibuted (Ca,U,Y,REE)-phosphates. This is supported by the elemental maps, as well as TEM results.

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