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The mineralogy of the supergene-weathered Bangombé natural fission reactor (RZB) and surrounding uranium deposit has been analyzed and compared with the thermodynamically predicted minerals based on groundwater compositions. The primary U-minerals are uraninite UO 2+x  and minor coffinite (U[SiO 4 ]· nH 2 O) ; sometimes with phosphorous. The U 6+  minerals include fourmarierite Pb 1−x [(UO 2 ) 4 O 3−2x (OH) 4+2x ]·4(H 2 O), bassetite (Fe 1−x  2+ Fe x  3+ [(UO 2 )(PO 4 )] 2 (OH) x ·8− x H 2 O); possibly associated with U(HPO 4 ) 2 ·2H 2 O and/or chernikovite ((H 3 O) 2 [(UO 2 )(PO 4 )] 2 ·6H 2 O); torbernite (Cu[(UO 2 )(PO 4 )] 2 · 8-12H 2 O), Ce-françoisite-(Nd) (REE(UO 2 ) 3 O(OH)(PO 4 ) 2 · 6H 2 O), and uranopilite ((UO 2 ) 6 (SO 4 )O 2 (OH) 6 (H 2 O) 6 ·8H 2 O). Autunite (Ca[(UO 2 )(PO 4 )] 2 ·10-12H 2 O) has also been reported. Thermodynamic equilibrium modeling was completed using Geochemist's Workbench¯ with an expanded data base and the groundwater composition (−112 mV ≤ E h  ≤ 143 mV; pH = 5.96) at the base of RZB in drill-hole BAX03. The new ΔG 0  f,298.15  data were obtained from the literature or estimated using the polyhedral contribution method. Based on the updated database,  E h -pH  diagrams predict that coffinite, U(HPO 4 ) 2 ·H 2 O and UOF 2 ·H 2 O are the only stable U 4+  phases and that uranopilite, torbernite and bassetite will become stable during oxidative alteration. Except from UOF 2 ·H 2 O, this is in accord with mineralogical observations. The role of Cu was predicted from log a Cu -pH diagrams, which predicts that torbernite is stable at log a Cu =3.98×10 −14  and pH≥2.2 at E h =143 mV in BAX03 groundwaters. At E h  = −112 mV, torbernite is stable at pH&gt;5.5. Soddyite ((UO 2 ) 2 SiO 4 ·2H 2 O) was predicted to form at the expense of coffinite, but soddyite has not been identified at Bangombé. Previous blind prediction modeling, often omitting P and S, had only predicted soddyite and haiweeite (Ca(UO 2 ) 2 [Si 5 O 12 (OH) 2 ]·4-5H 2 O) and hence, failed to predict the U 6+  minerals observed at Bangombé. The results stress the importance of SO 4  2−  and PO 4  3−  resulting from dissolution of accessory apatite, monazite and sulfides in the retardation of U owing to the formation of U 6+  sulfates and phosphates.

U 6+ phases in the weathering zone of the Bangombé U-deposit: observed and predicted mineralogy