Uranyl (U6+) phases in the weathering zone of the Bangombe U-deposit: Observed and predicted mineralogy.

Jensen, K.A., Palenik, C.S. and Ewing, R.C. (2002) U6+-phases in the weathering zone of the Bangombe U-deposit: Observed and predicted mineralogy. . Radiochimica Acta v90, 1-9.

Published on: 4/11/2002

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 UO2+x and minor coffinite (U[SiO4]·nH2O); sometimes with phosphorous. The U6+ minerals include fourmarierite Pb1−x[(UO2)4O3−2x(OH)4+2x]·4(H2O), bassetite (Fe1−x2+Fex3+[(UO2)(PO4)]2(OH)x·8−xH2O); possibly associated with U(HPO4)2·2H2O and/or chernikovite ((H3O)2[(UO2)(PO4)]2·6H2O); torbernite (Cu[(UO2)(PO4)]2· 8-12H2O), Ce-françoisite-(Nd) (REE(UO2)3O(OH)(PO4)2· 6H2O), and uranopilite ((UO2)6(SO4)O2(OH)6(H2O)6·8H2O). Autunite (Ca[(UO2)(PO4)]2·10-12H2O) has also been reported. Thermodynamic equilibrium modeling was completed using Geochemist’s Workbench¯ with an expanded data base and the groundwater composition (−112 mV ≤ Eh ≤ 143 mV; pH = 5.96) at the base of RZB in drill-hole BAX03. The new ΔG0f,298.15 data were obtained from the literature or estimated using the polyhedral contribution method. Based on the updated database, Eh-pH diagrams predict that coffinite, U(HPO4)2·H2O and UOF2·H2O are the only stable U4+ phases and that uranopilite, torbernite and bassetite will become stable during oxidative alteration. Except from UOF2·H2O, this is in accord with mineralogical observations. The role of Cu was predicted from log aCu-pH diagrams, which predicts that torbernite is stable at log aCu=3.98×10−14 and pH≥2.2 at Eh=143 mV in BAX03 groundwaters. At Eh = −112 mV, torbernite is stable at pH>5.5. Soddyite ((UO2)2SiO4·2H2O) 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(UO2)2[Si5O12(OH)2]·4-5H2O) and hence, failed to predict the U6+ minerals observed at Bangombé. The results stress the importance of SO42− and PO43− resulting from dissolution of accessory apatite, monazite and sulfides in the retardation of U owing to the formation of U6+ sulfates and phosphates.

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