Mineral replacement reactions in naturally occurring hydrated uranyl phosphates from the Tarabau deposit: Examples in the Cu–Ba uranyl phosphate system

Abstract

Uranyl phosphates are a mineral group which include a wide range of different species, each containing specific cations within the hydrated interlayer, and often display a geochemical/mineralogical relationship with Fe(III) oxy-hydroxides. The environmental relevance of these U-phases arises from their low solubility at most surface and groundwater conditions, where they can ultimately control aqueous U levels. In the present work, samples of naturally occurring uranyl phosphates from the Tarabau site, included in the Nisa deposit, located in central-eastern Portugal, are studied with X-ray diffraction (XRD), Electron Microprobe (EMP) and Scanning Electron Microscopy, with the purpose of i) identifying uranyl phosphate mineral paragenesis, ii) assessing chemical homogeneity and stoichiometry of the most relevant phases and iii) unraveling possible textural features of mineral reequilibration processes. XRD studies revealed that the analyzed samples comprehend metatorbernite-like structures, consistent with Cu(UO2)2(PO4)2·8H2O formula. Further EMP determinations allowed the definition of nearly stoichiometric Cu and Ba hydrated uranyl phosphates; CuxBa1−x(UO2)2(PO4)2·nH2O intermediate compositions and interlayer cation-depleted phases. The obtained results, combined with textural observations, allowed us to decipher mineral reequilibration reactions affecting the studied samples. Thus, reactive paths involving the replacement of Cu-bearing by Ba-bearing uranyl phosphates, cation-bearing uranyl phosphate by cation-depleted uranyl phosphate and cation-bearing uranyl phosphate by Fe, Al oxy-hydroxides have been defined. However, the studied textural features point toward two different mechanisms of mineral replacement, with superimposed expressions. On one hand, the replacement of Cu by Ba uranyl phosphate phases, and these last by oxy-hydroxides, takes place by coupled dissolution–precipitation reactions. On the other hand, cation depletion affecting uranyl phosphates occurs by a cation exchange process, possibly giving rise to increasing mineral porosity.