RT Journal Article
T1 Mineral replacement reactions in naturally occurring hydrated uranyl phosphatesfrom the Tarabau deposit: Examples in the Cu–Ba uranyl phosphate system
A1 Pinto, André Filipe Jorge
A1 Conçalves, Mário A.
A1 Praceres, Cátia
A1 Astilleros García-Monge, José Manuel
A1 Batista, Maria Joao
AB Uranyl phosphates are a mineral group which include a wide range of different species, each containing specificcations within the hydrated interlayer, and often display a geochemical/mineralogical relationship withFe(III) oxy-hydroxides. The environmental relevance of these U-phases arises from their low solubility atmost 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 theNisa 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 mineralparagenesis, ii) assessing chemical homogeneity and stoichiometry of the most relevant phases and iii) unravelingpossible textural features of mineral reequilibration processes. XRD studies revealed that the analyzedsamples comprehend metatorbernite-like structures, consistent with Cu(UO2)2(PO4)2·8H2O formula. FurtherEMP 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 obtainedresults, combined with textural observations, allowed us to decipher mineral reequilibration reactions affectingthe 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 mechanismsof mineral replacement, with superimposed expressions. On one hand, the replacement of Cu by Ba uranylphosphate 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, possiblygiving rise to increasing mineral porosity.
PB Elsevier Science B.V., Amsterdam.
SN 0009-2541
YR 2012
FD 2012
LK https://hdl.handle.net/20.500.14352/42528
UL https://hdl.handle.net/20.500.14352/42528
LA eng
DS Docta Complutense
RD 29 abr 2024