Synthesis and sructure investigation of USiO4 - comparison between local structure and bulk

Coffinite is a natural mineral with general composition USiO4 [1]. The structure (SG I 41/a m d) with tetravalent uranium in an eightfold coordination with oxygen and isolated silicate tetrahedra is isostructural to zircon, ZrSiO4, and thorite, ThSiO4. Coffinite is the second most important uranium ore. Under reducing conditions with high silica concentrations (c(SiO4) ≧10 -4 mol/L) the reaction of UO2 to USiO4 is favoured (Langmuir's criterion) [2]. Coffinite therefore may be a potential secondary phase in a deep geological repository for spent nuclear fuel. Estimates on the solubility of coffinite indicate a significantly lower uranium solubility compared to that of UO2, thus reducing the uranium source term significantly. Unfortunately, reliable and valdidated thermodynamic data are missing. However, the use of natural samples is not feasible, as these always are associated with significant amounts of impurities like ThO2, UO2, selenides, oxides of REE and organic matter. Because of the high alpha radiation damage, natural coffinit is also usually amorphous. The synthesis of coffinite proves to be rather challenging. Recently Pointeau et al. [3] reported the successful synthesis of coffinite by applying the protocol developed by Hoekstra and Fuchs [4]. However, the obtained products always contained amounts of nanocrystalline UO2. In contrast to recent studies [3,5] we were able to synthesize coffinite, USiO4, without impurities of UO2.

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