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In situ study of the synthesis of thorite (ThSiO4) under environmental representative conditions

Estevenon, Paul; Causse, Jeremy; Szenknect, Stéphanie; Welcomme, Eléonore; Mesbah, Adel; Moisy, Philippe; Poinssot, Christophe; Dacheux, Nicolas

Thorite, ThSiO4 with Zircon structure type, is one of the most abundant natural source of thorium on earth. While actinides are known to form nanoparticles in silicate medium, no direct link between those colloids and crystalline form of thorite was evidenced until now. Here we show that thorite can be produced in experimental conditions close to environmental pH and temperature. Thanks to in-situ Small and Wide Angle X-rays Scattering (SWAXS) measurements, colloids of a few nanometers are first evidenced for low reaction time. These colloids exhibit elongated shapes and finally tend to aggregate after the size has reached 10 nm. Once aggregated, the system goes through a maturation step finishing with the emergence of nanocrystallites presenting thorite zircon structure. This maturation step is longer when the reaction temperature is decreased highlighting kinetic considerations. The conclusions of this article have potential implications in the paragenesis of Th minerals deposits, but also in the behaviour of Th and, by analogy, tetravalent actinides in the environment. The Th-silicate colloids evidenced in this work have, at low temperature and at near neutral pH, a long-term stability and a morphology in favor of a high mobility in groundwaters. If these species are formed in more diluted media, this could be problematic regarding to the spreading of Th and, by analogy of others tetravalent actinides in the environment.

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