3D Electron Diffraction as a powerful tool to study the earliest nanocrystalline weathering products: An uraninite

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Marco E. Ciriotti
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3D Electron Diffraction as a powerful tool to study the earliest nanocrystalline weathering products: An uraninite

Messaggio da Marco E. Ciriotti » ven 22 lug, 2022 9:17

Referenza:
▪ Plášil, J., Steciuk, G., Majzlan, J., Škoda, R., Filip, J., Petr, M., Kolařík, J., Klementová, M., Bähre, O., Klöß, G., Lapčák, L. (2022): 3D Electron Diffraction as a Powerful Tool to Study the Earliest Nanocrystalline Weathering Products: A Case Study of Uraninite Weathering. Earth and Space Chemistry, 6, 1250-1258.

Abstract:
Weathering of ore minerals proceeds through initial transient products to many crystalline secondary minerals. However, the initial products are usually poorly characterized or overlooked because of their extremely small particle size, poor crystallinity, and chemical variability. Here, we document the strength of the precession-assisted three-dimensional (3D) electron diffraction in the characterization of such nanocrystalline phases in a case study on uraninite-sulfide weathering in Jáchymov (Czech Republic). The glassy, yellow-to-green near-amorphous coatings on the ore fragments contain at least two phases. 3D electron diffraction identified K0.268[(U6+O2)2O(OH)2.25](H2O)0.676 as the dominant phase, yet unknown from nature, with fourmarierite topology of its uranyl sheets. The minor phase was characterized as K-rich fourmarierite, but its crystallinity was too low for complete structure refinement. Glassy and brownish coatings occur on samples that are not rich in uraninite. They are mainly composed of schwertmannite, i.e., iron oxides with structural sulfate and, in the case of our material, with a substantial amount of adsorbed uranium. This material contains up to 17 wt % of UO3,total and 0.5–1.4 wt % of CuO according to the WDS study. Surprisingly, X-ray photoelectron spectroscopy showed that the adsorbed uranium is a mixture of U(IV) and U(VI), the reduced species formed most probably during Fe(II) oxidation to Fe(III) and coeval precipitation of schwertmannite. Hence, here, uraninite weathering produces initial nanocrystalline phases with fourmarierite-sheet topology. In the abundance of iron, schwertmannite forms instead and adsorbs much uranium, both tetra- and hexavalent. This study demonstrates the power of 3D electron diffraction techniques, such as precession electron diffraction tomography, to study the alteration nanosized phases. Such nanocrystalline phases and minerals should be expected in each weathering system and may impart significant control over the fate of metals and metalloids in such systems.
Marco E. Ciriotti

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