3D Electron Diffraction as a Powerful Tool to Study the Earliest Nanocrystalline Weathering Products: A Case Study of Uraninite Weathering

Weathering of ore minerals proceeds through initialtransient products to many crystalline secondary minerals. However,the initial products are usually poorly characterized or overlookedbecause of their extremely small particle size, poor crystallinity, andchemical variability. Here, we document the strength of theprecession-assisted three-dimensional (3D) electron diffraction inthe characterization of such nanocrystalline phases in a case study onuraninite-sulfide weathering in Ja??chymov (Czech Republic). Theglassy, yellow-to-green near-amorphous coatings on the ore fragmentscontain at least two phases. 3D electron diffraction identifiedK0.268[(U6+O2)2O(OH)2.25](H2O)0.676as the dominant phase, yetunknown from nature, with fourmarierite topology of its uranylsheets. 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 materialcontains up to 17 wt % of UO3,totaland 0.5-1.4 wt % of CuO according to the WDS study. Surprisingly, X-ray photoelectronspectroscopy showed that the adsorbed uranium is a mixture of U(IV) and U(VI), the reduced species formed most probably duringFe(II) oxidation to Fe(III) and coeval precipitation of schwertmannite. Hence, here, uraninite weathering produces initialnanocrystalline phases with fourmarierite-sheet topology. In the abundance of iron, schwertmannite forms instead and adsorbs muchuranium, both tetra- and hexavalent. This study demonstrates the power of 3D electron diffraction techniques, such as precessionelectron diffraction tomography, to study the alteration nanosized phases. Such nanocrystalline phases and minerals should beexpected in each weathering system and may impart significant control over the fate of metals and metalloids in such systems.

Automated summary

The weathering of ore minerals produces initial products that are often poorly characterized but can significantly affect the fate of metals in the system. In this study, 3D electron diffraction techniques were used to characterize nanocrystalline phases in the weathering of uraninite-sulfide. The study identified previously unknown phases and revealed that the abundance of iron controls the formation of different weathering products.