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

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.

Capturing ammonium nitrate in a synthetic uranium oxide hydrate phase: revealing the role of ammonium ions and anion inclusions

“…UOH minerals are abundant products of the oxidation-hydration weathering of uraninite and they commonly occur in the mineral associations involving sulfate minerals formed under acidic conditions. These mineral associations are related to the acid-mine-drainage (AMD) environments, 57 where microbial activity often occurs and thus, we can expect nitrogen to be present in the alteration system. Furthermore, several minerals and unnamed phases have been reported, 57 with particularly suspicious low-cation populations (cation-deficiencies).…”

“…These mineral associations are related to the acid-mine-drainage (AMD) environments, 57 where microbial activity often occurs and thus, we can expect nitrogen to be present in the alteration system. Furthermore, several minerals and unnamed phases have been reported, 57 with particularly suspicious low-cation populations (cation-deficiencies). Such observations most commonly concern minerals containing K + as a dominant cation, and thus, it is highly probable that NH 4 + might be present to compensate for the cation deficiency.…”