Crystal Chemistry and Structural Complexity of the Uranyl Carbonate Minerals and Synthetic Compounds

Uranyl carbonates are one of the largest groups of secondary uranium(VI)-bearing natural phases being represented by 40 minerals approved by the International Mineralogical Association, overtaken only by uranyl phosphates and uranyl sulfates. Uranyl carbonate phases form during the direct alteration of primary U ores on contact with groundwaters enriched by CO2, thus playing an important role in the release of U to the environment. The presence of uranyl carbonate phases has also been detected on the surface of “lavas” that were formed during the Chernobyl accident. It is of interest that with all the importance and prevalence of these phases, about a quarter of approved minerals still have undetermined crystal structures, and the number of synthetic phases for which the structures were determined is significantly inferior to structurally characterized natural uranyl carbonates. In this work, we review the crystal chemistry of natural and synthetic uranyl carbonate phases. The majority of synthetic analogs of minerals were obtained from aqueous solutions at room temperature, which directly points to the absence of specific environmental conditions (increased P or T) for the formation of natural uranyl carbonates. Uranyl carbonates do not have excellent topological diversity and are mainly composed of finite clusters with rigid structures. Thus the structural architecture of uranyl carbonates is largely governed by the interstitial cations and the hydration state of the compounds. The information content is usually higher for minerals than for synthetic compounds of similar or close chemical composition, which likely points to the higher stability and preferred architectures of natural compounds.

Supergénne minerály z U-Cu rudného výskytu Východná-Nižný Chmelienec v Nízkych Tatrách (hronikum, Slovensko)

An association of supergene U-Cu and Y/REE minerals was found in a relic of old ore dump at the abandoned U deposit occurrence Východná-Nižný Chmelienec, the northern slopes of the Nízke Tatry Mts., Slovakia. They have partially recent origin, since exploration of the locality took place between 1965 and 1966. The studied mineral assem- blage is represented by goethite, malachite, uranophane and (meta)zeunerite, in a lesser extent baryte and rare zálesíite. Uranophane appears separately (globular aggregates, thin coatings) and it also forms the main part of the yellow to yellow-green crystalline crusts on the rock cracks. The chemical composition of the uranophane was determined by electron microprobe analyses and it is close to its ideal chemical formula Ca(UO2)2(SiO3OH)2·5H2O. The average chemical composition of the studied uranophane can be expressed by an empirical formula (Ca1.0Mg0.02K0.01Fe0.01Ba0.01)Σ1.05 (UO2)2.08(SiO3OH)1.84·5H2O. The infrared vibrational spectra of the studied uranophane show 3 (UO2)2+ at 850-760 cm-1; the 3 (SiO4)4- antisymmetric stretching vibration at 1000-900 cm-1; the 1 (SiO4)4- symmetric stretching vibration at 1150-1199 cm-1; the  H2O bending vibration at 1800-1600 cm-1 and OH stretching vibrations at 3407; 3408 and 3409 cm-1. The weak bands 2648; 2646 and 2651 cm-1 may be assigned to organic impurities. The calculated U-O bond length 1.83 Å corresponds to short U-O bonds in uranophane. The accessory admixtures of uranophane coatings are (meta)zeunerite and zálesíite. (Meta)zeunerite occasionally forms thin coatings of light green to emerald green tabular crystals (up tu 0.5 mm) on the surface of the rocks. Chemical analyses of (meta)zeunerite correspond to the empirical formula (Cu0.66K0.03Fe0.01Ca0.01)Σ0.71(UO2)2.11[(AsO4)1.96(PO4)0.01]Σ1.97·12H2O. Zálesíite occurs as crystalline aggregates, nests, formed by tiny acicular crystals, up to 100 µm in length. This is the second finding (occurrence) of this mineral in Slovakia. An average zálesíite chemical composition is (Ca0.83REE0.18U0.05Al0.03Ti0.01)Σ1.10(Cu5.81Fe0.06Zn0.02)Σ5.90[(AsO4)2.75 (SiO4)0.21(PO4)0.02(SO4)0.03]Σ3.01(OH)5.10·3H2O. Malachite, which has been also found in the association, is only a minor mineral in the studied locality. The formation of uranyl silicates (uranophane) and minerals of the mixite group (zálesíite), present at the studied locality, points to neutralization of acidic supergene fluids in the mine dumps. Possibly, this environment later (precipitation of baryte) passed to neutral or slightly basic conditions (precipitation of carbonates - malachite). The identified uranyl phosphates/arsenates (zeunerite/metazeunerite), typical of an acidic environment, are therefore rare.

Dissolution of poorly soluble uranyl phosphate phases in the Metaautunite Subgroup under uranyl peroxide cage cluster forming conditions

Uranyl phosphate minerals are widespread in uranium deposits and normally exhibit very low solubility in aqueous systems. Uranyl phosphates of the autunite group and metaautunite subgroup impact the mobility of uranium in the environment and have inspired groundwater remediation strategies that emphasize their low solubility. The importance of soluble uranium-bearing macro-anions, including nanoscale uranyl peroxide cage clusters, is largely unexplored relative to solubilization of normally low-solubility uranium minerals. Eight synthetic analogs of metaautunite subgroup minerals have been prepared and placed in various alkaline aqueous solutions containing hydrogen peroxide and tetraethylammonium hydroxide. Each uranyl phosphate studied has a topologically identical anionic sheet of uranyl square bipyramids and phosphate tetrahedra combined with various cations (Li+, Na+, K +, Rb+, Cs+, Mg2+, Ca2+, Ba2+) and water in the interlayer. Uranyl peroxides formed under many of the experimental conditions examined, including solid studtite [(UO2)(O2)(H2O)2](H2O)2 and soluble uranyl peroxide cage clusters containing as many as 28 uranyl ions. Uranyl phosphate solids in contact with solutions in which uranyl peroxide cage clusters formed dissolved extensively or completely. The greatest dissolution of uranyl phosphates occurred in systems that contained cations with larger hydrated radii, Li+ and Na+. The details of the uranium speciation in solution depended on the pH and counter cations provided from the interlayers of the uranyl phosphate solids.