Search Results (9)

A structurally complex Paleozoic succession in southwestern Sinai hosts uranium and associated metals, and brittle deformation controls fluid flow and ore localization. The study integrates structural mapping with mineralogical, geochemical, and radiometric data to evaluate how fault architecture controls uranium and polymetallic mineral occurrences in the east Abu Zeneima area. Eleven representative samples were collected from major fault zones and host lithofacies, and 652 ground gamma-ray spectrometric measurements were acquired across mineralized localities and Paleozoic stratigraphic units. Heavy mineral separation, SEM–BSE/EDX, X-ray diffraction, and whole-rock geochemistry were used to identify ore and accessory phases and quantify their elemental composition. The middle carbonate member of the Um Bogma Formation is the primary host lithology and contains primary U dispersed within carbonaceous sandy dolostone and locally abundant secondary U phases coexisting with Cu–Fe–Mn phases and REE-bearing silicates and phosphates. Uranium enrichment (locally >2900 ppm eU) in the targeted anomalous samples shows a positive association with P₂O₅ and a weaker positive association with ΣREEs. Together with SEM–BSE/EDX and XRD identification of uranyl phosphates and REE-bearing accessory minerals, these observations suggest that phosphate-bearing secondary phases and REE-rich accessories locally contributed to uranium hosting. Seventy-four radioactive anomalies are predominantly associated with normal faults and are concentrated along fault cores and highly fractured downthrown blocks, especially along a NW–SE trend that forms the main mineralized corridor. The study findings emphasize the importance of fault zone architecture for targeting new uranium resources in Paleozoic basins. Full article

The subject of this work was supergene uranium mineralization and the YREE concentrations within. YREE differentiation patterns were used to recreate the prevailing crystallization conditions of abandoned mine dumps in Kromnów, Kopaniec, and Radoniów, located in the Izera Metamorphic Complex, Sudetes Mts. The collected samples were investigated using PXRD, SEM-EDS, and EPMA. YREE concentrations were measured using LA-ICP-MS. The secondary uranium mineralization from these locations consists of phosphates (meta-autunite, meta-torbernite, metauranocircite-I, saleéite, bassetite, phosphuranylite), arsenates (zeunerite), silicates (uranophane, sklodowskite), and uranyl hydroxides (likely becquerelite). Moreover, in Radoniów, phosphuranylite was found; it had not been found in Poland previously. Uranyl mineral assemblages indicate the diversity of chemistry of their mother solutions and suggest their weakly acidic character. The YREE content in secondary uranium minerals also reflects the pore solutions’ chemistry variation. The negative Y anomaly is observed in all uranyl phases. Similar behavior of Sm is also noted, excluding metatorbernite and torbernite. Among the uranyl minerals studied, only metatorbernite from Kromnów showed a positive Nb anomaly, which was probably related to proximity to weathering in YREE-breeding phases. Nevertheless, the YREE and chemical results suggest that this mineralization originated from the oxidizing solutions generated during the weathering of primary hydrothermal mineralization. In order to better understand the weathering zones in these locations, more detailed studies on pore solution chemistry are needed. Full article

To date, uranyl silicates are mostly represented by minerals in nature. However, their synthetic counterparts can be used as ion exchange materials. A new approach for the synthesis of framework uranyl silicates is reported. The new compounds Rb₂[(UO₂)₂(Si₈O₁₉)](H₂O)_2.5 (1), (K,Rb)₂[(UO₂)(Si₁₀O₂₂)] (2), [Rb₃Cl][(UO₂)(Si₄O₁₀)] (3) and [Cs₃Cl][(UO₂)(Si₄O₁₀)] (4) were prepared at harsh conditions in “activated” silica tubes at 900 °C. The activation of silica was performed using 40% hydrofluoric acid and lead oxide. Crystal structures of new uranyl silicates were solved by direct methods and refined: 1 is orthorhombic, Cmce, a = 14.5795(2) Å, b = 14.2083(2) Å, c = 23.1412(4) Å, V = 4793.70(13) ų, R1 = 0.023; 2 is monoclinic, C2/m, a = 23.0027(8) Å, b = 8.0983(3) Å, c = 11.9736(4) Å, β = 90.372(3) °, V = 2230.43(14) ų, R1 = 0.034; 3 is orthorhombic, Imma, a = 15.2712(12) Å, b = 7.9647(8) Å, c = 12.4607(9) Å, V = 1515.6(2) ų, R1 = 0.035, 4 is orthorhombic, Imma, a = 15.4148(8) Å, b = 7.9229(4) Å, c = 13.0214(7) Å, V = 1590.30(14) ų, R1 = 0.020. Their framework crystal structures contain channels up to 11.62 × 10.54 Å filled by various alkali metals. Full article

The introduction of the hydrogen economy, despite its obvious technological problems, creates a need for a significant number of niche-focused solutions, such as small-sized (10–100 W) fuel cells able to run on hydrogen of lesser purity than what is considered a standard in the case of PEMFCs. One of the solutions can be derived from the fact that an increase in the operational temperature of a cell significantly decreases its susceptibility to catalyst poisoning. Electrolytes suitable for the so-called medium temperature operational range of 120–400 °C, hence developed, are neither commercialized nor standardized. Among them, phosphate silicate protonically conductive glasses were found not only to reveal interestingly high levels of operational parameters, but also, to exhibit superior chemical and electrochemical stability over their polymeric counterparts. On the other hand, their mechanical properties, including cracking fragility, still need elaboration. Initial studies of the composite phosphate silicate glasses with uranyl-based protonic conductors, presented here, proved their value both in terms of application in fuel cell systems, and in terms of understanding the mechanism governing the charge transport mechanism in these and similar systems. It was found that whereas systems containing 10–20 wt% of the crystalline additive suffer from significant instability, materials containing 45–80 wt% (with an optimum at 60%) should be examined more thoughtfully. Moreover, the uranyl hydrogen phosphate was found to surpass its arsenate counterpart as an interesting self-healing behavior of the phase structure of the derived composite was proved. Full article

Crystals of two new inorganic uranyl silicates, Cs[(UO₆)₂(UO₂)₉(Si₂O₇)F] (1) and Rb₂[(PtO₄)(UO₂)₅(Si₂O₇)] (2), were produced from melts in evacuated silica tubes. Their structures have been solved by direct methods: 1 is trigonal, P-31c, a = 10.2040(3), c = 17.1278(5) Å, V = 1544.45(10) ų, R₁ = 0.042; 2 is tetragonal, P4/mbm, a = 16.0400(24), c = 3.9231(6) Å, V = 1009.34(10) ų, R₁ = 0.045. 1 is the first example of cation–cation interactions between the uranyl polyhedra in uranyl silicates. Therein, U^VI adopts three coordination modes, UO₆ octahedra, UO₆F, and UO₇ pentagonal bipyramids, with the latter sharing common edges to form U₂O₁₂ dimers. Three dimers associate into six-membered rings via cation–cation interactions. The structure of 1 can be described as a complex uranyl fluoride silicate framework with channels filled by the U1 atoms and disordered Cs⁺ cations. 2 represents a new type of topology never observed before among the structures of uranyl compounds; it is also a first complex uranium platinum oxide. Therein, the UO₆ tetragonal bipyramids share edges to form chains. Five such chains are stitched into a complex ribbon via the silicon polyhedra. The ribbons are connected into a framework by the PtO₄ squares; rubidium atoms are located in the channels of the framework. Full article

The Podgórze uranium deposit is located near Kowary in the Sudetes Mountains, SW Poland. The mine is located in the Karkonosze-Izera block, largely comprising Cambrian to Devonian metamorphic rocks intruded by the Variscan Karkonosze granite. Uranyl minerals from the Podgórze mine can be divided into three assemblages. The first one is associated with heavily ventilated mining galleries. The next assemblage is related to the outflow of water from fissures in the walls of the mine galleries. The last assemblage appears in the mine dump, where there is increased activity of other weathering products. The main purpose of this paper is to determine the mineralogical characteristics of uranyl minerals from the abandoned Podgórze uranium mine and reconstruct the physicochemical crystallization conditions based on the concentrations of rare earth elements (REEs) in these minerals. The results of thermodynamic modeling show that the aqueous species of uranyl ion in the mine water are represented by UO₂HAsO₄ (aq), UO₂CO₃(OH)₃⁻, UO₂CO₃ (aq), and UO₂OH⁺. The content of REEs and their distribution patterns were used to determine the crystallization conditions of uranyl minerals. Uranyl carbonates and arsenates have generally low concentrations of REEs compared to uranyl silicates, phosphates, and hydroxides, which have higher concentrations. The differences in REE concentration patterns may be related with the oxidizing nature of water circulating in the subsurface part of the deposit. Full article

Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl-silicate complexes), in the agates confirm their contemporaneous supply with SiO₂ and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO₂ with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A₁ modes) of alpha-quartz at 465 cm⁻¹ and moganite at 502 cm⁻¹, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO₂ under varying physico-chemical conditions and a discontinuous silica supply. Full article

Naturally occurring uranyl silicates are common constituents of the oxidized parts (i.e., supergene zone) of various types of uranium deposits. Their abundance reflects the widespread distribution of Si⁴⁺ in the Earth’s crust and, therefore, in groundwaters. Up to date, 16 uranyl silicate minerals are known. Noteworthy is that the natural uranyl silicates are not extremely diverse regarding their crystal structures; it is a result of possible concentrations (activity) of Si⁴⁺ in aqueous solutions derived from dissolution of primary Si minerals or the composition of late hydrothermal fluids. Therefore, in natural systems, we distinguish in fact among two groups of uranyl silicate minerals: uranophane and weeksite-group. They differ in U:Si ratio (uranophane, 1:1; weeksite, 2:5) and they form under different conditions, reflected in distinctive mineral associations. An overview of crystal-chemistry is provided in this paper, along with the new structure data for few members of the uranophane group. Calculations of the structural complexity parameters for natural uranyl silicates are commented about as well as other groups of uranyl minerals; these calculations are also presented from the point of view of the mineral paragenesis and associations. Full article

Incorporation reactions play an important role in dictating immobilization and release pathways for chemical species in low-temperature geologic environments. Quantum-mechanical investigations of incorporation seek to characterize the stability and geometry of incorporated structures, as well as the thermodynamics and kinetics of the reactions themselves. For a thermodynamic treatment of incorporation reactions, a source of the incorporated ion and a sink for the released ion is necessary. These sources/sinks in a real geochemical system can be solids, but more commonly, they are charged aqueous species. In this contribution, we review the current methods for ab initio calculations of incorporation reactions, many of which do not consider incorporation from aqueous species. We detail a recently-developed approach for the calculation of incorporation reactions and expand on the part that is modeling the interaction of periodic solids with aqueous source and sink phases and present new research using this approach. To model these interactions, a systematic series of calculations must be done to transform periodic solid source and sink phases to aqueous-phase clusters. Examples of this process are provided for three case studies: (1) neptunyl incorporation into studtite and boltwoodite: for the layered boltwoodite, the incorporation energies are smaller (more favorable) for reactions using environmentally relevant source and sink phases (i.e., ΔE_rxn(oxides) > ΔE_rxn(silicates) > ΔE_rxn(aqueous)). Estimates of the solid-solution behavior of Np⁵⁺/P⁵⁺- and U⁶⁺/Si⁴⁺-boltwoodite and Np⁵⁺/Ca²⁺- and U⁶⁺/K⁺-boltwoodite solid solutions are used to predict the limit of Np-incorporation into boltwoodite (172 and 768 ppm at 300 °C, respectively); (2) uranyl and neptunyl incorporation into carbonates and sulfates: for both carbonates and sulfates, it was found that actinyl incorporation into a defect site is more favorable than incorporation into defect-free periodic structures. In addition, actinyl incorporation into carbonates with aragonite structure is more favorable than into carbonates with calcite structure; and (3) uranium incorporation into magnetite: within the configurations tested that preserve charge neutrality (U⁶⁺ → 2Fe³⁺_oct/tet or U⁴⁺ → Fe²⁺_oct), uranium incorporation into magnetite is most favorable when U⁶⁺ replaces octahedral Fe³⁺ with charge balancing accomplished by an octahedral Fe³⁺ iron vacancy. At the end of this article, the limitations of this method and important sources of error inherent in these calculations (e.g., hydration energies) are discussed. Overall, this method and examples may serve as a guide for future studies of incorporation in a variety of contexts. Full article