Using high-precision scanning electron microscopy and energy dispersive X-ray spectroscopy, this study comprehensively investigated highly heterogeneous solid radioactive wastes stored within a near-surface tailings facility at Chepetsky Mechanical Plant JSC. Surface screening and micro-chemical mapping were systematically performed on representative specimens to determine the composition of the sludge matrices and individual inclusions. Fine particles with chemical compositions consistent with gypsum, calcium carbonate, uranium oxides, ammonium uranyl carbonate, pyrolusite, barium sulfate, iron, silicon, zirconium, and lead oxides were successfully identified. The analysis provided direct mineralogical confirmation of the presence of insoluble residues originating from historical uranium ore leaching and enrichment operations. Crucially, the scientific novelty of this work lies in establishing that uranium is ubiquitously distributed across the waste surfaces and is primarily fixed within the calcium-bearing matrix phases (CaSO4·2H2O, CaCO3 and CaF2) via an adsorption–coprecipitation mechanism, while also forming discrete micro-scale ammonium uranyl carbonate and oxide inclusions up to 2.5 µm that bypass bulk detection. The integrated SEM–EDS approach proved to be a powerful tool for micro-chemical characterization, effectively complementing conventional bulk X-ray diffraction and chemical analyses. These insights into uranium speciation provide an essential scientific foundation for developing efficient hydrometallurgical processing and environmental remediation technologies for complex industrial tailing facilities.
A high-quality single crystal of Mg2Cr2O5 was synthesized at 16 GPa and 2000 K using multi-anvil techniques. Its crystal structure and high-pressure behavior were investigated up to 45.5 GPa by means of single-crystal synchrotron X-ray diffraction combined with a diamond anvil cell. Lattice parameters and atomic positions were determined under various pressure conditions. The pressure-volume relationship is well described by a second-order Birch-Murnaghan equation of state, yielding an isothermal bulk modulus K0 = 227(3) GPa. Detailed crystal structure analysis reveals that the compression mechanism is primarily governed by polyhedral tilting.
The structural evolution of forsterite (Mg₂SiO₄) upon heterovalent coupled substitution with Li⁺ and Fe³⁺, forming the solid solution Mg(2-2x)LixFe³⁺xSiO₄, has been investigated up to ≈ 0.30. Single crystals were synthesized via high-temperature flux growth and characterized by single-crystal X-ray diffraction. Crystal structure refinements confirm a 2Mg²⁺ = Li⁺ + Fe³⁺ substitution, where Li⁺ occupies the smaller, less distorted M1 octahedron, while Fe³⁺ occupies the larger M2 octahedron, in agreement with charge-balance and bond-valence constraints. This substitution induces a linear contraction of the unit-cell volume, driven primarily by a decrease in the c-parameter, while concurrently causing a systematic expansion of the M1 octahedron and a contraction of the M2 octahedron. All measured structural parameters— such as mean bond distances, polyhedral volumes, and distortion indices—vary linearly with composition, indicating a continuous virtual solid solution up to x = 1.00. Extrapolation of the trends and subsequent Distance Least-Squares modelling predict a structurally viable atomic arrangement for the hypothetical LiFe³⁺SiO₄ endmember, which is crystal-chemically feasible as well as comparable to the known LiScSiO₄ olivine-type structure. These findings address conflicting literature reports by demonstrating that the olivine structure can accommodate the complete Li⁺ + Fe³⁺ ↔ 2Mg²⁺ substitution, thereby supporting the potential of LiFeSiO₄-type compounds as candidate cathode materials for lithium-ion batteries.
The Raman spectrum of lawsonite is characterized in situ at high temperatures through its dehydration temperature under vacuum. The structural reorganization associated with dehydration appears to initiate in the Si and Al-dominated layer of the structure, and proceeds between 525–550 and 725 °C. The loss of water from the structure appears complex, with the first structural reorganization associated with dehydration occurring at 550 °C, coexistence of lawsonite and the initial dehydration products up to 700 °C, and rapid disappearance of lawsonite between 675 and 725 °C. The dehydration is likely preceded by increased disorder of the hydroxyls within the structure. Indications of enhanced disorder of hydrogen ions at high temperatures include a dramatic decrease in intensity of an O–H librational vibration, large peak broadening of the principal hydroxyl stretch, and the disappearance of a shoulder on the strongest hydroxyl stretching vibration. The thermal anharmonicity of a range of Raman active modes is determined, and a revision to mode assignments is suggested based on the thermal shifts. Under low pressure conditions and our laboratory timescale, the observed product of lawsonite dehydration is the metastable pseudo-hexagonal polymorph of CaAl2Si2O8, dmisteinbergite. Production of dmisteinbergite through intermediate temperature lawsonite dehydration represents a previously unrecognized pathway for its formation, which previously has been limited to low-temperature hydrothermal environments, pyrometamorphic environments, materials rapidly quenching from melts (including in pseudotachylites), and as a retrograde phase from high grade metamorphic environments. The formation of metastable dmisteinbergite from lawsonite is likely facilitated by a topotactic relation between its layered structure and that of lawsonite.
This study investigated the enrichment factor (EF) and geochemical relationships of rare earth elements (REEs) in different grain size fractions of soils collected from Başören village, Kuluncak district, Malatya province, Türkiye. A total of 22 surface soil samples were collected and separated into five grain size fractions (< 75, 75–150, 150–180, 180–250, and > 250 μm), yielding 110 samples. Concentrations of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, Lu, and Y were determined by ICP-MS. EF values were calculated using Al as the reference element, and Pearson correlation was applied to evaluate the relationships between REEs. Strong positive correlations were observed among most REEs (r = 0.999–1.000, p < 0.01), whereas weaker correlations were shown between Eu and Gd elements and La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Yb, Lu, and Y elements. Moderately positive relationships were determined between Al and REEs, with coefficients of determination (R2) ranging from approximately 0.33 to 0.54, indicating that REE distributions are primarily controlled by aluminosilicate minerals and fine-grained fractions. EF values varied between sampling areas and grain size fractions. While negligible to moderate enrichment was observed in most samples, significant to very high enrichment was observed in some samples, particularly for Gd, Tb, Dy, Ho, and Yb. Samples T16 and T20 showed the highest enrichment levels, with Gd reaching very high enrichment classes (EF = 20–40). The results show that the spatial distribution and enrichment patterns of rare earth elements are largely influenced by lithogenic sources and grain size-dependent mineralogical characteristics.
