Uranium–Calcium–Silicate Glass with Anomalous Uranium Content as a Natural Analogue of Waste Matrices for Radioactive Waste Immobilization
DOI:
https://doi.org/10.17308/geology/1609-0691/2026/3/92-100Keywords:
mineraloid, X-ray amorphous glass, uranium (VI), fluorescence spectroscopy, haiweeite, gamma-ray spectrometry, secular equilibrium, skarns, BeshtauAbstract
Introduction: natural uranium-bearing glasses are extremely rare and are of considerable interest for mineralogy and uranium geochemistry. A new mineraloid – uranium–calcium–silicate glass – has been discovered in carbonate skarns of the southeastern exocontact of the Beshtaugorskii massif (Northern Caucasus). The aim of this work is its comprehensive mineralogical study.
Methods and factual material: five samples were examined; chemical composition was determined by SEM-EDS (20 analyses), structure was studied by X-ray diffraction and optical microscopy. Fluorescence spectra were recorded using an Ocean Optics USB 4000 FL spectrometer (λexc= 365 nm). Gamma-spectrometric analysis was performed on a NaI(Tl) scintillation detector.
Results and discussion: the material was found to be X-ray amorphous, optically isotropic, and exhibits bright yellow-green fluorescence characteristic of the uranyl ion (UO₂²⁺). The composition (wt.%): SiO₂ – 34.31; CaO – 6.56; UO₃ – 55.05; H₂O – 4.09. The empirical formula is 7CaO·12UO₃·33SiO₂·10H₂O. The UO₃ content is the highest ever reported for natural glasses: it is orders of magnitude higher than uranium concentrations in known hyalites and exceeds the maximum loading of industrial silicate glasses. Microimpurities of Na, Al, Ti, S (total 0.5–1 wt.%) stabilize the amorphous matrix.
The fluorescence spectrum (508.0, 526.9, 548.9 nm) matches the peak positions of crystalline uranium silicate haiweeite, indicating the identity of the short-range order in the arrangement of uranyl groups and silicon–oxygen tetrahedra. The redistribution of relative intensities (dominance of the 508 nm peak) is a spectroscopic signature of the amorphous state.
The gamma-ray spectrum recorded secular radioactive equilibrium in the ²³⁸U decay chain, indicating the closed-system behavior of the matrix and its age of >800 000 years. The formation of the mineraloid is interpreted as resulting from rapid cooling of a uranium–silicon melt in the skarn substrate, which prevented crystallization and led to the "freezing" of the structure at the short-range order stage.
Conclusion: a new mineraloid is described – a natural uranium–calcium–silicate glass with a record UO₃ content (56.97 wt.%) and the empirical formula 7CaO·12UO₃·33SiO₂·10H₂O. 2. The mineraloid is X-ray amorphous and isotropic; its fluorescence matches the peak positions of haiweeite (deviation <0.3 nm), proving the preservation of short-range order. 3. Gamma-ray spectrometry confirms secular equilibrium in the ²³⁸U decay chain; the age of the mineraloid is >800 000 years, the matrix is sealed. 4. The discovered object represents a unique natural formation, demonstrating the ability of an amorphous silicate matrix to retain uranium over geological time.
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