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A Raman spectroscopic study of high-uranium zircon from the Chernobyl "lava"

¹ Institut für Mineralogie, University of Münster, Corrensstr. 24, 48149 Münster, Germany
² Laboratory of Applied Mineralogy and Radiogeochemistry, V.G. Khlopin Radium Institute, 28, 2-nd Murinskiy ave., St. Petersburg, 194021, Russia

^* Corresponding author, e-mail: tgeisler{at}nwz.uni-muenster.de

We have studied technogenic, high-uranium zircon, which crystallised from melt formed during the accident at the Chernobyl Nuclear Power Plant in 1986, by confocal Raman spectroscopy, electron microprobe, and backscattered electron imaging. The correlation between Raman and electron microprobe measurements allowed studying mode behaviour as a function of U content of the strongly zoned zircon crystals. The USiO₄ content in the crystals ranges between 0.6 and 11.6 mol. %, corresponding to 0.8 and 15.8 wt. % UO₂, respectively. The frequency of the v₁(SiO₄) symmetrical and v₃(SiO₄) anti-symmetrical stretching mode decreases by 0.67(3) and 0.75(3) cm^-1 per mol. % USiO₄, respectively, which is a result of an increasing Si-O bond length with increasing U content. The lattice modes show a comparable shift to lower frequencies, whereas the internal v₂(SiO₄) and v₄(SiO₄) bending modes exhibit no or only a small shift to lower frequencies (< 0.12 cm^-1 per mol. % U). Only the frequency of the lowest energy band (E_g) near 202 cm^-1 increases slightly with increasing U content, which is unexpected and indicates that the cation-(SiO₄)^4- potentials, the electron orbitals, and/or the cation radius have a strong effect on this mode. The line broadening, reflecting the range of local distortions (i.e., microscopic strain), is most pronounced for the lattice modes, in agreement with the large size difference of both cations. We found that the E_g lattice modes, involving the movement of the SiO₄ tetrahedron and the cation within the a(b) plane, show significantly larger line broadening with increasing U concentration than the B_1g modes, involving lattice vibrations along the c axis. This suggests that the microscopic strain is significant larger in the a(b) plane than along the c axis, which can be explained by the structural properties of zircon.

Key-words: zircon, uranium, Raman spectroscopy, Chernobyl, solid solution, elastic strain..

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