Oxidation state of uranium in metamict and annealed zircon: near-infrared spectroscopic quantitative analysis
نویسندگان
چکیده
Radiation and thermally induced changes in the oxidation state of uranium in metamict zircon have been systematically analysed, for the first time, using polarized near-infrared spectroscopy. The results showed that in damaged zircon U ions in crystalline domains exhibited relatively sharp, anisotropic signals from tetravalent and pentavalent U ions in crystalline domains (U4+ crystal and U5+ crystal). The linewidths and peak positions of the 4834 cm −1 band (U4+ crystal,E ‖ c) and the 6668 cm−1 band (U5+ crystal,E ⊥ c) are a nonlinear function of the self-radiation dose. They reach nearly constant values at doses greater than ∼3.5 × 1018 α-events g−1. Quantitative analysis of U4+ crystal and U 5+ crystal signals revealed that the intensity ratio (U 4+ crystal/U 5+ crystal) exhibited a nearly linear increase as a function of dose and decreased on heating. This suggests that radiation leads to an alteration of the oxidation states of U and the tetravalent state is more preferable in radiation-damaged zircons. U ions associated with amorphous materials (Uamorphous) gave rise to broad and isotropic signals and they were mainly in the tetravalent state (U4+ amorphous), although small amounts of pentavalent U ions (U 5+ amorphous) may exist. The dose dependence of the intensity ratio of Uamorphous/Utotal did not follow that of the reported fraction of the amorphous domain, but gave clearly lower values. This implies the potential preferential occurrence of U ions or possible U enrichment in the crystalline regions. Annealing intermediately and highly damaged zircon had different impacts on the oxidation state of U. An intermediately damaged zircon (with a dose of ∼5.2× 1018 α-events g−1), annealed at high temperatures between 500 and 1800 K, showed a systematic increase in the signals of U4+ crystal and U 5+ crystal above 700 K while U 4+ crystal bands disappeared at temperatures above 1600 K. In contrast, a highly damaged zircon 0953-8984/03/203445+26$30.00 © 2003 IOP Publishing Ltd Printed in the UK 3445
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