Oxidation state of uranium in metamict and annealed zircon: near-infrared spectroscopic quantitative analysis

Abstract

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 ( Ucrystal4+ and Ucrystal5+). The linewidths and peak positions of the 4834 cm−1 band (Ucrystal4+, E ∥ c) and the 6668 cm−1 band (Ucrystal5+, E ⊥ c) are a non-linear 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 Ucrystal4+ and Ucrystal5+ signals revealed that the intensity ratio (Ucrystal4+ /U crystal5+) 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 (Uamorphous4+), although small amounts of pentavalent U ions (Uamorphous5+) 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 Ucrystal4+ and Ucrystal5+ above 700 K while Ucrystal4+ bands disappeared at temperatures above 1600 K. In contrast, a highly damaged zircon (with a dose of ∼ 15.9 × 1018 α-events g−1) exhibited the presence of Ucrystal4+ and Ucrystal5+ above 1000 K, with a dramatic increase in signals near 1400 K.