Applications of near-infrared FT-Raman spectroscopy in metamict and annealed zircon: oxidation state of U ions

We report a near-infrared Fourier-transform (FT) Raman spectroscopic method to characterize the electronic transitions of U ions and the alpha-decay damage in natural zircon. The application is demonstrated by analyzing metamict and annealed zircons from Sri Lanka. The data from crystalline zircon reveal a relatively sharp spectral feature appearing near 2733 cm in Stokes spectra with a laser excitation of 1064 nm. The feature is assigned as signals related to the previously reported U absorption near 6668 cm. With increasing self-irradiation dose, the feature shows a systematic decrease in intensity, accompanied by a gradual development of a broad feature between 3000 and 3400 cm. On heating for 1 h, the U feature shows an increase in intensity starting near 700 K for partially metamict zircon, whereas for highly damaged zircon the first recovery of the feature takes place near 1000 K, accompanied by a decrease in the radiationinduced broad band. The changes observed in the present study reflect the variations of local environments of U ions in natural zircon during metamictization and thermal annealing.