Infrared spectra of Si-O overtones, hydrous species, and U ions in metamict zircon: radiation damage and recrystallization

Abstract

Radiation damage and recrystallization in natural zircons have been studied by analysing Si-O stretching overtones/combinations, hydrous species, and U-ion spectra in the frequency region between 1200 and 11 000 cm -1. The effects of radiation are characterized by a dramatic variation of intensity, a decrease in frequencies of multi-phonon bands (e.g., Si-O stretching overtones), a change of spectral profile of OH species, a formation of new OH species, and new signals related to U ions. The formation of new anisotropic OH species in the crystalline regions of metamict zircon is observed and this could account for the different thermal behaviour of OH species between metamict zircon and titanite during high-temperature annealing. The results imply systematic modifications of the local environments of the OH and U ions in the damage process. Both U 4+ and U 5+ spectra show dramatic variations during metamictization. We observe, for the first time, that as a result of radiation damage, the U 5+ signals near 6668 and 9030 cm -1 become undetectable at a dose of around 1.5×1018 α-events g -1 while extra lines near 6650 and 8969 cm -1 appear. These variations are interpreted as radiation-induced local modifications in crystalline regions. The general shape of the U-ion spectrum of the crystalline zircon is somehow still preserved in highly damaged zircon. A decomposed zircon, consisting of ZrO2, SiO2, and ZrSiO4, shows spectral features different from those of metamict zircon samples. Thermal annealing of a highly damaged zircon leads to recovery of the structure of zircon, indicated by spectral changes of multi-phonon bands and U ions, accompanied with the appearance of new OH species. The results confirm that the recrystallization process in heavily damaged zircon involves the decomposition of metamict ZrSiO4 into SiO2 and ZrO2 near 1100 K and the significant crystal growth of ZrSiO4 near 1400 K as indicated by the recovery of Si-O stretching overtones and U 4+ and U 5+ bands.