Infraredspectra of Si-O overtones, hydrous species, and U ions inmetamict zircon: radiation damage and recrystallization

Abstract

Radiation damage and recrystallization in natural zircons havebeen studied by analysing Si-O stretchingovertones/combinations, hydrous species, and U-ion spectra inthe frequency region between 1200 and 11 000 cm-1. Theeffects of radiation are characterized by a dramatic variationof intensity, a decrease in frequencies of multi-phonon bands (e.g., Si-O stretching overtones), a change of spectral profileof OH species, a formation of new OH species, and new signalsrelated to U ions. The formation of new anisotropic OH speciesin the crystalline regions of metamict zircon is observed andthis could account for the different thermal behaviour of OHspecies between metamict zircon and titanite during high-temperature annealing. The results imply systematicmodifications of the local environments of the OH and U ions inthe damage process. Both U4+ and U5+ spectra showdramatic variations during metamictization. We observe, for thefirst time, that as a result of radiation damage, the U5+signals near 6668 and 9030 cm-1 become undetectable at a dose of around 1.5×1018 α-events g-1while extra lines near 6650 and 8969 cm-1 appear. These variations are interpreted as radiation-induced localmodifications in crystalline regions. The general shape of the U-ionspectrum of the crystalline zircon is somehow stillpreserved 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 recoveryof the structure of zircon, indicated by spectral changes ofmulti-phonon bands and U ions, accompanied with the appearanceof new OH species. The results confirm that therecrystallization process in heavily damaged zircon involves thedecomposition of metamict ZrSiO4 into SiO2 andZrO2 near 1100 K and the significant crystal growth ofZrSiO4 near 1400 K as indicated by the recovery of Si-Ostretching overtones and U4+ and U5+ bands.