Alpha-decay damage and recrystallization in zircon: evidence for an intermediate state from infrared spectroscopy

α-decay damage and recrystallization in natural zircon (with dose ranging from 0.06 to 23.3x1018 α-events g-1) were studied using polarized reflection infrared spectroscopy. The experimental results show that α-decay damage leads to a gradual decrease in reflectivity and a loss of anisotropy of IR spectra. Recrystallization of damaged zircon is found as a multi-stage process with a strong dependence on the initial degree of damage. In weakly and moderately damaged samples the major recrystallization takes place near 1000 K. Annealed samples recrystallize epitaxially along their original crystallographic orientations. A highly damaged zircon with radiation dose of 15.9x1018 α-events g-1 decomposes into SiO2 and ZrO2 near 1100 K. In this sample the growth of ZrSiO4 from the binary oxides occurs between 1400 and 1500 K. An additional IR signal peaked near 790 cm-1 is detected in moderately damaged samples annealed at temperatures higher than 800 K. This peak is sharp and isotropic. The peak tends to disappear at temperatures above 1400 K. This signal may be related to an unknown intermediate phase caused by heating of radiation-damaged zircon. Alternatively, the signal may be due to the structural distortions near the boundaries between the amorphized and crystalline regions. (author)