Metamictization of zircon: Raman spectroscopic study

Raman spectroscopy of radiation-damaged natural zircon samples shows increased line broadening and shifts of phonon frequencies with increasing radiation dose. Stretching and bending frequencies of SiO₄tetrahedra soften dramatically with increasing radiation damage. The frequency shifts can be used to determine the degree of radiation damage. Broad spectral bands related to Si-O stretching vibrations between 900 and 1000 cm^-1were observed in metamict/amorphous zircon. The radiation-dose-independent spectral profiles and the coexistence of this broad background and relative sharp Raman modes in partially damaged samples indicate that these bands are correlated with amorphous domains in zircon. The spectral profiles of metamict zircon suggest that in comparison with silica, the SiO₄tetrahedra are less polymerized in metamict zircon. This study also shows that ZrO₂and SiO₂are not the principal products of metamictization in zircon. No indication of bulk chemical unmixing of zircon into ZrO₂and SiO₂was found in 26 samples with a large variation of radiation damage (maximum dose: 23.5 × 10¹⁸ -eventsg^-1 ). Only one sample showed clearly, in all measured sample areas, extra sharp lines at 146, 260, 312, 460 and 642 cm^-1characteristic of tetragonal ZrO₂ . The geological (and possibly artificial heating) history of this sample is not known. It is concluded that radiation damage without subsequent high temperature annealing does not cause unmixing of zircon into constituent oxides.