Metamictization of zircon: Raman spectroscopic study

Raman spectroscopy of radiation-damaged natural zircon samples shows increa sed line broadening and shifts of phonon frequencies with increasing radiat ion dose. Stretching and bending frequencies of SiO4 tetrahedra soften dram atically 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(-1) were observed in metamict/amorphous zircon. The radiation-dose-independent spectral profiles and the coexistence of this broad background and relative sharp Raman mode s in partially damaged samples indicate that these bands are correlated wit h amorphous domains in zircon. The spectral profiles of metamict zircon sug gest that in comparison with silica, the SiO4 tetrahedra are less polymeriz ed in metamict zircon. This study also shows that ZrO2 and SiO2 are not the principal products of metamictization in zircon. No indication of bulk che mical unmixing of zircon into ZrO2 and SiO2 was found in 26 samples with a large variation of radiation damage (maximum dose: 23.5 x 10(18) alpha-even ts g(-1). Only one sample showed clearly, in all measured sample areas, ext ra sharp lines at 146, 260, 312, 460 and 642 cm(-1) characteristic of tetra gonal ZrO2. The geological land possibly artificial heating) history of thi s sample is not known. It is concluded that radiation damage without subseq uent high temperature annealing does not cause unmixing of zircon into cons tituent oxides.