Kinetics of thermal recovery and recrystallization of partially metamict zircon: a Raman spectroscopic study

Isothermal annealing of two partially metamict zircon from Sri Lanka were c arried out at 870 K, 968 K, 1071 K, and 1166 K for 0.5 min up to similar to 210 hours. Raman spectroscopy was used to monitor the amorphous-to-crystal line transformation. The evolution of the phonon frequency and the linewidt h of the nu (3) Si-O stretching band in zircon with annealing time clearly shows two recovery stages within the temperature and time ranges of the exp eriments. Both annealing stages form distinct linear segments in the freque ncy vs. linewidth plot, which are clearly separated from the trend defined by untreated metamict zircons. The first stage is characterized by the reco very of the short-range order, i.e. by the recovery of pre-existing, disord ered crystalline domains as indicated by a fast recovery rate of the phonon frequency at the beginning of the transformation. This process dominates a t temperature below similar to 1000 K. In the case of the less metamict zir con, first stage recovery was activated after significant incubation period s, which follow an Arrhenius relationship. The second stage involves epitax ial recrystallization, which is activated within the first few minutes at t emperatures > 1000 K (e.g., after similar to 1-2 min at 1166 K). At this st age, the decrease of the linewidth is most probably related to the relaxati on of phonon confinement associated with the growth of crystalline domains. We have estimated empirical activation energies of E-A = 2.24 +/- 0.04 eV and E-A = 2.6 +/- 0.2 eV for the first stage from the isothermal frequency and linewidth transformation curves. These activation energies are most lik ely related to recombination of point defects in the crystalline domains of partially metamict zircon. A single activation energy of E-A = 3.8 +/- 0.4 eV was obtained for epitaxial recrystallization by assuming Johnson-Mehl-A vrami growth kinetics as a first approximation. We suggest that a compariso n of frequency versus linewidth relationships observed for natural zircons with experimental trends provides a potential means for recognizing an anne aling history of natural zircons, provided that post-annealing radiation da mage did not completely obscure the annealing effect.