Annealing of alpha-decay damage in zircon: a Raman spectroscopic study

Recrystallization and structural recovery in cr-decay damage in zircon samp les have been studied using Raman spectroscopy. Fifteen zircon samples with different degrees of radiation damage have been thermally annealed between 600 K and 1800 K for up to 28 days and 8 hours. The experimental results f rom this study reveal that recrystallization in the damaged zircon samples is a multi-stage process that depends on the degree of initial damage of th e samples. In partially damaged samples the lattice recovery of damaged cry stalline ZrSiO4 takes place at temperatures as low as about 700 K, as shown by a remarkable band-sharpening and a significant increase in the frequenc ies of v(1) and v(3) Si-O stretching vibrations together with the external band near 357 cm(-1) with increasing temperature. A dramatic increase of Ra man scattering intensity of ZrSiO4 occurs in partially damaged samples near 1000 K due to a recrystallization process involving epitaxial growth. Heav ily damaged samples tend to decompose into ZrO2 and SiO2 at high temperatur es. Tetragonal ZrO2 has been observed under annealing between 1125 K and ab out 1600 K in heavily damaged samples while monoclinic ZrO2 appears above 1 600 K. Weak signals from ZrSiO4 were detected at 1125 K in highly metamict zircon although the main recrystallization appears to occur near 1500 K acc ompanied by a decrease of the volumes of ZrO2 as well as SiO2. This suggest s that this recrystallization is associated with the reaction of ZrO2 with SiO2 to form ZrSiO4. A possible intermediate phase has been observed, for the first time, by Ram an spectroscopy in damaged zircons annealed at temperatures between 800 K a nd 1400 K. This phase is characterized by strong, broad Raman signals near 670, 798 and 1175 cm(-1). Prolonged isothermal annealing at 1050 K results in a decrease of these characteristic bands and eventually the disappearanc e of this intermediate phase.