Intermediate structures in radiation damaged titanite (CaTiSiO5): a Raman spectroscopic study.

Effects of radiation damage and thermal annealing on the crystal structure of natural titanite (CaTiSiO(5)) were studied using Raman spectroscopy. The results show that well crystallized natural titanites generally have the P2(1)/a structure at the unit cell level, in contrast to the A2/a symmetry reported previously. Radiation caused by naturally incorporated impurities (such as U and Th) leads to structural damage and amorphization in titanite, as evidenced by a significant loss of band intensity, spectral broadening and frequency shifts. Additional bands (e.g. near 574 and 650 cm(-1)) occur in weakly or partially metamict titanite due to the formation of an intermediate phase (with the A2/a symmetry). Raman spectra of titanite thermal glasses showed features different from those of metamict titanite, especially in the Ti-O and Si-O stretching regions. The effect of thermal annealing is strongly affected by the initial degrees of damage that the sample experienced. Weakly damaged titanite samples showed that annealing leads to a structural recovery, and the spectral patterns of these recovered crystals are consistent with the P2(1)/a symmetry. Highly damaged titanite starts to recrystallize into an A2/a phase near 700-800 K, and additional structural modification occurs when annealed at 1300-1400 K, which involves significant change in broad Ti-O features. However, in terms of bandwidths, the metamict samples are far from fully recovered even on being annealed at 1300-1400 K.