TEMPERATURE-INDUCED P21/c TO C2/c PHASE TRANSITION IN PARTIALLY AMORPHOUS (METAMICT) TITANITE REVEALED BY RAMAN SPECTROSCOPY

In situ temperature-dependent Raman spectroscopic measurements of two partially amorphous (metamict) titanite samples with accumulated radiation doses similar to 1.2 x 10(18) alpha-events/g (E2335) and similar to 2.2 x 10(18) alpha-events/g (M28696) and amorphous fractions of 0.24 and 0.5, respectively, provide evidence for a thermally induced transformation analogous to the phase transition P2(1)/c <-> C2/c, which is characteristic of endmember titanite (CaTiSiO5). Quantitative analysis of the temperature evolution of the wavenumbers and widths of Raman-active vibrations in both partially amorphous titanites reveals an anomaly near 500 K which is consistent with the P2(1)/c <-> C2/c phase-transition temperature of titanite close to its endmember composition. The observed spectral changes are not due to structural recovery of the radiation-damaged structure because synchrotron X-ray diffraction experiments of M28696 titanite show that relevant recrystallization occurs at annealing temperatures above 600 K. The structural transformation near 500 K is observed by Raman spectroscopy even in heavily radiation-damaged and chemically inhomogeneous titanite (M28696). The microstructure of the samples consists of coexisting crystalline and amorphous domains, with the P2(1)/c to C2/c phase-transition occurring in the latter.