Structural phenomena of metamict titanite: a synchrotron, X-ray diffraction and vibrational spectroscopic study

Titanite, CaTiSiO5, occurs in nature in crystalline and metamict modification. Its structure consists of corner-linked TiO6 octahedra, SiO4 tetrahedra and sevenfold coordinated Ca. Metamictization occurs when a mineral contains impurities like U and Th and is exposed to radioactive irradiation. Recoil processes due to alpha radiation change over geological time scales the structure to a state with persisting short-range order but violated long-range order. Raman and infrared spectra and X-ray diffraction data of metamict as well as crystalline and titanite glass reveal local structural phenomena. Raman spectra of metamict titanite differ from spectra of well-crystalline titanite, indicating the breakdown of selection rules. Raman scattering reveals that anisotropy is preserved upon metamictization and that the structural state of highly metamict titanite should not be considered as quasi-amorphous. The metamict state partially recovers its disorder via heat treatment. Annealing-induced shrinking of unit cell volume is related to stiffening and improved local symmetry of SiO4 tetrahedra.