Partially amorphous (metamict) titanite: a silicate mineral in a natural radiation induced non-equilibrium phase

The present review will give a brief overview mainly focused on studies during the past 5 years on the mineral titanite (end-member composition CaTiSiO5) which could incorporate radioactive elements in its crystal structure. Aspects of investigations were the structural damage resulting from the radioactive decay, the thermally induced recrystallization process and the elastic properties of the mineral. During the radiation induced damage process, namely the metamictization, titanite becomes partially amorphous with coexisting defect enriched crystalline and aperiodic regions. Within these crystalline regions a P21/c to C2/c phase transition (∼ at 500 K) occurs. Resulting from radiation induced structural damage, titanite shows elastic properties close to those of titanite glass. During recrystallization on annealing an initial softening of the titanite structure takes place (up to temperature ∼950 K) resulting from the formation of soft interfaces. Thermal annealing at higher temperatures leads to increasing structural order and the hardening of the mineral.