Thermal annealing of metamict titanite: A synchrotron radiation and optical birefringence study

The thermal annealing behaviour of metamict titanite from Cardiff, Canada, which shows moderate structural damage from the alpha -decay of radiogenic elements (U, Th) has been studied using single-crystal syn chrotron radiation and optical-birefringence techniques. Progressive and isothermal annealing experiments yield a temperature-dependent response in the diffraction and optical signals. The low-temperature regime (<673 K) is characterized by a preferred annealing of Frenkel-defects which results in small changes of the unit-cell volume and the Bragg-intensities on a time-scale of several hours. On constant heating at higher temperatures, metamict titanite displays two-stage kinetics with most rapid effects during the first two hours of annealing and less significant changes after the initial annealing period. Fast annealing of Frenkel defects occurs first, followed by recrystallization of more damaged regions. Single-crystal studies reveal lower temperatures for pronounced annealing effects than previously reported in powder-diffraction studies (ca. 100 K).