Alpha-recoil damage in zirconolite (CaZrTi₂O₇)

Radiation effects in a natural, metamict zirconolite from Sri Lanka that has received an alpha-decay dose greater than 10²⁶ alpha decays/m³ have been studied using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance spectroscopy (EPR), extended x-ray absorption fine structure spectroscopy (EXAFS), and x-ray absorption near edge structure spectroscopy (XANES). The same techniques were applied to the sample annealed between 1000° and 1100 °C. The heat of recrystallization was measured by differential thermal analysis (DTA) with values of 40 to 50 J/g. In contrast to previous work [A. E. Ringwood, Am. Scientist 70, 201 (1982); Mineralog. Mag. 49, 159 (1985)], these results demonstrate that there are fundamental differences at the atomic level between the annealed, crystalline, and the natural, fully damaged zirconolite. We suggest that the most likely structure for the fully damaged state is that of a random, three-dimensional network with no atomic periodicity extending beyond the first coordination sphere. Even within the first coordination sphere, there is a reduction in coordination number and an associated decrease in bond length for principal cations (Ca and Ti). Despite this structural modification and the great age of the specimen (~550 million years), the natural zirconolite shows only minor signs of geochemical alteration. The metamict structure is readily annealed, initially to a disordered, flourite-type structure, and finally, at higher temperatures, to a highly twinned, monoclinic zirconolite structure.