Annealing metamict zircon: A powder X-ray diffraction study of a highly defective phase

The transition from highly defective (metamict) to crystalline zircon is characterized by short-range structural order, which follows a different kinetic behavior than the corresponding long-range order. Experimental results from in situ powder X-ray diffraction experiments are analyzed using Rietveld refinement and direct Fourier analysis. The Rietveld refinement leads to a detailed assessment of the structural contraction of the crystalline portion of the same during thermal annealing, The Fourier analysis shows a highly heterogeneous distribution of interatomic bond distances. The main effect of structural annealing is a change of the distances between Si and O atoms and the average Si-Zr distance. The Zr-Zr distance seems to be less affected by this thermal treatment. Short-range order is recovered at lower temperatures than that of long-range order. Crystallinity is recovered anisotropically with slower annealing along the crystallographic c-direction than along the a-direction.