Fluctuation electron microscopy of medium-range order in ion-irradiated zircon

A fluctuation electron microscopy study of amorphous zircons is reported. A natural metamict zircon sample was compared with amorphous samples prepared by irradiating a gem-quality zircon crystal with a 1.0 MeV beam of N²⁺ ions over a range of fluences up to 1.5 × 10¹⁷ ions/cm ². The two types of amorphous material exhibit similar selected-area diffraction patterns. However, the statistics of the scattering from 1 nm wide volumes indicates that they have significantly different normalised intensity variance plots. Modelling of the variance suggests that the natural metamict zircon consists of paracrystalline grains of zircon embedded within an amorphous ZrSiO₄ matrix. The ion-irradiated samples are better modelled as segregated paracrystalline grains of ZrO₂ composition within an amorphous SiO₂-rich matrix. Although both samples have been rendered disordered by radiation damage, there are two important differences. First, the ion-irradiated materials experience significantly higher fluence rates compared with the metamict materials. Second, the metamict zircon has had geologic time to anneal. The experiments show that zircon, freshly-amorphised by ion-irradiation, exhibits different medium-range order to that in natural metamict zircon.