Transmission Electron Microscope Study of Natural Radiation Damage in Zircon (ZrSiO4)

Zircons from different localities show considerable variation in physical properties, and it is generally accepted that these variations are the result of α‐radiation damage due to the presence of radioactive impurities, in particular, uranium and thorium. Transmission electron microscopy has been used to study the radiation damage as a function of α‐dose (D) in a number of zircons. No damage was observed in crystals for which D < 1014 α/mg. However, black spot contrast was observed in crystals for which D = 1015 α/mg. Annealing experiments and electron spin resonance studies suggest that the black spots are due to clusters of oxygen interstitials. Crystals for which D > 1016 α/mg were, on X‐ray evidence, glassy (metamict). However, the electron diffraction patterns and micrographs showed that these crystals were composed of slightly misoriented zircon crystallites about 100 Å in size. After annealing, these metamict crystals showed dislocation loops and networks and the single crystal character was restored. The dislocation loops appear to lie on {101} planes and have Burgers vectors parallel to 〈101〉. Inspection of the crystal structure of zircon suggests that this is a likely slip system for zircon. Both interstitial and vacancy dislocation loops were observed. These results suggest a mechanism for the transformation from normal to metamict zircon and this is discussed.