Application of lattice imagery to radiation damage investigation in natural zircon

High-, intermediate-, and low-type zircon crystals of natural origin were investigated using a 1,000 kV high-resolution electron microscope. The lattice images obtained successfully for high zircon were in good accordance with computer simulated ones, and 1.5 Å separations, the nearest distance between zirconium atoms projected along the a axis, were clearly resolved under a certain instrumental condition. The images of fission tracks and surrounding areas show nearly perfect lattice images and that within the fission tracks, with a width of 20 ～ 30 Å and length of ca. 1,000 Å, the structure is heavily disordered, almost amorphous; that both sides of the tracks the lattices are displaced or dislocated, and that in the area adjacent to the tracks, bright and dark spot images occur, corresponding to vacancies and their interstitial atoms. In low zircon, the structure is completely destroyed to show an entirely amorphous state, whereas an intermediate type consists of domains of the order of 50 ～ 100 Å across with nearly regular lattices, along whose boundaries strongly disordered areas with widths of few tens of angstroms appear, but the relative orientations of the neighbouring domains are almost continuous. Thus a whole process of metamictization is visualized on a lattice scale. Metamictization proceeds principally by the formation of fission tracks, the direct result of fast movement of nuclear particles; recoil nuclei therefrom seem to play a less important role in the destruction of the structure.