High Resolution Study of the Metamict State in Zircon

Abstract

Ceramic waste forms are being developed as solid hosts for radioactive waste disposal, Of concern is the effect of long-term radiation damage on crystalline integrity and subsequent leachability of the waste form. In preparation for understanding radiation damage effects, we have performed a high resolution study of the metamict state in natural zircons (ZrSiO4). Metamict minerals are a class of “amorphous” minerals, originally crystalline which have suffered extensive structural degradation over time from decay of internal radioactive impurities, principally U and Th. From x-ray diffraction evidence it is widely believed that the metamict state is amorphous in the sense of an isotropic glass. However, an electron microscope study performed some 15 years ago suggested that metamict zircons possess a structure composed of slightly misoriented crystallites about 100 Å in size, a structure indistinguishable from an isotropic glass by x-ray diffraction. We felt that high resolution electron microscopy could clearly delineate between the isotropic glass and misoriented microcrystalline concepts.