1993 Volume 22 Issue 1 Pages 3-20
Intensive efforts have been made to understand metamictization of minerals for the last ten years by using modern techniques such as transmission electron microscopy, infrared spectroscopy, X-ray absorption spectroscopy, Rutherford backscattering spectrometry. Ion-bombardment and short half-life actinide doping have been successfully applied to the studies of metamict minerals.This paper reviews data on metamict minerals and materials recently accumulated. Metamictization, the alpha-decay induced transformation from a periodic, crystalline mineral to an aperiodic, metamict mineral occurs in a very narrow range of alpha-decay dose (e.g., between 1015 and 1016 alphadecay events/mg in zircon). The damage ingrowth process can be divided into three stages: In Stage I, the mineral is essentially crystalline, with an expansion of the unit cell, a decrease in the Bragg diffraction intensity and an increase in the strain, which are caused mainly by the interstitial defects. Isolated, aperiodic domains resulting from alpha-recoil tracks are seen, but small in amount. Alpha-particles play a main role in the metamictization in Stage I. In Stage II, the damage ingrowth is characterized by a significant increase in the aperiodic domains resulting from alpha-recoil tracks. The crystalline domains become smaller in volume and size. In Stage III, the mineral is essentially X-ray and electron diffraction amorphous, and does not have any long-range periodicity. The decrease in the density through Stages II and III suggests a variation in the structure of the aperiodic region, the metamict structure.