- © The Mineralogical Society Of America
The widespread distribution of zircon in the continental crust, its tendency to concentrate trace elements, particularly lanthanides and actinides, its use in age-dating, and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies. Because zircon is highly refractory, it also has important industrial applications, including its use as a lining material in high-temperature furnaces. However, during the past decade, zircon has also been proposed for advanced technology applications, such as a durable material for the immobilization of plutonium (Ewing et al. 1995) or, when modified by ion-beam irradiation, as an optic waveguide material (Babsail et al. 1991). In all of these applications, the change in properties as a function of increasing radiation dose is crucial (see for example, Lumpkin 2001). In this chapter, we summarize the state-of-knowledge on the radiation damage accumulation process in zircon.
Although the concentrations of uranium and thorium are generally low (typically less than 5,000 ppm) in natural zircon, some zircon crystals are of great age (the oldest dated zircon grains are >4 Ga) and, thus, have calculated doses of >1019 α-decay events/g, well beyond the dose required for the radiation-induced transformation to the aperiodic, metamict state. In an α-decay event, the α-particle dissipates most of its energy (4.0 to 6.0 MeV for actinides) by ionization processes over a range of 10 to 20 μm, but undergoes enough elastic collisions along its path to produce approximately one hundred isolated atomic displacements. The largest number of displacements occurs near the end of the α-particle trajectory. The more massive, but lower energy, α-recoil (70 keV 234Th-recoil from decay of 238U) dissipates nearly all of its energy in elastic collisions over only 30 to 40 nm, transferring enough kinetic energy to cause ~1,000 atomic displacements according to …