Thermoelastic properties of partially metamict zircon: Indication of multi-stage recrystallization processes

Radioactive decay of unstable isotopes causes damage to zircon, which significantly reduces its elastic stiffnesses (e.g., Özkan, 1976). These damages can be partially healed by temperature treatment of the zircon crystal. In order to study in situ the recrystallization of metamict zircon, thermoelastic properties, and thermal expansion data were collected between 100 K and 1600 K utilizing resonant ultrasound spectroscopy and dilatometry, respectively. The investigated samples of natural gem-quality zircon belong to the damage stage I introduced by Holland & Gottfried, 1955, i.e. the damage in the crystal structure is mainly dominated by the accumulation of isolated point defects.

While non-metamict zircon samples display a linear decrease in elastic stiffnesses, the partially metamict zircon samples undergo strong irreversible effects detected in both utilized methods. The elastic stiffnesses increase starting at about 600 K, while the thermal expansion decreases. The severity of this effect becomes more pronounced with an increasing initial state of damage and thus can be related to the healing of defects induced by metamictization. A second effect sets in at about 1200 K, likely related to a transition from static to dynamic behavior. This supports the idea that reducing radiation damage is a multi-stage process, including point defect healing and recrystallization of an amorphous fraction.

H. Özkan, J. Appl. Phys. 47, 4772 (1976)

H. Holland, D. Gottfried, Acta Cryst. 8, 291 (1955)