Is "metamictization" of zircon a phase transition?

Diffuse X-ray scattering from single crystals of metamict zircon reveals re sidual crystallinity even at high fluences (up to 7.2 x 10(18) alpha-decay events/g). The experimental evidence does not suggest that radiation-induce d amorphization is a "phase transition." The observations are in good agree ment with a nonconvergent, heterogeneous model of amorphization in which da mage production is a random process of cascade formation and overlap at inc reasing fluence. Instead of an amorphization transition, the existence of a percolation tran sition is postulated. At the level of radiation damage near the percolation point, the heterogeneous strain broadening of X-ray diffraction profiles i s reduced whereas the particle-size broadening increases. Simultaneously, t he macroscopic swelling of the zircon becomes larger than the maximum expan sion of the unit-cell parameters. A suitable empirical parameter that chara cterizes this transition is the flux, D-s, at which the macroscopic expansi on is identical to the maximum expansion of the crystallographic unit cell. In zircon, D-s = 3.5 . 10(18) alpha-decay events/g.