Alpha-decay--induced fracturing in zircon: the transition from the crystalline to the metamict state.

A natural single crystal of zircon, ZrSiO(4,) from Sri Lanka exhibited zonation due to alpha-decay damage. The zones vary in thickness on a scale from one to hundreds of micrometers. The uranium and thorium concentrations vary from zone to zone such that the alpha-decay dose is between 0.2 x 10(16) and 0.8 x 10(16) alpha-events per milligram (0.15 to 0.60 displacement per atom). The transition from the crystalline to the aperiodic metamict state occurs over this dose range. Differential expansion of individual layers due to variations in their alpha-decay dose caused a systematic pattern of fractures that do not propagate across aperiodic layers. High-resolution transmission electron microscopy revealed a systematic change in the microstructure from a periodic atomic array to an aperiodic array with increasing alpha-decay dose. At doses greater than 0.8 x 10(16) alpha-events per milligram there is no evidence for long-range order. This type of damage will accumulate in actinide-bearing, ceramic nuclear waste forms. The systematic pattern of fractures would occur in crystalline phases that are zoned with respect to actinide radionuclides.