Radiation damage in zircons

Metamictization effects have been investigated in zircons having different uranium and thorium contents and different degrees of metamictization. U and Th contents range from 57 to 2890 ppm and from 27 to 420 ppm, respectively. Parallel to the U and Th increase, density decreases from 4.80 to 4.10 (g/cm(3)) and change in X-ray powder diffraction are noticed. Powder diffraction maxima decrease in intensity, shift to lower 2 theta values, broaden and become asymmetrical with the increment of diffuse-scattering component. Unit cell parameters systematically increase (a = 6.60-6.68 Angstrom, c = 5.97-6.11 Angstrom, V = 260-272 Angstrom(3)) The powder IR spectra show similar variations as X-ray diffraction patterns. There are only minor differences in the two sequences. On the other hand, powder X-ray diffraction patterns and IR spectra give different results during annealing: after heating at 950 degrees C, X-ray diffraction patterns become sharp and well resolved and display a nearly complete structural recovery; IR spectra show that the recovery in the structure in the two metamict zircons is significant, but seems incomplete if compared with the more crystalline samples.