RADIATION-DAMAGE IN ZIRCONS

Metamictization effects have been investigated in zircons having diffe rent uranium and thorium contents and different degrees of metamictiza tion. U and Th contents range from 57 to 2890 ppm and from 27 to 420 p pm, 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 ar e noticed. Powder diffraction maxima decrease in intensity, shift to l ower 2 theta values, broaden and become asymmetrical with the incremen t 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 d iffraction patterns. There are only minor differences in the two seque nces. On the other hand, powder X-ray diffraction patterns and IR spec tra give different results during annealing: after heating at 950 degr ees C, X-ray diffraction patterns become sharp and well resolved and d isplay 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.