Ion irradiation effects in natural garnets: Comparison with zircon

Abstract

The behavior of garnet ( A 3 B 2( XO 4) 3; I a3d; Z=8 ) under ion-beam irradiation was investigated in order to compare its radiation susceptibility to another orthosilicate: zircon, ZrSiO 4. Five natural end-member compositions were examined by in situ transmission electron microscopy during irradiation with 1.0 MeV Kr 2+ over the temperature range of 50–1070 K. The critical amorphization temperature, above which amorphization does not occur, was 1030 K for andradite, but could not be determined for the other garnet composition because the T c was higher than the highest temperature of the experiment. Based on topologic criterion, the degree of structural freedom in garnet is ∼−2.25 and for zircon ∼−1.5. Based on topology the critical amorphization dose for garnet should be higher than that of zircon; however, the average amorphization dose of garnet (0.20 dpa) is lower than that of zircon (0.37 dpa) at room temperature. This may be the result of the assumed value for the displacement energies, E d, used in the calculation of dpa. Garnet did not decompose, while zircon decomposes to SiO 2+ZrO 2 during the ion irradiation at high temperature. This behavior may be related to the phase relations of garnet which melts congruently and zircon which decomposes to ZrO 2+SiO 2.