No abstract

O 4 ), and phenakite structures have been investigated using in situ and high-resolution transmission electron microscopy (HRTEM) over a wide temperature range (20-873 K). At low temperatures (<200 K), the olivine and phenakite are susceptible to radiation-induced amorphization with a critical amorphization dose of 0.2-0.5 displacement per atom (dpa). The critical amorphization dose increases with increasing irradiation temperature at varying rates for the various phases, resulting in a distinct critical amorphization temperature for each phase. For the Mg-Fe series of olivine, the susceptibility to amorphization at higher temperatures (room temperature or above) increases with increasing Fe content. Although the spinel phases are, in general, much more resistant to amorphization, a highpressure metastable spinel phase, ␥-SiFe 2 O 4 , is easily amorphized at doses <0.2 dpa at temperatures below 723 K. This phase decomposes after irradiation at 873 K. At 20 K, complete amorphization of the FeCr 2 O 4 spinel (chromite) is achieved at ∼4 dpa, but no evidence of amorphization is observed in MgAl 2 O 4 spinel after 5.4 dpa. The distinct differences in the relative susceptibility of these phases to amorphization are discussed in terms of the structural and chemical controls on the amorphization process.K. E. Sickafus-contributing editor Manuscript No. 189659.