INSITU TEM STUDY OF ION-BEAM-INDUCED AMORPHIZATION OF COMPLEX SILICATE STRUCTURES

The in situ transmission electron microscopy (TEM) with ion-irradiation technique has been used for the first time to study the radiation-induced amorphization (metamictization) process of naturally occurring silicates: neptunite [Na2KLi(Fe,Mn)2Ti2(SiO3)8], titanite (CaTiSiO5), gadolinite (REE2FeBe2Si2O10), zircon (ZrSiO4) and olivine [(Mg,Fe)2SiO4]. These phases were irradiated with 1.5 MeV Kr+ ions in the Argonne HVEM-Tandem facility at room temperature with the electron diffraction pattern monitored in situ. The critical doses required for amorphization of the electron transparent thickness of neptunite, titanite, gadolinite, zircon and olivine have been determined to be 1.7, 2.0, 2.3, 4.8 and 6.0 x 10(14) ions/cm2, respectively. The results show a correlation between amorphization dose and the chemical and structural complexity of these five phases. The most complex structure (e.g. neptunite) becomes amorphous at the lowest critical dose. The critical amorphization dose also increases with the increasing melting temperature of the minerals.