Ray K. Eby

The amorphization of complex silicates by ion-beam irradiation

Twenty-five silicates were irradiated at ambient temperature conditions with 1.5 MeV Kr[sup +]. Critical doses of amorphization were monitored [ital in] [ital situ] with transmission electron microscopy. The doses required for amorphization are compared with the structures, bond-types, compositions, and physical properties of the silicates using simple correlation methods and more complex multivariate statistical analysis. These analyses were made in order to determine which properties most affect the critical amorphization dose. Simple two-variable correlations indicate that melting point, efficiency of atomic packing, the dimensionality of SiO[sub 4] polymerization (DOSP), and bond ionicity have a relationship with critical amorphization dose. However, these relationships are evident only in selected portions of the data set; that is, for silicate phases with a common structure type.

Ion-Irradiation Study of the “Exotic” Mineral Neptunite: LiNa 2 K(Fe,Mg,Mn) 2 Ti 2 Si 8 O 24

Single crystals of the silicate neptunite were irradiated with 600 keV Ar 2+ and 1.5 MeV Kr + and analysed by transmission electron microscopy. Amorphization was observed in a surface layer several hundred angstroms thick following Ar 2+ irradiations up to 5.0×l0 13 Ar/cm 2 , yet the Ar 2+ ions travelled an average of 1/2 μm in depth. The microstructure of the amorphous surface layer depends on the ion fluence, but the amorphous layer thickness remained constant. At the highest fluence, a narrow region below the amorphous layer shows a brittle-to-ductile strain transition, due to tensional volume-expansion of the adjacent ductile amorphous layer. With 1.5 MeV Kr1 + , amorphization of the electron transparent region was completed after a fluence of 1.7×l0 14 Kr + /cm 2 , and no further damage was observed up to 5.1×10 15 Kr + /cm 2 . However, following a low fluence of 2.0×10 11 Kr + /cm 2 , a single crystal of neptunite became a polycrystalline aggregate (grain size 10 nm) within 7 days of room temperature aging.