Ion-Irradiation Study of the Exotic'' Mineral Neptunite: Lina Sub 2 K(Fe,Mg,Mn) Sub 2 Ti Sub 2 Si Sub 8 O Sub 24

Single crystals of the rare silicate neptunite were irradiated with 600 keV Ar{sup 2+} and 1.5 MeV Kr{sup +}, and analyzed by transmission electron microscopy (TEM). Amorphization was observed in a surface layer only several hundred angstroms thick following Ar{sup 2+} irradiations up to 5.0 {times} 10{sup 13} Ar/cm{sup 2}, yet the Ar{sup 2+} ions traveled an average of 1/2 {mu}m in depth. Ionization is the dominant energy-loss process in the near surface region, and thus may contribute to the formation of the amorphous surface layer. 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. Using 1.5 MeV Kr{sup +}, amorphization of the electron transparent region was completed after a fluence of 1.5 {times} 10{sup 15} Kr{sup +}/cm{sup 2}, and no further damage was observed up to 5.1 {times} 10{sup 15} Kr{sup +}/cm{sup 2}. Then, using a low fluence of 2.0 {times} 10{sup 11} Kr{sup +}/cm{sup +}, a single crystal of neptunite became a polycrystalline aggregate (grain size 10 nm) after aging 10 days at room temperature. 6 refs., 6 figs