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Ion-Irradiation Study of the “Exotic” Mineral Neptunite: LiNa2K(Fe,Mg,Mn)2Ti2Si8O24

  • Ray K. Eby (a1), L. M. Wang (a1), G. W. Arnold (a2) and R. C. Ewing (a1)


Single crystals of the silicate neptunite were irradiated with 600 keV Ar2+ and 1.5 MeV Kr+ and analysed by transmission electron microscopy. Amorphization was observed in a surface layer several hundred angstroms thick following Ar2+ irradiations up to 5.0×l013 Ar/cm2, yet the Ar2+ 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×l014 Kr+/cm2, and no further damage was observed up to 5.1×1015 Kr+/cm2. However, following a low fluence of 2.0×1011 Kr+/cm2, a single crystal of neptunite became a polycrystalline aggregate (grain size 10 nm) within 7 days of room temperature aging.



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