Experimental High Resolution Dark Field Electron Microscopy

High resolution dark field (DF) images of the superstructures of the pyrrhotite (Fe1-xS) and bornite-digenite (Cu5FeS4-Cu9S5) series can be related to structure. Further, they provide more detail than bright field (BF) images. The same objective aperture size and stigmater settings were used for DF as for BF imaging; symmetrical arrangements of diffracted beams in the objective aperture were used. Images that can be related to structure were obtained at the defocus value giving the greatest image contrast, thereby enabling proper defocusing without requiring extensive through-focus series.For the minerals of interest, diffraction patterns consist of many superstructure reflections and a few subcell reflections. BF images contain primarily features of the superstructure, presumably because the subcell reflections fall far from the axis of the objective lens and thus are affected by spherical and chromatic aberrations and beam divergence. Likewise, DF images formed with a similar arrangement of beams as that in BF contain only features of superstructure, but with reverse contrast to BF.