A transmission electron microscope was used to characterize a powder form of hexagonallyordered mesoporous silica material. The structural symmetry built into this amorphous material allowed one to obtain three characteristic images, i.e. a hexagonal honey-comb structure and wide/narrow parallel lines. These images were found to originate primarily from phase contrast, which changed sensitively with defocusing. To further understand the contrast behaviour of these images, an analytical form of

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... he defocus contrast theory was developed and applied to the simulation of the characteristic wide/narrow parallel line images. The simulation was found to be in good qualitative agreement with experiments, where changes in focus conditions and specimen thickness were predicted to alter the contrast in the resulting parallel-line type images.