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Interstellar dust in the foreground of bright point sources will scatter X-rays over arcminute scales, producing a diffuse `halo' image. The scattering halo intensity is strongly sensitive to the dust grain size distribution, spatial distribution, and composition of dust on the sight line. Currently, Chandra's spatial resolution makes it the best detector available for studying X-ray scattering halos from the diffuse ISM. A Chandra successor with similar resolution but larger effective area will lend a greater opportunity to study scattering echoes from low column densities of the diffuse interstellar or intergalactic medium. More importantly, the combination of high resolution spectroscopy with imaging is uniquely suited to studying the composition of dust grains. At soft energies in particular, the spectrum of scattered light is likely to have significant features at the 0.3 keV (C-K) and 0.5 (O-K) photoelectric absorption edges. This direct probe of ISM dust grain elements will be important for (i) understanding the relative abundances of graphitic grains or PAHs versus silicates, and (ii) measuring the depletion of gas phase elements into solid form.