  Interstellar abundance determinations from fits to X-ray absorption edges
often rely on the incorrect assumption that scattering is insignificant and can
be ignored. We show instead that scattering contributes significantly to the
attenuation of X-rays for realistic dust grain size distributions and
substantially modifies the spectrum near absorption edges of elements present
in grains. The dust attenuation modules used in major X-ray spectral fitting
programs do not take this into account. We show that the consequences of
neglecting scattering on the determination of interstellar elemental abundances
are modest; however, scattering (along with uncertainties in the grain size
distribution) must be taken into account when near-edge extinction fine
structure is used to infer dust mineralogy. We advertise the benefits and
accuracy of anomalous diffraction theory for both X-ray halo analysis and near
edge absorption studies. An open source Fortran suite, General Geometry
Anomalous Diffraction Theory (GGADT), is presented that calculates X-ray
absorption, scattering, and differential scattering cross sections for grains
of arbitrary geometry and composition.
