  Gamma-ray bursts (GRBs) usually occurs in a dense star-forming region with
massive circum-burst medium. The small-angle scattering of intense prompt X-ray
emission off the surrounding dust grains will have observable consequences, and
sometimes can dominate the X-ray afterglow. In most of the previous studies,
only Rayleigh-Gans (RG) approximation is employed for describing the scattering
process, which works accurately for the typical size of grains (with radius
$a\leq 0.1\,{\rm \mu m}$) in the diffuse interstellar medium. When the size of
the grains may significantly increase as in a more dense region where GRBs
would occur, the RG approximation may not be valid enough for modeling detailed
observational data. In order to study the temporal and spectral properties of
the scattered X-ray emission more accurately with potentially larger dust
grains, we provide a practical approach using the series expansions of
anomalous diffraction (AD) approximation based on the complicated Mie theory.
We apply our calculations to understanding the puzzling X-ray afterglow of
recently observed GRB~130925A which showed a significant spectral softening. We
find that the X-ray scattering scenarios with either AD or RG approximation
adopted could both well reproduce the temporal and spectral profile
simultaneously. Given the plateau present in early X-ray light curve, a typical
distribution of smaller grains as in the interstellar medium would be suggested
for GRB 130925A.
