  We present theoretical calculations of the X-ray scattering properties of
porous grain aggregates with olivine monomers. The small and large angle
scattering properties of these aggregates are governed by the global structure
and substructure of the grain, respectively. We construct two diagnostics, R_X
and T_X, based on the optical and X-ray properties of the aggregates, and apply
them to a Chandra measurement of the dust halo around the Galactic binary
GX13+1. Grain aggregates with porosities higher than about 0.55 are ruled out.
Future high-precision observations of X-ray dust haloes together with detailed
modeling of the X-ray scattering properties of porous grain mixtures will
further constrain the presence of porous grain aggregates in a given dust
population.
