  Understanding the chemistry of the interstellar medium (ISM) is fundamental
for the comprehension of the Galactic and stellar evolution. X-rays provide an
excellent way to study the dust chemical composition and crystallinity along
different sight-lines in the Galaxy. In this work we study the dust grain
chemistry in the diffuse regions of the interstellar medium in the soft X-ray
band (<1 keV). We use newly calculated X-ray dust extinction cross sections,
obtained from laboratory data, in order to investigate the oxygen K and iron L
shell absorption. We explore the XMM-Newton and Chandra spectra of 5 low-mass
X-ray binaries located in the Galactic plane, and we model the gas and dust
features of oxygen and iron simultaneously. The dust samples used for this
study include silicates with different Mg:Fe ratios, sulfides, iron oxides and
metallic iron. Most dust samples are in both amorphous and crystalline lattice
configuration. The extinction cross sections have been computed using Mie
scattering approximation and assuming a power law dust size distribution. We
find that the Mg-rich amorphous pyroxene (Mg0.75Fe0.25SiO3) represents the
largest fraction of dust towards most of the X-ray sources, about 70% on
average. Additionally, we find that ~15% of the dust column density in our
lines of sight is in Fe metallic. We do not find strong evidence for
ferromagnetic compounds, such as Fe3O4 or iron sulfides (FeS, FeS2). Our study
confirms that the iron is heavily depleted from the gas phase into solids; more
than 90% of iron is in dust. The depletion of neutral oxygen is mild, between
10-20% depending on the line of sight.
