Magnesium and silicon in interstellar dust: an X-ray overview
  The dense Galactic environment is a large reservoir of interstellar dust.
Therefore, this region represents a perfect laboratory to study the properties
of the cosmic dust grains. X-rays are the most direct way to detect the
interaction of light with dust present in these dense environments. The
interaction between the radiation and the interstellar matter imprints specific
absorption features in the X-ray spectrum. We study them with the aim of
defining the chemical composition, the crystallinity and structure of the dust
grains which populate the inner regions of the Galaxy. We investigate the
magnesium and the silicon K-edges detected in the Chandra/HETG spectra of eight
bright X-ray binaries, distributed in the neighbourhood of the Galactic centre.
We model the two spectral features using accurate extinction cross sections of
silicates, that we have measured at the synchrotron facility Soleil, France.
Near the Galactic centre magnesium and silicon show abundances similar to the
solar ones and they are highly depleted from the gas phase
($\delta_{\rm{Mg}}>0.90$ and $\delta_{\rm{Si}}>0.96$). We find that amorphous
olivine with a composition of $\rm MgFeSiO_{4}$ is the most representative
compound along all lines of sight according to our fits. The contribution of
Mg-rich silicates and quartz is low (less than $10\%$). On average we observe a
percentage of crystalline dust equal to $11\%$. For the extragalactic source
LMC X-1, we find a preference for forsterite, a magnesium-rich olivine. Along
this line of sight we also observe an underabundance of silicon $A_{\rm
Si}/A_{\rm LMC} = 0.5\pm0.2$.
