Mid-infrared extinction and fresh silicate dust towards the Galactic
  Center
  We interpret the interstellar extinction observed towards the Galactic Center
(GC) in the wavelength range $\lambda = 1 - 20\,\mkm$. Its main feature is the
flat extinction at $3 - 8\,\mkm$ whose explanation is still a problem for the
cosmic dust models. We search for structure and chemical composition of dust
grains that could explain the observed extinction. In contrast to earlier works
we use laboratory measured optical constants and consider particles of
different structure. We show that a mixture of compact grains of aromatic
carbon and of some silicate is better suited for reproducing the flat
extinction in comparison with essentially porous grains or aliphatic carbon
particles. Metallic iron should be located inside the particle, i.e. cannot
form layers on silicate grains as the extinction curves become then very
peculiar. We find a model including aromatic carbonaceous particles and
three-layered particles with an olivine-type silicate core, a thin very porous
layer and a thin envelope of magnetite that provides a good (but still not
perfect) fit to the observational data. We suggest that such silicate dust
should be fresh, i.e. recently formed in the atmospheres of late-type stars in
the central region of the Galaxy. We assume that this region has a radius of
about 1 kpc and produces about a half of the observed extinction. The remaining
part of extinction is caused by a "foreground" material being practically
transparent at $\lambda = 4 - 8\,\mkm$.
