A hidden reservoir of Fe/FeS in interstellar silicates?
  The depletion of iron and sulphur into dust in the interstellar medium and
the exact nature of interstellar amorphous silicate grains is still an open
question. We study the incorporation of iron and sulphur into amorphous
silicates of olivine- and pyroxene-type and their effects on the dust
spectroscopy and thermal emission. We used the Maxwell-Garnett effective-medium
theory to construct the optical constants for a mixture of silicates, metallic
iron, and iron sulphide. We also studied the effects of iron and iron sulphide
in aggregate grains. Iron sulphide inclusions within amorphous silicates that
contain iron metal inclusions shows no strong differences in the optical
properties of the grains. A mix of amorphous olivine- and pyroxene-type
silicate broadens the silicate features. An amorphous carbon mantle with a
thickness of 10 nm on the silicate grains leads to an increase in absorption on
the short-wavelength side of the 10 $\mu$m silicate band. The assumption of
amorphous olivine-type and pyroxene-type silicates and a 10 nm thick amorphous
carbon mantle better matches the interstellar silicate band profiles. Including
iron nano-particles leads to an increase in the mid-IR extinction, while up to
5 ppm of sulphur can be incorporated as Fe/FeS nano inclusions into silicate
grains without leaving a significant trace of its presence.
