  Comets can be divided into two groups: type I, characterized by high gas/dust
ratio, low polarization, and a weak or absent 10 micron silicate feature, and
type II, for which a low gas/dust ratio, high polarization, and strong silicate
feature are typical. We show that the low polarization is the apparent result
of depolarization by gas contamination at low dust concentration, which, in
turn, results from the dust in type I comets being concentrated near the
nucleus. The simulations of thermal emission show that for more porous
particles (BCCA), the silicate feature is more pronounced than more compact
ones (BPCA), for which it even vanishes as the particles become larger. We also
show that in both types of comets the main contribution to light scattering and
emission comes from particles larger than 10 micron. Conclusions: .The strength
of the silicate feature in the cometary infrared spectra suggests that the dust
in type II comets consists of high-porosity aggregates, whereas the dust of
type I comets contains low-porosity ones. This is consistent with the
polarimetric features of these comets, which indicate that the dust in type I
comets tends to concentrate near the nucleus. This may result from the
predominance of highly processed particles in type I comets, whereas in type II
comets we see pristine or slightly-processed dust. This conclusion is in
accordance with the orbital characteristics of the comets. We have found that
the strength of the silicate feature correlates with the semi-major axis of
periodic comets and, for short-period comets, with the perihelion distance.
Thus, the silicate feature weakens due to compaction of aggregate particles if
a comet spends more time in the vicinity of the Sun, which allows the comet to
accumulate a mantle on the surface of its nucleus.
