  The recent Spitzer detections of the 9.7 micron Si--O silicate emission in
type 1 AGNs provide support for the AGN unification scheme. The properties of
the silicate dust are of key importance to understanding the physical, chemical
and evolutionary properties of the obscuring dusty torus around AGNs. Compared
to that of the Galactic interstellar medium (ISM), the 10 micron silicate
emission profile of type 1 AGNs is broadened and has a clear shift of peak
position to longer wavelengths. In literature this is generally interpreted as
an indication of the deviations of the silicate composition, size, and degree
of crystallization of AGNs from that of the Galactic ISM. In this Letter we
show that the observed peak shift and profile broadening of the 9.7 micron
silicate emission feature can be explained in terms of porous composite dust
consisting of ordinary interstellar amorphous silicate, amorphous carbon and
vacuum. Porous dust is naturally expected in the dense circumnuclear region
around AGNs, as a consequence of grain coagulation.
