  We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron
ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K)
in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195
micron SWS and LWS spectra are dominated by strong emission from circumstellar
dust. We find that the dust consists of amorphous silicates, with a minor but
significant contribution from crystalline silicates. The amorphous silicates
consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be
excluded but if present they contain much less Fe than the amorphous olivines.
Comparison with laboratory data shows that the pure Mg-end members of the
crystalline olivine and pyroxene solid solution series are present. In
addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as
crystalline H2O ice. Several narrow emission features remain unidentified.
Modelling of the dust emission using a dust radiation transfer code shows that
large grains (~1 micron) must be present and that the abundance of the
crystalline silicates is between 7 and 15% of the total dust mass, depending on
the assumed enstatite to forsterite ratio, which is estimated to be between 1
and 3. The amorphous and crystalline dust components in the shell do not have
the same temperature, implying that the different dust species are not
thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a
period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106
binary is likely a post-red-supergiant in transition to a blue supergiant or WR
phase.
