We present infrared spectroscopy and imaging of AFGL 4106 .
The 2.4-5 \mu m ISO-SWS spectrum reveals the presence of a cool , luminous star ( T _ { \mbox { eff } } \approx 3750 K ) in addition to an almost equally luminous F star ( T _ { \mbox { eff } } \approx 7250 K ) .
The 5 – 195 \mu m 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 can not 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 Al _ { 2 } O _ { 3 } ) as well as crystalline H _ { 2 } O ice .
Several narrow emission features remain unidentified .

Modelling of the dust emission using a dust radiation transfer code shows that large grains ( \approx 1 \mu m ) 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 \approx 3.9 \cdot 10 ^ { -2 } M _ { \odot } expelled over a period of \approx 4 \cdot 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 .