  We investigate the composition of the solid-state materials in the winds
around S-type AGB stars. The S stars produce dust in their wind that bears a
resemblance to the dust produced in some O-rich AGB stars. However, the
reported resemblance is mostly based on IRAS/LRS spectra with limited spectral
resolution, sensitivity, and wavelength coverage. We investigate the dust
composition around S stars using ISO/SWS data that surpass the previous studies
in terms of spectral resolution and wavelength coverage. We compare the dust
spectra from the 9 sources with the O-rich AGB spectra and a subset of M
super-giants. We constructed average dust emission spectra of the different
categories. We report the discovery of several previously unreported dust
emission features in the S star spectra. The long wavelength spectra of W Aql
and pi1 Gru exhibit the "30" micrometer feature attributed to MgS. Two sources
exhibit a series of emission bands between 20 and 40 micrometer that we
tentatively ascribe to Diopside. We show that the 10-20 micrometer spectra of
the S stars are significantly different from the O-rich AGB stars. The O-rich
stars exhibit a structured emission feature that is believed to arise from
amorphous silicate and aluminium-oxide. The S stars lack the substructure found
in the O-rich stars. Instead they show a smooth peak with a varying
peak-position from source to source. We suggest that this feature is caused by
a family of related material, whose exact composition determines the peak
position. The observed trend mimics the laboratory trend of non-stoichiometric
silicates. In this scenario the degree of non-stoichiometry is related to the
Mg to SiO4 ratio, in other words, to the amount of free O available during the
dust grain growth.
