  Studying the composition of dust in the interstellar medium (ISM) is crucial
in understanding the cycle of dust in our galaxy. The mid-infrared spectral
signature of amorphous silicates, the most abundant dust species in the ISM, is
studied in different lines-of-sight through the Galactic plane, thus probing
different conditions in the ISM. We have analysed 10 spectra from the Spitzer
archive, of which 6 lines-of-sight probe diffuse interstellar medium material
and 4 probe molecular cloud material. The 9.7 um silicate absorption features
in 7 of these spectra were studied in terms of their shape and strength. In
addition, the shape of the 18 um silicate absorption features in 4 of the
diffuse sightline spectra were analysed. The 9.7 um silicate absorption bands
in the diffuse sightlines show a strikingly similar band shape. This is also
the case for all but one of the 18 um silicate absorption bands observed in
diffuse lines-of-sight. The 9.7 um bands in the 4 molecular sightlines show
small variations in shape. These modest variations in the band shape are
inconsistent with the interpretation of the large variations in
{\tau}_9.7/E(J-K) between diffuse and molecular sightlines in terms of silicate
grain growth. Instead, we suggest that the large changes in {\tau}_9.7 / E(J-K)
must be due to changes in E(J-K).
