The 9.7 and 18 μm silicate absorption profiles towards diffuse and molecular cloud lines-of-sight

Context. Studying the composition of dust in the interstellar medium (ISM) is crucial for understanding the cycle of dust in our galaxy. Aims. 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. Methods. We have analysed ten spectra from the Spitzer archive, of which six lines-of-sight probe diffuse interstellar medium material and four probe molecular cloud material. The 9.7 mu m silicate absorption features in seven of these spectra were studied in terms of their shape and strength. In addition, the shape of the 18 mu m silicate absorption features in four of the diffuse sightline spectra were analysed. Results. The 9.7 mu m 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 mu m silicate absorption bands observed in diffuse lines-of-sight. The 9.7 mu m bands in the four 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).