Is the silicate emission feature only influenced by grain size ?

The flattening of the 10 µm silicate emission feature observed in the spectra of T Tauri and Herbig Ae/Be stars is usually interpreted as an indicator of grain growth. We show in this paper that a similar behaviour of the feature shape occurs when the porosity of composite grains varies. We modelled the fluffy aggregates with inclusions of different sizes with multi-layered spheres consisting of amorphous carbon, amorphous silicate, and vacuum. We also found that the inclusion of crystalline silicates in composite porous particles can lead to a shift of the known resonances and production of new ones.