Spectra of a population of amorphous Al₂O₃, MgSiO₃ or Mg₂SiO₄ grains compared to the observed ISO spectrum (Sloan et al. 2003). We considered an MRN grain radii distribution for all models shown here. The different lines are for grains consisting of aerosil Al₂O₃ particles (green dashed), Al₂O₃ porous particles (light blue long dashed-dotted), Al₂O₃ compact particles (brown short dashed-dotted), MgSiO₃ (purple dotted) and Mg₂SiO₄ (short dashed blue).