Spectrum of an M-giant taken with the Spitzer Space Telescope (Chiar, unpublished Spitzer data). The extinction towards this star is A_V ~ 10 mag. Plotted is the observed flux (in Jy) of the star multiplied by the wavelength (in μm) squared (F_ν × λ²) versus the wavelength (λ). This choice implies that a horizontal line is expected for an unreddened photosphere. The two most prominent absorption bands at 9.7 and 18 μm are due to interstellar silicate absorption. The smaller band at 8 μm is caused by gas phase SiO in the photosphere of the M-giant itself and clearly affects the 9.7 μm silicate feature, thus hampering the detailed analysis of the shape of the interstellar silicate band.