Average Class D profile spectrum (in Wm⁻² µm⁻¹), normalised to the flux of the 7.7 µm feature (solid black line). The vertical line marks the average peak wavelength of the 7.7 µm complex for Class D spectra (i.e. 7.8 µm). The three Class D spectra that showed the most deviation from the average are shown in red, blue, and green. Aliphatic bands are present at 6.9 and 7.25 µm.