Upper: the τ_9.7 versus E(J − K) relationship for the chemical composition of the best fit Min et al. (2007) found for the Galactic centre together with a mass fraction of 31% of graphite (solid line). The dashed and dotted lines represent a model with the same amount of graphite together with a pure magnesium rich amorphous pyroxene (i.e. MgSiO₃) and magnesium rich amorphous olivine (i.e. Mg₂SiO₄), respectively. lower: the 9.7 μm silicate absorption profiles for the above models. The left panel shows the calculated 9.7 μm silicate absorption profiles in terms of the mass extinction coefficient and the right panel shows the same profiles normalised to 1 at 10.5 μm as was done with the observations. The solid line in both panels represent the best fit for the Galactic centre feature from Min et al. (2007).