Histograms of the mass column densities of amorphous olivine (N_ol; left), amorphous pyroxene (N_pyr; center), and crystalline forsterite (N_cry; right). The optical depths at the peak wavelengths corresponding to the mass column densities are shown in the upper x-axes. The peak wavelengths used for calculating the optical depths of amorphous olivine, amorphous pyroxene, and crystalline forsterite are 9.8, 9.2, and 11.0 μm, respectively. Black and white bars indicate the sources whose 5.3–12 μm spectral fits are accepted and those rejected, respectively, on the basis of the χ² statistics with a significance level of 0.05.