The opacity in the 9.7 μm silicate absorption feature vs. near-IR color excess for spherical grains. The colors represent the material: open symbols represent silicates, black symbols represent graphite, and gray models represent a mix of 2/3 silicate and 1/3 graphite grains. Models involving ice-coated grains are indicated by circles and the diamond indicate an MRN distribution of spherical grains. The 0.1(sil,gra) model denotes the opacities of a 0.1 μm sphere that consist for 2/3 out of silicates and 1/3 of graphite. The solid line of slope q = 0.34 corresponds to the diffuse ISM. Σ_d is the total dust column in units of g cm^-2.