Peak positions, λ₁₀ and λ₁₈ (first and third rows), full widths at half maximum, FWHM₁₀ and FWHM₁₈ (second and fourth rows), and ratios of the dust mass opacities of the mid-IR silicate features, R_10∕18 (bottom row), for isolated grains (left column) and aggregates (right column). λ, FWHM, andR_10∕18 are plotted as a function of the radius of a compact sphere with same mass for compact spheres (black circles), porous spheres (fuchsia squares), prolate grains (blue triangles), oblate grains (coral triangles), and aggregates of compact spheres with a₀ = 0.05 (red circles), 0.1 (orange circles), 0.5 (brown circles), 1 μm (purple circles), aggregates of porous spheres with P = 20% and a₀ = 0.1 μm (fuchsia squares), aggregates of prolate monomers with a₀ = 0.05 μm (blue triangles), and aggregates of oblate monomers with a₀ = 0.05 μm (coral triangles). The horizontal black dashed lines show λ, FWHM, andR_10∕18 of the smallest isolated compact sphere.