Spectra of the imaginary part of the refractive index (lower limit values) for forsterite in a-polarization (solid lines, upper panel) and enstatite in c-polarization (circles and solid lines, lower panel) compared to the best-fit continuum absorption model (see also Fig. 6). The dashed lines represent the full model (single-phonon plus phonon difference-process contributions); the dash-dotted lines are the single-phonon components alone for θ = 300 K.