A smooth surface layer of highly disordered olivine, (Mg, Fe)2SiO4, has been produced by exposure of polished, natural olivine to a dose of 5×1016 cm−2 of 1.5 MeV neon ions from a Van de Graaff accelerator. The dielectric functions of the disordered silicate in the wavelength range from 8 to 30 μm have been determined from analysis of specular reflectance data, and extinction for Rayleigh particles of such disordered olivine has been calculated. Extinction measurements for amorphous olivine smoke collected on a substrate are also presented. The small particle extinctions of both kinds of structurally disordered olivine are shown to agree well with the main features of the absorption and emission spectra from interstellar grains in the 10 and 20 μm region.