Evolution of spectral slopes, s, of (a) olivineand (b) pyroxene with respect to the energy density, E. Each curve corresponds to one type of irradiation and connects points with increasing irradiation. H⁺ stands for hydrogen, He⁺ for helium, and Ar⁺ for argon ions. Spectral slopes are divided by the corresponding spectral slope of the fresh material, s_f, and include error bars.