Spectral energy distribution as a function of wavelength. The red curves shows the observed ISO/SWS spectra of R Aqr and R Cas (Kraemer et al., 2002; Sloan et al., 2003). The black solid curve shows the spectra from a dynamical model using pure Mg₂SiO₄ grains and the grey curves show the spectra when assuming a core of Mg₂SiO₄ and a mantle of MgFeSiO₄ in the a posteriori spectral calculations. The dashed, dot-dashed and dotted lines show the spectra setting the mantle size to 1%, 5% and 10% of the grain radius, respectively.