Energetic processing of interstellar silicate grains by cosmic rays

While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively low'' energy, heavy- ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single- crystal synthetic forsterite (Mg2SiO4) amorphizes when irradiated by 10 MeV Xe ions at large enough fluences. Using modeling, we extrapolate these results to show that 0.1-5.0 GeV heavy-ion cosmic rays can rapidly (similar to 70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium.