Structural processing of enstatite by ion bombardment

During their lifetime, cosmic dust silicates suffer from a continuous processing by annealing, cosmic ray and UV irradiation, destruction and possibly also interstellar recondensation. Since the discovery that a significant proportion of stardust silicates leaves their star in crystalline form, the question arose as to why the interstellar silicate dust component does not show any indication of crystallinity. Amorphization due to ion irradiation is one possible explanation for the effect. In this paper, the results of irradiation experiments of submicrometre-sized clinoenstatite (MgSiO3) particles with 400 keV Ar+ and 50 keV He+ ions are presented. The irradiation doses have been varied between 1x1016 and 1x1018 ions/cm2 for He+ ions and 1x1014 up to 5x1014 ions/cm2 for Ar+ ions. These doses are comparable to those values that an interstellar silicate grain should be exposed to during its average life-time of 4x108 years. Threshold values for amorphization have been amounted to 1x1017 and 3x1014 ions/cm2 for 50 keV He+ and 400 keV Ar+ ions. Besides the structural changes in the microcrystallites morphological modifications in the grains, but no change of the chemical composition are found. Conclusions of potential astrophysical relevance have been drawn