Low-temperature crystallization of MgSiO3 glasses under electron irradiation:: Possible implications for silicate dust evolution in circumstellar environments

Synthetic MgSiO3 glasses were irradiated at room temperature by 300 keV electrons in a transmission electron microscope (TEM). One of the samples had been previously irradiated by 50 keV He+ ions. Electron irradiation induces the nucleation and growth of randomly oriented nanometersized crystallites. The crystallites first consist of MgO and subsequently of forsterite (Mg2SiO4). Both are seen to form within an amorphous SiO2 matrix. The rate of crystallization of the samples has been monitored by conventional TEM imaging and electron diffraction. The sample that had been pre-irradiated with He+ ions is found to transform faster than the as-quenched glass. The crystallization of metastable MgSiO3 glasses is explained by ionizing radiation-induced elemental diffusion that allows the reorganization of matter into a more favourable thermodynamic state. These results show that ionizing radiation interactions could account for crystal formation as observed in infrared spectroscopy in some young stellar environments.