- Effects of the impact of natural long-term irradiation with alpha particles in one chamosite and one cordierite sample were characterised in detail using electron microprobe, Raman microprobe, optical absorption spectroscopy (cordierite only), and transmission electron microscopy (TEM; cordierite only) analysis. In both cases, the impact of He- 4 cores (alpha particles) that were emitted from actinide-bearing mineral inclusions has caused the formation of radiation damage haloes in the host mineral. These haloes have maximum radii of about 33 mu m (chamosite) and 47 mu m (cordierite). They show notably changed optical properties, i.e., intensified absorption of light as recognised by brown (chamosite) and yellow (cordierite) pleochroism and enhanced or even anomalous interference colours. In spite of the significant disturbance of their short range order, alpha particle haloes are characterised by generally low degrees of structural radiation damage. This is indicated by rather moderate broadening of vibrational bands and, in the caseEffects of the impact of natural long-term irradiation with alpha particles in one chamosite and one cordierite sample were characterised in detail using electron microprobe, Raman microprobe, optical absorption spectroscopy (cordierite only), and transmission electron microscopy (TEM; cordierite only) analysis. In both cases, the impact of He- 4 cores (alpha particles) that were emitted from actinide-bearing mineral inclusions has caused the formation of radiation damage haloes in the host mineral. These haloes have maximum radii of about 33 mu m (chamosite) and 47 mu m (cordierite). They show notably changed optical properties, i.e., intensified absorption of light as recognised by brown (chamosite) and yellow (cordierite) pleochroism and enhanced or even anomalous interference colours. In spite of the significant disturbance of their short range order, alpha particle haloes are characterised by generally low degrees of structural radiation damage. This is indicated by rather moderate broadening of vibrational bands and, in the case of cordierite, apparently undisturbed electron diffraction patterns in the TEM. Intensive damage, virtually close to an amorphous state, was only found in cordierite up to a few tens of nanometres away from actinide-bearing inclusions. This damage is mainly assigned to recoils of heavy nuclei upon emission of an alpha particle, which have particle trajectory lengths that are three orders of magnitude shorter than those of the alpha particles. Similar to observations on biotite, alpha particle haloes in chamosite and cordierite as observed in the optical microscope may be considered as representative of a very early stage of the metamictisation process…
