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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 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
The metastable paragenesis of corundum and quartz is rare in nature but common in laboratory experiments where according to thermodynamic predictions aluminum-silicate polymorphs should form. We demonstrate here that the existence of a hydrous, silicon-bearing, nanometer-thick layer (called "HSNL") on the corundum surface can explain this metastability in experimental studies without invoking unspecific kinetic inhibition. We investigated experimentally formed corundum reaction products synthesized during hydrothermal and piston-cylinder experiments at 500-800 degrees C and 0.25-1.8 GPa and found that this HSNL formed inside and on the corundum crystals, thereby controlling the growth behavior of its host. The HSNL represents a substitution of Al with Si and H along the basal plane of corundum. Along the interface of corundum and quartz, the HSNL effectively isolates the bulk phases corundum and quartz from each other, thus apparently preventing their reaction to the stable aluminum silicate. High temperatures and prolonged experimental duration lead to recrystallization of corundum including the HSNL and to the formation of quartz + fluid inclusions inside the host crystal. This process reduces the phase boundary area between the bulk phases, thereby providing further opportunity to expand their coexistence. In addition to its small size, its transient nature makes it difficult to detect the HSNL in experiments and even more so in natural samples. Our findings emphasize the potential impact of nanometer-sized phases on geochemical reaction pathways and kinetics under metamorphic conditions in one of the most important chemical systems of the Earth's crust.