Generation of UV-induced radiation defects in calcite

The effect of UV light on one of the most common minerals on Earth - calcite (CaCO3), has been little investigated, despite its significant implications for many areas of research, such as dating and dosimetry, photodegradation of CaCO3-containing materials, and conservation of historical objects. In this study, conducted on nano- and micro-grained calcite samples using Electron Paramagnetic Resonance spectroscopy, we provide a detailed analysis of the radiation defects (paramagnetic centers) generated by UV radiation, including their identification, location (surface- or bulk-related) and stability during the period up to 4 years after irradiation. The observed paramagnetic centers, such as isotropic and anisotropic CO2-, CO3-, SO2-, SO3-, NO32- and NO22- ions, are analogous to gamma-induced species, which contradicts a widely-held belief that low-energy radiation cannot create radiation defects in wide band-gap materials, such as calcite, and has significant implications for EPR and luminescence dating of carbonates. We present a mechanism of the UV-induced generation of carbonate paramagnetic species in calcite based on retrapping the 'secondary electrons', i.e. the electrons released from non-carbonate traps by UV light. The efficiency of this process is strongly connected with the size and morphology of calcite grains, which can be especially relevant, considering the variety of CaCO3 particles available in nature and used in the industry.