Hydroxyapatite is used as a model for studying radical formation in the mineral compartment of irradiated calcified tissues. Treating this material with ¹³C- enriched CO₂ confirms that radiogenic long-lived radicals correspond to carbon centred species. It is shown, however, that these radicals are not located on the carbonate ions which substitute either the phosphate or the hydroxyl groups. The conditions which allow the formation and the trapping of these radicals are investigated (role of humidity, CO₂ and temperature) and this paramagnetic species is identified as CO⁻₂ adsorbed at the surface of apatite crystallites.