Radiation-Induced Defects in Nonradioactive Natural Minerals: Mineralogical and Environmental Significance.

Short-lived radionuclides generate electronic defects, trapped electrons and positive holes, often associated to element impurities, which act as final traps over geological periods. Two main examples will be illustrated. Natural fluorites (CaF2) exhibit positive holes trapped on rare earths and oxygen impurities, Y-associated F-centers and Ca colloids formed under severe irradiation. The thermal stability of radiation defects gives constraints on the evolution of fluorites as a function of temperature and time. Other minerals, as apatite, confirm the importance of impurities in the trapping of radiation-induced defects over geological periods. Point defects are also observed in several clay minerals. The high specific surface area makes clay minerals sensitive to the geochemical radiation background and provides a record of the past occurrence of radionuclides in geosystems. Point defects in kaolinite are hole-centers associated to oxygen atoms linked to Si- and Al-sites. An experimental dosimetry gives the paleodose, which can be used either to derive mean past U-concentration or to estimate the age of kaolinite, according to the geochemical constraints on the geosystem. The detection of past migrations of radioelements in natural analogues may be used in the safety assessment of radioactive waste disposals