Radiation-induced Defects in Nonradioactive Natural Minerals: Mineralogical and Environmental Significance
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Radiation-induced Defects in Nonradioactive Natural Minerals: Mineralogical and Environmental Significance Georges Calas, Thierry Allard, Etienne Balan, Guillaume Morin and Stéphanie Sorieul Laboratoire de Minéralogie-Cristallographie, UMR CNRS 7590, Universités Paris 6 et 7 and IPGP, Campus Boucicaut, Case 115, 4 place Jussieu, 75252 Paris Cedex 05, France-
ABSTRACT Natural short-lived radionuclides generate electronic defects in minerals, such as trapped electrons and positive holes, often associated with element impurities, which act as final traps over geological periods. Two main examples will be illustrated. The first example will concern the point defects, which are observed in 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 geological systems. In kaolinite, three types of hole-centers are trapped by oxygen atoms linked to Si- or Al-sites. An experimental dosimetry gives the paleodose, which can be used either to assess mean past U-concentration or for kaolinite dating, depending on the available geochemical parameters. The detection of past migrations of radioelements in natural analogues may be used in the safety assessment of radioactive waste disposals. The second example will concern the role of mineral impurities in defect formation and stabilization. Natural fluorites (CaF2) exhibit hole-and electron-centers trapped on several rare earths and oxygen impurities, often present at the ppm level, which are responsible for the wide range of coloration observed in natural fluorites. Ca colloids may form under severe irradiation and give rise to a characteristic absorption. The thermal stability of radiation-induced defects gives constraints on the evolution of fluorites as a function of temperature and time. Other minerals, such as apatite, confirm the importance of impurities in stabilizing radiationinduced defects over geological periods.
INTRODUCTION Radionuclide-free minerals often exhibit native radiation-induced damages of their crystal lattice. Unpaired electrons or positive holes may be produced by high-energy radiation (α-, γ- or x-rays, electrons, neutrons) and trapped by structural or chemical defects [1, 2]. These defects may cause the coloration of minerals, such as quartz, fluorite, calcite or apatite. They may be used for dating in the Gy range or for assessing thermal events. The well-known coloration of smoky quartz (Al-center) and amethyst (Fe4+) or the great diversity of fluorite colors arise from trapped electrons and positive holes, which create new energy states, and cause specific optical transitions. The detection, structural analysis and measurement of defect concentration may be made by solid-state spectroscopy, such as Electron Paramagnetic Resonance (EPR), UV-visible absorption spectroscopy and luminescence methods. On account of their mutual annihilation, the concentration of unpaired electrons and positive holes created during irradiation
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