Nature and thermal stability of paramagnetic defects in natural clay: a study by electron spin resonance

A natural clay of South of France is studied by electron spin resonance (ESR). Two radiation-induced defects are revealed, certainly localized in the quartz contained in clay. The first is the peroxy-center, associated with oxygen, formed by the action of an O-2(-) ion on a silicon atom to lead to Si-O-O-.. The second is the E'-center, where an electron was trapped in oxygen vacancy to give equivalent to Si-.. The spectroscopic parameters were determined. The thermal stability of these defects is studied by isothermal annealing experiments. Second order kinetics best explain the results, although more complex mechanisms must occur. The two defects have very different behaviors: the peroxy-center seems more stable at high temperature than the E'-center, although its activation energy is much weaker. They could be both used for the ESR dating of old clay. (C) 2000 Elsevier Science Ltd. All rights reserved.