Electron paramagnetic resonance of the E'1 center in quartz. Basic aspect and interest for uranium prospection and geology

Natural irradiation due to the presence of radioactve elements in the rocks leads to the formation of defects in the crystal lattice of minerals exposed to this irradiation. These irradiation-induced defects can subsist for a variable time period after the radioactive source has disappeared. They then act as a memory recording the irradiation of the mineral in its environment. The study of these defects allows one to extract this information and, at the same time, provides information about the radiation history of the rock or deposit. We have chosen to study quartz, a mineral found to be associated with the majority of uranium deposits and for which the behaviour under irradiation has been studied by physicists. The method of analysis used - Electron Paramagnetic Resonance (EPR) - is an extremely sensitive spectroscopic technique which provides unambiguous determination of the nature of an impurity or a paramagnetic defect in a solid. This technique, routinely used for the characterisation of defects, is relatively simple and rapid to apply when the observation conditions for the defect considered have been determined. By numerous analyses of samples of many origins we have been able to demonstrate the existence of a radiation-induced centre which is well adapted to recording of natural radioactivity and the passage of uranium mineralization events. This centre, called the E'1 centre, is an intrinsic defect made up of an oxygen vacancy with a trapped electron. An experimental study of this defect has enabled us to determine its principal characteristics. Two uranium deposits have been studied in particular detail by this method: the Cluff D deposit (Saskatchewan, Canada) and the deposit at Bernardan in la Marche region, France