Nature and Stability of Radiation-Induced Defects in Natural Kaolinites: New Results and a Reappraisal of Published Works

Blandine Clozel1, *, Thierry Allard1 and Jean-Pierre Muller1, 2
1 Laboratoire de Minéralogie-Cristallographie, UA CNRS 09 Universités Paris 6 et 7 4 Place Jussieu, 75252 Paris Cedex 05, France
2 O.R.S.T.O.M., Département T.O.A., 75480 Paris Cedex 10, France
* Present address: BRGM, DRIGGP, Research Division, Geotechnical Engineering and Mineral Technology, Avenue de Concyr. B.P. 6009, 45060 Orleans Cedex 2, France.

Abstract: A new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented kaolinite samples originating from various environments. Three different centers were identified. Two of the centers, A- and A′-centers, are trapped holes on oxygen from Si-O bonds. They have a distinct signature and orthogonal orientation, i.e., perpendicular and parallel to the (ab) plane, respectively. The third center, the B-center, is a hole trapped on the oxygen bonding Al in adjacent octahedral positions (AlVI-O--AlVI bridge). This confirmed some previous assignments from the literature, some others are no longer considered as valid.

A least squares fitting procedure is proposed to assess the RID concentration in any kaolinite. It allows a quantitative approach of the thermal stability of RID. Isochronal annealing shows that the thermal stability of the centers decreases in the order A, A′, B over the temperature range 0–450°C (1) B-center is completely annealed above 300°C (2) A′-center can be annealed by heating at 400°C for more than two hours; (3) A-center is stable up to 450°C. The activation energy and the magnitude of the mean half-life for A-center is evaluated through isothermal annealing at 350, 375 and 400°C, with Ea = 2.0 eV ± 0.2, and t½, > 1012 years at 300 K. The stability of A-center seems to decrease with increasing crystalline disorder. Nevertheless, it is high enough for radiation dosimetry using kaolinites from any environment on the Earth's surface.

Key Words: EPR • Kaolinite • Radiation induced defects • RID

Clays and Clay Minerals; December 1994 v. 42; no. 6; p. 657-666; DOI: 10.1346/CCMN.1994.0420601
© 1994, The Clay Minerals Society
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