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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 A1 in adjacent octahedral positions (Alv~-O--Alw 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~ (1) B-center is completely annealed above 300~ (2) A'-center can be annealed by heating at 400~ for more than two hours; (3) A-center is stable up to 450~ The activation energy and the magnitude of the mean halflife for A-center is evaluated through isothermal annealing at 350, 375 and 400*(2, with Ea = 2.0 eV + 0.2, and tv, > l0 ~2 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.

The aim of this work was to determine the importance of nodule formation on the dynamics of major and trace elements (TEs) along a Planosol toposequence developed in metamorphic parent material at La Châtre (Massif Central, France). The different horizons were sampled within three pits and analysed for major and trace element contents. The nodule-rich horizon was studied more closely. A simplified sequential extraction scheme, X-ray diffraction (XRD) and microscopic approaches were used in order to determine the individual phases containing TE in nodules.Along the slope, the nodule-rich horizon varies in thickness, is composed of different oxide fractions and has different scavenging efficiencies according to the TE considered. Iron was found to accumulate in the middle of the slope, while Mn accumulated at the base. The scavenging effect is only evident for Ni in profile 1. For Fe and Cu, it is maximal in profile 2 where the nodule-rich horizon is the thickest. For Pb and Mn, maximal scavenging effect is recorded for both profiles 2 and 3, in the lowest part of the slope. Cr is not accumulated at all. This was related to the water dynamic and the hydromorphic conditions prevailing along the slope.Results obtained by sequential extractions and associated X-ray diffraction on the different nodule size fractions and those obtained by electron microprobe allow inference of the TE distribution in nodules. Nodules were mainly composed of three to four types of cements surrounding grains of quartz, feldspars, micas and accessory minerals: iron-rich cements, Si-and Al-rich cements, Mn-rich cements and Ti-rich cements in places.The iron-rich cements consist of poorly crystalline goethite and possibly some ferrihydrite. Ferrihydrite is associated with Cr as demonstrated by extractions. Goethite contained Mn and most of the TE extracted except for Ni and Pb.Fine-grained Si-and Al-rich cements were also observed. They contain variable amounts of Ti and Mn. 0016-7061/$ -see front matter D Geoderma 125 (2005) 11 -24 www.elsevier.com/locate/geodermaMn-rich cements were not present in all the nodules and were mainly linked to the dark zones of the nodules. The nature of these Mn oxides could not be determined. They were found to contain Co, Ni, Cu and probably Pb. D

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