Tracing kaolinites through their defect centers; kaolinite paragenesis in a laterite (Cameroon)

Abstract

Stable structural point defects (A-centers) have been studied in kaolinite using electron paramagnetic resonance (EPR). The variation in the concentration of defect centers (S(A)) has been systematically investigated along seven lateritic weathering profiles from Cameroon. Petrological and geochemical data have been obtained on the same samples. A-centers result from an external irradiation of kaolinite. There is no relation between defect center concentration and textural changes, the Fe content of the kaolinite lattice, and the crystalline order of this mineral. On the contrary there does exist a relationship between the integrated intensity of the EPR signal on one hand and the Fe and the U-Th content of the bulk samples on the other.A possible radiation source has to be sought in the radioactive elements disseminated through the weathered materials and mainly accumulated in ferruginous materials. However, the present-day U-Th content is too low to explain the measured concentration of defect centers. By taking into account the efficient chemisorption of radioactive elements on poorly crystallized iron oxides, the A-center defects are attributed to an inherited irradiation of the kaolinites during the first stages of rock weathering and their subsequent crystallization. The major implication of the proposed model is that iron oxide gels precipitate at the weathering front and adsorb the radionuclides released by primary minerals. The evolution of iron oxides toward a more crystalline structure leads to a release of the radioactive elements, and the A-centers are thus the memory of their transit. At the same time, the concentration of defect centers gives an efficient fingerprint of the successive generations of kaolinites during weathering processes.