Abstract Paramagnetic impurities, Fe 3+ , Mn 2+ and vanadyl (VO 2+ ) ions, along with paramagnetic radiation-induced defect centers, trapped holes, have been investigated with electron paramagnetic resonance (EPR) in more than 350 kaolin samples from different localities. The trace elements determined are either substituted (Fe 3+ ) or adsorbed (Mn 2+ and VO 2+ ). The thermal stability of the radiation-induced defects has been determined. The genetic significance of the EPR spectra has been considered on the basis of previous data from the literature and new informations obtained in this study. It is shown that paramagnetic centers: (1) can be used to differentiate kaolinites from the three major environments at the Earth’s surface, hydrothermal, weathering and sediments; (2) are indicators of growth conditions of kaolinite; (3) provide a basis for interpreting conditions of kaolinite formation in different environments; and (4) fingerprint accurately several stages of kaolinite growth and/or successive geochemical events.

Abstract In lateritic profiles from Cameroon, the relationships between ferruginous nodules, in which the texture has been inherited from the parent rock, and surrounding red and yellow matrices, in which the original rock texture has disappeared, were investigated by optical and scanning electron microscopy and by electron microprobe, X-ray powder diffraction, infrared, and electron spin resonance analyses. An orderly succession of changes was found for kaolinite from the ferruginous nodules to the clayey matrices as follows: (1) successive generations of the clay, each of smaller particle size; (2) concomitant decrease in degree of crystallinity; (3) increase in amount of iron substitution; and (4) decrease in the degree of orientation relative to the foliate texture of the parent gneiss. Correlative changes were also observed for associated iron oxides: (1) progressive decrease in the overall content of iron oxides; (2) decrease in the content of hematite (containing a minor amount of Al substitution); and (3) an increase in the content of Al-substituted goethite. These data suggest successive transformations from the ferruginous nodules to the surrounding clayey matrices, although progressive recrystallization may have taken place due to variation of the geochemical conditions of weathering of the original rock.