Identification of clay coatings in an older quaternary terrace of the Allier, Limagne, France

Micromorphologically three types of coatings were observed in B horizons of two planosol profiles (Albaqualfs): (a) nonlaminated isotropic limpid clay coatings, (b) nonlaminated anisotropic limpid clay coatings, and (c) microlaminated anisotropic dusty clay coatings. Micromorphological observations suggested that coating types (a) and (b) were genetically similar, resulting from weathering and neoformation. Coating type (c) originated from clay illuviation. Elemental compositions of clay coatings were obtained with a scanning electron microscope-energy dispersive X-ray analyzer technique. Alumina contents decreased from 38% in isotropic coatings to about 28% in illuviation coatings. The Al content was inversely related to contents of K, Mg, Ca, and Fe. Factor analysis indicated that two factors accounted for 56.5 and 22.8% of the total elemental variance. The first factor associated with coating type revealed the antithetic behavior of Al vs. K, Mg, Ca, and Fe, and accounted for about 90% in the variability of Al. The second factor associated with the sample location explained greater than 70% of the variability in Ti and Si. Chemical classification of investigated samples was based on elemental similarity. Cluster analysis revealed that coatings of the same type were more alike than coatings originating from the same profile. About 83% of grouped samples were classified correctly as isotropic and anisotropic weathering coatings, and anisotropic illuviation coatings. Clay minerals associated with individual coatings were identified with an X-ray step-scan method after sampling microquantities of these coatings with a microdrill. Anisotropic illuviation and weathering coatings consisted of illite and kaolinite. Anisotropic weathering coatings, however, were found to contain less illite than illuviation coatings. Isotropic coatings were X-ray amorphous, suggesting the presence of Al-Fe-Si gel.