Morphology, Chemistry, and Mineralogy of Isotropic Aluminosilicate Coatings in a Guadeloupe Andisol

Abstract

Abstract Few micromorphological and in situ submicroscopical studies exist of neoformed amorphous and crystalline clay coatings, and little is known about the genesis and distribution patterns of these coatings and their dependence on site conditions. Our study describes the distribution and composition of isotropic coatings, infillings, and pseudomorphs after roots in a Hapludand on Holocene andesitic pyroclastics in humid tropical Guadeloupe. Field observations show the occurrence of fine‐textured, very pale brown to yellow (10YR 8/3‐8/6) coatings, infillings, and pseudomorphs after roots in large pores in the Bw2 and 2C horizons of the soil. Thin‐section analyses demonstrated that these features are isotropic, translucent, nonlaminated, and appear pale yellow in plane polarized light, indicating that they consist of amorphous material. In situ submicroscopical analyses revealed that the coatings in the Bw2 horizon consist of allophane and imogolite with an Al/Si ratio of 2, whereas those in the 2C horizon consist exclusively of allophane with an Al/Si ratio of 1.4. The fine groundmass adjacent to the coatings in the Bw2 horizon has an Al/Si ratio of 1.4. The coatings resulted from precipitation of weathering products of mainly volcanic glass and plagioclase. The observed differences in composition of the coatings in the Bw2 and 2C horizons are thought to be the result of different leaching conditions.