MORPHOLOGY, CHEMISTRY, AND MINERALOGY OF ISOTROPIC ALUMINOSILICATE COATINGS IN A GUADELOUPE ANDISOL

Few micromorphological and in situ submicroscopical studies exist of n eoformed amorphous and crystalline clay coatings, and little is known about the genesis and distribution patterns of these coatings and thei r dependence on site conditions. Our study describes the distribution and composition of isotropic coatings, infillings, and pseudomorphs af ter roots in a Hapludand on Holocene andesitic pyroclastics in humid t ropical Guadeloupe. Field observations show the occurrence of fine-tex tured, 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 a re isotropic, translucent, nonlaminated, and appear pale yellow in pla ne polarized tight, 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 wi th 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 f rom precipitation of weathering products of mainly volcanic glass and plagioclase. The observed differences in composition of the coatings i n the Bw2 and 2C horizons are thought to be the result of different le aching conditions.