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In geologically stable areas in the tropics, climatic changes and geomorphic cycles give origin to polygenetic soils. Polygenesis involves new soil formation phases taking place on preweathered materials from previous phases, resulting in soils with rather similar chemical and mineralogical properties. Polygenetic soils from Minas Gerais, Brazil, were investigated with the purpose of distinguishing mineralogical aspects in relation to polygenesis. The soil materials were studied by micromorphology and electron microscope and analyzed by XRD. All soils are strongly weathered and even show weathered grains of ilmenite and quartz. The soils show two phases of kaolinite formation (pseudomorphs after mica and precipitation from solution) and of accumulation of iron compounds. Desilication causes gibbsite formation from kaolinite, occasionally even in the deep saprolite. Superposed processes can be due to progressively deeper weathering or to changes of the external environment.

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The occurrence of 2:1 phyllosilicates in Andisols is variously ascribed to in situ pedogenic origin, aeolian addition, or the presence of hydrothermally altered rock fragments. We studied the origin of 2:1 phyllosilicates that occur in Holocene Hapludands on andesitic, sandy beach ridges in Costa Rica by micromorphological, mineralogical, and submicroscopical techniques. The 2:1 phyllosilicates also occur as pseudomorphs after primary minerals in fresh rock of the inland volcanoes, from which the parent material of the beach ridges was mainly derived. Hydrothermal processes are most likely responsible for the formation of such pseudomorphs. Rock weathering produces sand‐sized rock fragments with clay pseudomorphs and also liberates individual pseudomorphs. Subsequent erosion and alluvial transport affect their shape, but not their internal fabric. In the beach ridges, clay pseudomorphs appear as individual, sand‐sized clay bodies, and inside sand‐sized andesitic rock fragments. Submicroscopical analyses of these individual clay bodies and andesitic rock fragments with clay pseudomorphs indicate a predominance of 2:1 phyllosilicates. This implies that they are inherited from the parent material and are not due to postdepositional soil formation in the beach ridges. Weathering and biological activity affect the clay bodies and rock fragments with clay pseudomorphs, leading to the formation of clay‐sized particles consisting of 2:1 phyllosilicates. Toward the soil surface, these particles are incorporated into the allophanic groundmass resulting from actual soil formation. The geographically extensive occurrence of 2:1 phyllosilicates in Andisols suggests that the genetic processes described here may have more than regional validity.

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.