INHERITANCE OF 2/1 PHYLLOSILICATES IN COSTA-RICAN ANDISOLS

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.