INHERITANCE OF 2 1 PHYLLOSILICATES IN COSTA-RICAN ANDISOLS/

The occurrence of 2:1 phyllosilicates in Andisols is variously ascribe d to in situ pedogenic origin, aeolian addition, or the presence of hy drothermally altered rock fragments. We studied the origin of 2:1 phyl losilicates that occur in Holocene Hapludands on andesitic, sandy beac h ridges in Costa Rica by micromorphological, mineralogical, and submi croscopical techniques. The 2:1 phyllosilicates also occur as pseudomo rphs after primary minerals in fresh rock of the inland volcanoes, fro m which the parent material of the beach ridges was mainly derived. Hy drothermal 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. Sub sequent erosion and alluvial transport affect their shape, but not the ir internal fabric. In the beach ridges, clay pseudomorphs appear as i ndividual, sand-sized clay bodies, and inside sand-sized andesitic roc k fragments. Submicroscopical analyses of these individual clay bodies and andesitic rock fragments with clay pseudomorphs indicate a predom inance of 2:1 phyllosilicates. This implies that they are inherited fr om the parent material and are not due to postdepositional soil format ion 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 all ophanic groundmass resulting from actual soil formation. The geographi cally extensive occurrence of 2:1 phyllosilicates in Andisols suggests that the genetic processes described here may have more than regional validity.