Mineralogy of a soil chronosequence on andesitic lava in humid tropical Costa Rica

We studied a chronosequence of three soils developed on andesitic lavas in humid tropical Costa Rica, in order to investigate mineralogical development and to evaluate the importance of time versus other soil forming factors. Morphological, chemical, and mineralogical methods were used to unravel evolution of the soils. Primary minerals in the <1-m deep, 2000-year-old Fulvudand show increasing weathering in the order: opaque minerals — pyroxene — plagioclase — volcanic glass. Coarse rock fragments are virtually unweathered. Secondary minerals are exclusively noncrystalline material and Al– and Fe–humus complexes. In the <18,190-year-old, about 3 m deep Fulvudand volcanic glass is absent in the groundmass of the A and B horizons. Plagioclase and pyroxene grains are strongly weathered, and coarse rock fragments have distinct weathering rinds. Secondary minerals are dominantly Al– and Fe–humus complexes in the upper horizon, while noncrystalline material is most important at greater depths. Furthermore, considerable amounts of gibbsite occur, and appear to form from disintegration of noncrystalline material and Al–humus complexes. Kaolin minerals (kaolinite and halloysite) occur throughout the profile. They presumably formed under dryer climates in the past. Finally, some goethite is present. The 450,000- year-old, and more than 6 m deep Haploperox is devoid of primary minerals except for opaques in the upper 5 m. Gibbsite, kaolin minerals and goethite are dominant among the secondary products. Gibbsite appears to form from disintegration of kaolin minerals, while kaolin minerals are thought to have formed under drier climatic conditions. In all soils, degree and depth of weathering appears to be primarily a function of time. Differences in clay mineralogy are believed to be mainly the result of differences in soil age too, although different climates to which soils have been exposed play a role as well.