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 developmen t and to evaluate the importance of time versus other soil forming factors. Morphological, chemical, and mineralogical methods were used to unravel ev olution of the soils. Primary minerals in the < 1-m deep, 2000-year-old Ful vudand show increasing weathering in the order: opaque minerals - pyroxene - plagioclase - volcanic glass. Coarse rock fragments are virtually unweath ered. Secondary minerals are exclusively noncrystalline material and Al- an d Fe-humus complexes. In the < 18,190-year-old, about 3 m deep Fulvudand vo lcanic glass is absent in the groundmass of the A and B horizons. Plagiocla se and pyroxene grains are strongly weathered, and coarse rock fragments ha ve 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 a nd Al-humus complexes. Kaolin minerals (kaolinite and halloysite) occur thr oughout the profile. They presumably formed under dryer climates in the pas t. 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 s econdary products. Gibbsite appears to form from disintegration of kaolin m inerals, while kaolin minerals are thought to have formed under drier clima tic conditions. In all soils, degree and depth of weathering ap;Fears 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 rate as well. (C) 2000 Els evier Science B.V. All rights reserved.