MINERALOGY OF A HOLOCENE CHRONOSEQUENCE ON ANDESITIC BEACH SEDIMENTS IN COSTA-RICA

We investigated weathering and neoformation of minerals in a Holocene (<5000 yr) soil chronosequence on sandy, andesitic, ocean beach ridges in humid tropical Costa Rica using micromorphological, mineralogical, and chemical analyses. Troposamments are present on the younger beach ridges and Hapludands are on the older ones. The parent materials of all soils are sands with similar mineralogical composition: andesitic rock fragments, plagioclase, and pyroxene dominate, with minor amounts of opaque minerals. None of the parent materials contained >13% (v/v) volcanic glass. Weathering and neoformation of minerals with increasi ng soil age is characterized by (i) increasing pellicular and linear a lteration of sand grains and (ii) decrease of the sand fraction and co ncomitant increase of finer material. Andesitic rock fragments weather more rapidly than plagioclase and pyroxene mineral grains. The altera tion rates of the latter two are similar. Clay content in the almost-e qual-to 2000-yr-old soil is several times higher than in soils develop ed in rhyolitic parent materials of similar age in New Zealand. Format ion of allophane with Al/Si ratios ranging from 1.9 to 3.8 takes place mainly in the B horizons. Aluminum-humus complexes, allophane, and Al oxides and hydroxides are mainly formed in the A horizons. Small amou nts of gibbsite were noticed in soils older than 2000 yr. Small amount s of 2:1 and 1:1 clay minerals present in the clay fraction of all soi ls are thought to be inherited from the parent material, which contain s sand-sized bodies of clay and andesitic rock fragments with clay pse udomorphs, both consisting of 2:1 and 1:1 clay minerals.