Mineralogy and Iron Characterization of Plinthitic Soils on Alluvial Landforms in Venezuela

Abstract

Abstract The iron oxides and clay mineralogy were characterized for several soils of Venezuela which contain plinthite and pseudo‐plinthite. Molar ratios of SiO 2 /Al 2 O 3 (Ki ratios) of the plinthite and pseudo‐plinthite zones range from below 2 to above 4 suggesting a wide range in mineral composition. Cation exchange capacity for these zones ranged from 14.6 to 65.6 meq/100 g of clay suggesting that these mottled zones are not always associated with low activity clays. Kaolinite dominated the clay minerals. Montmorillonite was identified in a few pedons. Goethite was the dominant iron oxide present in the plinthite. The percentage of iron was determined by extracting the soil with citrate‐bicarbonate‐dithionite and by extracting with ammonium oxalate. These two extractions were ratioed (OX/CBD) yielding values of >0.05 for pseudo‐plinthite and values of <0.05 for plinthite in most cases. Soils with plinthite contained most of the iron in the sand‐ and silt‐size fraction following one drying cycle while soils with pseudo‐plinthite contained most of the iron in the clay‐size fraction.