The temporal evolution of pedogenic Fe-smectite to Fe-kaolin via interstratified kaolin-smectite in a moist tropical soil chronosequence

Fe-rich kaolins in soils of a terrace chronosequence in the moist tropics of Costa Rica are derived from the transformation of smectite via interstratified kaolin-smectite (K-S). The smectite is ferruginous smectite (10.1 +/- 3.7% Fe2O3) that forms early during pedogenesis from basaltic-andesitic parent material and is the dominant mineral in < 5 ka soils. Interstratified K-S with 60-90% kaolin layers is the dominant mineral in 10 ka soils, while 125 ka soils contain a physical mixture of Fe-kaolin (6.0 +/- 1.9% Fe2O3) and K-S with > 80% kaolin layers. K-S is indicated by X-ray diffraction (XRD) data that reveal asymmetric 001 peaks between 17.5 and 17.8 angstrom (ethylene glycol-solvated) and irrational 001 peaks that exhibit Wring-like behaviour, and by transmission electron microscopy-analytical electron microscopy (TEM-AEM) data that document single crystals of K-S with compositions intermediate to those of end-member crystals of smectite and kaolin. Lattice fringe images from high resolution TEM (HRTEM) show lateral transitions from smectite layers to kaolin layers via a cell-preserved, layer-by-layer transformation mechanism, and changes in the compositions of tetrahedral and octahedral sheets, notably decreases in octahedral Fe and Mg and tetrahedral Al, indicate that the reaction is accompanied by localized dissolution of smectite 2:1 layers that proceeds laterally. Fourier transform infrared (FTIR) and XRD data indicate that kaolin layers are a mixture of kaolinite and halloysite, and differential thermal analysis-thermogravimetry (DTA-TG) combined with data from XRD and TEM-AEM indicate the development of Al-hydroxy-interlayers in K-S with time. The soil-forming factors that appear to control formation of K-S are (1) basaltic-andesitic parent that produces high concentrations of base cations and Si in young (< 5 ka) soils, and (2) moist tropical climate (3085 mm MAP, 27.3 degrees C MAT) with a short dry season that fosters formation of smectite in < 5 ka soils. By 125 ka, soils are dominated by Fe-kaolin and K-S, helping to constrain reaction rate. The relatively high cation exchange capacity (CEC) of the Fe-rich kaolins (18 +/- 12 cmol(c)/kg) relative to many tropical kaolinites is attributed to small amounts of octahedral Mg and tetrahedral Al inherited from precursor ferruginous smectite layers. Accordingly, the origin (from smectite) and relatively high CEC of the K-S and Fe-kaolin has potentially important implications related to clay mineral reactions and elemental cycling in moist tropical soils. (C) 2009 Elsevier B.V. All rights reserved.