Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)

Formation of kaolinite-smectite (K-S) mixed layers in a soil toposequence d eveloped from basaltic parent material was examined. The soil formed in a t emperate climate with alternating dry and wet seasons in Sardinia (Italy). Chemical composition and charge characteristics of the smectite component i n the K-S mixed layers were analyzed to help determine a mechanism for form ation. Soils were sampled at the top, intermediate, and base of a steep (35 %) slope. As indicated by X-ray diffraction data, the fine clay fraction (< 0.1 mu m) in the soils is dominated by K-S with a decreasing proportion of kaolinite from the top (70%) to the base (30%). Rapid internal drainage ind uced by the slope is probably the major factor responsible for the formatio n of K-S. Chemical composition and charge characteristics of the smectite c omponent in the K-S were analyzed by X-ray diffraction (intercalation with alkylammonium ions), cation exchanged capacity (CEC) and surface area measu rements, and infrared spectroscopy. Results indicate that the smectite comp onent is nearly identical over the soil toposequence. The smectite componen t is the same with respect to charge magnitude and chemical composition, in dependent of the proportion of kaolinite and smectite components. This sugg ests the pedogenic formation of K-S by transformation of smectite through d issolution of some smectite layers and subsequent crystallization of kaolin ite between the layers of the remaining smectite crystallites.