Pedogenic Formation of Kaolinite-Smectite Mixed Layers in a Soil Toposequence Developed from Basaltic Parent Material in Sardinia (Italy)

Dominique Righi1, Fabio Terribile2 and Sabine Petit1
1 UMR-CNRS 6532 “Hydrogéologie, Argiles, Sols et Altérations” Faculté des Sciences, 86022 Poitiers Cedex, France
2 CNR-ISPAIM, PO Box 101, 80040 San Sebastiano al Vesuvio, Napoli, Italy

Abstract: Formation of kaolinite-smectite (K-S) mixed layers in a soil toposequence developed from basaltic parent material was examined. The soil formed in a temperate climate with alternating dry and wet seasons in Sardinia (Italy). Chemical composition and charge characteristics of the smectite component in the K-S mixed layers were analyzed to help determine a mechanism for formation. 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 µm) 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 induced by the slope is probably the major factor responsible for the formation of K-S. Chemical composition and charge characteristics of the smectite component in the K-S were analyzed by X-ray diffraction (intercalation with alkylammonium ions), cation exchanged capacity (CEC) and surface area measurements, and infrared spectroscopy. Results indicate that the smectite component is nearly identical over the soil toposequence. The smectite component is the same with respect to charge magnitude and chemical composition, independent of the proportion of kaolinite and smectite components. This suggests the pedogenic formation of K-S by transformation of smectite through dissolution of some smectite layers and subsequent crystallization of kaolinite between the layers of the remaining smectite crystallites.

Key Words: Basaltic Rock (Weathering) • Kaolinite-Smectite Mixed Layers • Sardinia • Smectite-Layer Charge • Soils

Clays and Clay Minerals; August 1999 v. 47; no. 4; p. 505-514; DOI: 10.1346/CCMN.1999.0470413
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)