Coexistence of allophane, gibbsite, kaolinite and hydroxy-Al-interlayered 2:1 clay minerals in a perudic Andosol

Allophane and halloysite are the most common secondary aluminosilicate minerals in Andosols developed in pyroclasts in humid tropical areas. We studied the composition and the charge properties of the clays extracted from the weathered B horizon of a perhydrated Andosol developed in andesitic ash in Guadeloupe, French West Indies, by selective dissolution, X-ray diffraction (XRD), thermal analysis and characterization of ion exchange properties. The Bw horizon contained weatherable minerals and exhibited an allophane content of 25%. Its clay fraction contained gibbsite, allophane, kaolinite and hydroxy-Al interlayered 2:1 clay minerals. The occurrence of gibbsite, allophane and kaolinite was linked with intense leaching of silica. After oxalate dissolution of allophane, the cation exchange capacity (CEC) was 11.8 cmolc kg−1 clay, i.e. a typical value for low-activity clays. Subsequent citrate treatment completely dissolved gibbsite, but partly extracted Al interlayers from the hydroxy-Al-interlayered phase. This extraction led to a 5.6-fold increase of the cation exchange capacity (62.3 cmolc kg−1 clay) and a 2.3-fold increase in K+ selectivity. The 2:1 clay minerals involved a combination or continuum of high-charge beidellite and vermiculite. From our data, we believe that the 2:1 clay minerals first acted as a sink for aluminium and might have expressed an anti-gibbsitic effect. After substantial Al interlayering of 2:1 clay minerals mimicking low-activity clay, gibbsite might have formed in a second stage.