Origin of Clay minerals in soils on pyroclastic deposits in the island of Lipari (Italy)

The island of Lipari (Italy) is characterized by calc-alkaline to potassic volcanism and a Mediterranean-type climate. The mineralogical and chemical features of two different soil profiles with ages of 92,000 and 10,000–40,000 y, respectively, have been investigated. There were no Andisols, but Vitric and Vertic Cambisols have developed at both sites. Although the morphology of the soils was similar, remarkable differences in the clay mineralogy between the two sites were observed. The site with the Vitric Cambisol was associated with the weathering sequence: glass → halloysite → kaolinite or interstratified kaolinite-2:1 clay minerals. Both sites had smectite in the clay fraction and, to a large extent, this smectite had a low charge and could be characterized as a dioctahedral montmorillonite. At the site with a Vertic Cambisol, smectite was the predominant mineral phase in the clay fraction. The smectites (predominantly montmorillonite) found in this soil were probably not of pedogenetic origin and are, therefore, inherited from the parent material. Their formation is due to hydrothermal alteration of glass particles during or immediately after the emplacement of the pyroclastic flow. The octahedral character of the smectites did not change from the C to the A horizon indicating that they are resistant to weathering processes. A high-charge expandable mineral was detected in small concentrations in the Vertic Cambisol and had a dioctahedral structure. In this case also, no signs of significant weathering or transformation could be detected in the soil profile. In contrast to many other investigations, no active smectite formation within the soil profiles could be measured. The subtropical and rather dry climate in Lipari might, therefore, favor the persistence of dioctahedral low-charge montmorillonites that are associated with a small amount of a high-charge expandable mineral in the soil.