WEATHERING OF BASALTIC PEBBLES IN A RED SOIL FROM SARDINIA: a microsite approach for the identification of secondary mineral phases

Mineral weathering microsites in a basalt rock-derived soil were investigated in the Mediterranean environment of south-western Sardinia. An in situ approach, frequently used for saprolites and rock investigations, was employed to examine the weathering microsites of a soil system. This study compared clay phases that formed from the weathering of mineral-bearing rock pebbles and soil crystal chemical composition, with the goal of evaluating the evolutionary trends of the secondary minerals. Micromorphological, mineralogical (XRD and FT-IR analyses) and microscopic (SEM–EDS) analyses were carried out on: i) plagioclase, enstatite and glassy features (weathered volcanic glass) microsystems, in rock pebbles embedded in the soil, ii) clay coatings microsystem, connecting the soil–rock environment and iii) the soil itself. EDS data were plotted on phase diagrams to evaluate developmental and evolutionary trends of the weathered products. The weathering of enstatite and volcanic glass resulted in, primarily, Fe-bearing montmorillonites, while a mixture of Fe-bearing smectite and kaolinite developed from plagioclases. Our results indicated formation of secondary phases in open microsystems, where transfers of elements among primary minerals and glass occur by slow diffusion along the network of inter-mineral micropores. Cracks that developed inside the soil–rock pebbles acted as direct connections between microsystems and the soil environment. Clay and Fe–Mn coatings deposited in the cracks are an indication of clay migration. The mineralogy of these clay coatings was similar to that of the soil matrix and the fine clays. Since it is likely that the clay migration processes remain active in Guspini soils, the further mineralogical similarities of clay coatings and soil fine clays suggest that the transformation of Fe-smectites into K–S MLMs occurs in the clay phases during transport, prior to deposition into the soil environment. Indeed, the similar mineralogy of the in situ analysed soil matrix and fine clays (the youngest soil fraction) is interpreted as due to same soil processes, suggesting stable environmental conditions in the Guspini area.