Soil genesis on trachytic and leucititic lavas of Cimini volcanic complex (Latium, Italy)

Abstract

Three distinct soils which are developing on volcanic effusive rocks of Mt. Fogliano, San Martino and Mt. Cimino, and form part of the Cimini volcanic complex, north of Rome, were analysed and their physico-chemical, mineralogical and micromorphological characteristics are reported. These soils are believed to have developed in a fairly recent time, beginning around 20,000 years ago, under similar climatic and vegetational conditions. The Mt. Fogliano soil, a Typic Hapludand, developed in a leucite tephritic lava, showed evident andic properties and a high content of amorphous short-order phases, such as Al-rich imogolite-like allophanes. The formation of crystalline clays is very limited, the main clay mineral being halloysite (0.7 nm). The leucite appeared to be almost completely weathered in the soil. The San Martino soil, an Andic Dystrochrept, developed from trachytic lavas, was affected by the addition of tephritic pyroclasts and showed less pronounced andic properties; in fact, the surface horizons exhibited a high content of allophanic materials, the (Al oAl p) /Si o molar ratio being around 2, but they did not meet the andic requirements due to their limited thickness. The main clay minerals were found to be kaolinite and illite. The lower horizon contained halloysite (0.7 nm), illite, and a low content of allophanic constituents. The Mt. Cimino soil, a Vitrandic Hapludalf, derived from latitic lavas, was characterized by a marked genesis and translocation of clay, showed very faint andic properties and was found to be very poor in extractable Al. This pedoenvironment favours the formation of crystalline clays, the main clay minerals being kaolinite, illite and vermiculite at a different degree of intercalation with Al hydroxy polymers. The nature of the rock seems to have played a significant role in determining the different pedogenetical trends of these soils. The feebly resistant leucite in the Mt. Fogliano soil has contributed to the formation of Al-rich allophane-like constituents and Al-humus complexes, the holocrystalline trachytic substratum of the San Martino soil, rich in feldspars, has promoted the production of halloysite over the formation of allophane in the deeper horizon, while the latitic rock of the Mt. Cimino soil, by the weathering of feldspars and biotite, present in a higher amount, has induced the formation of crystalline clay minerals and favoured the formation of a non-allophanic soil in a pedoenvironment where Andisols are usually found.