SOIL GENESIS ON TRACHYTIC AND LEUCITITIC LAVAS OF CIMINI VOLCANIC COMPLEX (LATIUM, ITALY)

Three distinct soils which are developing on volcanic effusive rocks o f Mt. Fogliano, San Martino and Mt. Cimino, and form part of the Cimin i volcanic complex, north of Rome, were analysed and their physico-che mical, mineralogical and micromorphological characteristics are report ed. These soils are believed to have developed in a fairly recent time , beginning around 20,000 years ago, under similar climatic and vegeta tional conditions. The Mt. Fogliano soil, a Typic Hapludand, developed in a leucite tephritic lava, showed evident andic properties and a hi gh content of amorphous short-order phases, such as Al-rich imogolite- like allophanes. The formation of crystalline clays is very limited, t he main clay mineral being halloysite (0.7 nm). The leucite appeared t o be almost completely weathered in the soil. The San Martino soil, an Andic Dystrochrept, developed from trachytic lavas, was affected by t he addition of tephritic pyroclasts and showed less pronounced andic p roperties; in fact, the surface horizons exhibited a high content of a llophanic 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 th ickness. The main clay minerals were found to be kaolinite and illite. The lower horizon contained halloysite (0.7 nm), illite, and a low co ntent of allophanic constituents. The Mt. Cimino soil, a Vitrandic Hap ludalf, derived from latitic lavas, was characterized by a marked gene sis and translocation of clay, showed very faint andic properties and was found to be very poor in extractable Al. This pedoenvironment favo urs the formation of crystalline clays, the main clay minerals being k aolinite, illite and vermiculite at a different degree of intercalatio n with Al hydroxy polymers. The nature of the rock seems to have playe d 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 constituen ts 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, w hile the latitic rock of the Mt. Cimino soil, by the weathering of fel dspars and biotite, present in a higher amount, has induced the format ion of crystalline clay minerals and favoured the formation of a non-a llophanic soil in a pedoenvironment where Andisols are usually found.