Soils under subalpine vegetation in the Mont Avic Natural Park (Aosta Valley, Western Italian Alps), developed on ophiolitic materials, show well developed signs of podzolization; this is in contrast with the base-rich soils usually found on ultramafic rocks. The high base status and the neutral pH should retard the translocation of metallo-humic compounds, which is the key process of podzolization. Chemical properties and clay mineralogy of three pedons (one on metamorphic gabbros and two on serpentinite) were studied in order to understand the particular pedogenetic process active in the area and the differences between the mineralogy and weathering pathways of the two rocks. According with the chemical properties, the soils were classified as Densic Podzol (WRB 1998 edition) on metagabbros and Spodi-Dystric Cambisol on serpentinite, due to the lack of Al, despite the morphology and the high metal redistribution between the E and Bs horizons. On serpentinite, the mineralogy of the clay fraction shows a high degree of alteration in the E horizons: the main minerals are talc, smectite, chlorite, serpentine and chlorite-illite interlayers, as well as vermiculite-illite interlayered. In the Bs horizon, however, serpentine minerals are the most common, followed by chlorite and some interleyered vermiculite-illite. There are no smectites. Weathering of the serpentine minerals to talc, smectites and interlayered clay minerals seems the most important clay-forming process, possibly linked with the release of amorphous Fe-oxides, which can be complexed by organic soluble acids and leached down the profile. On metagabbros, the minerals are similar, but there is less serpentinite and talc and more chlorite and interstratified vermiculite-illite and vermiculite-chlorite. The illuvial Bs horizon is enriched in “imogolite-type material”.
Clay formation and podzolization on mafic and ultramafic rocks in Mont Avic Natural Park (Ao, Italy)
D'AMICO, MICHELE;BONIFACIO, Eleonora;
2006-01-01
Abstract
Soils under subalpine vegetation in the Mont Avic Natural Park (Aosta Valley, Western Italian Alps), developed on ophiolitic materials, show well developed signs of podzolization; this is in contrast with the base-rich soils usually found on ultramafic rocks. The high base status and the neutral pH should retard the translocation of metallo-humic compounds, which is the key process of podzolization. Chemical properties and clay mineralogy of three pedons (one on metamorphic gabbros and two on serpentinite) were studied in order to understand the particular pedogenetic process active in the area and the differences between the mineralogy and weathering pathways of the two rocks. According with the chemical properties, the soils were classified as Densic Podzol (WRB 1998 edition) on metagabbros and Spodi-Dystric Cambisol on serpentinite, due to the lack of Al, despite the morphology and the high metal redistribution between the E and Bs horizons. On serpentinite, the mineralogy of the clay fraction shows a high degree of alteration in the E horizons: the main minerals are talc, smectite, chlorite, serpentine and chlorite-illite interlayers, as well as vermiculite-illite interlayered. In the Bs horizon, however, serpentine minerals are the most common, followed by chlorite and some interleyered vermiculite-illite. There are no smectites. Weathering of the serpentine minerals to talc, smectites and interlayered clay minerals seems the most important clay-forming process, possibly linked with the release of amorphous Fe-oxides, which can be complexed by organic soluble acids and leached down the profile. On metagabbros, the minerals are similar, but there is less serpentinite and talc and more chlorite and interstratified vermiculite-illite and vermiculite-chlorite. The illuvial Bs horizon is enriched in “imogolite-type material”.
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