Fractionnement des argiles par ultracentrifugation en continu : évolution des illites en milieu podzollque / Clay fractionation by continuous flow uitracentrifugation : evolution of illites in a podzolic soil

Abstract

A fractionation method of soil clays is described. The separation of the clay fraction into size classes (< 0. 1/µm ; 0. 1 -0.5 /µm ; 0.5 -2 µm) is made by repeated dispersion and ultracentrifugation with a Sharpies centrifuge. The fractionation quality depends on the dispersion quality of the clay suspension. The use of cation exchange resins (Na+ form) improves the dispersion. The method applied to a podzol developed on triassic sandstone gives evidence of accumulation of small clays (< 0. 1 µm) in the spodic horizons. This accumulation is certainly old and would result in clay illuviation that happened during the deposit of the colluvium along the slopes. During the podzolization phase, mixed-layer clay minerals (I — Sm) and smectites were formed from weathered and fragmented illites. The smectites tend to disapear from the eluvial A2 horizon : a part was translocated toward the Bh horizon, the greatest part of them being melted. The released aluminum was distributed in the spodic Bs horizon, where its hydroxi-polymerization occured. This provokes the formation of amorphous alumina, mixed silica-aluminous gels like allophane and hydroxi-interlayered smectite and vermiculite.