Transformations mineralogiques dans les podzols et les sols podzoliques de Galice (NW Espagne)

Abstract

The weathering and formation of minerals in Podzols and the hypotheses on the mechanism of podzolization are discussed in accord with the composition of active fraction (colloids and soil solution) and physico-chemical parameters (pe-pH) of horizons of Ortic Podzols from Galicia (NW Spain). Two different geochemical environments are established in Ortic Podzols. The first one, that includes surface horizons (A and E), is characterized by the predominance of dissolution of primary minerals through a mechanism of "acid-complexolysis” and by the nearly total absence of mineral formation. Under these conditions, only kaolinite can be stable, being promoted by high temperatures and a lowering in the activity of organic matter. lmogolite, halloysite and gibbsite are always unstable. In the second environment, represented by the spodic horizons (Bh and Bs), the conditions are controlled by weaker acids than above (CO2, hydroxy-AI ions .... ), the weathering mechanisms range from hydrolysis to moderately acid hydrolysis and the geochemical processes are defined as “aluminosiallitization” or “monosiallltization”. Kaolinite is always a stable mineral, and metastable phases like imogolite, gibbsite and sometimes halloysite can be precipitated from soil solution or formed as the mineralization of organic ligands proceeds. The mechanism of podzolization supported by experimental data is the migration of organo-metallic complexes, and there are not evidences that justify the presence of protoimogolite in surface horizons. lmogolite or protoimogolite must not be considered as typical phases of surface horizons during podzolization, but as characteristic components of environments with moderate acidity and low organic matter activity. This kind of conditions occurs in spodic horizons of Ortic Podzols and in surface (A) and weathering (B) horizons of acid soils without albic horizons.