Weathering of sandstone clasts in a forest soil in Tuscany (Italy)

Abstract

The effects of weathering on clasts (rock fragments) in soil have not been investigated as widely as those on the less than 2-mm soil fraction (“fine earth”) or on rock surfaces exposed to the atmosphere. In this work, a scanning electron microscope/X-ray diffraction investigation, integrated with some chemical analyses, was made on coarse (4–8 cm as maximum length) clasts of sandstone from the three main horizons (A, Bw, and BC) of a sandstone-derived forest soil. The degree of alteration of the clasts varied considerably according to depth. Those collected from the A horizon were strongly colonised by fungal hyphae and showed abundant micro-cracks into which the hyphae often extended. Additionally, the surfaces of clasts appeared somewhat smoothed with only resistant grains of quartz and feldspars standing proud. Exposed particles of mica and chlorite often showed disrupted/exfoliated shapes. Evidence of alteration at the surface of clasts diminished with depth in the soil profile; in particular, little or no biological forms were detected by scanning electron microscopy in the BC horizon. In the A horizon, X-ray diffraction analysis on the external about 1-mm-thick yellowish layer of clasts and their grey interior, indicated some loss in quantity of micas in the surface rind. The alteration of the phyllosilicates was studied in more detail in the <20-μm fraction, which was known from visual inspection to include most of these minerals. Unexpectedly, there were no substantial differences in the degree of alteration of mica and chlorite between the interior and exterior of the clasts in any single soil horizon. There were, however, major differences in the degree of alteration of mica and chlorite between clasts from different horizons. These differences were consistent with the alteration of mica and chlorite by weathering throughout the whole volume of the clasts during soil profile development. Thus, from the base to the top of the soil profile, the clasts showed a marked decrease in chlorite content and the transformation of both mica and chlorite to vermiculite via mixed-layer phases. The presence in the external rind of the clasts from the topsoil of an interstratified chlorite/vermiculite with a higher degree of vermiculisation than in the interior was the only evidence of a stronger weathering confined to the surface of clasts. The fact that the effects of mineral alteration were rather homogeneously distributed throughout the volume of the clasts may be related to the porous nature of the original lithology but emphasises that rock fragments are actively involved in soil processes.