Formation of weathering rinds on andesite

Abstract

AbstractTo explore the mechanisms of weathering‐rind formation, rock properties of an andesite cobble were investigated. The andesite cobble studied in this paper was taken from a fluvial terrace which formed around 290 ka BP in the Nasuno‐ga‐hara area, central Japan. The sampled cobble has a double weathering rind comprising an outer brown zone (brown band) and an inner pale grey zone (white band) with thicknesses of 3 mm and 7 mm, respectively. The boundary between the outer brown band and the inner white band is clearly defined, whereas the boundary between the white band and the grey fresh‐looking interior of the rock is not so clearly defined.Mineralogical, chemical, chromatic and mechanical properties of the cobble were investigated in the laboratory. The outer zone, the brown band, is characterized by high brittleness, high FeO+Fe2O3 content, and low contents of CaO, MgO, Na2O and K2O. The oxidation of iron accounts for the brown colour of the rind. This outer zone is considered to have experienced both oxidation and dissolution. The inner zone, the white band, is characterized by hardness reduction and the dissolution of alkali/alkaline earth metals towards the surface of the cobble. This inner band is considered to have experienced only dissolution.The formation of the weathering rinds can be explained by changes in water‐movement directions in terrace deposits. Alkali/alkaline earth metals dissolve when water is supplied to a cobble from the surrounding matrix. Subsequently, the dissolved metals move out of the cobble accompanied by outward water movement when the subsurface water table is low. The concentrations of CaO, MgO, Na2O,K2O and Al2O3, therefore, decrease in both weathering bands, but iron tends to remain as ferric hydroxides in the outer brown band. Processes of dissolution and oxidation have produced the brown band. Copyright © 2001 John Wiley & Sons, Ltd.