Experimental study on alteration characteristics during chemical weathering of granite

Abstract

This study aimed to elucidate the alteration characteristics during chemical weathering under acidic conditions for a better understanding of granite landform evolution. The 56-day alteration experiment was carried out in a closed system, where the effect of solution pH on reaction was examined using acidic solutions of various pH values. The chemical composition of the altered surface was determined using the X-ray photoelectron spectroscopy technique. The specific characteristics of granite as a rock were investigated by a comparison with the results of its constituent minerals (single crystals of annite, albite, microcline, and quartz). During the surface alteration of granite, Fe-oxides and Si-oxides were formed. Under neutral to slightly acidic conditions, plagioclase dissolved, and then pH increased. Dissolved iron from biotite smoothly precipitated as Fe-oxides. Under low pH conditions, the dissolution of biotite was predominant, and Si-oxides were formed. In the alteration of granite under neutral to slightly acidic conditions, two effects were seen: 1) the increase in pH due to plagioclase dissolution promoted Fe-oxides precipitation; and 2) the presence of some species in aqueous solution due to biotite dissolution probably inhibited Al-oxides precipitation. These effects increase the reactivity of biotite and allow easier formation of iron minerals during granite weathering in the natural environment.