2011 Volume 84 Issue 2 Pages 131-144
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.
Geographical Review of Japa,. Ser. A, Chirigaku Hyoron