SILICA ADSORPTION AND DISSOLUTION PROPERTIES OF ANDOSOLS FROM NORTHEASTERN JAPAN AS RELATED TO THEIR NONCRYSTALLINE CLAY MINERALOGICAL COMPOSITION

Abstract

We studied silica adsorption and dissolution properties for three groups of Andosols having contrasting clay mineralogical compositions. Our results indicated that these properties are highly useful to characterize the soil environments as related to the formation of noncrystalline clay minerals. A1 horizons of young soils (Group 1), containing abundant laminar opaline silica and 2:1 minerals, showed very low Si adsorption and very high Si dissolution, representing a zero-order reaction. This fact indicated the prevalance of amorphous Si surfaces and the virtual absence of Si-adsorbing hydrous A1 surfaces that are favorable for the formation of laminar opaline silica. In contrast, we observed very high Si adsorption and low silica dissolution for B horizon soils of old soils (Group 2). These properties indicated the predominance of amorphous hydrous A1 surfaces that have a great affinity for Si, leading to the formation of allophane and imogolite in these soils. A horizons of buried soils (Group 3) having an intermediate clay mineralogy between those of Groups 1 and 2 soils showed intermediate properties in Si adsorption and in rates of Si dissolution. The results implied that the burial of these soils has significantly changed their surface weathering condition to subsoil weathering condition. Laminar opaline silica is considered to be a relic mineral formed by surface weathering.