Abstract
Allophane and imogolite are frequently found as main clay components in soils developed on pyroclastic materials such as volcanic ash and pumice. The relative contents of both components are different between soils, but genetic factors affecting the relative contents are not clear. Thes Study was conducted in an attempt to analyze the factors by means of a model experiment and the molecular orbital method. A mixture of aluminum chloride and orthosilicic acid solution (Si/Al=1:2) was titrated with sodium hydroxide solution for preparing a solution having an OH/Al ratio of 2:1 Various amounts of metal ions (Na, K, Ca, Mg) were added to the solution as chloride, and heated at 90 ℃ for 110 h. The main reaction product in the solution without adding the metal ions was imogolite. The amount of imogolite in the reaction products decreased as the concentration of metal ions was increased. However, allophane was found as a product when metal ions were added, and its amount increased as the concentration of metal ions was increased. The effects of metal ions on inhibiting imogolite formation and on facilitating allophane formation were great in the following order : Na, K < Ca, Mg. These metal ions affect dissociation of the silanol group of orthosilicic acid. The higher the concentration of metal ions, the greater the dissociation. The degree of dissociation is higher by alkaline earth-metal ions than by alkali metal ions under the same ion charge concentration. This indicates that the dissociation or non-dissociation of orthosilicic acid causes differential formations of imogolite and allophane. Molecular orbital calculation (ab initio) proved that non-dissociated orthosilicic acid induces the formation of imogolite with tubular morphology because this form of acid is asymmetrical in molecular configuration. The calculation, on the other hand, showed that dissociated orthosilicic acid gives rise to the formation of allophane with hollow spherical morphology because of the symmetrical configuration of the acid.
