Chemical Properties of Allophane from Hawaiian and Japanese Soils

Abstract

AbstractFour moderately to strongly weathered soils from Hawaii and one moderately weathered soil from Japan, all derived from volcanic ash, were treated to separate out the allophane. The separates are described. With one exception, the fine clay fractions (<0.1µ) were almost X‐amorphous with only weak bands of 2:1 lattice clay minerals. The exception, the Waikaloa soil, contained much halloysite. Coarse‐clay fractions (2‐0.1µ) were mostly crystalline mica, kaolin and gibbsite. Differential thermograms of the amorphous fine clays were characteristic of allophane. The area of the low temperature endotherm correlated with the degree of crystallinity estimated from X‐ray data. Silica‐alumina molar ratios for the fine clays, varying from 0.927 to 1.50, were larger than the ratios for whole soils. The ratios for the three fine clays fractions which appeared least crystalline were all close to 1.0. Loss on ignition was inversely correlated with the silica‐alumina ratios. The allophane in these soils was fibrous and sponge‐like. The main infrared absorption band shifted from 9.70 to 10.29µ with increasing weight ratio of Al2O3/(Al2O3 + SiO2) and apparently reflected a change in state of coordination. Cation exchange capacities (CEC) of the separated fractions increased with increasing pH. There was a correlation between the apparent degree of crystallinity in X‐ray patterns of fine clays and the total change in CEC from pH 4 to pH 8.5. The CEC and infrared data together suggested that some stabilized AlIV‐O‐AlIV bonds occur in the allophane minerals.