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A B S T R A C T :The unique properties of imogolite are closely related to its structure, which is a tube of 23-27 /~ outer diameter and ~ 10 A inner diameter, with an AIOH outer surface and SiOH inner surface. Acid dispersions contain the long, positively-charged tubes as isolated units or small bundles, which form bulky gels in alkali, and flocculate with negatively-charged colloids, polyvalent anions, and long-chain anionic detergents. Sorption properties are associated with the 10 A intra-tube pores and with inter-tube channels of variable dimensions. Surface acidity is less than that of layer-silicate clays. The chemical and mechanical stability, biological activity, filmand fibre-forming characteristics, and conditions of synthesis are reviewed, on the basis of both new and published findings. Areas of potential application are indicated.Imogolite is a hydrous aluminium silicate with a unique tubular structure and novel properties; as such, it has no immediate application in any commercial process where layer-silicate clays now play a role. The properties of the isolated product and of its dispersions can be rationally related to its structure, and this relationship is reviewed here. Emphasis is placed on those features which might find practical applications. Problems of synthesis and isolation of imogolite are also discussed.

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Adsorption isotherms were obtained for fulvic and humic acids on synthetic gibbsite and goethite and on a natural imogolite. The results were interpreted with the help of spectra of the adsorption complexes, and measurements of hydroxyl displaced. The mechanism of adsorption involved ligand exchange with the surface hydroxyl groups and hydrogen bonding. The hydroxyl groups displaced included singly coordinated species on the (100) face of goethite and on the edge fams of gibbsite. The doubly coordinated hydroxyl groups on the (001) face of gibbsite were not replaced, and this face adsorbed only un-ionized fulvic acid probably by a hydrogen bonding mechanism. Imoglite adsorbed most fulvic acid because of its high porosity. Only isolated places on its surface reacted with fulvic acid to form carboxylate groups.

A berdeen) WITH PLATE SummaryExamination by infrared spectroscopy and electron microscopy of the fine clays (<0.5 pm) dispersed at pH 3.5 from HzO,-treated soil indicates that imogolite and proto-imogolite allophanes are concentrated in podzolic Bz and B, horizons, and make up at least 6 percent of one B, horizon soil, which contains virtually no layer silicate clays. It is argued here that imogolite-type components are the principal source of extractable aluminium and silicon in such horizons, that they may act as cementing agents in indurated horizons, and that proto-imogolite, a soluble aluminium-silicate complex, is the predominant mobile form in which aluminium is transported to B, and lower horizons of podzols. Comparison of the amounts of aluminium extracted by acetic acid with those extracted by EDTA indicates that extractable aluminium in Bhg, Bh, and organic-rich A, horizons is present principally in organic complexes. It is proposed that the aluminium fulvates concentrated in these horizons are formed in situ.

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