SMALL-ANGLE X-RAY POWDER DIFFRACTION, MORPHOLOGY, AND STRUCTURE OF ALLOPHANE AND IMOGOLITE

Abstract--Small-angle X-ray powder diffraction analyses and high-resolution electron microscopy of allophane samples (SiOJAlzO3 ratio, 1.12 to 1.68) showed that allophanes consist of nearly identical spherical particles with diameters of about 40 A and retain their characteristic "hollow " spherical mor-phology at different ambient moisture and even after dehydroxylation by heating at 500 * to 6000C. Unheated allophane samples gave another X-ray powder diffraction band whose maximum position varied from 12.3 to 14.5 A depending on their SIO2/A1203 ratio. The appearance ofthis band may denote some long-range ordering in the structure of allophane. Unlike the spherical particles of allophane, the tube unit of imogolite collapsed on dehydroxylation. This observation suggests hat imogolite and allo-phane are different in their framework structures and that a Si- or Si(A1)-tetrahedral sheet rather than an Al-octahedral sheet constitutes the framework structure ofallophane, irrespective of its SiO~/A12Oa r tio