Small-angle x-ray powder diffraction, morphology, and structure of allophane and imogolite

TLDR: In this paper, the authors showed that imogolite and allophane 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 of allophanes, irrespective of its SiO~/A12Oa ratio.

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 of this 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 that 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 ratio. 500 ~;WU 600~ ~,~/I~ OH /~G~'~X*o, ~�9 "~" ~r162 ~-~L~?Co }J~,DCg)o~o?~Y~" Y-~-2~d~O)~J<zfY .~ 12.3 ;b~ 14.5h~L�9