Small-Angle X-Ray Powder Diffraction, Morphology, and Structure of Allophane and Imogolite

Abstract

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 morphology 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 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 ofallophane, irrespective of its SiO~/A12Oa ratio.

Structure and Thermal Transformations of Imogolite Studied by 29Si and 27Al High-Resolution Solid-State Nuclear Magnetic Resonance

“…The infrared spectrum supplied by K. Wada is similar to published spectra (Russell et al, 1969), and the XRD pattern is identical to that reported for imogolite from the same location (Brown, 1980), with the same low-angle features as found by Van der Gaast et al (1985).…”

“…Such a mechanism, however, de- pends on cleavage of the tubes at an early stage, although this is incompatible with the observation of Russell et al (1969) that the tubular structure survives dehydroxylation at 700~ More recent work (Van der Gaast et aL, 1985, Wilson et aL, 1988 suggests that breakdown of the tubes may have begun at about the onset of dehydroxylation, even though the threadlike habit was preserved in the early stages. CONCLUSIONS 1.…”

“…The collection and purification of this material, designated Ki-G, has been described elsewhere (Van der Gaast et al, 1985). Chemical analysis by atomic absorption spectrometry gave the following composition: SiO2, 23.61%; A1203, 36.00%; TiO2, 0.06%; Fe203, 0.20%; MgO, 0.06%; CaO, 0.18%; MnO, 0.01%; Na20, 1.49%; K20, 0.10%, H20 (total), 37.6%.…”

“…Although Russell et al (1969) reported that the tubular morphology is unchanged by heating to 700~ Van der Gaast et al (1985) observed that dehydroxylation at 500~ was accompanied by a breakdown of the tubes.…”

27Al Mas NMR and Aluminum X-Ray Absorption Near Edge Structure Study of Imogolite and Allophanes

“…Si-rich allophanes with Al/Si ratios close to unity are represented by the Ok, VA, and Kk samples. Ok and VA originated from weathered andesitic ashes (Yoshinaga and Aomine, 1962;van der Gaast et al, 1985), and Kk from weathered andesitic lapilli (Parfitt et al, 1980). Natural imogolite KiG has been sampled with the KiP allophane (van der Gaast et at., 1985).…”