Effect of SiO₂/Al₂O₃ Ratio on the Thermal Reactions of Allophane. II. Infrared and X-Ray Powder Diffraction Data

Abstract

Abstract--Allophanes with SiOJAI203 molar ratios from 1.38 to 1.92 were heated at temperatures up to 500~ and the changes induced were investigated by means of infrared spectroscopy (IR) and X-ray powder diffraction (XRD). Heat treatment caused the IR absorption band near 1000 cm -1 due to Si-O stretching to shift towards higher frequencies, and the band near 450 cm -1 due to O-Si-O bending to increase markedly in intensity. These results are probably due to condensation of SiO tetrahedra, following breakdown of Si-O-AI linkages and dehydroxylation of SiOH groups. Concurrent intensity increase of the IR absorption at about 700 cm 1 suggested an increase in the amount of 4-coordinated AI in the heated materials. 'Imogolite structures' were decomposed progressively at higher temperatures and were almost absent at about 400~ as indicated by the weakening and disappearance of the IR band at 348 cm -1 and of the XRD reflections at 2.25 and 1.40 A. The XRD reflection at 3.3-3.45 A shifted to about 3.6 A_, probably as a result of thermal condensation of the silica component. The observed thermal changes were much greater for samples with low SiOJAI203 ratios (1.38-1.51) than for those with high ratios (1.81-1.92), indicating a lower thermal stability for the former materials. Thus, the thermal stability of allophanes appears to be related to the content of 'imogolite structures' and to the polymerization status of the silica component.

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

“…The position of the first band did not change, but its intensity was increased by heating and attained its m a x i m u m at 400 ~ to 500~ suggesting that the dehydroxylated allophane still consisted of spherical particles similar to those found in unheated allophane. This finding is surprising because Henmi and Wada (1976) found that the "hollow" allophane spherical particles broke down very easily in the electron beam, and other workers found that heating caused substantial changes on the infrared spectra of allophane (Kitagawa, 1974;Egashira and Aomine, 1974;Henmi et al, 1981). As illustrated in Figure 4 for the Ki-P sample, however, electron microscopy confirmed that allophane heated at 500~ still consisted of the spherical particles with diameters of 35 to 40 A.…”

Cation and Anion Retention by Natural and Synthetic Allophane and Imogolite

“…The Egmont sample (Eg) contains "proto-imogolite" allophane and Copyright 9 1984, The Clay Minerals Societyis comparable with sample 923. The properties of the Te Akatea (Te Ak) and Kakino (KnP) allophanes were reported by Perrott (1977) and Henmi et al (1981), respectively. The clays (< 2 #m) were isolated by centrifugation after overnight mechanical dispersion in distilled water, and used without further chemical pretreatment.…”

Kinetics of Dissolution of Noncrystalline Oxides and Crystalline Clay Minerals in a Basic Tiron Solution

“…4.6 <2-~tm fraction, from Kuzuu mine, Tochigi, Japail. Henmi et al (1981) Henmi et al (1983 Hayashi (1963) Oinuma and Hayashi (1968) Sakamoto and Sudo (1958) Imai et al (1969) studied in a basic tiron solution. To explain these dissolution behaviors, a mass-balance equation of the specified cation in liquid phases has been proposed.…”