Effect of SiO₂/Al₂O₃ Ratio on the Thermal Reactions of Allophane

Abstract

Abstract--Differences were found in the differential thermal analysis curves and in the temperatures of new-phase development between allophanes of high (1.91-1.99) and low (1.47-1.53) SiO2/A1~O3 ratios. The endothermic peak due to continuous dehydration and dehydroxylation was at higher temperatures (153 ~ 185~ for allophanes with high SiOJA1203 ratios and at lower temperatures (148~176 for those with low SiO2/A1203 ratios. The temperature of the exothermic peak was lower and the height affected more by the exchangeable cation content for allophanes with high ratios than for those with low ratios. New phases did not develop in allophanes having high SiOz/AI203 ratios even after they were heated to 1000~ above the temperature of the exotbermic peak. In contrast, a symptomatic development of new phases was noted in allophanes with low SiO~/AlzQ ratios at 900~ below the temperature of the exothermic peak. The effect of SiO~/A120.~ ratio in the thermal behavior of allophane strongly suggests that differences in the structure are closely associated with the chemical composition of this material.

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

“…Less than 0.5% imogolite tubes could be seen by TEM. The composition and the SiOz/Al~O3 ratios (Table 1) of four samples (KIP, KyP, VA, and PA) were somewhat different from those reported previously for clay fractions from the same sources (Aomine and Wada, 1962;Yoshinaga, 1%6;Henmi and Wada, 1976;Henmi, 1980). This is presumably due to differences in sample preparation, particle size, or analytical procedure.…”

“…In Part I of this study, Henmi (1980) found significant differences in the differential thermal analysis (DTA) curves and in the temperature of new-phase development between two groups of allophanes with SiOJ A12Oz molar ratios of 1.91-1.99 and 1.47-1.53. The purpose of this paper is to examine differences in thermal stability among allophanes with different SiOJA1203 ratios by means of X-ray powder diffraction and infrared techniques.…”

“…Allophane clays (<0.4/~m) separated from the same ash and pumice samples used in Part I of the study (Henmi, 1980) were used along with a clay sample obtained from pumice from Kakino, Kumamoto Prefecture. The Kakino pumice was collected at an escarpment where the pumice bed, about 3 m thick, is overlain by a 4-m thick series of weathered ash layers.…”

Studies on allophane and imogolite by high-resolution solid-state 29Si- and 27Al-NMR and ESR.

“…The imogolite sample was separated from gel films which filled the in terstices of pumice beds. The detailed sample preparation methods and some mineralogical features of the samples used in this study have been published by Henmi and his co-workers (Henmi, 1977(Henmi, , 1980Henmi et al, 1981Henmi et al, , 1982Henmi et al, , 1983. Transmission electron microscopic observation showed that each of those samples is composed of "pure" allophane, imogolite or halloysite.…”

“…The samples used in this study have been characterized very carefully by some mineralogical methods such as electron microscopy, infrared absorption, X-ray diffrac tion, X-ray fluorescence and thermal analysis (Henmi, 1977;1980;1985;1986). Structural differences of allophane depending on the Si02/A12O3 ratio will be reported here.…”