High Resolution Electron Micrographs of 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 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.

Section: Allophanesupporting

confidence: 83%

“…Imogolite is a unique paracrystalline clay mineral that consists of bundles of a long tube unit with an external diameter of about 20/~ (Wada et al, 1970). It occurs with allophane in many soils derived from volcanic ash and weathered pumices.…”

Section: Introductionmentioning

confidence: 99%

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

Gaast

1985

“…Many small particles, probably consisting of hollow spherules in three dimensions, were seen as ring shapes with diameters of 35 to 50 A in electron micrographs (see also, Kitagawa, 1971;Wada et al, 1972;Henmi and Wada, 1976;Wada and Wada, 1977;Shoji and Saigusa, 1977). Only small amounts of imogolite fibers (Yoshinaga et al, 1968;Wada et al, 1970) were recognized in the clays; imogolite is commonly admixed in allophanes from weathered volcanic ash and pumice (Henmi and Wada, 1976). The clays were treated with sodium dithionite-citrate-bicarbonate (Mehra and Jackson, 1960) and 2% Nar (Jackson, 1956a) and washed with I N NaCI soIution to exchange the clay with Na.…”

Section: Methodsmentioning

confidence: 98%

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

Henmi

1980

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.

“…et al, 1972) and mixed layered kaolinite-montmorillonite (Lee et al, 1973) were observed under HREM after thin sectioning with UM. The crystal structures of montmorillonite (Barclay and Thomson, 1969), organo-montmorillonite (Suito et al, 1972), imogolite (Wada et al, 1970), garnierites (Uyeda et al, 1973) and interstratified mica-smectite (Yoshida, 1973) also were observed in portions of curved crystals with HREM.…”

Section: Introductionmentioning

confidence: 99%

Micaceous Occlusions in Kaolinite Observed by Ultramicrotomy and High Resolution Electron Microscopy

Lee

1975

Abstract--The layer structure of kaolinite from Twiggs, Georgia and fire-clay type kaolinite (Frantex B, from France), particle size separates 2-0-2 #m was studied by high resolution electron microscopy after embedment in Spurr low-viscosity Epoxy media and thin sectioning normal to the (001) planes by an ultramicrotome. Images of the (001) planes (viewed edge-on) of both kaolinites were spaced at 7 .~ and generally aligned in parallel, with occasional bending into more widely spaced images of about l0 • interval. Some of the 10 A images converged to 7 /~ at one or both ends, forming ellipse-shaped islands 80 to 130 ~ thick and 300 to 500 ~, long. The island areas and interleaved 10 ~ layers between 7 A layers may represent a residue of incomplete weathering of mica to kaolinite.The proportions of micaceous occlusions were too small and the layer sequences too irregular to be detected by X-ray diffraction. The lateral continuity of the layers through the 7-10-7/~ sequence in a kaolinite particle would partially interrupt or prevent expansion in dimethyl sulfoxide (DMSO) and other kaolinite intercalating media. Discrete mica particles were also observed with parallel images at 10 A, as impurities in both kaolinites. The small K content of the chemical analyses of the kaolinite samples is accounted for as interlayer K, not only in discrete mica particles but also in the micaceous occlusions.