Allophanes electron

Allophane occurs in both a spherical and a fibrous form. The limited chemical data are not sufficient to show that morphology is related to composition. Electron micrographs of microtome sections of an Australian allophane (John Brown, personal communication) show that many of the spheres are made up of stacks of thin concentric sheets. Kitagawa (1971) showed high-magnification electron micrographs to... 

Allophane. Allophane is a hydrous aluminosilicate with a variable composition (Wada and Harward, 1974 Wada, 1977, 1985). Electron micrographs show that aUophanes are made of "hollow spherules" with a diameter of 3.5-5 nm (Wada,... 

Due to the electron withdrawing effect of the carbonyl groups, the urethane group has a much lower reactivity than the aminic -N-H groups and in order to promote the allophanate formation higher temperatures are necessary greater than 110 °C. It is important to mention that the allophanate formation is a reversible reaction. 

The heretofore uncharacterized allophanate ion adopts a planar configuration that facilitates the formation of an intermolecular N-H- O hydrogen bond (Figure 8.26). The relative instability of the patent aUophanic acid is consistent with this hydrogen-bonded ring stracture, as protonation at the exocyclic carboxyl oxygen would result in an overall reduction of -electron delocalization. 

Transmission electron microscopy shows that allophane particles are nearly spherical with diameters in the range of 3.5-5 nm (by the way of comparison of the platelet-like... 

The power law part appearing as linear in a log-log scale has a slope related to -D, D the fractal dimension, which expresses the cluster compactness. The position of the two intersects (black and white arrows) allows to calculate the cluster size and the particle size a. For q higher than 2 nm the curve shows a hump which can be attributed to the particle size (3-4 nm, white arrow). This description is in agreement with the transmission electronic microscopy results [4, 17, 23, 24]. Table 12.2 lists the evolution of D and la (the fractal extent) versus allophane content drawn from the experimental curves. The fractal extent can be considered as the size of a tortuous nano labyrinth. Our results show that the size of the fractal labyrinth increases with the allophane content which suggests that the accessibility inside the soil microstructure will decrease. 

If much well-ordered kaolinite is present, the assymmetric peaks are not prominent in the patterns from random samples, and the basal reflections are sharper and much enhanced in intensities in patterns from oriented samples. If much disordered kaolinite is present, the assymmetric peaks are prominent in the first patterns, and the basal reflections are much enhanced in the second. Chemical pretreatments prior to X-ray diffraction, such as those proposed by Wada [1965] and Alexiades and Jackson [1965], are sometimes useful in determining amounts of kaolinite and halloysite. Where the halloysite is tubular, it is easily detected in electron micrographs, although the amount can seldom be determined. Amounts of hydrated halloysite can be determined if allophane is not present in differential thermal analysis by calibrating and measuring the low-temperature endothermic peak. 

The availability of such instrumental techniques as X-ray powder diffraction, differential thermal analysis, electron micrography, and infrared absorption spectrophotometry, together with refinements in measurements of surface properties, particularly since 1950, has improved methods of defining allophane. It is, therefore, necessary to consider the nature of the evidence of these and other techniques with respect to forms of allophane occurring in soils, in order to appreciate the range of soil properties conferred by this material. 

Figure 4. Electron micrographs of soil clays showing the sequence Allophane B— Allophane AB— Allophane A—Metahalloysite. (a) Clay from Kaharoa soil showing allophane B (b) clay from Gisborne soil showing allophane AB (c) clay from Egmont soil showing allophane A (d) clay from Hamilton soil showing metahalloysite tubes.

The following section reviews evidence concerning allophane provided by the techniques of X-ray powder diffraction, electron diffraction differential thermal analysis, infrared absorption, and electron micrographic methods of analysis. 

In electron micrographs of clay fractions of some soils many discrete particles of allophane are seen to be of the order of 50 A in diameter. Such particles might contain approximately 10,000 oxygen atoms, and even if these particles had ordered crystal structure, they would be too... 

X-ray diffraction patterns of ordered structure in allophanic soil clays are evidence of the presence of crystalline impurities. It is, however, possible that finely divided particles showing some evidence of crystallinity by X-ray or electron diffraction patterns may have allophanic surface properties, and such materials (e.g., hydrous feldspars) may be conveniently regarded as special cases of allophane. 

Figure 11. Electron micrographs of allophane separated from the basal beds of the Hamilton ash, Te Akatea, New Zealand, (a) Clay separated from moist soil at pH 3.5 with HCl (b) clay separated from moist soil at pH 9 with NaOH (c) clay separated from air dry, deferrated soil at pH 3.5 with HCl (d) clay separated from air dry, deferrated soil at pH 9 with NaOH.

Material of similar appearance in the electron microscope may be prepared by the precipitation of gels of alumina and silica, van Reeuwuk [1967] has shown that spherical bodies similar to those seen in some natural allophane sometimes appear in synthetic gels, while Watanabe [1963] compares micrographs of allophane with those of siUca and alumina gels. Some synthetic alumina gels even have a fibrous appearance. 

Figure 12. Electron micrographs of some allophanic clays separated from New Zealand soils, (a) Clay dispersed in sodium carbonate at pH 9 from Kaharoa soil (b) clay dispersed in sodium carbonate at pH 9 from Egmont Ash soil (c) fibrous clay dispersed with acid from Egmont Ash soil (d) coarse glassy particles in clay dispersed with alkali from Egmont Ash soil.

The conslusion that can be drawn from all the evidence of electron microscopy is the same as that given by the evidence of all other physical techniques that allophane is an amorphous aluminosilicate gel of fine particle size. 

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