Allophane dehydration

Hashimoto, 1. and 3ackson, M.L., 1960. Rapid dissolution of allophane and Kaolinite-Halloysite after dehydration. Clay and Clay Minerals, 7th Conf., pp. 102-113. 

The dehydration of allophanes can begin at about 100°C and continue up to about 900°C. More structural organization can also result in a rapid loss of hydroxyls in the range of 400-600°C. 

Allophane and halloysite were the subject of detailed consideration by Ross and Kerr [1934]. They employed optical. X-ray, thermal dehydration, and chemical analysis to investigate a number of selected samples of allophane from different localities. They found wide ranges of chemical compositions (Si02, 25.19 to 33.96% AI2O3, 30.41 to 36.53% H20 , 12.84 to 21.20% H20, 14.43 to 20.28%) and of indices of refraction (1.472 to 1.496). From these results and from those of other publications they concluded that allophane is an amorphous material commonly associated with halloysite. It has no crystal structure and no definite chemical composition. The name allophane should be restricted to mutual solid solutions of silica, alumina, water, and minor amounts of bases, but the specimen should include all such materials, even though the proportions of these constituents may differ. This broad definition by Ross and Kerr of the range of properties of allophane is generally accepted. 

Dehydration curves of some allophanes in New Zealand soils are shown in Figure 10. Three of these curves are obtained from clays consisting solely of allophane and illustrate the features already described. The fourth curve, obtained from a strongly weathered soil from basalt, contains kaolin and gibbsite. At the decomposition temperatures of the latter two minerals, a rapid loss in weight takes place. 

IiMURA [1961a], however, attributed anion exchange to the dissociation of hydroxide from aluminum hydroxide. In a further paper (Iimura [1961b]) he showed that drying allophane at 105°C results in an increase in the capacity to adsorb ammonium, but also in a remarkable decrease in anion adsorption, presumably due to the dehydration of the hydroxyl group attached to the aluminum of allophane. 

Egawa, T., 1961. Infrared absorption spectra of allophane and their changes after dehydration by heating. Adv, Clay Sci, 3 103-110. 

Hashimoto, L, and M. L. Jackson, 1958. Rapid dissolution of allophane and kaolinite after dehydration. Proc. 7th U.S. Nat. Conf. Clays Clay Afm. 102-113. 

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