Allophanes infrared absorption

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. 

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. 

The presence of absorption bands at particular wavelengths can be related to the presence of particular interatomic bonds. Thus infrared absorption spectrophotometry provides evidence of the structure of materials such as allophane that are amorphous to X-ray diffraction. 

The infrared absorption spectra of allophanes are characterized by a broad absorption band between 800 and 1300 cm an absorption maximum near 1660 cm", and a broad absorption band between 3000 and 3800 cm . Mitchell, Farmer and McHardy [1964] have related the infrared absorption spectra of allophane and synthetic gels of alumina and silica. The broad absorption between 3000 and 4000 cm is due to OH stretching vibrations of SiOH, AlOH, and associated adsorbed water and the position of the maximum shifts to a higher frequency as the proportion of aluminum in the gel increases. The absorption maximum... 

The study of infrared absorption patterns of allophane clays shows that they have properties consistent with their being disordered hydrous aluminosilicates. Compositional differences are reflected in the observed patterns. Patterns in some cases also indicate trends toward ordered structures or the presence of associated crystalline material. The presence of other constituents, i.e., organic matter, may also be revealed. 

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

---