Chemical analyses allophane

The reporting of mica in soil clays depends somewhat on the method of detection. Jackson and Mackenzie [1964] state that some soil clays, which show no indication of mica based on X-ray diffraction, may contain from 5 to 20 % or more of micas based on chemical analysis and on the basis of 10% K2O in mica. According to Schuffelen and van der Marel [1955], soils high in allophane fix very considerable quantitities of potassium. Thus, potassium does not necessarily reside altogether in micas and feldspars in soils. Some of it may be in amorphous material. However, some of the potassium may be in micalike zones of particles, which are largely montmorillonite or vermiculite and have weathered from micas. Such zones may be too small to be detected by X-ray diffraction (Knibbe and Thomas [1972]). 

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. 

Sanchez-Calvo, M. C., 1961. Allophane and other colloids in the brown loam clays and their changes in the Western Canary Islands. I. Chemical analysis, DTA (differential thermal analysis), and X-ray diffraction. Anales. Edafol. Agrobiol. (Madrid) 20 189. 

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. 

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