Chemical composition allophanes

Solubility calculations were added for two allophanes, for which the equilibrium constants and formulae are a function of pH. Paces (74) found cold ground waters collected from springs in granitic rocks of the Bohemian Massif of Czechoslovakia to be supersaturated with respect to kaolinite while being unsaturated with respect to amorphous silica. He interpreted this as an indication that a metastable aluminosilicate more soluble than kaolinite was controlling the concentrations of alumina and silica in these waters. This aluminosilicate was further hypothesized to be of varied chemical composition, controlled by the mole... 

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. 

It will be seen that, for geological specimens, allophane is a term applying to a group of homogeneous amorphous hydrous aluminosilicate materials, individual members of which may be further characterized by their detailed chemical composition or by other special properties. Ross and Kerr suggested the useful term mineraloids for such homogeneous amorphous materials to distinguish them from crystalline minerals with definite limits of composition. 

Because allophanes have a range of chemical compositions, their physical properties also range widely, but within certain characteristic limits. 

Wada [1967] also proposed a structural scheme for allophane in somewhat more definite terms, based largely on chemical composition and the relationship between allophane, imogolite, and halloysite observed in Japanese soils. 

Yoshinaga, N., 1966. Chemical composition and some thermal data of eighteen allophanes from Ando soils and weathered pumices. Soil Sci. Plant Nutr. (Tokyo) 12 47. 

Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

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... 

Since that time it has been established that amorphous hydrous aluminosilicates related to allophane are major constituents of certain soils. They are highly reactive and their presence in even small amounts may strongly influence chemical, physical, and colloidal properties of many soils. Although they are indefinite in composition, the general properties of amorphous hydrous aluminosilicates are similar. They differ greatly from those of other soil constituents. It is therefore possible to treat the amorphous hydrous aluminosilicates as allophanes and to consider them as a group as allophane. ... 

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