Validity of Proposed Model for the Chemical Structure of Allophane with Nano-ball Morphology(Soil and Non-Crystalline Clays,Proceedings of the 13^<th> International Clay Conference)

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Nano-ball allophane is hydrous aluminum silicate of hollow spherical morphology with diameter of 3.5 to 5.0nm, but its chemical structure has not been clarified completely. An electron microscopic observation showed that allophane aggregate was composed of unit particles with uniform size. It must be considered that allophane has definite structural arrangement and therefore can not be considered as 'amorphous'. We found that regularity of polyhedron could be used to build up basic structure of nano-ball allophane. This polyhedron is composed of eight hexagonal gibbsite sheets with orthosilicic acid bonded to it. The bond lengths of Al-O and Si-O are 0.192 and 0.162nm. The molecular weight is 46510 and Si/Al atomic ratio is 0.574. The model has a diameter of 4.25nm and contains six holes. By using calculated specific gravity of 2.75g cm^<-3>, diameter of 4.25nm and wall thickness of 0.7nm, the amount of water retention in the space within the wall and specific surface area were calculated as 0.16% and 1114m^2 g^<-1>. These calculation results are in good agreement with experimental data, and confirm that our proposed model is the chemical structure of nano-ball allophane in the natural environment.
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