Abstract
A structure model of allophane was proposed from results obtained by X-ray radial distribution, X-ray fluorescence and nuclear magnetic resonance analyses. Al atoms in allophane were in four-fold coordination as well as in six-fold coordination. The linkages of Sitetrahedra, Al-tetrahedra and Al-octahedra in allophane were considered to be two-dimensional by radial distribution analysis. This two-dimensional arrangement was further confirmed by the fact that the differential radial distribution curve of allophane fitted very closely to the theoretical differential radial distribution curve of kaolin. The authors gave a structual scheme for allophane, which consists of a kaolin-like sheet structure with Si atoms in the tetrahedral layer being partially substituted by Al atoms. This kaolin-like structure has cation vacant sites in the tetrahedral layer in Al2O3-rich allophane and it has those in the octahedral layer in SiO2-rich allophane. The OH contents calculated from the structure models are in good agreement with those estimated by nuclear magnetic resonance analysis.
