The structural formulas were determined of very pure specimens of montmorillonite, beidellite, saponite, nontronite, hectoritc, wolchonskoite, vermiculite, and batavite. The lattice perfection of these minerals and the nature and variation of their crystalline swelling depend significantly on the surface density of the exchangeable cations between the silicate sheets. In a neutral solution of NH4F, OH ions were replaced by F ions. The exchange capacity is smaller the greater the diameter of the

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... silicate layers of the various montmorillonite minerals. Since the anion-exchange capacity of nontronite can be increased through oxidation of Fe 2+ it is probable that the exchangeable OH ions are located on the boundaries of the octahedrat layers. With kaolinite, the catlon-exchange capacity decreases with increasing thickness of ~he crystal plates. The OH ions of the basal surface can be exchanged with F ions, which leads to the destruction of the lattice. A one-dimensional Fourier synthesis of K-batavite, like the investigations of Walker on vermiculite and of Brown and also M6ring on montmorillonite, provides confirmation of the structure and enables the lattice positions of the K ions to be determined.