The structure contains pairs of mica-like layers linked by 0.7 (K, Ca) ions/unit cell. These pairs of layers are separated by 0, 1, 2 or 3 layers of water molecules each containing 2 molecules/unit cell. One-dimensional fourier syntheses of electron density reveal the number and position of the interlayer cations and water molecules. Comparison with vermiculite shows significant differences. At high temperatures, around 900° C., the loss of (OH) groups without disruption of the layer structure is demonstrated. Organic complexes of allevardite are compared with those of montmorillonite. A fourier synthesis of the ethylene glycol complex suggests the orientation of the organic molecules between the silicate layers.