Careful (00l) fourier syntheses have been made on muscovite, pyrophyllite, and montmorillonite (dehydrated), in an effort to discover the cause of the different (00l) spacings of these minerals. The oxygens of the outer (tetrahedral) layer superpose exactly in these syntheses, those of the inner (octahedral) layer almost exactly. Thus the differences are entirely in the inter-layer spaces, and are probably due to electrostatic repulsion of the negatively charged multiple layers. As the negative charge increases (and with it the repulsion, and hence the inter-layer spacing), in the series pyrophyllite-montmorillonite-mica, the net energy of binding of the sheets increases also. The two variations are not in contradiction.

It follows from this work that the shifts in cell size due to ionic substitutions are entirely taken up in the plane of the layer, its thickness remaining the same.