Celadonite and glauconite samples heated at different temperatures were studied by X-ray and electron diffraction. For dioctahedral micas the in-plane component of the translation between layers (ccosβ/a), which is strongly dependent on the position of the vacant octahedral site, significantly decreases at temperatures greater than the temperature of maximum dehydroxylation. The simulation of XRD patterns from different structural models reveals the actual crystal structure of dehydroxylated samples as well as the dynamics of the structural transformations. In the non-heated state the samples consist of tv (trans-vacant) 2:1 layers. During dehydroxylation, cations migrate from cis- into trans-octahedra and have 5-fold coordination. In the averaged unit-cell the ‘residual’ anions formed after the dehydroxylation reaction occupy the former OH sites with probability equal to 0.5. The migration of octahedral cations is accompanied by the transformation of the C-centred layer unit-cells into primitive ones. In contrast to Fe, Al and Mg cations have a greater ability to migrate.