Growth spirals and complex polytypism in micas. I. Polytypic structure generation

The possible polytypic structures of a mica monocrystal are generated, using a single screw dislocation emerging on both (001) faces. The discussion takes into account: (1) each basic structure the crystallite may have during its previous layer-by-layer growth, (2) the number and position of the elementary layers of the crystal when the spiral becomes active, (3) the Burgers vector strength of the dislocation. Deduced new polytypic structures, belonging to different structural series, are described and their positions with respect to the different basic structures are discussed. Some well-known growth features in micas such as 'epitaxic overgrowths' and 'macroscopic' twinning may, in some cases, be connected with a spiral growth mechanism. The layer-stacking sequences of complex polytypic structures of natural and synthetic micas reported thus far in the literature can be compared with the predictions of the screw dislocation theory of polytypism on perfect or more or less faulted basic structures.