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

Abstract

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. 345

Polytypism of Micas. I. MDO Polytypes and Their Derivation

“…The polytypes A, however, are much more common. The review of non-refined complex mica polytypes by Baronnet (1975) also supports this rule. Apparently only two structures are known (Takeda, 1967;Christie, 1978) that violate this rule, but the level of their refinement does not warrant definite conclusions.…”

Experimental study of disordered mica structures by high-resolution electron microscopy

“…The structures of the mica minerals are complex and varied, although based on a relatively simple stacking arrangement. There is a sixfold multiplicity for the stacking of adjacent layers, resulting in a large number of possible stacking sequences and thus polytypes (Smith & Yoder, 1956;Ross, Takeda & Wones, 1966;Takeda, 1967;Baronnet, 1975). X-ray diffraction techniques have been a powerful means of determining average stacking sequences and have solved the problems of mica polytypes, but they are not suitable for studying heavily disordered stacking sequences and their variations within a given crystal.…”

Polytypism of Micas. II. Classification and Abundance of MDO Polytypes

“…Conclusion (1) supports convincingly the rationality of the MDO concept; however, this statement is not a denial of the existence of non-MDO mica polytypes described in the literature (e.g., Ross et al, 1966;Takeda, 1967;Rieder, 1970;Baronnet, 1975;Christie, 1978). These papers unfortunately do not contain the decisive details necessary to judge the reliability of the described structures.…”