According to Rodriguez Gallego (1985), structural imperfections in phyllosilicates can be classified into the following groups: 1) point defects; 2) line defects or dislocations; 3) plane defects (polytypism etc.); 4) volume defects (intergrowth, exsolutions, etc.).
Polytypism of phyllosilicates is a phenomenon where fundamental 1:1 (kaolinite minerals, serpentines), 2:1 (micas, vermiculites, smectites, pyrophyllite, talc) or 2:1:1 (chlorites) structural layers of identical structure, symmetry, and composition reveal different stacking along the c axis, namely by an axial or rotational shifting.
The discovery of disorder and polytypism in the chemically simple kaolinite subgroup is a good example of specialized crystallographic research starting in 1938 and continuing for decades.
Modern geologists and sedimentary petrologists wishing to study some parts of argillosphere in more detail should have in their research team a specialized crystallographer for the investigation of order-disorder states and polytypism of phyllosilicates.
Already in 1959-1963, Radoslovich with his coworkers drew the crystallographers' attention (quoted in detail above, and by Konta, 2005) to the fact that the energetic state expressed in "local balance of forces" in layer structures may provide a means of structural control over polytypism. The first cause of order or disorder in the structure of phyllosilicates and the resulting polymorphs and polytypes as well as of the interstratification phenomenon lies in the chemical micromilieu acting during the crystallization.
Mixed layered crystals built of the same structural layers but differing in polytypism are called homotypic interstratifications.
The first cause of order/disorder states in the structure of phyllosilicates and resulting polymorphism, polytypism or interstratifications lies in the chemical micromilieu acting during the crystallization.
However, the authors were unable to distinguish among several hypotheses to explain the observed polytypism
. The hypotheses (see also Seutin 1990) ranged from the traditional idea that two species are involved, to an allelic polymorphism at a major locus with pleiotropic or epistatic effects on many structural genes, and to various ecophenotypic explanations (e.g., niche polymorphism, threshold response mechanisms, phenotypic modulation, and developmental conversion; sensu Smith-Gill 1983).