This chapter aims at describing first the common and specific characters of polytypes and polysomes in minerals. Then internal microstructural features that play a role at different stages of the description and of the genesis of polytypes and polysomes are described. The classical case of crystals with a translational lattice will be extended to that of conical and cylindrical lattices as well. We will subsequently address the longstanding question if we can consider polytypes and polysomes as real phases and, further on, if so, if such phases could be seen as stable (thermodynamic phases), metastable or instable (kinetic phases). Theories attempting to account for the equilibrium and kinetic aspects have mostly been developed for polytypes, but some of them may presumably apply for polysomes as well. After recalling equilibrium models, we intend to place stress especially on some kinetic models that seem each to account for a part of experimental and observational evidences. The conclusion of this chapter will emphasize the limits of validity of such models and on keypoint observations to be conducted in the near future to document this discussion.
Polytypes and polymorphs are various crystal structures at constant or nearly constant chemical composition (or formula unit). While polymorphs admit changes in atomic structure without restriction, polytype structures differ in the arrangement of atoms, ions or molecules in one direction only. Accordingly, polytype structures may be seen as made of true layers (e.g. micas and other layer silicates with planar structures) or “conceptual” layers as for close-packed structures
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Modular Aspects of Minerals
Since the first beginning of the crystal chemical study of the inorganic compounds, a simple modular approach was developed, by looking at the crystal structures as built up through the assembling of simple polyhedral units. This approach was no only useful for a vivid and insightful description of the complex atomic arrangements of natural and synthetic compounds, but, through the use of simple and powerful rules for assembling polyhedral units, it displayed an extraordinary heuristic power, suggesting reliable models for many complex structural assemblages. The polyhedral approach also laid the basis for meaningful classifications which were applied to all the classes of inorganic compounds.