The phenomenon of polytypism was first recognized by Baumhauer (1912, 1915) as a result of his investigations of many SiC single crystals by optical goniometry. Among these, he discovered three types and his observations were formulated in five statements:
– all three types originate simultaneously in the same melt and seemingly also under the same, or nearly the same conditions,
– they can be related in a simple way to the same axial ratio (within an individual primary series each),
– any two types (I and II, II and III) have certain faces in common but, except the basal face, there is no face occurring simultaneously in all three types,
– the crystals belonging to different, but also to all three, types often form inter-growths with parallel axes,
– any of the three types exhibits a typical X-ray diffraction pattern and thus also an individual molecular or atomic structure.
Baumhauer recognized the special role of these types among modifications of the same substance and called this phenomenon polytypism – a special case of polymorphism. The later determination of the crystal structures of Baumhauer’s three types indicated that his results can be interpreted by a series of structures consisting of identical layers with hexagonal symmetry and differing only in their stacking mode.
The stipulation that the individual polytypes grow from the same system and under (nearly) the same conditions, influenced for years the investigation of polytypes because it
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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.