Abstract
Polytypes are stacking variations that occur in crystals, forming periodic extended defects, without changing the overall composition. Mineral examples are the different polytypes of micas, kaolinite, talc, graphite. None of the theories that have been proposed to explain the occurrence of short and long period superstructures have met with universal success in explaining their occurrences. Polysomatism is defined as the stacking of compositionally different modules, similar to polytypism. Minerals belonging to a polysomatic series have compositions that are linear combinations of the end members. Structures derived from the bixbyite (Mn2O3) structure are used as examples of a polysomatic series and a number of polysomes in single crystals have been described using HRTEM. The relative stabilities of different polysomes have been modelled with a lattice energy minimization programme. The results show that certain polysomes are significantly more stable than others, and that the contribution of next-nearest neighbour interactions seems to be an important factor in determining polysome stability.
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