Polyhedral crystals bounded by flat faces can exhibit various Tracht and Habitus because they result from the interconnection of internal structural factors and the external factors involved during crystal growth. The concepts of the structural form and the equilibrium form were first suggested in an attempt to provide a greater understanding of the origin of the morphology of polyhedral crystals. Furthermore, extensive experiments on and observations of natural minerals were carried out in an attempt to clarify the origin of morphological variation of growth forms. The results obtained from the many investigations performed in the twentieth century are summarized in this chapter.

Forms of polyhedral crystals

The various morphologies, such as polyhedral, hopper, dendritic, and spherulitic, that are exhibited by single crystals and polycrystalline aggregates, have been discussed in relation to the driving force in the preceding chapters.

Among all of the various morphologies exhibited by crystals, it is the problem of variations in Tracht and Habitus exhibited by polyhedral single crystals that has attracted the deepest concern. Polyhedral single crystals are expected when crystals grow by the spiral growth mechanism under a driving force of Δµ/κT. There are four logical routes that we can take to understand this problem.

The first is the prediction of the Habitus made from the characteristics of the crystal structure, entirely neglecting the effect of growth conditions. We will call this the “structural form” or “abstract form.”