Crystals are solid materials having regular arrangements of atoms, ions, or molecules. Crystal forms are determined not only by structure but also by the factors involved in growth. The same crystal species may therefore appear in various forms. In this chapter, the external forms of real crystals are systematically classified.

Morphology of crystals-the problems

The morphology of crystals is the central theme of this book. Our intention is to present systematically the fundamental concepts that allow us to analyze the factors that determine the various forms of crystals. We may then deduce and analyze the phenomena and processes that we cannot observe in situ, such as those occurring in the depths of the Earth or in the animate world.

When there is no distortion in the structure or no change in orientation throughout a crystal, we refer to the structure as a single crystal. A solid consisting of many single crystals with different orientations is called a polycrystalline aggregate. There are also polycrystalline aggregates of multiple phases, as in metals, rocks, and ceramics.

Since real crystals may contain various defects of lattice order, we may regard these crystals as nearly perfect single crystals, and we accept the defects as part of the original system. Assuming this to be the case, individual crystals may be classified as single crystals, no matter how big or small they are, and a combination of individual crystals is termed a polycrystalline aggregate.