Habit

Habitus (Fr). Habitus (Ge). Hábito (Sp). Abito (It). 晶癖 (Ja).

The crystal habit is the characteristic external shape of an individual crystal or groups of crystals. For a single crystal, the habit is a description of its general shape and forms, as well as the description of their actual development in the crystal. On the basis of the development of its faces, a crystal is classified in three categories:

• euhedral, when the faces are well developed due to uncrowded growth;
• subhedral, when it shows only partially developed faces;
• anhedral, when the faces are not developed and cannot be indexed without ambiguity.

When the habit is controlled only by the internal crystal structure, the crystal is called idiomorph. The opposite term is xenomorph (or allotriomorph), which indicates that the typical habit has not developed because of late crystallization between earlier formed crystals. Typical examples of xenomorph crystals are matrix minerals in rapidly crystallizing volcanic lavas and shallow igneous intrusions, or interstitial or cementing minerals formed during the diagenesis of sedimentary rocks.