Crystal habit
In mineralogy, crystal habit is the characteristic external shape of an individual crystal or aggregate of crystals. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to limited space in the crystallizing medium.
Crystal forms
Recognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of the internal ordered atomic arrangement. Most natural crystals, however, do not display ideal habits and are commonly malformed. The quality of the shape of a crystal in a mineral specimen can be described as one of the following:- Euhedral: a crystal that is completely bounded by its characteristic faces, well-formed. Synonymous terms: idiomorphic, automorphic;
- Subhedral: a crystal partially bounded by its characteristic faces and partially by irregular surfaces. Synonymous terms: hypidiomorphic, hypautomorphic;
- Anhedral: a crystal that lacks any of its characteristic faces, completely malformed. Synonymous terms: allotriomorphic, xenomorphic.
Altering factors
Some minerals may replace other existing minerals while preserving the original's habit, i.e. pseudomorphous replacement. A classic example is tiger's eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.