Crystal habit

In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals.

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The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.

Factors influencing a crystal's habit include: a combination of two or more forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space). Minerals belonging to the same crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while most sapphires form elongate barrel-shaped crystals, those found in Montana form stout tabular crystals. Ordinarily, the latter habit is seen only in ruby. Sapphire and ruby are both varieties of the same mineral; corundum.

Some minerals may replace other existing minerals while preserving the original's habit: this process is called pseudomorphous replacement. A classic example is tiger's eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms euhedral (well-formed), prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.

List of crystal habits