Crystal habit

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

The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognising 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 which are influenced by certain factors. Crystal habit may mislead the inexperienced as a mineral's 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.

The crystals of some minerals form long needles, tapered or pointed at the ends. We say they have an acicular habit. Those which are elongated, but not tapered, are said to be prismatic. Other crystals are stubbier and resemble a barrel, they are said to have a barrel-shaped, stout or equant habit. If crystal are long and flat, they are bladed. Very long and thin-hair like crystals are called filiform.

Aggregates of crystals may adopt forms that are specific of certain minerals. Hematite for example, is found often in colloform or reniform groupings. Other terms used are botryoidal, mammillary, and tuberose. Some minerals are said to be foliated or micaceous: these minerals also tend to part along cleavages to form thin sheets. The ones that break into fibres are said to have fibrous habit, those that come apart as granules are said to be granular.

A dendritic or arborescent habit describes minerals whose crytals branch like trees from a central trunk, usually in two or more directions. Feathery crytals are said to be plumose. Needlelike converging in many directions is described as stellate or star-shaped. Nodular deposits have a roughly spherical form, such as geodes. Some minerals (which are usually polycrystalline) do not have a distinct shape: these are termed massive.