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.

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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.

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List of crystal habits

Habit: Description: Example:
Acicular - Needle-like, slender and/or tapered - Rutile in quartz
Amygdaloidal - Almond-shaped - Heulandite
Anhedral - Poorly formed, distorted - Olivine
Bladed - Blade-like, slender and flattened - Kyanite
Botryoidal or globular - Grape-like, hemispherical masses - Smithsonite
Columnar - Similar to fibrous: Long, slender prisms often with parallel growth - Calcite
Coxcomb - Aggregated flaky or tabular crystals closely spaced. - Barite
Dendritic or arborescent - Tree-like, branching in one or more direction from central point - Magnesite in opal
Dodecahedral - Dodecahedron, 12-sided - Garnet
Drusy or encrustation - Aggregate of minute crystals coating a surface - Uvarovite
Enantiomorphic - Mirror-image habit and optical characteristics; right- and left-handed crystals - Quartz
Equant, stout, stubby or blocky - Squashed, pinnacoids dominant over prisms - Zircon
Euhedral - Well-formed, undistorted - Spinel
Fibrous or columnar - Extremely slender prisms - Tremolite
Filiform or capillary - Hair-like or thread-like, extremely fine - Natrolite

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Foliated or micaceous - Layered structure, parting into thin sheets - Mica
Granular - Aggregates of anhedral crystals in matrix - Scheelite
Hemimorphic - Doubly terminated crystal with two differently shaped ends. - Hemimorphite
Mamillary - Breast-like: intersecting large rounded contours - Malachite
Massive or compact - Shapeless, no distinctive external crystal shape - Serpentine
Nodular or tuberose - Deposit of roughly spherical form with irregular protuberances - Geodes
Octahedral - Octahedron, eight-sided (two pyramids base to base) - Magnetite
Plumose - Fine, feather-like scales - Mottramite
Prismatic - Elongate, prism-like: all crystal faces parallel to c-axis - Tourmaline
Pseudo-hexagonal - Ostensibly hexagonal due to cyclic twinning - Aragonite
Pseudomorphous - Occurring in the shape of another mineral through pseudomorphous replacement - Tiger's eye
Radiating or divergent - Radiating outward from a central point - Pyrite suns
Reniform or colloform - Similar to mamillary: intersecting kidney-shaped masses - Hematite
Reticulated - Acicular crystals forming net-like intergrowths - Cerussite
Rosette - Platy, radiating rose-like aggregate - Gypsum
Sphenoid - Wedge-shaped - Sphene
Stalactitic - Forming as stalactites or stalagmites; cylindrical or cone-shaped - Rhodochrosite
Stellate - Star-like, radiating - Pyrophyllite
Striated/striations - Surface growth lines parallel or perpendicular to c-axis - Chrysoberyl
Tabular or lamellar - Flat, tablet-shaped, prominent pinnacoid - Ruby
Wheat sheaf - Aggregates resembling hand-reaped wheat sheaves - Zeolites