Crystal habit

This article is about the descriptive term used in mineralogy. For the addictive drug, see crystal methamphetamine.

File:Pyrite sun.jpg

Pyrite sun (or dollar) in laminated shale matrix. Between tightly spaced layers of shale, the aggregate was forced to grow in a laterally compressed, radiating manner. Under normal conditions, pyrite would form cubes or pyritohedrons

Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates.

In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen).

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.

File:PyOx.JPG

Goethite replacing pyrite cubes

Factors influencing a crystal's habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space); and specific growth tendencies like growth striations. 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 prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.

The names of crystal habits are derived from:

Predominant crystal faces (prism - prismatic, pyramid - pyramidal and pinacoid - platy). Crystal forms (cubic, octahedral, dodecahedral). Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive). Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped)).

List of crystal habits

Habit	Image	Description	Common Example(s)
Acicular	250px	Needle-like, slender and/or tapered	Natrolite, Rutile
Amygdaloidal	250px	Almond-shaped	Heulandite, subhedral Zircon
Bladed	250px	Blade-like, slender and flattened	Actinolite, Kyanite
Botryoidal or globular	250px	Grape-like, hemispherical masses	Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite
Columnar	250px	Similar to fibrous: Long, slender prisms often with parallel growth	Calcite, Gypsum/Selenite
Coxcomb	250px	Aggregated flaky or tabular crystals closely spaced.	Barite, Marcasite
Cubic	250px	Cube shape	Pyrite, Galena, Halite
Dendritic or arborescent	250px	Tree-like, branching in one or more direction from central point	Pyrolusite and other Mn-oxide minerals, Magnesite, native copper
Dodecahedral	250px	Dodecahedron, 12-sided	Garnet
Drusy or encrustation	250px	Aggregate of minute crystals coating a surface or cavity	Uvarovite, Malachite, Azurite
Enantiomorphic	250px	Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals	Quartz, Plagioclase, Staurolite
Equant, stout	250px	Length, width, and breadth roughly equal	Olivine, Garnet
Fibrous	250px	Extremely slender prisms	Serpentine group, Tremolite (i.e. Asbestos)
Filiform or capillary	250px	Hair-like or thread-like, extremely fine	many Zeolites
Foliated or micaceous or lamellar (layered)	250px	Layered structure, parting into thin sheets	Mica (Muscovite, Biotite, etc.)
Granular	250px	Aggregates of anhedral crystals in matrix	Bornite, Scheelite
Hemimorphic	250px	Doubly terminated crystal with two differently shaped ends.	Hemimorphite, Elbaite
Hexagonal	250px	Hexagon shape, six-sided	Quartz, Hanksite
Hopper crystals	250px	Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity	Halite, Calcite, synthetic Bismuth
Mamillary	250px	Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates	Malachite, Hematite
Massive or compact	250px	Shapeless, no distinctive external crystal shape	Limonite, Turquoise, Cinnabar, Realgar
Nodular or tuberose	250px	Deposit of roughly spherical form with irregular protuberances	Chalcedony, various Geodes
Octahedral	250px	Octahedron, eight-sided (two pyramids base to base)	Diamond, Magnetite
Plumose	250px	Fine, feather-like scales	Aurichalcite, Boulangerite, Mottramite
Prismatic	250px	Elongate, prism-like, semi-cylidrical: crystal faces parallel to c-axis well-developed	Tourmaline, Beryl
Pseudo-hexagonal		Hexagonal appearance due to cyclic twinning	Aragonite, Chrysoberyl
Radiating or divergent	250px	Radiating outward from a central point	Wavellite, Pyrite suns
Reniform or colloform	250px	Similar to botryoidal/mamillary: intersecting kidney-shaped masses	Hematite, Pyrolusite, Greenockite
Reticulated	250px	Crystals forming net-like intergrowths	Cerussite
Rosette or lenticular (lens shaped crystals)	250px	Platy, radiating rose-like aggregate	Gypsum, Barite (i.e. Desert rose)
Sphenoid	250px	Wedge-shaped	Sphene
Stalactitic		Forming as stalactites or stalagmites; cylindrical or cone-shaped	Calcite, Goethite
Stellate	250px	Star-like, radiating	Pyrophyllite, Aragonite
Striated	250px	Not a habit per se, but a condition of lines that can grow on certain crystal faces on certain minerals	Tourmaline, Pyrite, Quartz, Feldspar, Sphalerite
Stubby or blocky or tabular	250px	More elongated than equant, slightly longer than wide, flat tablet shaped	Feldspar, Topaz
Platy	250px	Flat, tablet-shaped, prominent pinnacoid	Wulfenite
Tetrahedral	250px	Tetrahedra-shaped crystals	Tetrahedrite, Spinel, Magnetite
Wheat sheaf	250px	Aggregates resembling hand-reaped wheat sheaves	Stilbite

References

bs:Kristalni habitus