Crystal habit

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.

In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. A single crystal's habit is a description of its general shape and its crystallographic forms, plus how well developed each are. Recognizing the habit may help in identifying a mineral. When the faces are well-developed due to uncrowded growth a crystal is called euhedral, one with partially developed faces is subhedral, and one with undeveloped crystal faces is called anhedral. The long axis of a euhedral quartz crystal typically has a six-sided prismatic habit with parallel opposite faces. Aggregates can be formed of individual crystals with euhedral to anhedral grains. The arrangement of crystals within the aggregate can be characteristic of certain minerals. For example, minerals used for asbestos insulation often grow in a fibrous habit, a mass of very fine fibers.^[1][2]

The terms used by mineralogists to report crystal habits describe the typical appearance of an ideal mineral. Recognizing the habit can aid in identification as some habits are characteristic. Most minerals, however, do not display ideal habits due to conditions during crystallization. Euhedral crystals formed in uncrowded conditions with no adjacent crystal grains are not common; more often faces are poorly formed or unformed against adjacent grains and the mineral's habit may not be easily recognized.^[1]

Goethite replacing pyrite cubes

Factors influencing 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 such as 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[3][4][5]	Image	Description	Common Example(s)
Acicular		Needle-like, slender and/or tapered	Natrolite, Rutile[6]
Amygdaloidal		Almond-shaped	Heulandite, subhedral Zircon
Bladed		Blade-like, slender and flattened	Actinolite, Kyanite
Botryoidal or globular		Grape-like, hemispherical masses	Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite
Columnar		Similar to fibrous: Long, slender prisms often with parallel growth	Calcite, Gypsum/Selenite
Coxcomb		Aggregated flaky or tabular crystals closely spaced.	Barite, Marcasite
Cubic		Cube shape	Pyrite, Galena, Halite
Dendritic or arborescent		Tree-like, branching in one or more direction from central point	Romanechite and other Mn-oxide minerals, magnesite, native copper
Dodecahedral		Rhombic dodecahedron, 12-sided	Garnet
Drusy or encrustation		Aggregate of minute crystals coating a surface or cavity	Uvarovite, Malachite, Azurite
Enantiomorphic		Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals	Quartz, Plagioclase, Staurolite
Equant, stout		Length, width, and breadth roughly equal	Olivine, Garnet
Fibrous		Extremely slender prisms	Serpentine group, Tremolite (i.e. Asbestos)
Filiform or capillary		Hair-like or thread-like, extremely fine	many Zeolites
Foliated or micaceous or lamellar (layered)		Layered structure, parting into thin sheets	Mica (Muscovite, Biotite, etc.)
Granular		Aggregates of anhedral crystals in matrix	Bornite, Scheelite
Hemimorphic		Doubly terminated crystal with two differently shaped ends.	Hemimorphite, Elbaite
Hexagonal		Hexagon shape, six-sided	Quartz, Hanksite
Hopper crystals		Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity	Halite, Calcite, synthetic Bismuth
Mammillary		Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates	Malachite, Hematite
Massive or compact		Shapeless, no distinctive external crystal shape	Limonite, Turquoise, Cinnabar, Realgar
Nodular or tuberose		Deposit of roughly spherical form with irregular protuberances	Chalcedony, various Geodes
Octahedral		Octahedron, eight-sided (two pyramids base to base)	Diamond, Magnetite
Plumose		Fine, feather-like scales	Aurichalcite, Boulangerite, Mottramite
Prismatic		Elongate, prism-like: crystal faces parallel to c-axis well-developed	Tourmaline, Beryl
Pseudo-hexagonal		Hexagonal appearance due to cyclic twinning	Aragonite, Chrysoberyl
Radiating or divergent		Radiating outward from a central point	Wavellite, Pyrite suns
Reniform or colloform		Similar to botryoidal/mamillary: intersecting kidney-shaped masses	Hematite, Pyrolusite, Greenockite
Reticulated		Crystals forming net-like intergrowths	Cerussite
Rosette or lenticular (lens shaped crystals)		Platy, radiating rose-like aggregate	Gypsum, Barite (i.e. Desert rose)
Sphenoid		Wedge-shaped	Sphene
Stalactitic		Forming as stalactites or stalagmites; cylindrical or cone-shaped	Calcite, Goethite
Stellate		Star-like, radiating	Pyrophyllite, Aragonite
Striated		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		More elongated than equant, slightly longer than wide, flat tablet shaped	Feldspar, Topaz
Platy		Flat, tablet-shaped, prominent pinnacoid	Wulfenite
Tetrahedral		Tetrahedra-shaped crystals	Tetrahedrite, Spinel, Magnetite
Wheat sheaf		Aggregates resembling hand-reaped wheat sheaves	Stilbite

See also

• Abnormal grain growth
• Grain growth
• Crystallization

References

1. Klein, Cornelis, 2007, Minerals and Rocks: Exercises in Crystal and Mineral Chemistry, Crystallography, X-ray Powder Diffraction, Mineral and Rock Identification, and Ore Mineralogy, Wiley, third edition, ISBN 978-0471772774
2. Wenk, Hans-Rudolph and Andrei Bulakh, 2004, Minerals: Their Constitution and Origin, Cambridge, first edition, ISBN 978-0521529587
3. What are descriptive crystal habits
4. Crystal Habit
5. Habit
6. Hanaor, D.A.H; Xu, W; Ferry, M; Sorrell, CC (2012). "Abnormal grain growth of rutile TiO₂ induced by ZrSiO₄". Journal of Crystal Growth. 359: 83–91. doi:10.1016/j.jcrysgro.2012.08.015.