Crystal habit

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 form is.

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

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.

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	Natrolite	Needle-like, slender and/or tapered	natrolite, rutile[6]
Amygdaloidal	Native copper	Like embedded almonds	heulandite, subhedral zircon
Bladed	Actinolite	Blade-like, slender and flattened	actinolite, kyanite
Botryoidal or globular	Malachite	Grape-like, hemispherical masses	hematite, pyrite, malachite, smithsonite, hemimorphite
Columnar	Selenite (gypsum)	Similar to fibrous: Long, slender prisms often with parallel growth	calcite, gypsum/selenite
Coxcomb	Marcasite	Aggregated flaky or tabular crystals closely spaced.	barite, marcasite
Cubic	Fluorite	Cube shape	pyrite, galena, halite
Dendritic or arborescent	Pyrolusite	Tree-like, branching in one or more direction from central point	romanechite, magnesite, native copper
Dodecahedral	Garnet	Rhombic dodecahedron, 12-sided	garnet
Drusy or encrustation	Quartz	Aggregate of minute crystals coating a surface or cavity	uvarovite, malachite, azurite
Enantiomorphic	Gypsum	Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals	quartz, plagioclase, staurolite
Equant, stout	Apophyllite	Length, width, and breadth roughly equal	olivine, garnet
Fibrous	Byssolite	Extremely slender prisms	serpentine group, tremolite (i.e. asbestos)
Filiform or capillary	Millerite	Hair-like or thread-like, extremely fine	many zeolites
Foliated or micaceous or lamellar (layered)	Lepidolite	Layered structure, parting into thin sheets	muscovite, biotite
Granular	Bornite	Aggregates of anhedral crystals in matrix	bornite, scheelite
Hemimorphic	Hemimorphite	Doubly terminated crystal with two differently shaped ends	hemimorphite, elbaite
Hexagonal	Corundum	Hexagon shape, six-sided	quartz, hanksite
Hopper crystals	Halite	Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity	halite, calcite, synthetic bismuth
Mammillary	Malachite	Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates	malachite, hematite
Massive or compact	Turquoise	Shapeless, no distinctive external crystal shape	limonite, turquoise, cinnabar, realgar
Nodular or tuberose	Chalcedony	Deposit of roughly spherical form with irregular protuberances	chalcedony
Octahedral	Diamond	Octahedron, eight-sided (two pyramids base to base)	diamond, magnetite
Platy	Wulfenite	Flat, tablet-shaped, prominent pinnacoid	wulfenite
Plumose	Aurichalcite	Fine, feather-like scales	aurichalcite, boulangerite, mottramite
Prismatic	Tourmaline	Elongate, prism-like: well-developed crystal faces parallel to the vertical axis	tourmaline, beryl
Pseudo-hexagonal	Aragonite	Hexagonal appearance due to cyclic twinning	aragonite, chrysoberyl
Radiating or radial or divergent	Stibnite	Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths)	stibnite
Reniform or colloform	Mottramite	Similar to botryoidal/mamillary: intersecting kidney-shaped masses	hematite, pyrolusite, greenockite
Reticulated	Cerussite	Crystals forming net-like intergrowths	cerussite
Rosette or lenticular (lens shaped crystals)	Desert rose (barite)	Platy, radiating rose-like aggregate	gypsum, barite (i.e. desert rose)
Sphenoid	Titanite	Wedge-shaped	sphene
Stalactitic	Malachite	Forming as stalactites or stalagmites; cylindrical or cone-shaped	calcite, goethite, malachite
Stellate	Pyrophyllite	Star-like, radial aggregates radiating from a "star"-like point to produce gross spheres (crystals are not or weakly separated and have similar lengths)	pyrophyllite, aragonite, wavellite, pyrite suns
Striated	Pyrite	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
Tabular (also stubby or blocky)	Oligoclase	More elongated than equant, slightly longer than wide, flat tablet-shaped	feldspar, topaz
Tetrahedral	Tetrahedrite	Tetrahedra-shaped crystals	tetrahedrite, spinel, magnetite
Wheat sheaf	Stilbite	Aggregates resembling hand-reaped wheat sheaves	stilbite

See also

• Abnormal grain growth
• Grain growth

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". Archived from the original on 2017-07-07. Retrieved 2009-04-06.
4. Crystal Habit Archived 2009-04-12 at the Wayback Machine
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. arXiv:1303.2761. Bibcode:2012JCrGr.359...83H. doi:10.1016/j.jcrysgro.2012.08.015.