Crystal habit

Short description: Mineralogical term for the visible shape of a mineral

Smoky quartz with spessartine on top of feldspar matrix, featuring different crystal habits (shapes)

In mineralogy, crystal habit is the characteristic external shape of an individual crystal or aggregate of crystals. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to limited space in the crystallizing medium (commonly in rocks).^[1]^[2]

Crystal forms

Recognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of the internal ordered atomic arrangement.^[1] Most natural crystals, however, do not display ideal habits and are commonly malformed. Hence, it is also important to describe the quality of the shape of a mineral specimen:

Euhedral: a crystal that is completely bounded by its characteristic faces, well-formed. Synonymous terms: idiomorphic, automorphic;
Subhedral: a crystal partially bounded by its characteristic faces and partially by irregular surfaces. Synonymous terms: hypidiomorphic, hypautomorphic;
Anhedral: a crystal that lacks any of its characteristic faces, completely malformed. Synonymous terms: allotriomorphic, xenomorphic.

Altering factors

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, i.e. 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.

List of crystal habits

^[3] ^[4]^{[better source needed]}</ref>^{[better source needed]}^[5]

Aggregate habits

Habit	Image	Description	Common example(s)
Acicular	Script error: No such module "Image array".	Needle-like, slender, and end-tapered prisms growing in a radial/globular fashion.	natrolite, scolecite, yuanfuliite
Arborescent	Script error: No such module "Image array".	Tree-like crystals growing similar to branches.	copper, gold, silver
Capillary/Filiform	Script error: No such module "Image array".	Hair-like or thread-like, extremely fine	byssolite, millerite
Colloform/Nodular/Tuberose	Script error: No such module "Image array".	Rounded, finely banded deposits with irregular concentric protuberances	agate, baryte, sphalerite
Concentric	Script error: No such module "Image array".	Circular ring aggregates around a center. This habit is found in cross-sections from reniform/mamillary habits, and also from elongated stalactites of amethyst (quartz), malachites, rhodocrosite, and others	agate, quartz, malachite, rhodocrosite
Dendritic	Script error: No such module "Image array".	Root-like, branching in one or more direction from central point	copper, gold, romanechite, magnesite, silver
Druse/Encrustation	Script error: No such module "Image array".	Aggregate of crystals coating a surface or cavity, usually found in geodes and some fossils	azurite, celestine, calcite, uvarovite, malachite, quartz
Fibrous/Asbestiform	Script error: No such module "Image array".	Extremely slender prisms forming muscle-like fibers	actinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine group
Foliated/Micaceous/Lamellar	Script error: No such module "Image array".	Layered crystal planes, parting into thin sheets	biotite, hematite, muscovite, lepidolite, molybdenite
Granular	Script error: No such module "Image array".	Aggregates of diminute anhedral crystals in matrix or other surface	andradite, bornite, scheelite, quartz, uvarovite
Hopper	Script error: No such module "Image array".	Outer portions of cubes grow faster than inner portions, creating a concavity similar to that of a hopper	bismuth (artificial), halite, galena
Oolithic	Script error: No such module "Image array".	Small cirumferences or grains (commonly flattened) that resemble eggs	aragonite, calcite
Pisolitic	Script error: No such module "Image array".	Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic	aragonite, bauxite, calcite, pisolite
Platy/Tabular/Blocky	Script error: No such module "Image array".	Flat, tablet-shaped, prominent pinnacoid	baryte, feldspar, topaz, vanadinite, wulfenite
Plumose	Script error: No such module "Image array".	Fine, feather-like scales	aurichalcite, okenite, mottramite
Radial/Radiating/Divergent	Script error: No such module "Image array".	Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths).	aenigmatite, atacamite, epidote, pyrophyllite, stibnite
Reticulated	Script error: No such module "Image array".	Crystals forming triangular net-like intergrowths.	cerussite, rutile
Rosette/Lenticular	Script error: No such module "Image array".	Platy, radiating rose-like aggregate (also lens shaped crystals)	gypsum, baryte, calcite
Stalactitic	Script error: No such module "Image array".	Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern	calcite, chalcedony, chrysocolla, goethite, malachite, romanechite
Stellate	Script error: No such module "Image array".	Star-like, radial fibers found inside spherical habits, such as mamillary or reniform.	hematite, pectolite, shattuckite, wavellite

Asymmetrical/Irregular habits

Habit	Image	Description	Common example(s)
Amygdaloidal	Script error: No such module "Image array".	Like embedded almonds	heulandite, stilbite, zircon
Hemimorphic	Script error: No such module "Image array".	Doubly terminated crystal with two differently shaped ends	elbaite, hemimorphite, olivine
Massive/Compact	Script error: No such module "Image array".	Shapeless, no distinctive external crystal shape	limonite, turquoise, cinnabar, quartz, realgar, lazurite
Sceptered	Script error: No such module "Image array".	Crystal growth stops and continues at the top of the crystal, but not at the bottom. Exceptional aggregates of this habit (such as quartz) are often referred as "Elestial".	baryte, calcite, marcasite, quartz

Symmetrical habits

Habit	Image	Description	Common example(s)
Cubic	Script error: No such module "Image array".	Cube-shaped	fluorite, pyrite, galena, halite
Dodecahedral	Script error: No such module "Image array".	Dodecahedron-shaped, 12-sided. Central facet can vary.	garnet, pyrite
Enantiomorphic	Script error: No such module "Image array".	Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals	aragonite, gypsum, quartz, plagioclase, staurolite
Hexagonal	Script error: No such module "Image array".	Hexagonal prism (six-sided)	beryl, galena, quartz, hanksite, vanadinite
Icositetrahedral	Script error: No such module "Image array".	Icositetrahedron-shaped, 24-faced	analcime, spessartine
Octahedral	Script error: No such module "Image array".	Octahedron-shaped, square bipyramid (eight-sided)	diamond, fluorine, fluorite, magnetite, pyrite
Prismatic	Script error: No such module "Image array".	Elongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axis	beryl, tourmaline, vanadinite
Rhombohedral	Script error: No such module "Image array".	Rhombohedron-shaped (six-faced rhombi)	calcite, magnesite, rhodochrosite, siderite
Scalenohedral	Script error: No such module "Image array".	Scalenohedron-shaped, pointy ends	calcite, rhodochrosite, titanite
Tetrahedral	Script error: No such module "Image array".	Tetrahedron-shaped, triangular pyramid (four-sided)	chalcopyrite, tetrahedrite, sphalerite, magnetite

Rounded/Spherical habits

Habit	Image	Description	Common example(s)
Botryoidal	Script error: No such module "Image array".	Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other	calcite, chalcedony, halite, plumbogummite, smithsonite
Globular	Script error: No such module "Image array".	Isolated hemispheres or spheres	calcite, fluorite, gyrolite
Mammillary	Script error: No such module "Image array".	Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates.	chalcedony, hematite, malachite
Reniform	Script error: No such module "Image array".	Irregular kidney-shaped spherical masses	cassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite

References

↑ ^1.0 ^1.1 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
↑ Wenk, Hans-Rudolph and Andrei Bulakh, 2004, Minerals: Their Constitution and Origin, Cambridge, first edition, ISBN:978-0521529587
↑ "What are descriptive crystal habits". http://www.galleries.com/minerals/property/habits.htm.
↑ Crystal Habit
↑ Hanaor (2012). "Abnormal grain growth of rutile TiO₂ induced by ZrSiO". Journal of Crystal Growth 359: 83–91. doi:10.1016/j.jcrysgro.2012.08.015. Bibcode: 2012JCrGr.359...83H.

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Original source: https://en.wikipedia.org/wiki/Crystal habit. Read more

[Klein-1] 1.0 ^1.1 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

[Wenk-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". http://www.galleries.com/minerals/property/habits.htm.

[4] Crystal Habit

[5] Hanaor (2012). "Abnormal grain growth of rutile TiO₂ induced by ZrSiO". Journal of Crystal Growth 359: 83–91. doi:10.1016/j.jcrysgro.2012.08.015. Bibcode: 2012JCrGr.359...83H.

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