Crystal habit

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]

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

Crystal formsEdit

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 factorsEdit

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 habitsEdit

^[3]^{[better source needed]}^[4]^{[better source needed]}^[5]^{[better source needed]}^[6]

Aggregate habitsEdit

Habit	Image	Description	Common example(s)
Acicular	Natrolite	Needle-like, slender and/or tapered	natrolite, rutile
Bladed	Quartz	Blade-like ends, slender and somewhat flattened	quartz, stilbite, kyanite
Columnar	Selenite (gypsum)	Similar to bladed and radial: Long, slender prisms often with parallel growth	calcite, gypsum/selenite
Concentric	Quartz	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	quartz, malachite, rhodocrosite
Coxcomb	Marcasite	Aggregated flaky or tabular crystals closely spaced.	barite, marcasite
Dendritic/Arborescent	Copper	Tree-like, branching in one or more direction from central point	romanechite, magnesite, native copper
Druse/Encrustation	Celestine	Aggregate of crystals coating a surface or cavity, usually found in geodes	azurite, celestine, calcite, uvarovite, malachite, quartz
Fibrous (including asbestiform)	Kyanite	Extremely slender prisms, muscle-like fibers	serpentine group, actinolite, kyanite, gypsum, nitratine, tremolite (i.e. asbestos)
Filiform or capillary	Byssolite	Hair-like or thread-like, extremely fine	many zeolites, byssolite, millerite, okenite
Foliated/Micaceous/Lamellar	Muscovite	Layered crystal structures, parting into thin sheets	muscovite, biotite, lepidolite, molybdenite
Granular	Quartz	Aggregates of diminute anhedral crystals in matrix or other surface	andradite, bornite, scheelite, quartz
Hopper	Halite	Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity	halite, calcite, synthetic bismuth
Oolithic	Calcite	Small cirumferences or grains (commonly flattened) that resemble eggs	aragonite, calcite
Pisolitic	Bauxite	Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic	bauxite, gibbsite
Platy	Wulfenite	Flat, tablet-shaped, prominent pinnacoid	wulfenite
Plumose	Aurichalcite	Fine, feather-like scales	aurichalcite, boulangerite, mottramite
Radial/Radiating/Divergent	Atacamite	Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths)	atacamite, stibnite
Reticulated	Cerussite	Crystals forming net-like intergrowths	cerussite
Rosette/Lenticular	Baryte	Platy, radiating rose-like aggregate (also lens shaped crystals)	gypsum, baryte, calcite
Stalactitic	Chrysocolla	Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern	calcite, chrysocolla, goethite, malachite
Stellate	Wavellite	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"
Tabular/Blocky/Stubby	Vanadinite	More elongated than equant, slightly longer than wide, flat tablet-shaped	feldspar, topaz, vanadinite
Wheat sheaf	Stilbite	Aggregates resembling hand-reaped wheat sheaves	stilbite

Asymmetrical/Irregular habitsEdit

Habit	Image	Description	Common example(s)
Amygdaloidal	Native copper	Like embedded almonds	heulandite, subhedral zircon
Hemimorphic	Hemimorphite	Doubly terminated crystal with two differently shaped ends	hemimorphite, elbaite
Massive/Compact	Turquoise	Shapeless, no distinctive external crystal shape	limonite, turquoise, cinnabar, quartz, realgar, lazurite
Nodular/Tuberose	Agate	Deposit of roughly spherical form with irregular protuberances	agate (and other chalcedony)
Sceptered	Quartz	Crystal growth stops and continues at the top of the crystal, but not at the bottom	hedenbergite, quartz

Symmetrical habitsEdit

Habit	Image	Description	Common example(s)
Cubic	Halite	Cube shape	fluorite, pyrite, galena, halite
Dodecahedral	Pyrite	Dodecahedron-shaped, 12-sided	garnet, pyrite
Enantiomorphic	Gypsum	Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals	gypsum, quartz, plagioclase, staurolite
Equant/Stout	Olivine	Length, width, and breadth roughly equal	apophyllite, olivine, garnet
Hexagonal	Emerald	Hexagonal prism (six-sided)	emerald, galena, quartz, hanksite, vanadinite
Icositetrahedral	Spessartine	Icositetrahedron-shaped, 24-faced	spessartine
Octahedral	Fluorine	Octahedron-shaped, square bipyramid (eight-sided)	diamond, fluorine, fluorite, magnetite, pyrite
Prismatic	Beryl	Elongate, prism-like: well-developed crystal faces parallel to the vertical axis	beryl, tourmaline, vanadinite, emerald
Pseudo-hexagonal	Aragonite	Hexagon-like appearance due to cyclic twinning	aragonite, chrysoberyl
Rhombohedral	Siderite	Rhombohedron-shaped (six-faced rhombi)	calcite, rhodochrosite, siderite
Scalenohedral	Rhodochrosite	Scalenohedron-shaped, pointy ends	calcite, rhodochrosite, titanite
Tetrahedral	Sphalerite	Tetrahedron-shaped, triangular pyramid (four-sided)	tetrahedrite, spinel, sphalerite, magnetite

Rounded/Spherical habitsEdit

Habit	Image	Description	Common example(s)
Botryoidal	Chalcedony	Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other	chalcedony, pyrite, smithsonite, hemimorphite
Colloform	Sphalerite	Rounded, finely banded	sphalerite, pyrite
Globular	Gyrolite	Isolated hemispheres or spheres	calcite, fluorite, gyrolite
Mammillary	Chalcedony	Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates. It is almost synonymous with reniform.	chalcedony, hematite, malachite
Reniform	Mottramite	Kidney-shaped masses	cassiterite, chalcedony, chrysocolla, hematite, fluorite, goethite, greenockite, malachite, wavellite, mottramite

ReferencesEdit

^ ^a ^b 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". Archived from the original on 2017-07-07. Retrieved 2009-04-06.
^ Crystal Habit Archived 2009-04-12 at the Wayback Machine
^ Habit
^ Hanaor, D.A.H; Xu, W; Ferry, M; Sorrell, CC (2012). "Abnormal grain growth of rutile TiO₂ induced by ZrSiO". Journal of Crystal Growth. 359: 83–91. arXiv:1303.2761. Bibcode:2012JCrGr.359...83H. doi:10.1016/j.jcrysgro.2012.08.015. S2CID 94096447.

[Klein-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". 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 ZrSiO". Journal of Crystal Growth. 359: 83–91. arXiv:1303.2761. Bibcode:2012JCrGr.359...83H. doi:10.1016/j.jcrysgro.2012.08.015. S2CID 94096447.

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