Physics:Crystal habit

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

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

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).

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. 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

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Aggregate habits

Habit	Image	Description	Common example(s)
Acicular		Needle-like, slender, and end-tapered prisms growing in a radial/globular fashion.	natrolite, scolecite, yuanfuliite
Arborescent		Tree-like crystals growing similar to branches.	copper, gold, silver
Capillary/Filiform		Hair-like or thread-like, extremely fine	byssolite, millerite
Colloform/Nodular/Tuberose		Rounded, finely banded deposits with irregular concentric protuberances	agate, baryte, sphalerite
Concentric		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		Root-like, branching in one or more direction from central point	copper, gold, romanechite, magnesite, silver
Druse/Encrustation		Aggregate of crystals coating a surface or cavity, usually found in geodes and some fossils	azurite, celestine, calcite, uvarovite, malachite, quartz
Fibrous/Asbestiform		Extremely slender prisms forming muscle-like fibers	actinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine group
Foliated/Micaceous/Lamellar		Layered crystal planes, parting into thin sheets	biotite, hematite, muscovite, lepidolite, molybdenite
Granular		Aggregates of diminute anhedral crystals in matrix or other surface	andradite, bornite, scheelite, quartz, uvarovite
Hopper		Outer portions of cubes grow faster than inner portions, creating a concavity similar to that of a hopper	bismuth (artificial), halite, galena
Oolithic		Small spheres or grains (commonly flattened) that resemble eggs	aragonite, calcite
Pisolitic		Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic	aragonite, bauxite, calcite, pisolite
Platy/Tabular/Blocky		Flat, tablet-shaped, prominent pinnacoid	baryte, feldspar, topaz, vanadinite, wulfenite
Plumose		Fine, feather-like scales	aurichalcite, okenite, mottramite
Radial/Radiating/Divergent		Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths).	aenigmatite, atacamite, epidote, pyrophyllite, stibnite
Reticulated		Crystals forming triangular net-like intergrowths.	cerussite, rutile
Rosette/Lenticular		Platy, radiating rose-like aggregate (also lens shaped crystals)	gypsum, baryte, calcite
Stalactitic		Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern	calcite, chalcedony, chrysocolla, goethite, malachite, romanechite
Stellate		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		Like embedded almonds	heulandite, stilbite, zircon
Hemimorphic		Doubly terminated crystal with two differently shaped ends	elbaite, hemimorphite, olivine
Massive/Compact		Shapeless, no distinctive external crystal shape	limonite, turquoise, cinnabar, quartz, realgar, lazurite
Sceptered		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		Cube-shaped	fluorite, pyrite, galena, halite
Dodecahedral		Dodecahedron-shaped, 12-sided. Central facet can vary.	garnet, pyrite
Enantiomorphic		Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals	aragonite, gypsum, quartz, plagioclase, staurolite
Hexagonal		Hexagonal prism (six-sided)	beryl, galena, quartz, hanksite, vanadinite
Icositetrahedral		Icositetrahedron-shaped, 24-faced	analcime, spessartine
Octahedral		Octahedron-shaped, square bipyramid (eight-sided)	diamond, fluorine, fluorite, magnetite, pyrite
Prismatic		Elongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axis	beryl, tourmaline, vanadinite
Rhombohedral		Rhombohedron-shaped (six-faced rhombi)	calcite, magnesite, rhodochrosite, siderite
Scalenohedral		Scalenohedron-shaped, pointy ends	calcite, rhodochrosite, titanite
Tetrahedral		Tetrahedron-shaped, triangular pyramid (four-sided)	chalcopyrite, tetrahedrite, sphalerite, magnetite

Rounded/Spherical habits

Habit	Image	Description	Common example(s)
Botryoidal		Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other	calcite, chalcedony, halite, plumbogummite, smithsonite
Globular		Isolated hemispheres or spheres	calcite, fluorite, gyrolite
Mammillary		Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates.	chalcedony, hematite, malachite
Reniform		Irregular kidney-shaped spherical masses	cassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite

See also

• Abnormal grain growth
• Grain growth
• Crystal structure

Bibliography

• Kostov, Ivan; Kostov, Ruslan I. (1999). Crystal Habits of Minerals. Sofia: Academic Publishing House "Prof. Marin Drinov". ISBN 9789546420602. OCLC 488807766.