Crystal habit

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

Rounded/Spherical habits

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

See also

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