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]

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

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

Natrolite

Scolecite

Needle-like, slender, and end-tapered prisms growing in a radial/globular fashion.

natrolite, scolecite, yuanfuliite

Arborescent

Silver

Gold

Tree-like crystals growing similar to branches.

copper, gold, silver

Capillary/Filiform

Byssolite

Millerite

Hair-like or thread-like, extremely fine

byssolite, millerite

Colloform/Nodular/Tuberose

Agate

Sphalerite

Rounded, finely banded deposits with irregular concentric protuberances

agate, baryte, sphalerite

Concentric

Amethyst

Rhodocrosite

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

Copper

Romanechite

Root-like, branching in one or more direction from central point

copper, gold, romanechite, magnesite, silver

Druse/Encrustation

Celestine

Calcite

Aggregate of crystals coating a surface or cavity, usually found in geodes and some fossils

azurite, celestine, calcite, uvarovite, malachite, quartz

Fibrous/Asbestiform

Tremolite

Baryte

Extremely slender prisms forming muscle-like fibers

actinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine group

Foliated/Micaceous/Lamellar

Molybdenite

Biotite

Layered crystal planes, parting into thin sheets

biotite, hematite, muscovite, lepidolite, molybdenite

Granular

Uvarovite

Quartz

Aggregates of diminute anhedral crystals in matrix or other surface

andradite, bornite, scheelite, quartz, uvarovite

Hopper

Halite

Bismuth

Outer portions of cubes grow faster than inner portions, creating a concavity similar to that of a hopper

bismuth (artificial), halite, galena

Oolithic

Calcite

Small cirumferences or grains (commonly flattened) that resemble eggs

aragonite, calcite

Pisolitic

Bauxite

Pisolite

Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic

aragonite, bauxite, calcite, pisolite

Platy/Tabular/Blocky

Baryte

Wulfenite

Flat, tablet-shaped, prominent pinnacoid

baryte, feldspar, topaz, vanadinite, wulfenite

Plumose

Aurichalcite

Okenite

Fine, feather-like scales

aurichalcite, okenite, mottramite

Radial/Radiating/Divergent

Atacamite

Pyrophyllite

Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths).

aenigmatite, atacamite, epidote, pyrophyllite, stibnite

Reticulated

Cerussite

Rutile

Crystals forming triangular net-like intergrowths.

cerussite, rutile

Rosette/Lenticular

Baryte

Gypsum

Platy, radiating rose-like aggregate (also lens shaped crystals)

gypsum, baryte, calcite

Stalactitic

Chrysocolla

Calcite

Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern

calcite, chalcedony, chrysocolla, goethite, malachite, romanechite

Stellate

Hematite

Wavellite

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

Heulandite

Stilbite

Like embedded almonds

heulandite, stilbite, zircon

Hemimorphic

Hemimorphite

Olivine

Doubly terminated crystal with two differently shaped ends

elbaite, hemimorphite, olivine

Massive/Compact

Turquoise

Quartz

Shapeless, no distinctive external crystal shape

limonite, turquoise, cinnabar, quartz, realgar, lazurite

Sceptered

Amethyst

Baryte

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

Halite

Pyrite

Cube-shaped

fluorite, pyrite, galena, halite

Dodecahedral

Pyrite

Almandine

Dodecahedron-shaped, 12-sided. Central facet can vary.

garnet, pyrite

Enantiomorphic

Aragonite

Staurolite

Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals

aragonite, gypsum, quartz, plagioclase, staurolite

Hexagonal

Vanadinite

Galena

Hexagonal prism (six-sided)

beryl, galena, quartz, hanksite, vanadinite

Icositetrahedral

Spessartine

Analcime

Icositetrahedron-shaped, 24-faced

analcime, spessartine

Octahedral

Spinel

Fluorite

Octahedron-shaped, square bipyramid (eight-sided)

diamond, fluorine, fluorite, magnetite, pyrite

Prismatic

Beryl

Tourmaline

Elongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axis

beryl, tourmaline, vanadinite

Rhombohedral

Siderite

Rhodochrosite

Rhombohedron-shaped (six-faced rhombi)

calcite, magnesite, rhodochrosite, siderite

Scalenohedral

Calcite

Rhodochrosite

Scalenohedron-shaped, pointy ends

calcite, rhodochrosite, titanite

Tetrahedral

Sphalerite

Tetrahedrite

Tetrahedron-shaped, triangular pyramid (four-sided)

chalcopyrite, tetrahedrite, sphalerite, magnetite

Rounded/Spherical habits

Habit

Image

Description

Common example(s)

Botryoidal

Chalcedony

Calcite

Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other

calcite, chalcedony, halite, plumbogummite, smithsonite

Globular

Gyrolite

Calcite

Isolated hemispheres or spheres

calcite, fluorite, gyrolite

Mammillary

Chalcedony

Hematite

Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates.

chalcedony, hematite, malachite

Reniform

Malachite

Shattuckite

Irregular kidney-shaped spherical masses

cassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite

References

^ ^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". Archived from the original on 2017-12-01. Retrieved 2009-04-06.
^ 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.

Bibliography

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