Crystal habit

Pyrite sun (or dollar) in laminated shale matrix. Between tightly spaced layers of shale, the aggregate was forced to grow in a laterally compressed, radiating manner. Under normal conditions, pyrite would form cubes or pyritohedrons.

In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. A single crystal's habit is a description of its general shape and its crystallographic forms, plus how well developed each form is. Recognizing the habit may help in identifying a mineral. When the faces are well-developed due to uncrowded growth a crystal is called euhedral, one with partially developed faces is subhedral, and one with undeveloped crystal faces is called anhedral. The long axis of a euhedral quartz crystal typically has a six-sided prismatic habit with parallel opposite faces. Aggregates can be formed of individual crystals with euhedral to anhedral grains. The arrangement of crystals within the aggregate can be characteristic of certain minerals. For example, minerals used for asbestos insulation often grow in a fibrous habit, a mass of very fine fibers.[1][2]

The terms used by mineralogists to report crystal habits describe the typical appearance of an ideal mineral. Recognizing the habit can aid in identification as some habits are characteristic. Most minerals, however, do not display ideal habits due to conditions during crystallization. Euhedral crystals formed in uncrowded conditions with no adjacent crystal grains are not common; more often faces are poorly formed or unformed against adjacent grains and the mineral's habit may not be easily recognized.[1]

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: this process is called 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.

The names of crystal habits are derived from:

Predominant crystal faces (prism – prismatic, pyramid – pyramidal and pinacoid – platy). Crystal forms (cubic, octahedral, dodecahedral). Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive). Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped)).

List of crystal habits

Habit[3][4][5] Image Description Common Example(s)
Acicular Needle-like, slender and/or tapered Natrolite, Rutile[6]
Amygdaloidal Almond-shaped Heulandite, subhedral Zircon
Bladed Blade-like, slender and flattened Actinolite, Kyanite
Botryoidal or globular Grape-like, hemispherical masses Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite
Columnar Similar to fibrous: Long, slender prisms often with parallel growth Calcite, Gypsum/Selenite
Coxcomb Aggregated flaky or tabular crystals closely spaced. Barite, Marcasite
Cubic Cube shape Pyrite, Galena, Halite
Dendritic or arborescent Tree-like, branching in one or more direction from central point Romanechite and other Mn-oxide minerals, magnesite, native copper
Dodecahedral Rhombic dodecahedron, 12-sided Garnet
Drusy or encrustation Aggregate of minute crystals coating a surface or cavity Uvarovite, Malachite, Azurite, Quartz
Enantiomorphic Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals Quartz, Plagioclase, Staurolite
Equant, stout Length, width, and breadth roughly equal Olivine, Garnet
Fibrous Extremely slender prisms Serpentine group, Tremolite (i.e. Asbestos)
Filiform or capillary Hair-like or thread-like, extremely fine many Zeolites
Foliated or micaceous or lamellar (layered) Layered structure, parting into thin sheets Mica (Muscovite, Biotite, etc.)
Granular Aggregates of anhedral crystals in matrix Bornite, Scheelite
Hemimorphic Doubly terminated crystal with two differently shaped ends. Hemimorphite, Elbaite
Hexagonal Hexagon shape, six-sided Quartz, Hanksite
Hopper crystals Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity Halite, Calcite, synthetic Bismuth
Mammillary Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates Malachite, Hematite
Massive or compact Shapeless, no distinctive external crystal shape Limonite, Turquoise, Cinnabar, Realgar
Nodular or tuberose Deposit of roughly spherical form with irregular protuberances Chalcedony, various Geodes
Octahedral Octahedron, eight-sided (two pyramids base to base) Diamond, Magnetite
Platy Flat, tablet-shaped, prominent pinnacoid Wulfenite
Plumose Fine, feather-like scales Aurichalcite, Boulangerite, Mottramite
Prismatic Elongate, prism-like: crystal faces parallel to c-axis well-developed Tourmaline, Beryl
Pseudo-hexagonal Hexagonal appearance due to cyclic twinning Aragonite, Chrysoberyl
Radiating or divergent Radiating outward from a central point Wavellite, Pyrite suns
Reniform or colloform Similar to botryoidal/mamillary: intersecting kidney-shaped masses Hematite, Pyrolusite, Greenockite
Reticulated Crystals forming net-like intergrowths Cerussite
Rosette or lenticular (lens shaped crystals) Platy, radiating rose-like aggregate Gypsum, Barite (i.e. Desert rose)
Sphenoid Wedge-shaped Sphene
Stalactitic Forming as stalactites or stalagmites; cylindrical or cone-shaped Calcite, Goethite, Malachite
Stellate Star-like, radiating Pyrophyllite, Aragonite
Striated Not a habit per se, but a condition of lines that can grow on certain crystal faces on certain minerals Tourmaline, Pyrite, Quartz, Feldspar, Sphalerite
Stubby or blocky or tabular More elongated than equant, slightly longer than wide, flat tablet shaped Feldspar, Topaz
Tetrahedral Tetrahedra-shaped crystals Tetrahedrite, Spinel, Magnetite
Wheat sheaf Aggregates resembling hand-reaped wheat sheaves Stilbite

See also


References

  1. 1 2 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
  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
  4. ↑ Crystal Habit
  5. ↑ Habit
  6. ↑ Hanaor, D.A.H; Xu, W; Ferry, M; Sorrell, CC (2012). "Abnormal grain growth of rutile TiO2 induced by ZrSiO4". Journal of Crystal Growth. 359: 83–91. doi:10.1016/j.jcrysgro.2012.08.015.
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