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Crystal habit

This article is about the descriptive term used in mineralogy. For the addictive drug, see [[methamphetamine#REDIRECTmw:Help:Magic words#Other
This page is a soft redirect.crystal methamphetamine]].
File:Pyrite sun.jpg
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 groups of crystals. A single crystal's habit is a description of its general shape and its crystallographic forms, plus how well developed each are. Recognizing the habit may help in identifying the 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]

File:PyOx.JPG
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 250px Needle-like, slender and/or tapered Natrolite, Rutile
Amygdaloidal 250px Almond-shaped Heulandite, subhedral Zircon
Bladed 250px Blade-like, slender and flattened Actinolite, Kyanite
Botryoidal or globular 250px Grape-like, hemispherical masses Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite
Columnar 250px Similar to fibrous: Long, slender prisms often with parallel growth Calcite, Gypsum/Selenite
Coxcomb 250px Aggregated flaky or tabular crystals closely spaced. Barite, Marcasite
Cubic 250px Cube shape Pyrite, Galena, Halite
Dendritic or arborescent 250px Tree-like, branching in one or more direction from central point Romanechite and other Mn-oxide minerals, magnesite, native copper
Dodecahedral 250px Rhombic dodecahedron, 12-sided Garnet
Drusy or encrustation 250px Aggregate of minute crystals coating a surface or cavity Uvarovite, Malachite, Azurite
Enantiomorphic 250px Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals Quartz, Plagioclase, Staurolite
Equant, stout 250px Length, width, and breadth roughly equal Olivine, Garnet
Fibrous 250px Extremely slender prisms Serpentine group, Tremolite (i.e. Asbestos)
Filiform or capillary 250px Hair-like or thread-like, extremely fine many Zeolites
Foliated or micaceous or lamellar (layered) 250px Layered structure, parting into thin sheets Mica (Muscovite, Biotite, etc.)
Granular 250px Aggregates of anhedral crystals in matrix Bornite, Scheelite
Hemimorphic 250px Doubly terminated crystal with two differently shaped ends. Hemimorphite, Elbaite
Hexagonal 250px Hexagon shape, six-sided Quartz, Hanksite
Hopper crystals 250px Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity Halite, Calcite, synthetic Bismuth
Mammillary 250px Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates Malachite, Hematite
Massive or compact 250px Shapeless, no distinctive external crystal shape Limonite, Turquoise, Cinnabar, Realgar
Nodular or tuberose 250px Deposit of roughly spherical form with irregular protuberances Chalcedony, various Geodes
Octahedral 250px Octahedron, eight-sided (two pyramids base to base) Diamond, Magnetite
Plumose 250px Fine, feather-like scales Aurichalcite, Boulangerite, Mottramite
Prismatic 250px Elongate, prism-like: crystal faces parallel to c-axis well-developed Tourmaline, Beryl
Pseudo-hexagonal 250px Hexagonal appearance due to cyclic twinning Aragonite, Chrysoberyl
Radiating or divergent 250px Radiating outward from a central point Wavellite, Pyrite suns
Reniform or colloform 250px Similar to botryoidal/mamillary: intersecting kidney-shaped masses Hematite, Pyrolusite, Greenockite
Reticulated 250px Crystals forming net-like intergrowths Cerussite
Rosette or lenticular (lens shaped crystals) 250px Platy, radiating rose-like aggregate Gypsum, Barite (i.e. Desert rose)
Sphenoid 250px Wedge-shaped Sphene
Stalactitic 250px Forming as stalactites or stalagmites; cylindrical or cone-shaped Calcite, Goethite
Stellate 250px Star-like, radiating Pyrophyllite, Aragonite
Striated 250px 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 250px More elongated than equant, slightly longer than wide, flat tablet shaped Feldspar, Topaz
Platy 250px Flat, tablet-shaped, prominent pinnacoid Wulfenite
Tetrahedral 250px Tetrahedra-shaped crystals Tetrahedrite, Spinel, Magnetite
Wheat sheaf 250px Aggregates resembling hand-reaped wheat sheaves Stilbite

See also


References

  1. ^ 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
  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
ro:Habitus