Crystal habit


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

Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates.

In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen).

The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.

Goethite replacing pyrite cubes

Factors influencing a crystal's 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 like 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[1][2][3] Image Description Common Example(s)
Acicular Natroliteinde1.jpg Needle-like, slender and/or tapered Natrolite, Rutile
Amygdaloidal HeulanditeLonavala.jpg Almond-shaped Heulandite, subhedral Zircon
Bladed Aktinolith Finnland.JPG Blade-like, slender and flattened Actinolite, Kyanite
Botryoidal or globular Hematite09.jpg Grape-like, hemispherical masses Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite
Columnar WLA hmns selenite.jpg Similar to fibrous: Long, slender prisms often with parallel growth Calcite, Gypsum/Selenite
Coxcomb MarcassiteII.jpg Aggregated flaky or tabular crystals closely spaced. Barite, Marcasite
Cubic Selpologne.jpg Cube shape Pyrite, Galena, Halite
Dendritic or arborescent Pyrolusite dendritic.jpg Tree-like, branching in one or more direction from central point Pyrolusite and other Mn-oxide minerals, Magnesite, native copper
Dodecahedral GarnetCrystalUSGOV.jpg Dodecahedron, 12-sided Garnet
Drusy or encrustation Azurite-velvet-beauty.jpg Aggregate of minute crystals coating a surface or cavity Uvarovite, Malachite, Azurite
Enantiomorphic KarlsbadDoubleFrance.jpg Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals Quartz, Plagioclase, Staurolite
Equant, stout Peridot2.jpg Length, width, and breadth roughly equal Olivine, Garnet
Fibrous Byssolite France.jpg Extremely slender prisms Serpentine group, Tremolite (i.e. Asbestos)
Filiform or capillary Millerite-Calcite.jpg Hair-like or thread-like, extremely fine many Zeolites
Foliated or micaceous or lamellar (layered) Lepidolite-208658.jpg Layered structure, parting into thin sheets Mica (Muscovite, Biotite, etc.)
Granular Mineraly.sk - bornit.jpg Aggregates of anhedral crystals in matrix Bornite, Scheelite
Hemimorphic HemimorphiteMexique.jpg Doubly terminated crystal with two differently shaped ends. Hemimorphite, Elbaite
Hexagonal Hanksite.JPG Hexagon shape, six-sided Quartz, Hanksite
Hopper crystals Bismuth crystal macro.jpg Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity Halite, Calcite, synthetic Bismuth
Mamillary Malachite 2(République Démocratique du Congo).jpg Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates Malachite, Hematite
Massive or compact Turquoise with quartz.jpg Shapeless, no distinctive external crystal shape Limonite, Turquoise, Cinnabar, Realgar
Nodular or tuberose Chalcedony geode.JPG Deposit of roughly spherical form with irregular protuberances Chalcedony, various Geodes
Octahedral Rough diamond.jpg Octahedron, eight-sided (two pyramids base to base) Diamond, Magnetite
Plumose Aurichalcite-24456.jpg Fine, feather-like scales Aurichalcite, Boulangerite, Mottramite
Prismatic Tourmaline02.jpg Elongate, prism-like, semi-cylidrical: crystal faces parallel to c-axis well-developed Tourmaline, Beryl
Pseudo-hexagonal Aragonite - Enguidanos.jpg Hexagonal appearance due to cyclic twinning Aragonite, Chrysoberyl
Radiating or divergent Wavellit, Polska G. Świętokrzyskie.jpg Radiating outward from a central point Wavellite, Pyrite suns
Reniform or colloform Greenockite-259580.jpg Similar to botryoidal/mamillary: intersecting kidney-shaped masses Hematite, Pyrolusite, Greenockite
Reticulated Cerusitetsumeb.jpg Crystals forming net-like intergrowths Cerussite
Rosette or lenticular (lens shaped crystals) Roses des Sables Tunisie.jpg Platy, radiating rose-like aggregate Gypsum, Barite (i.e. Desert rose)
Sphenoid Titanit-Doppelender, other view - Ochtendung, Eifel, Germany.jpg Wedge-shaped Sphene
Stalactitic Labeled speleothems.jpg Forming as stalactites or stalagmites; cylindrical or cone-shaped Calcite, Goethite
Stellate Pyrophyllite-118706.jpg Star-like, radiating Pyrophyllite, Aragonite
Striated Tourmaline.xtal.750pix.jpg 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 Oligoclase-4jg47a.jpg More elongated than equant, slightly longer than wide, flat tablet shaped Feldspar, Topaz
Platy Wulfenite from Arizona.jpg Flat, tablet-shaped, prominent pinnacoid Wulfenite
Tetrahedral Tetrahedrite-Chalcopyrite-Sphalerite-251531.jpg Tetrahedra-shaped crystals Tetrahedrite, Spinel, Magnetite
Wheat sheaf Stilbite-20450.jpg Aggregates resembling hand-reaped wheat sheaves Stilbite

References


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