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Crystal habit

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In , crystal habit is the characteristic external shape of an individual 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 . When the faces are well-developed due to uncrowded growth a crystal is called , 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 insulation often grow in a fibrous habit, a mass of very fine fibers.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, Wenk, Hans-Rudolph and Andrei Bulakh, 2004, Minerals: Their Constitution and Origin, Cambridge, first edition,

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 . 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. Factors influencing habit include: a combination of two or more crystal forms; trace impurities present during growth; 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 form elongate barrel-shaped crystals, those found in form stout tabular crystals. Ordinarily, the latter habit is seen only in . Sapphire and ruby are both varieties of the same mineral: .

Some minerals may replace other existing minerals while preserving the original's habit: this process is called . A classic example is tiger's eye , replaced by . 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
AcicularNeedle-like, slender and/or taperednatrolite,

Like embedded , subhedral
BladedBlade-like, slender and flattenedactinolite,
or globularGrape-like, hemispherical masses, , malachite, ,
ColumnarSelenite ()Similar to fibrous: Long, slender prisms often with parallel growth, gypsum/selenite
CoxcombAggregated flaky or tabular crystals closely spaced., marcasite
Cubic shape, ,
Dendritic or arborescentTree-like, branching in one or more direction from central point, , native
DodecahedralRhombic dodecahedron, 12-sidedgarnet
Drusy or encrustationAggregate of minute crystals coating a surface or cavity, ,
EnantiomorphicMirror-image habit (i.e. ) and optical characteristics; right- and left-handed crystalsquartz, ,
Equant, stoutLength, width, and breadth roughly equal,
FibrousExtremely slender prisms, (i.e. )
Filiform or capillaryHair-like or thread-like, extremely finemany
Foliated or micaceous or lamellar (layered)Layered structure, parting into thin sheets,
GranularAggregates of anhedral crystals in matrixbornite,
HemimorphicDoubly terminated crystal with two differently shaped endshemimorphite,
Hexagonal shape, six-sided,
Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavityhalite, , synthetic
MammillaryBreast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregatesmalachite,
Massive or compactShapeless, no distinctive external crystal shape, turquoise, ,
Nodular or tuberoseDeposit of roughly spherical form with irregular protuberanceschalcedony
Octahedral, eight-sided (two pyramids base to base)diamond,
PlatyFlat, tablet-shaped, prominent pinnacoidwulfenite
PlumoseFine, feather-like scalesaurichalcite, ,
PrismaticElongate, prism-like: well-developed crystal faces parallel to the vertical axistourmaline,
Pseudo-hexagonalHexagonal appearance due to cyclic twinningaragonite,
Radiating or radial or divergentRadiating outward from a central point without producing a star (crystals are generally separated and have different lengths)stibnite
Reniform or colloformSimilar to botryoidal/mamillary: intersecting kidney-shaped masses, ,
ReticulatedCrystals forming net-like intergrowthscerussite
Rosette or lenticular (lens shaped crystals)Desert rose ()Platy, radiating rose-like aggregate, barite (i.e. desert rose)
StalactiticForming as stalactites or stalagmites; cylindrical or cone-shaped, , malachite
StellateStar-like, radial aggregates radiating from a "star"-like point to produce gross spheres (crystals are not or weakly separated and have similar lengths)pyrophyllite, , , suns
StriatedNot a habit per se, but a condition of lines that can grow on certain crystal faces on certain minerals, pyrite, , ,
Tabular (also stubby or blocky)More elongated than equant, slightly longer than wide, flat tablet-shaped,
Tetrahedral-shaped crystalstetrahedrite, ,
Wheat sheafAggregates resembling hand-reaped wheat sheavesstilbite

See also

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