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).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,
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. Most natural crystals, however, do not display ideal habits and are commonly malformed. The quality of the shape of a crystal in a mineral specimen can be described as one of the following:
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 Crystal Habit
Aggregate habits HabitImageDescriptionCommon example(s)AcicularNeedle-like, slender, and end-tapered prisms growing in a radial/globular fashion.natrolite, scolecite, yuanfuliiteArborescentTree-like crystals growing similar to branches.Native copper, gold, native silverCapillary/Filiform Hair-like or thread-like, extremely fine byssolite, milleriteColloform/Nodular/TuberoseRounded, finely banded deposits with irregular concentric protuberances agate, baryte, sphaleriteConcentricCircular 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 othersagate, quartz, malachite, rhodocrositeDendriticRoot-like, branching in one or more direction from central point Native copper, gold, romanechite, magnesite, native silverDruse/EncrustationAggregate of crystals coating a surface or cavity, usually found in geodes and some fossilsazurite, celestine, calcite, uvarovite, malachite, quartzFibrous/AsbestiformExtremely slender prisms forming muscle-like fibersactinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine groupFoliated/Micaceous/LamellarLayered crystal planes, parting into thin sheetsbiotite, hematite, muscovite, lepidolite, molybdeniteGranularAggregates of diminute anhedral crystals in matrix or other surfaceandradite, bornite, scheelite, quartz, uvaroviteHopperOuter portions of cubes grow faster than inner portions, creating a concavity similar to that of a hopperbismuth (artificial), halite, galenaOolithicSmall spheres or grains (commonly flattened) that resemble eggsaragonite, calcitePisoliticRounded concentric nodules often found in sedimentary rocks. Much larger than oolithic aragonite, bauxite, calcite, pisolitePlaty/Tabular/BlockyFlat, tablet-shaped, prominent pinnacoidbaryte, feldspar, topaz, vanadinite, wulfenitePlumoseFine, feather-like scalesaurichalcite, okenite, mottramiteRadial/Radiating/DivergentRadiating outward from a central point without producing a star (crystals are generally separated and have different lengths).aenigmatite, atacamite, epidote, pyrophyllite, stibniteReticulatedCrystals forming triangular net-like intergrowths.cerussite, rutileRosette/LenticularPlaty, radiating rose-like aggregate (also lens shaped crystals)gypsum, baryte, calciteStalactiticForming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" patterncalcite, chalcedony, chrysocolla, goethite, malachite, romanechiteStellateStar-like, radial fibers found inside spherical habits, such as mamillary or reniform. hematite, pectolite, shattuckite, wavellite
Asymmetrical/Irregular habits HabitImageDescriptionCommon example(s)AmygdaloidalLike embedded almondsheulandite, stilbite, zirconHemimorphicDoubly terminated crystal with two differently shaped endselbaite, hemimorphite, olivineMassive/CompactShapeless, no distinctive external crystal shapelimonite, turquoise, cinnabar, quartz, realgar, lazuriteScepteredCrystal 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 HabitImageDescriptionCommon example(s)CubicCube-shapedfluorite, pyrite, galena, haliteDodecahedralDodecahedron-shaped, 12-sided. Central facet can vary.garnet, pyriteEnantiomorphicMirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystalsaragonite, gypsum, quartz, plagioclase, stauroliteHexagonalHexagonal prism (six-sided)beryl, galena, quartz, hanksite, vanadiniteIcositetrahedralIcositetrahedron-shaped, 24-facedanalcime, spessartineOctahedralOctahedron-shaped, square bipyramid (eight-sided)diamond, fluorine, fluorite, magnetite, pyritePrismaticElongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axisberyl, tourmaline, vanadiniteRhombohedralRhombohedron-shaped (six-faced rhombi)calcite, magnesite, rhodochrosite, sideriteScalenohedralScalenohedron-shaped, pointy endscalcite, rhodochrosite, titaniteTetrahedralTetrahedron-shaped, triangular pyramid (four-sided)chalcopyrite, tetrahedrite, sphalerite, magnetite
Rounded/Spherical habits HabitImageDescriptionCommon example(s)BotryoidalGrape-like, large and small hemispherical masses, nearly differentiated/separated from each othercalcite, chalcedony, halite, plumbogummite, smithsoniteGlobularIsolated hemispheres or spherescalcite, fluorite, gyroliteMammillaryBreast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates.chalcedony, hematite, malachiteReniformIrregular kidney-shaped spherical massescassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite
See also Abnormal grain growth Grain growth Crystal structure
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