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.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 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.
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 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 Pseudomorph. 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:
List of crystal habits
Acicular Natrolite Needle-like, slender and/or tapered natrolite, rutile
Amygdule Native copper Like embedded heulandite, subhedral zircon Bladed Actinolite Blade-like, slender and flattened actinolite, kyanite Botryoidal or globular Malachite Grape-like, hemispherical masses hematite, pyrite, malachite, smithsonite, hemimorphite Columnar Selenite (gypsum) Similar to fibrous: Long, slender prisms often with parallel growth calcite, gypsum/selenite Coxcomb Marcasite Aggregated flaky or tabular crystals closely spaced. barite, marcasite Cubic Fluorite Cube shape pyrite, galena, halite Dendritic or arborescent Pyrolusite Tree-like, branching in one or more direction from central point romanechite, magnesite, native copper Dodecahedral Garnet Rhombic dodecahedron, 12-sided garnet Drusy or encrustation Quartz Aggregate of minute crystals coating a surface or cavity uvarovite, malachite, azurite Enantiomorphic Gypsum Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals quartz, plagioclase, staurolite Equant, stout Apophyllite Length, width, and breadth roughly equal olivine, garnet Fibrous Byssolite Extremely slender prisms serpentine group, tremolite (i.e. asbestos) Filiform or capillary Millerite Hair-like or thread-like, extremely fine many Foliated or micaceous or lamellar (layered) Lepidolite Layered structure, parting into thin sheets muscovite, biotite Granular Bornite Aggregates of anhedral crystals in matrix bornite, scheelite Hemimorphic Hemimorphite Doubly terminated crystal with two differently shaped ends hemimorphite, elbaite Hexagonal Corundum Hexagon shape, six-sided quartz, hanksite Hopper crystals Halite Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity halite, calcite, synthetic bismuth Mammillary Malachite Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates malachite, hematite Massive or compact Turquoise Shapeless, no distinctive external crystal shape limonite, turquoise, cinnabar, realgar Nodular or tuberose Chalcedony Deposit of roughly spherical form with irregular protuberances chalcedony Octahedral Diamond Octahedron, eight-sided (two pyramids base to base) diamond, magnetite Platy Wulfenite Flat, tablet-shaped, prominent pinnacoid wulfenite Plumose Aurichalcite Fine, feather-like scales aurichalcite, boulangerite, mottramite Prismatic Tourmaline Elongate, prism-like: well-developed crystal faces parallel to the vertical axis tourmaline, beryl Pseudo-hexagonal Aragonite Hexagonal appearance due to cyclic twinning aragonite, chrysoberyl Radiating or radial or divergent Stibnite Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths) stibnite Reniform or colloform Mottramite Similar to botryoidal/mamillary: intersecting kidney-shaped masses hematite, pyrolusite, greenockite Reticulated Cerussite Crystals forming net-like intergrowths cerussite Rosette or lenticular (lens shaped crystals) Desert rose (barite) Platy, radiating rose-like aggregate gypsum, barite (i.e. desert rose) Sphenoid Titanite Wedge-shaped sphene Stalactitic Malachite Forming as stalactites or stalagmites; cylindrical or cone-shaped calcite, goethite, malachite Stellate Pyrophyllite Star-like, radial aggregates radiating from a "star"-like point to produce gross spheres (crystals are not or weakly separated and have similar lengths) pyrophyllite, aragonite, wavellite, pyrite suns Striated Pyrite 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 Tabular (also stubby or blocky) Oligoclase More elongated than equant, slightly longer than wide, flat tablet-shaped feldspar, topaz Tetrahedral Tetrahedrite Tetrahedra-shaped crystals tetrahedrite, spinel, magnetite Wheat sheaf Stilbite Aggregates resembling hand-reaped wheat sheaves stilbite
See also
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