Crystal habit

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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. 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 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.

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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. [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]

Goethite replacing pyrite cubes Goethite-Pyrite-44341.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:[ citation needed ]

List of crystal habits

Habit [3] [4] [5] ImageDescriptionCommon example(s)
Acicular Natroliteinde1.jpg Natrolite Needle-like, slender and/or taperednatrolite, rutile [6]
Amygdaloidal Large native copper amygdule (Mesoproterozoic, 1.05-1.06 Ga; Ahmeek Mine, Ahmeek, Upper Peninsula of Michigan, USA) 1 (17307955385).jpg Native copper Like embedded almonds heulandite, subhedral zircon
Bladed
Actinolite-247712.jpg
Actinolite
Blade-like, slender and flattenedactinolite, kyanite
Botryoidal or globular
Malachite-47496.jpg
Malachite
Grape-like, hemispherical masses hematite, pyrite, malachite, smithsonite, hemimorphite
Columnar
Gfp-Gypsum-v-selenite.jpg
Selenite (gypsum)
Similar to fibrous: Long, slender prisms often with parallel growth calcite, gypsum/selenite
Coxcomb
MarcassiteII.jpg
Marcasite
Aggregated flaky or tabular crystals closely spaced. barite, marcasite
Cubic
Fluorite and sphalerite J1.jpg
Fluorite
Cube shape pyrite, galena, halite
Dendritic or arborescent
Pyrolusite dendritic.jpg
Pyrolusite
Tree-like, branching in one or more direction from central point romanechite, magnesite, native copper
Dodecahedral
GarnetCrystalUSGOV.jpg
Garnet
Rhombic dodecahedron, 12-sidedgarnet
Drusy or encrustation
Apophyllite-(KF)-Stilbite-Ca-Heulandite-Ca-226327.jpg
Quartz
Aggregate of minute crystals coating a surface or cavity uvarovite, malachite, azurite
Enantiomorphic
Gypsum-251119.jpg
Gypsum
Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystalsquartz, plagioclase, staurolite
Equant, stout
Natroapophyllite-Stilbite-Ca-pkn64c.jpg
Apophyllite
Length, width, and breadth roughly equal olivine, garnet
Fibrous
Byssolite France.jpg
Byssolite
Extremely slender prisms serpentine group, tremolite (i.e. asbestos)
Filiform or capillary
Millerite in geode (Hall's Gap, Kentucky, USA).jpg
Millerite
Hair-like or thread-like, extremely finemany zeolites
Foliated or micaceous or lamellar (layered)
Lepidolite-208658.jpg
Lepidolite
Layered structure, parting into thin sheets muscovite, biotite
Granular
Mineraly.sk - bornit.jpg
Bornite
Aggregates of anhedral crystals in matrix bornite, scheelite
Hemimorphic
HemimorphiteMexique.jpg
Hemimorphite
Doubly terminated crystal with two differently shaped endshemimorphite, elbaite
Hexagonal
Corundum-243453.jpg
Corundum
Hexagon shape, six-sided quartz, hanksite
Hopper crystals
Halite 1.jpg
Halite
Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavityhalite, calcite, synthetic bismuth
Mammillary
Malachite 2(Republique Democratique du Congo).jpg
Malachite
Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregatesmalachite, hematite
Massive or compact
Turquoise with quartz.jpg
Turquoise
Shapeless, no distinctive external crystal shape limonite, turquoise, cinnabar, realgar
Nodular or tuberose
Quartz-282339.jpg
Chalcedony
Deposit of roughly spherical form with irregular protuberanceschalcedony
Octahedral
Rough diamond.jpg
Diamond
Octahedron, eight-sided (two pyramids base to base)diamond, magnetite
Platy
Wulfenite-Mimetite-232971.jpg
Wulfenite
Flat, tablet-shaped, prominent pinnacoidwulfenite
Plumose
Aurichalcite-24456.jpg
Aurichalcite
Fine, feather-like scalesaurichalcite, boulangerite, mottramite
Prismatic
Tourmaline02.jpg
Tourmaline
Elongate, prism-like: well-developed crystal faces parallel to the vertical axistourmaline, beryl
Pseudo-hexagonal
Aragonite - Enguidanos.jpg
Aragonite
Hexagonal appearance due to cyclic twinningaragonite, chrysoberyl
Radiating or radial or divergent
Barite-Stibnite-tmu27a.jpg
Stibnite
Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths)stibnite
Reniform or colloform
Mottramite-Cuprite-133541.jpg
Mottramite
Similar to botryoidal/mamillary: intersecting kidney-shaped masses hematite, pyrolusite, greenockite
Reticulated
Cerussite - Nakhlak mine, Anarak, Esfahan, Iran.jpg
Cerussite
Crystals forming net-like intergrowthscerussite
Rosette or lenticular (lens shaped crystals)
Roses des Sables Tunisie.jpg
Desert rose (barite)
Platy, radiating rose-like aggregate gypsum, barite (i.e. desert rose)
Sphenoid
Titanite crystals on Amphibole - Ochtendung, Eifel, Germany.jpg
Titanite
Wedge-shaped sphene
Stalactitic
Malachite-mals03a.jpg
Malachite
Forming as stalactites or stalagmites; cylindrical or cone-shaped calcite, goethite, malachite
Stellate
Pyrophyllite-118706.jpg
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-43055.jpg
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-4jg47a.jpg
Oligoclase
More elongated than equant, slightly longer than wide, flat tablet-shaped feldspar, topaz
Tetrahedral
Tetrahedrite-Chalcopyrite-Sphalerite-251531.jpg
Tetrahedrite
Tetrahedra-shaped crystalstetrahedrite, spinel, magnetite
Wheat sheaf
Stilbite-20450.jpg
Stilbite
Aggregates resembling hand-reaped wheat sheaves stilbite

See also

Related Research Articles

Mineral Element or chemical compound that is normally crystalline, formed as a result of geological processes

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.

Rutile

Rutile is a mineral composed primarily of titanium dioxide (TiO2), and is the most common natural form of TiO2. Other rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.

Amphibole

Amphibole is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO
4
tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles can be green, black, colorless, white, yellow, blue, or brown. The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.

Strontianite

Strontianite (SrCO3) is an important raw material for the extraction of strontium. It is a rare carbonate mineral and one of only a few strontium minerals. It is a member of the aragonite group.

Selenite (mineral) Mineral variety of gypsum

Selenite, satin spar, desert rose, gypsum flower are crystal habit varieties of the mineral gypsum.

Epidote Sorosilicate mineral

Epidote is a calcium aluminium iron sorosilicate mineral.

Phenakite

Phenakite or phenacite is a fairly rare nesosilicate mineral consisting of beryllium orthosilicate, Be2SiO4. Occasionally used as a gemstone, phenakite occurs as isolated crystals, which are rhombohedral with parallel-faced hemihedrism, and are either lenticular or prismatic in habit: the lenticular habit is determined by the development of faces of several obtuse rhombohedra and the absence of prism faces. There is no cleavage, and the fracture is conchoidal. The Mohs hardness is high, being 7.5 – 8; the specific gravity is 2.96. The crystals are sometimes perfectly colorless and transparent, but more often they are greyish or yellowish and only translucent; occasionally they are pale rose-red. In general appearance the mineral is not unlike quartz, for which indeed it has been mistaken. Its name comes from Ancient Greek: φέναξ, romanized: phénax, meaning "deceiver" due to its close visual similarity to quartz, named by Nils Gustaf Nordenskiöld in 1833.

Natrolite Zeolite mineral

Natrolite is a tectosilicate mineral species belonging to the zeolite group. It is a hydrated sodium and aluminium silicate with the formula Na2Al2Si3O10 • 2H2O. The type locality is Hohentwiel, Hegau, Germany.

Riebeckite A sodium-rich member of the amphibole group of silicate minerals

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na2(Fe2+3Fe3+2)Si8O22(OH)2. It forms a solid solution series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.

Arfvedsonite

Arfvedsonite is a sodium amphibole mineral with composition: [Na][Na2][(Fe2+)4Fe3+][(OH)2|Si8O22]. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms.

Olivenite

Olivenite is a copper arsenate mineral, formula Cu2AsO4OH. It crystallizes in the monoclinic system (pseudo-orthorhombic), and is sometimes found in small brilliant crystals of simple prismatic habit terminated by domal faces. More commonly, it occurs as globular aggregates of acicular crystals, these fibrous forms often having a velvety luster; sometimes it is lamellar in structure, or soft and earthy.

Todorokite

Todorokite is a rare complex hydrous manganese oxide mineral with the formula (Na,Ca,K,Ba,Sr)1-x(Mn,Mg,Al)6O12·3-4H2O. It was named in 1934 for the type locality, the Todoroki mine, Hokkaido, Japan. It belongs to the prismatic class 2/m of the monoclinic crystal system, but the angle β between the a and c axes is close to 90°, making it seem orthorhombic. It is a brown to black mineral which occurs in massive or tuberose forms. It is quite soft with a Mohs hardness of 1.5, and a specific gravity of 3.49 - 3.82. It is a component of deep ocean basin manganese nodules.

Asbestiform is a crystal habit. It describes a mineral that grows in a fibrous aggregate of high tensile strength, flexible, long, and thin crystals that readily separate. The most common asbestiform mineral is chrysotile, commonly called "white asbestos", a magnesium phyllosilicate part of the serpentine group. Other asbestiform minerals include riebeckite, an amphibole whose fibrous form is known as crocidolite or "blue asbestos", and brown asbestos, a cummingtonite-grunerite solid solution series.

Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

Prism (geology)

In sedimentology, a prism is a long, narrow, wedge-shaped sedimentary body. These types of sediments are typically formed during orogenic deformation; for example, the arkose detrital sedimentary rock found in fault troughs.

Acicular (crystal habit) Crystal habit

Acicular, in mineralogy, refers to a crystal habit composed of slender, needle-like crystals. Crystals with this habit tend to be fragile. Complete, undamaged acicular specimens are uncommon.

Simpsonite

Simpsonite has a general formula of Al4(Ta,Nb)3O13(OH). It occurs as euhedral to subhedral tabular to short and prismatic crystals, commonly in subparallel groups. Under the petrographic microscope it has a very high relief.

Euhedral and anhedral

Euhedral crystals are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral : a rock with an anhedral texture is composed of mineral grains that have no well-formed crystal faces or cross-section shape in thin section. Anhedral crystal growth occurs in a competitive environment with no free space for the formation of crystal faces. An intermediate texture with some crystal face-formation is termed subhedral.

Mottramite

Mottramite is an orthorhombic anhydrous vanadate hydroxide mineral, PbCu(VO4)(OH), at the copper end of the descloizite subgroup. It was formerly called cuprodescloizite or psittacinite (this mineral characterized in 1868 by Frederick Augustus Genth). Duhamelite is a calcium- and bismuth-bearing variety of mottramite, typically with acicular habit.

Mammothite

Mammothite is a mineral found in the Mammoth mine in Tiger, Arizona and also in Laurium, Attika, Greece. This mineral was named in 1985 by Donald R. Peacor, Pete J. Dunn, G. Schnorrer-Köhler, and Richard A. Bideaux, for the Mammoth vein (one of the two main veins in the mine) and the town of Mammoth, Arizona, which was named for the mine. The mammothite that is found in Arizona exist as euhedral crystals imbedded in micro granular, white colored anglesite with a sacharoidal texture. The associated minerals include phosgenite, wulfenite, leadhillite and caledonite. In Greece, the mammothite exists as small euhedral crystals and also as microscopic rock cavities lined with projecting crystals within the slags. The associated minerals here are cerussite,phosgenite and matlockite. The ideal chemical formula for mammothite is Pb6Cu4AlSb5+O2(OH)16Cl4(SO4)2.

References

  1. 1 2 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". Archived from the original on 2017-07-07. Retrieved 2009-04-06.
  4. Crystal Habit Archived 2009-04-12 at the Wayback Machine
  5. Habit
  6. Hanaor, D.A.H; Xu, W; Ferry, M; Sorrell, CC (2012). "Abnormal grain growth of rutile TiO2 induced by ZrSiO4". Journal of Crystal Growth. 359: 83–91. arXiv: 1303.2761 . Bibcode:2012JCrGr.359...83H. doi:10.1016/j.jcrysgro.2012.08.015. S2CID   94096447.