Crystal habit

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.[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-Pyrite-44341
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:

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)).

Pyrite sun
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.

List of crystal habits

Habit[3][4][5] Image Description Common Example(s)
Acicular Natroliteinde1 Needle-like, slender and/or tapered Natrolite, Rutile[6]
Amygdaloidal Large native copper amygdule (Mesoproterozoic, 1.05-1.06 Ga; Ahmeek Mine, Ahmeek, Upper Peninsula of Michigan, USA) 1 (17307955385) Like embedded almonds Heulandite, subhedral Zircon
Bladed Actinolite-247712 Blade-like, slender and flattened Actinolite, Kyanite
Botryoidal or globular Malachite-47496 Grape-like, hemispherical masses Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite
Columnar Gfp-Gypsum-v-selenite Similar to fibrous: Long, slender prisms often with parallel growth Calcite, Gypsum/Selenite
Coxcomb MarcassiteII Aggregated flaky or tabular crystals closely spaced. Barite, Marcasite
Cubic Fluorite and sphalerite J1 Cube shape Pyrite, Galena, Halite
Dendritic or arborescent Pyrolusite dendritic Tree-like, branching in one or more direction from central point Romanechite and other Mn-oxide minerals, magnesite, native copper
Dodecahedral GarnetCrystalUSGOV Rhombic dodecahedron, 12-sided Garnet
Drusy or encrustation Apophyllite-(KF)-Stilbite-Ca-Heulandite-Ca-226327 Aggregate of minute crystals coating a surface or cavity Uvarovite, Malachite, Azurite, Quartz
Enantiomorphic Gypsum-251119 Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals Quartz, Plagioclase, Staurolite
Equant, stout Natroapophyllite-Stilbite-Ca-pkn64c Length, width, and breadth roughly equal Olivine, Garnet
Fibrous Byssolite France Extremely slender prisms Serpentine group, Tremolite (i.e. Asbestos)
Filiform or capillary Millerite in geode (Hall's Gap, Kentucky, USA) Hair-like or thread-like, extremely fine many Zeolites
Foliated or micaceous or lamellar (layered) Lepidolite-208658 Layered structure, parting into thin sheets Mica (Muscovite, Biotite, etc.)
Granular Mineraly.sk - bornit Aggregates of anhedral crystals in matrix Bornite, Scheelite
Hemimorphic HemimorphiteMexique Doubly terminated crystal with two differently shaped ends Hemimorphite, Elbaite
Hexagonal Corundum-243453 Hexagon shape, six-sided Quartz, Hanksite
Hopper crystals Bismuth crystal macro Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity Halite, Calcite, synthetic Bismuth
Mammillary Malachite 2(République Démocratique du Congo) 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 Shapeless, no distinctive external crystal shape Limonite, Turquoise, Cinnabar, Realgar
Nodular or tuberose Quartz-282339 Deposit of roughly spherical form with irregular protuberances Chalcedony, various Geodes
Octahedral Rough diamond Octahedron, eight-sided (two pyramids base to base) Diamond, Magnetite
Platy Wulfenite-Mimetite-232971 Flat, tablet-shaped, prominent pinnacoid Wulfenite
Plumose Aurichalcite-24456 Fine, feather-like scales Aurichalcite, Boulangerite, Mottramite
Prismatic Tourmaline02 Elongate, prism-like: well-developed crystal faces parallel to the vertical axis Tourmaline, Beryl
Pseudo-hexagonal Aragonite - Enguidanos Hexagonal appearance due to cyclic twinning Aragonite, Chrysoberyl
Radiating or divergent Pyrite-200582 Radiating outward from a central point Wavellite, Pyrite suns
Reniform or colloform Mottramite-Cuprite-133541 Similar to botryoidal/mamillary: intersecting kidney-shaped masses Hematite, Pyrolusite, Greenockite
Reticulated Cerusitetsumeb Crystals forming net-like intergrowths Cerussite
Rosette or lenticular (lens shaped crystals) Roses des Sables Tunisie Platy, radiating rose-like aggregate Gypsum, Barite (i.e. Desert rose)
Sphenoid Titanite crystals on Amphibole - Ochtendung, Eifel, Germany Wedge-shaped Sphene
Stalactitic Malachite-mals03a Forming as stalactites or stalagmites; cylindrical or cone-shaped Calcite, Goethite, Malachite
Stellate Pyrophyllite-118706 Star-like, radiating Pyrophyllite, Aragonite
Striated Pyrite-43055 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 More elongated than equant, slightly longer than wide, flat tablet shaped Feldspar, Topaz
Tetrahedral Tetrahedrite-Chalcopyrite-Sphalerite-251531 Tetrahedra-shaped crystals Tetrahedrite, Spinel, Magnetite
Wheat sheaf Stilbite-20450 Aggregates resembling hand-reaped wheat sheaves Stilbite

See also

References

  1. ^ a b 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
  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.
Acicular (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.

The term "acicular" derives from the Late Latin "acicula" meaning "little needle". Strictly speaking, the word refers to a growth habit that is slender and tapering to a point. Prismatic crystals are not acicular; however, colloquial usage has altered the commonly understood meaning of the word. When writing for mineralogical publications, authors should restrict their usage of "acicular" to crystals with the tapering growth habit.

To add to the confusion, some minerals are described with various morphological terms. For example, natrolite is often described as slender prismatic and millerite is often described as filiform or capillary.

Anatase

Anatase is a mineral form of titanium dioxide (TiO2). The mineral is almost always encountered as a black solid, although the pure material is colorless or white. Two other mineral forms of TiO2 are known, brookite and rutile.

Anatase is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but, although the degree of symmetry is the same for both, there is no relation between the interfacial angles of the two minerals, except in the prism-zone of 45° and 90°. The common pyramid of anatase, parallel to the faces of which there are perfect cleavages, has an angle over the polar edge of 82°9', the corresponding angle of rutile being 56°52½'. It was on account of this steeper pyramid of anatase that the mineral was named, by René Just Haüy in 1801, from the Greek anatasis, "extension", the vertical axis of the crystals being longer than in rutile. There are also important differences between the physical characters of anatase and rutile: the former is less hard (5.5–6 vs. 6–6.5 Mohs) and dense (specific gravity about 3.9 vs. 4.2). Also, anatase is optically negative whereas rutile is positive, and its luster is even more strongly adamantine or metallic-adamantine than that of rutile.

Arsenopyrite

Arsenopyrite is an iron arsenic sulfide (FeAsS). It is a hard (Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it produces poisonous sulfur and arsenic fumes which can be fatal if inhaled in large quantities. With 46% arsenic content, arsenopyrite, along with orpiment, is a principal ore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, the mineral will slowly oxidize, converting the arsenopyrite into an iron arsenate, a relatively stable compound. Arsenopyrite is generally an acid consuming sulfide mineral unlike iron pyrite which can lead to acid mine drainage.

The crystal habit, hardness, density, and garlic odor when struck are diagnostic. Arsenopyrite in older literature may be referred to as mispickel, a name of German origin.Arsenopyrite also can be associated with significant amounts of gold. Consequently, it serves as an indicator of gold bearing reefs. Many arsenopyrite gold ores are refractory, i.e. the gold is not easily cyanide leached from the mineral matrix.

Arsenopyrite is found in high temperature hydrothermal veins, in pegmatites, and in areas of contact metamorphism or metasomatism.

Beryl

Beryl ( BERR-əl) is a mineral composed of beryllium aluminium cyclosilicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring, hexagonal crystals of beryl can be up to several meters in size, but terminated crystals are relatively rare. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, red (the rarest), and white. Beryl is also an ore source of beryllium.

Chalcophyllite

Chalcophyllite is a rare secondary copper arsenate mineral occurring in the oxidized zones of some arsenic-bearing copper deposits. It was first described from material collected in Germany. At one time chalcophyllite from Wheal Tamar in Cornwall, England, was called tamarite, but this name is now discredited (not to be confused with the amphibole mineral taramite, which is quite different). At Wheal Gorland a specimen exhibiting partial replacement of liriconite, Cu2Al(AsO4)(OH)4·4(H2O), by chalcophyllite has been found. The mineral is named from the Greek, chalco "copper" and fyllon, "leaf", in allusion to its composition and platy structure. It is a classic Cornish mineral that can be confused with tabular spangolite.

Cummingtonite

Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2, magnesium iron silicate hydroxide.

Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being metastable.

Cummingtonite shares few compositional similarities with alkali amphiboles such as arfvedsonite, glaucophane-riebeckite. There is little solubility between these minerals due to different crystal habit and inability of substitution between alkali elements and ferro-magnesian elements within the amphibole structure.

Demesmaekerite

Demesmaekerite is a rare uranium selenite mineral with the chemical formula: Pb2Cu5(UO2)2(SeO3)6(OH)6·2H2O.

It is named after the Belgian mineralogist Gaston Demesmaeker, who worked at the Musonoi Mine in Katanga. It is a secondary mineral which contains lead, copper and selenium, it is a bottle green to brown/yellow color, its crystal habit varies depending on where it is found.

It can be found associated with other rare selenium-bearing uranium ores, such as haynesite, guilleminite, marthozite and piretite.

Desert rose (crystal)

Desert rose is the colloquial name given to rose-like formations of crystal clusters of gypsum or baryte which include abundant sand grains. The 'petals' are crystals flattened on the c crystallographic axis, fanning open in radiating flattened crystal clusters.

The rosette crystal habit tends to occur when the crystals form in arid sandy conditions, such as the evaporation of a shallow salt basin. The crystals form a circular array of flat plates, giving the rock a shape similar to a rose blossom. Gypsum roses usually have better defined, sharper edges than baryte roses. Celestine and other bladed evaporite minerals may also form rosette clusters. They can appear either as a single rose-like bloom or as clusters of blooms, with most sizes ranging from pea sized to 4 inches (10 cm) in diameter.

The ambient sand that is incorporated into the crystal structure, or otherwise encrusts the crystals, varies with the local environment. If iron oxides are present, the rosettes take on a rusty tone.

The desert rose may also be known by the names: sand rose, rose rock, selenite rose, gypsum rose and baryte (barite) rose.

Edenite

Edenite is a double chain silicate mineral of the amphibole group with the general chemical composition NaCa2Mg5(Si7Al)O22(OH)2. Edenite is named for the locality of Edenville, Orange County, New York, where it was first described.

Eosphorite

Eosphorite is a brown (occasionally pink) manganese hydrous phosphate mineral with chemical formula: MnAl(PO4)(OH)2·H2O. It is used as a gemstone.Eosphorite crystallizes in the monoclinic crystal system. It forms slender prismatic crystals which often form radiating or spherical clusters. The crystals often show pseudo–orthorhombic forms due to twinning.Eosphorite forms a series with childrenite, the iron rich member, with divalent iron replacing most of the manganese in the crystal lattice. The two endmembers are isostructural but differ in their properties, such as crystal habit, coloration, and optical properties.

It was first described in 1878 for an occurrence in the Branchville Mica Mine in Branchville, Fairfield County, Connecticut, US. Its name is derived from the Greek έωσφορος for "dawn-bearing," because of its pink color. It occurs worldwide typically as a secondary mineral in phosphate rich granitic pegmatites in association with rhodochrosite, lithiophilite, triphylite, triploidite, dickinsonite, albite, cookeite, apatite, beryllonite, hydroxyl-herderite, and tourmaline. An attractive combination of eosphorite and rose quartz occurs at Taquaral, Minas Gerais, Brazil.

Eudialyte

Eudialyte, whose name derives from the Greek phrase Εὖ διάλυτος eu dialytos, meaning "well decomposable", is a somewhat rare, nine member ring cyclosilicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites. Its name alludes to its ready solubility in acid.Eudialyte was first described in 1819 for an occurrence in nepheline syenite of the Ilimaussaq intrusive complex of southwest Greenland.

Gonnardite

Gonnardite is a comparatively rare, fibrous zeolite, natrolite subgroup. Older papers claim that a complete solid solution exists between tetranatrolite and gonnardite, but tetranatrolite was discredited as a separate species in 1999. A series, based on the disorder of the silicon-aluminum in the framework, appears to exist between Na-rich gonnardite and natrolite, Na2(Si3Al2)O10·2H2O.Gonnardite was named in 1896 after Ferdinand Pierre Joseph Gonnard (1833–1923), who was Professor of Mining Engineering at the University of Lyon, France.

Guilleminite

Guilleminite (Ba(UO2)3(SeO3)2(OH)4•3H2O) is a uranium mineral named by R. Pierrot, J. Toussaint, and T. Verbeek in 1965 in honor of Jean Claude Guillemin (1923–1994), a chemist and mineralogist. It is a rare uranium/selenium mineral found at the Musonoi Mine in the Katanga Province of the Democratic Republic of the Congo.This secondary mineral also includes barium in its structure, in addition to selenium and uranium. It is bright yellow in colour and usually has an acicular crystal habit. It has a Mohs hardness of 2–3.

Hellyerite

Hellyerite, NiCO3·6(H2O), is an hydrated nickel carbonate mineral. It is light blue to bright green in colour, has a hardness of 2.5, a vitreous luster, a white streak and crystallises in the monoclinic system. The crystal habit is as platy and mammillary encrustations on its matrix.

Marrite

Marrite (mar'-ite) is depicted by the chemical formula PbAgAsS3. It is the arsenic equivalent of mineral freieslebenite (PbAgSbS3), but also displays close polyhedral characteristics with sicherite and diaphorite. Marrite was named in honor of geologist John Edward Marr (1857–1933) of Cambridge, England.

Millerite

Millerite is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates. It can be distinguished from pentlandite by crystal habit, its duller colour, and general lack of association with pyrite or pyrrhotite.

Quartz

Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K). Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature threshold.

There are many different varieties of quartz, several of which are semi-precious gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia.

Selenite (mineral)

Selenite, also known as satin spar, desert rose, or gypsum flower are four crystal structure varieties of the mineral gypsum. These four varieties of gypsum may be grouped together and called selenite.

All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no significant selenium, the similarity of the names of the substances coming from the Ancient Greek word for the Moon.

Some of the largest crystals ever found are of selenite, the largest specimen found in the Naica Mine's Cave of the Crystals being 12 metres long and weighing 55 tons.

Zimbabweite

Zimbabweite is a mineral; formula (Na,K)2PbAs4(Nb,Ta,Ti)4O18. It is generally classed as an arsenite but is notable for also containing niobium and tantalum. A yellow brown mineral with orthorhombic crystal habit and a hardness of 5. It was discovered in 1986 in kaolinized pegmatite, i.e. weathered to clay, in Zimbabwe.

Mineral identification

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