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.
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical properties of minerals and mineralized artifacts. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.
A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.
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.
Asbestos is a set of six naturally occurring silicate minerals, which all have in common their asbestiform habit: i.e., long, thin fibrous crystals, with each visible fiber composed of millions of microscopic "fibrils" that can be released by abrasion and other processes. The minerals are chrysotile, amosite, crocidolite, tremolite, anthophyllite, and actinolite.
Asbestos insulating board, also known as "AIB" or by the trade name "Asbestolux", was an asbestos containing board used in the building trade because of its excellent fire resistance and heat insulating properties. These boards were commonly used in the UK from the 1930s up until production was terminated in 1980. AIB tended to contain 25-40% asbestos, with amosite being the most common form of asbestos used, although a mixture of amosite and chrysotile was also common, and crocidolite was sometimes used in older boards and some marine boards.
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.
Crystallization is the process by which a solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some of the ways by which crystals form are precipitating from a solution, freezing, or more rarely deposition directly from a gas. Attributes of the resulting crystal depend largely on factors such as temperature, air pressure, and in the case of liquid crystals, time of fluid evaporation.
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.
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. The surface along which the lattice points are shared in twinned crystals is called a composition surface or twin plane.
In structural geology, striations are linear furrows, or linear marks, generated from fault movement. The striation's direction reveal the movement directions in the fault plane.
Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide (α-Al2O3) with trace amounts of elements such as iron, titanium, chromium, copper, or magnesium. It is typically blue, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. The only color corundum stone that the term sapphire is not used for is red, which is called a ruby. Pink colored corundum may be either classified as ruby or sapphire depending on locale. Commonly, natural sapphires are cut and polished into gemstones and worn in jewelry. They also may be created synthetically in laboratories for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires – 9 on the Mohs scale (the third hardest mineral, after diamond at 10 and moissanite at 9.5) – sapphires are also used in some non-ornamental applications, such as infrared optical components, high-durability windows, wristwatch crystals and movement bearings, and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (especially integrated circuits and GaN-based LEDs).
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.
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form, resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another. The name literally means "false form".
Tiger's eye is a chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre. As members of the quartz group, tiger's eye and the related blue-coloured mineral hawk's eye gain their silky, lustrous appearance from the parallel intergrowth of quartz crystals and altered amphibole fibres that have mostly turned into limonite.
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.
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)).
|Acicular||Needle-like, slender and/or tapered||Natrolite, Rutile|
|Amygdaloidal||Like embedded almonds||Heulandite, subhedral Zircon|
|Bladed||Blade-like, slender and flattened||Actinolite, Kyanite|
|Botryoidal or globular||Grape-like, hemispherical masses||Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite|
|Columnar||Similar to fibrous: Long, slender prisms often with parallel growth||Calcite, Gypsum/Selenite|
|Coxcomb||Aggregated flaky or tabular crystals closely spaced.||Barite, Marcasite|
|Cubic||Cube shape||Pyrite, Galena, Halite|
|Dendritic or arborescent||Tree-like, branching in one or more direction from central point||Romanechite and other Mn-oxide minerals, magnesite, native copper|
|Dodecahedral||Rhombic dodecahedron, 12-sided||Garnet|
|Drusy or encrustation||Aggregate of minute crystals coating a surface or cavity||Uvarovite, Malachite, Azurite, Quartz|
|Enantiomorphic||Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals||Quartz, Plagioclase, Staurolite|
|Equant, stout||Length, width, and breadth roughly equal||Olivine, Garnet|
|Fibrous||Extremely slender prisms||Serpentine group, Tremolite (i.e. Asbestos)|
|Filiform or capillary||Hair-like or thread-like, extremely fine||many Zeolites|
|Foliated or micaceous or lamellar (layered)||Layered structure, parting into thin sheets||Mica (Muscovite, Biotite, etc.)|
|Granular||Aggregates of anhedral crystals in matrix||Bornite, Scheelite|
|Hemimorphic||Doubly terminated crystal with two differently shaped ends||Hemimorphite, Elbaite|
|Hexagonal||Hexagon shape, six-sided||Quartz, Hanksite|
|Hopper crystals||Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity||Halite, Calcite, synthetic Bismuth|
|Mammillary||Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates||Malachite, Hematite|
|Massive or compact||Shapeless, no distinctive external crystal shape||Limonite, Turquoise, Cinnabar, Realgar|
|Nodular or tuberose||Deposit of roughly spherical form with irregular protuberances||Chalcedony, various Geodes|
|Octahedral||Octahedron, eight-sided (two pyramids base to base)||Diamond, Magnetite|
|Platy||Flat, tablet-shaped, prominent pinnacoid||Wulfenite|
|Plumose||Fine, feather-like scales||Aurichalcite, Boulangerite, Mottramite|
|Prismatic||Elongate, prism-like: well-developed crystal faces parallel to the vertical axis||Tourmaline, Beryl|
|Pseudo-hexagonal||Hexagonal appearance due to cyclic twinning||Aragonite, Chrysoberyl|
|Radiating or divergent||Radiating outward from a central point||Wavellite, Pyrite suns|
|Reniform or colloform||Similar to botryoidal/mamillary: intersecting kidney-shaped masses||Hematite, Pyrolusite, Greenockite|
|Reticulated||Crystals forming net-like intergrowths||Cerussite|
|Rosette or lenticular (lens shaped crystals)||Platy, radiating rose-like aggregate||Gypsum, Barite (i.e. Desert rose)|
|Stalactitic||Forming as stalactites or stalagmites; cylindrical or cone-shaped||Calcite, Goethite, Malachite|
|Stellate||Star-like, radiating||Pyrophyllite, Aragonite|
|Striated||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||More elongated than equant, slightly longer than wide, flat tablet shaped||Feldspar, Topaz|
|Tetrahedral||Tetrahedra-shaped crystals||Tetrahedrite, Spinel, Magnetite|
|Wheat sheaf||Aggregates resembling hand-reaped wheat sheaves||Stilbite|
Rutile is a mineral composed primarily of titanium dioxide (TiO2).
Amphibole is an important 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 (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.
Epidote is a calcium aluminium iron sorosilicate mineral.
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, translit. phénax, meaning "deceiver" due to its close visual similarity to quartz, named by Nils Gustaf Nordenskiöld in 1833.
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 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 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 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.
Sperrylite is a platinum arsenide mineral with formula PtAs2 and is an opaque metallic tin white mineral which crystallizes in the isometric system with the pyrite group structure. It forms cubic, octahedral or pyritohedral crystals in addition to massive and reniform habits. It has a Mohs hardness of 6 - 7 and a very high specific gravity of 10.6.
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.
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, 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 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 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 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.
Bobdownsite is the fluorine bearing mineral of the whitlockite group of phosphate minerals whose formula is Ca9(Mg)(PO4)6(PO3F). It is isotypic with whitlockite and was misidentified as such until proper chemical analysis. Whitlockites structure and relationships with other phosphate compounds has been extensively studied. Bobdownsite was first recovered from Big Fish River, Yukon, Canada from a Lower Cretaceous outcrop of bedded ironstones and shales. Bobdownsite is named after Robert Terrace Downs a professor of mineralogy in the Department of Geosciences at the University of Arizona, who lived and worked in the Yukon Territory in the 1970s. Bobdownsite is unique because it is the first known naturally forming phosphate to contain a P-F bond.
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.