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A crystal or crystalline solid is a material whose constituents (such as , , or ) are arranged in a highly ordered microscopic structure, forming a that extends in all directions. In addition, macroscopic are usually identifiable by their , consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as . The process of crystal formation via mechanisms of is called or .

The word crystal derives from the word κρύσταλλος (), meaning both "" and "rock crystal", κρύσταλλος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library from κρύος (), "icy cold, frost". κρύος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library

Examples of large crystals include , , and . Most inorganic solids are not crystals but , i.e. many microscopic crystals fused together into a single solid. Polycrystals include most , rocks, , and . A third category of solids is , where the atoms have no periodic structure whatsoever. Examples of amorphous solids include , , and many .

Despite the name, , and related products are not crystals, but rather types of glass, i.e. amorphous solids.

Crystals, or crystalline solids, are often used in practices such as , and, along with , are sometimes associated with spellwork in beliefs and related religious movements.Regal, Brian. (2009). Pseudoscience: A Critical Encyclopedia. Greenwood. p. 51.

Crystal structure (microscopic)
The scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement. ( are an exception, see below).

Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a structure. In the final block of ice, each of the small crystals (called "" or "grains") is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the . Most macroscopic solids are polycrystalline, including almost all , , , rocks, etc. Solids that are neither crystalline nor polycrystalline, such as , are called , also called , vitreous, or noncrystalline. These have no periodic order, even microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does.

A crystal structure (an arrangement of atoms in a crystal) is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are stacked in three-dimensional space to form the crystal.

The symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries (230 is commonly cited, but this treats chiral equivalents as separate entities), called . These are grouped into 7 , such as cubic crystal system (where the crystals may form cubes or rectangular boxes, such as halite shown at right) or hexagonal crystal system (where the crystals may form hexagons, such as ).

Crystal faces, shapes and crystallographic forms
Crystals are commonly recognized, macroscopically, by their shape, consisting of flat faces with sharp angles. These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see.

crystals are those that have obvious, well-formed flat faces. Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid.

The flat faces (also called ) of a crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of relatively low . The surface science of metal oxides, by Victor E. Henrich, P. A. Cox, page 28, google books link This occurs because some surface orientations are more stable than others (lower ). As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. (See diagram on right.)

One of the oldest techniques in the science of consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlying .

A crystal's crystallographic forms are sets of possible faces of the crystal that are related by one of the symmetries of the crystal. For example, crystals of often take the shape of cubes, and the six faces of the cube belong to a crystallographic form that displays one of the symmetries of the isometric crystal system. Galena also sometimes crystallizes as octahedrons, and the eight faces of the octahedron belong to another crystallographic form reflecting a different symmetry of the isometric system. A crystallographic form is described by placing the Miller indices of one of its faces within brackets. For example, the octahedral form is written as {111}, and the other faces in the form are implied by the symmetry of the crystal.

Forms may be closed, meaning that the form can completely enclose a volume of space, or open, meaning that it cannot. The cubic and octahedral forms are examples of closed forms. All the forms of the isometric system are closed, while all the forms of the monoclinic and triclinic crystal systems are open. A crystal's faces may all belong to the same closed form, or they may be a combination of multiple open or closed forms.

(1964). 9780442276249, Van Nostrand.

A is its visible external shape. This is determined by the crystal structure (which restricts the possible facet orientations), the specific crystal chemistry and bonding (which may favor some facet types over others), and the conditions under which the crystal formed.

Occurrence in nature

By volume and weight, the largest concentrations of crystals in the Earth are part of its solid . Crystals found in rocks typically range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are occasionally found. , the world's largest known naturally occurring crystal is a crystal of from Malakialina, , long and in diameter, and weighing .G. Cressey and I. F. Mercer, (1999) Crystals, London, Natural History Museum, page 58

Some crystals have formed by and processes, giving origin to large masses of crystalline rock. The vast majority of are formed from molten magma and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as , which have cooled very slowly and under great pressures, have completely crystallized; but many kinds of were poured out at the surface and cooled very rapidly, and in this latter group a small amount of amorphous or matter is common. Other crystalline rocks, the metamorphic rocks such as , and , are recrystallized. This means that they were at first fragmental rocks like , and and have never been in a condition nor entirely in solution, but the high temperature and pressure conditions of have acted on them by erasing their original structures and inducing recrystallization in the solid state.

Other rock crystals have formed out of precipitation from fluids, commonly water, to form druses or veins. such as halite, and some limestones have been deposited from aqueous solution, mostly owing to in arid climates.

Water-based in the form of , , and are common crystalline/polycrystalline structures on Earth and other planets.Yoshinori Furukawa, "Ice"; Matti Leppäranta, "Sea Ice"; D.P. Dobhal, "Glacier"; and other articles in Vijay P. Singh, Pratap Singh, and Umesh K. Haritashya, eds., Encyclopedia of Snow, Ice and Glaciers (Dordrecht, NE: Springer Science & Business Media, 2011). , 9789048126415 A single is a single crystal or a collection of crystals,
(2015). 9781627887335, Voyageur Press. .
while an is a .
(2017). 9781351416238, Routledge. .
Ice crystals may form from cooling liquid water below its freezing point, such as ice cubes or a frozen lake. , snowflakes, or small ice crystals suspended in the air () more often grow from a gaseous-solution of water vapor and air, when the temperature of the air drops below its , without passing through a liquid state. Another unusual property of water is that it expands rather than contracts when it crystallizes. Nucleation of Water: From Fundamental Science to Atmospheric and Additional Applications by Ari Laaksonen, Jussi Malila -- Elsevier 2022 Page 239--240

Organigenic crystals
Many living are able to produce crystals grown from an , for example and in the case of most or in the case of and in .

Polymorphism and allotropy
The same group of atoms can often solidify in many different ways. Polymorphism is the ability of a solid to exist in more than one crystal form. For example, water is ordinarily found in the hexagonal form , but can also exist as the cubic , the , and many other forms. The different polymorphs are usually called different phases.

In addition, the same atoms may be able to form noncrystalline phases. For example, water can also form , while SiO2 can form both (an amorphous glass) and (a crystal). Likewise, if a substance can form crystals, it can also form polycrystals.

For pure chemical elements, polymorphism is known as . For example, and are two crystalline forms of , while is a noncrystalline form. Polymorphs, despite having the same atoms, may have very different properties. For example, diamond is the hardest substance known, while graphite is so soft that it is used as a lubricant. can form six different types of crystals, but only one has the suitable hardness and melting point for candy bars and confections. Polymorphism in is responsible for its ability to be , giving it a wide range of properties.

is a similar phenomenon where the same atoms can exist in more than one form.

Crystallization is the process of forming a crystalline structure from a fluid or from materials dissolved in a fluid. (More rarely, crystals may be deposited directly from gas; see: and .)

Crystallization is a complex and extensively-studied field, because depending on the conditions, a single fluid can solidify into many different possible forms. It can form a , perhaps with various possible phases, , impurities, defects, and . Or, it can form a , with various possibilities for the size, arrangement, orientation, and phase of its grains. The final form of the solid is determined by the conditions under which the fluid is being solidified, such as the chemistry of the fluid, the , the , and the speed with which all these parameters are changing.

Specific industrial techniques to produce large single crystals (called boules) include the Czochralski process and the Bridgman technique. Other less exotic methods of crystallization may be used, depending on the physical properties of the substance, including hydrothermal synthesis, sublimation, or simply solvent-based crystallization.

Large single crystals can be created by geological processes. For example, selenite crystals in excess of 10  are found in the Cave of the Crystals in Naica, Mexico. For more details on geological crystal formation, see above.

Crystals can also be formed by biological processes, see above. Conversely, some organisms have special techniques to prevent crystallization from occurring, such as antifreeze proteins.

Defects, impurities, and twinning
An ideal crystal has every atom in a perfect, exactly repeating pattern. However, in reality, most crystalline materials have a variety of crystallographic defects, places where the crystal's pattern is interrupted. The types and structures of these defects may have a profound effect on the properties of the materials.

A few examples of crystallographic defects include (an empty space where an atom should fit), interstitial defects (an extra atom squeezed in where it does not fit), and (see figure at right). Dislocations are especially important in materials science, because they help determine the mechanical strength of materials.

Another common type of crystallographic defect is an , meaning that the "wrong" type of atom is present in a crystal. For example, a perfect crystal of would only contain atoms, but a real crystal might perhaps contain a few atoms as well. These boron impurities change the to slightly blue. Likewise, the only difference between and is the type of impurities present in a crystal.

In , a special type of impurity, called a , drastically changes the crystal's electrical properties. Semiconductor devices, such as , are made possible largely by putting different semiconductor dopants into different places, in specific patterns.

is a phenomenon somewhere between a crystallographic defect and a . Like a grain boundary, a twin boundary has different crystal orientations on its two sides. But unlike a grain boundary, the orientations are not random, but related in a specific, mirror-image way.

is a spread of crystal plane orientations. A consists of smaller crystalline units that are somewhat misaligned with respect to each other.

Chemical bonds
In general, solids can be held together by various types of , such as , , , van der Waals bonds, and others. None of these are necessarily crystalline or non-crystalline. However, there are some general trends as follows:

crystallize rapidly and are almost always polycrystalline, though there are exceptions like and single-crystal metals. The latter are grown synthetically, for example, fighter-jet turbines are typically made by first growing a single crystal of titanium alloy, increasing its strength and melting point over polycrystalline titanium. A small piece of metal may naturally form into a single crystal, such as Type 2 , but larger pieces generally do not unless extremely slow cooling occurs. For example, iron are often composed of single crystal, or many large crystals that may be several meters in size, due to very slow cooling in the vacuum of space. The slow cooling may allow the precipitation of a separate phase within the crystal lattice, which form at specific angles determined by the lattice, called Widmanstatten patterns. Encyclopedia of the Solar System by Tilman Spohn, Doris Breuer, Torrence V. Johnson -- Elsevier 2014 Page 632

typically form when a metal reacts with a non-metal, such as sodium with chlorine. These often form substances called salts, such as sodium chloride (table salt) or potassium nitrate (), with crystals that are often brittle and cleave relatively easily. Ionic materials are usually crystalline or polycrystalline. In practice, large salt crystals can be created by solidification of a fluid, or by crystallization out of a solution. Some ionic compounds can be very hard, such as oxides like found in many gemstones such as and synthetic sapphire.

solids (sometimes called covalent network solids) are typically formed from one or more non-metals, such as carbon or silicon and oxygen, and are often very hard, rigid, and brittle. These are also very common, notable examples being and Angelo State University: Formulas and Nomenclature of Ionic and Covalent Compounds

Weak van der Waals forces also help hold together certain crystals, such as crystalline , as well as the interlayer bonding in . Substances such as , and form molecular bonds because the large molecules do not pack as tightly as atomic bonds. This leads to crystals that are much softer and more easily pulled apart or broken. Common examples include chocolates, candles, or viruses. Water ice and are examples of other materials with molecular bonding. Science for Conservators, Volume 3: Adhesives and Coatings by Museum and Galleries Commission -- Museum and Galleries Commission 2005 Page 57 materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization—and sometimes polymers are completely amorphous.

A consists of arrays of atoms that are ordered but not strictly periodic. They have many attributes in common with ordinary crystals, such as displaying a discrete pattern in x-ray diffraction, and the ability to form shapes with smooth, flat faces.

Quasicrystals are most famous for their ability to show five-fold symmetry, which is impossible for an ordinary periodic crystal (see crystallographic restriction theorem).

The International Union of Crystallography has redefined the term "crystal" to include both ordinary periodic crystals and quasicrystals ("any solid having an essentially discrete diagram").

Quasicrystals, first discovered in 1982, are quite rare in practice. Only about 100 solids are known to form quasicrystals, compared to about 400,000 periodic crystals known in 2004. The 2011 Nobel Prize in Chemistry was awarded to for the discovery of quasicrystals.

Special properties from anisotropy
Crystals can have certain special electrical, optical, and mechanical properties that and normally cannot. These properties are related to the of the crystal, i.e. the lack of rotational symmetry in its atomic arrangement. One such property is the , where a voltage across the crystal can shrink or stretch it. Another is , where a double image appears when looking through a crystal. Moreover, various properties of a crystal, including electrical conductivity, electrical permittivity, and Young's modulus, may be different in different directions in a crystal. For example, crystals consist of a stack of sheets, and although each individual sheet is mechanically very strong, the sheets are rather loosely bound to each other. Therefore, the mechanical strength of the material is quite different depending on the direction of stress.

Not all crystals have all of these properties. Conversely, these properties are not quite exclusive to crystals. They can appear in or that have been made by or stress—for example, .

is the science of measuring the crystal structure (in other words, the atomic arrangement) of a crystal. One widely used crystallography technique is X-ray diffraction. Large numbers of known crystal structures are stored in crystallographic databases.

Image gallery
File:Insulincrystals.jpg| crystals grown in earth orbit. File:Hoar frost macro2.jpg|: A type of ice crystal (picture taken from a distance of about 5 cm). File:Gallium crystals.jpg|, a metal that easily forms large crystals. File:Apatite-Rhodochrosite-Fluorite-169799.jpg|An apatite crystal sits front and center on cherry-red rhodochroite rhombs, purple fluorite cubes, quartz and a dusting of brass-yellow pyrite cubes. File:Monokristalines Silizium für die Waferherstellung.jpg|Boules of , like this one, are an important type of industrially-produced . File:Bornite-Chalcopyrite-Pyrite-180794.jpg|A specimen consisting of a bornite-coated chalcopyrite crystal nestled in a bed of clear quartz crystals and lustrous pyrite crystals. The bornite-coated crystal is up to 1.5 cm across. File:Calcite-millerite association.jpg|Needle-like crystals partially encased in crystal and oxidized on their surfaces to ; from the Milwaukee Formation of File:Crystallized sugar, multiple crystals and a single crystal grown from seed.jpg|Crystallized sugar. Crystals on the right were grown from a sugar cube, while the left from a single seed crystal taken from the right. Red dye was added to the solution when growing the larger crystal, but, insoluble with the solid sugar, all but small traces were forced to precipitate out as it grew.

See also

Further reading
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