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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 geometrical shape, 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. Examples of 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 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, 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 and shapes
Crystals are commonly recognized 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 with 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 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. .

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

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 wildly different properties. For example, diamond is among the hardest substances known, while graphite is so soft that it is used as a lubricant.

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 thin-film deposition 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.

are almost always polycrystalline, though there are exceptions like and single-crystal metals. The latter are grown synthetically. (A microscopically-small piece of metal may naturally form into a single crystal, but larger pieces generally do not.) 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. solids (sometimes called covalent network solids) are also very common, notable examples being and . Weak van der Waals forces also help hold together certain crystals, such as crystalline , as well as the interlayer bonding in . 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

See also

Further reading
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