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A crystal of amethyst quartz.

Microscopically, a single crystal has atoms in a near-perfect periodic arrangement; a polycrystal iscomposed of many microscopic crystals (called"crystallites" or "grains"); and an amorphous solid(such as glass) has no periodic arrangement evenmicroscopically.

CrystalFrom Wikipedia, the free encyclopedia

A crystal or crystalline solid is a solid materialwhose constituent atoms, molecules or ions et ceteraare arranged in an ordered pattern extending in allthree spatial dimensions. In addition to theirmicroscopic structure, large crystals are usuallyidentifiable by their macroscopic geometrical shape,consisting of flat faces with specific, characteristicorientations.

The scientific study of crystals and crystal formationis known as crystallography. The process of crystalformation via mechanisms of crystal growth is calledcrystallization or solidification. The word crystal isderived from the Ancient Greek word κρύσταλλος(krustallos), meaning both “ice” and “rock crystal”,[1]

from κρύος (kruos), "icy cold, frost".[2][3]

Examples of large crystals include snowflakes,diamonds, and table salt. Most inorganic solids are notcrystals but polycrystals, i.e. many microscopiccrystals fused together into a single solid. Examples ofpolycrystals include most metals, rocks, ceramics, andice. A third category of solids is amorphous solids,where the atoms have no periodic structurewhatsoever. Examples of amorphous solids includeglass, wax, and many plastics.

Contents

1 Crystal structure (microscopic)2 Crystal faces and shapes3 Occurrence in nature

3.1 Rocks3.2 Ice3.3 Organigenic crystals

4 Polymorphism and allotropy5 Crystallization6 Defects, impurities, and twinning7 Chemical bonds

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Microscopic structure ofa halite crystal. (Purple issodium ion, green ischlorine ion.) There iscubic symmetry in theatoms' arrangement.

Macroscopic (~16cm)halite crystal. The right-angles between crystalfaces are due to the cubicsymmetry of the atoms'arrangement.

Halite (table salt, NaCl): Microscopic andmacroscopic

7.1 Properties8 Quasicrystals9 Special properties from anisotropy10 Crystallography11 Gallery12 See also13 References

13.1 Further reading

Crystal structure (microscopic)

The scientific definition of a "crystal" is based on themicroscopic arrangement of atoms inside it, called thecrystal structure. A crystal is a solid where the atomsform a periodic arrangement. (Quasicrystals are anexception, see below.)

Not all solids are crystals. For example, when liquidwater starts freezing, the phase change begins withsmall ice crystals that grow until they fuse, forming apolycrystalline structure. In the final block of ice, eachof the small crystals (called "crystallites" or "grains")is a true crystal with a periodic arrangement of atoms,but the whole polycrystal does not have a periodicarrangement of atoms, because the periodic pattern isbroken at the grain boundaries. Most macroscopicinorganic solids are polycrystalline, including almostall metals, ceramics, ice, rocks, etc. Solids that areneither crystalline nor polycrystalline, such as glass,are called amorphous solids, also called glassy,vitreous, or noncrystalline. These have no periodicorder, 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 crystaldoes.

A crystal structure (an arrangement of atoms in a crystal) is characterized by its unit cell, a small imaginarybox 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 crystallographic space groups. These are grouped into 7crystal systems, such as cubic crystal system (where the crystals may form cubes or rectangular boxes, suchas halite shown at right) or hexagonal crystal system (where the crystals may form hexagons, such asordinary water ice).

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As a halite crystal is growing, new atoms can very easilyattach to the parts of the surface with rough atomic-scalestructure and many dangling bonds. Therefore these parts ofthe crystal grow out very quickly (yellow arrows).Eventually, the whole surface consists of smooth, stablefaces, where new atoms cannot as easily attach themselves.

Ice crystals.

Crystal faces and shapes

Crystals are commonly recognized by theirshape, consisting of flat faces with sharp angles.These shape characteristics are not necessaryfor a crystal—a crystal is scientifically definedby its microscopic atomic arrangement, not itsmacroscopic shape—but the characteristicmacroscopic shape is often present and easy tosee.

Euhedral crystals are those with obvious, well-formed flat faces. Anhedral crystals do not,usually because the crystal is one grain in apolycrystalline solid.

The flat faces (also called facets) of a euhedralcrystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They areplanes of relatively low Miller index.[4] This occurs because some surface orientations are more stable thanothers (lower surface energy). As a crystal grows, new atoms attach easily to the rougher and less stableparts of the surface, but less easily to the flat, stable surfaces. Therefore, the flat surfaces tend to growlarger 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 crystallography consists of measuring the three-dimensionalorientations of the faces of a crystal, and using them to infer the underlying crystal symmetry.

A crystal's habit is its visible external shape. This is determined by the crystal structure (which restricts thepossible facet orientations), the specific crystal chemistry and bonding (which may favor some facet typesover others), and the conditions under which the crystal formed.

Occurrence in nature

Rocks

By volume and weight, the largest concentrations of crystals in theearth are part of the Earth's solid bedrock.

Some crystals have formed by magmatic and metamorphicprocesses, giving origin to large masses of crystalline rock. The vastmajority of igneous rocks are formed from molten magma and thedegree of crystallization depends primarily on the conditions underwhich they solidified. Such rocks as granite, which have cooled veryslowly and under great pressures, have completely crystallized; butmany kinds of lava were poured out at the surface and cooled veryrapidly, and in this latter group a small amount of amorphous or glassy matter is common. Other crystallinerocks, the metamorphic rocks such as marbles, mica-schists and quartzites, are recrystallized. This means

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Fossil shell with calcite crystals.

that they were at first fragmental rocks like limestone, shale and sandstone and have never been in a moltencondition nor entirely in solution, but the high temperature and pressure conditions of metamorphism haveacted on them by erasing their original structures and inducing recrystallization in the solid state.[5]

Other rock crystals have formed out of precipitation from fluids,commonly water, to form druses or quartz veins. The evaporitessuch as halite, gypsum and some limestones have been depositedfrom aqueous solution, mostly owing to evaporation in aridclimates.

Ice

Water-based ice in the form of snow, sea ice and glaciers is a verycommon manifestation of crystalline or polycrystalline matter onEarth. A single snowflake is typically a single crystal, while an icecube is a polycrystal.

Organigenic crystals

Many living organisms are able to produce crystals, for example calcite and aragonite in the case of mostmolluscs or hydroxylapatite in the case of vertebrates.

Polymorphism and allotropy

The same group of atoms can often solidify in many different ways. Polymorphism is the ability of a solidto exist in more than one crystal form. For example, water ice is ordinarily found in the hexagonal form IceIh, but can also exist as the cubic Ice Ic, the rhombohedral ice II, and many other forms. The differentpolymorphs are usually called different phases.

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

For pure chemical elements, polymorphism is known as allotropy. For example, diamond and graphite aretwo crystalline forms of carbon, while amorphous carbon is a noncrystalline form. Polymorphs, despitehaving the same atoms, may have wildly different properties. For example, diamond is among the hardestsubstances known, while graphite is so soft that it is used as a lubricant.

Polyamorphism is a similar phenomenon where the same atoms can exist in more than one amorphous solidform.

Crystallization

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

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Vertical coolingcrystallizer in a beetsugar factory.

Two types of crystallographic defects. Topright: edge dislocation. Bottom right: screwdislocation.

Crystallization is a complex and extensively-studied field, because depending on the conditions, a singlefluid can solidify into many different possible forms. It can form a single crystal, perhaps with variouspossible phases, stoichiometries, impurities, defects, and habits. Or, it can form a polycrystal, with variouspossibilities for the size, arrangement, orientation, and phase of its grains. Thefinal form of the solid is determined by the conditions under which the fluid isbeing solidified, such as the chemistry of the fluid, the ambient pressure, thetemperature, 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 exoticmethods of crystallization may be used, depending on the physical propertiesof the substance, including hydrothermal synthesis, sublimation, or simplysolvent-based crystallization.

Large single crystals can be created by geological processes. For example,selenite crystals in excess of 10 meters are found in the Cave of the Crystals inNaica, Mexico.[6] 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 fromoccurring, such as antifreeze proteins.

Defects, impurities, and twinning

An ideal crystal has every atom in a perfect, exactly repeatingpattern. However, in reality, most crystalline materials have avariety of crystallographic defects, places where the crystal'spattern is interrupted. The types and structures of thesedefects may have a profound effect on the properties of thematerials.

A few examples of crystallographic defects include vacancydefects (an empty space where an atom should fit), interstitialdefects (an extra atom squeezed in where it does not fit), anddislocations (see figure at right). Dislocations are especiallyimportant in materials science, because they help determinethe mechanical strength of materials.

Another common type of crystallographic defect is animpurity, meaning that the "wrong" type of atom is present in a crystal. For example, a perfect crystal ofdiamond would only contain carbon atoms, but a real crystal might perhaps contain a few boron atoms aswell. These boron impurities change the diamond's color to slightly blue. Likewise, the only differencebetween ruby and sapphire is the type of impurities present in a corundum crystal.

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

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Twinned pyrite crystal group.

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

Mosaicity is a spread of crystal plane orientations. A mosaic crystalis supposed to consist of smaller crystalline units that are somewhatmisaligned with respect to each other.

Chemical bonds

Crystalline structures occur in all classes of materials, with all typesof chemical bonds. Almost all metal exists in a polycrystalline state;amorphous or single-crystal metals must be produced synthetically,often with great difficulty. Ionically bonded crystals can form uponsolidification of salts, either from a molten fluid or uponcrystallization from a solution. Covalently bonded crystals are alsovery common, notable examples being diamond, silica, andgraphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usuallyprevent complete crystallization. Weak van der Waals forces can also play a role in a crystal structure; forexample, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite.

Properties

Crystal Particles Attractive forces Meltingpoint Other properties

Ionic Positive andnegative ions Electrostatic attractions High Hard, brittle, good electrical conductor in

molten state

Molecular Polarmolecules

London force anddipole-dipole attraction Low Soft, non-conductor or extremely poor

conductor of electricity in liquid state

Molecular Non-polarmolecules London force Low Soft conductor

Quasicrystals

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

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

The International Union of Crystallography has redefined the term "crystal" to include both ordinaryperiodic crystals and quasicrystals ("any solid having an essentially discrete diffraction diagram"[7]).

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The material Ho-Mg-Zn formsquasicrystals, which can take on themacroscopic shape of adodecahedron. (Only a quasicrystal,not a normal crystal, can take thisshape.) The edges are 2mm long.

Quasicrystals, first discovered in 1982, are quite rare in practice.Only about 100 solids are known to form quasicrystals, compared toabout 400,000 periodic crystals measured to date.[8] The 2011 NobelPrize in Chemistry was awarded to Dan Shechtman for thediscovery of quasicrystals.[9]

Special properties from anisotropy

Crystals can have certain special electrical, optical, and mechanicalproperties that glass and polycrystals normally cannot. Theseproperties are related to the anisotropy of the crystal, i.e. the lack ofrotational symmetry in its atomic arrangement. One such property isthe piezoelectric effect, where a voltage across the crystal can shrinkor stretch it. Another is birefringence, where a double image appearswhen looking through a crystal. Moreover, various properties of acrystal, including electrical conductivity, electrical permittivity, andYoung's modulus, may be different in different directions in acrystal. For example, graphite crystals consist of a stack of sheets, and although each individual sheet ismechanically very strong, the sheets are rather loosely bound to each other. Therefore, the mechanicalstrength 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 glasses or polycrystals that have been made anisotropic by working or stress—forexample, stress-induced birefringence.

Crystallography

Crystallography 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 knowncrystal structures are stored in crystallographic databases.

Gallery

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Insulin crystals grown in earth orbit.

Hoar frost: A type of ice crystal(picture taken from a distance ofabout 5 cm).

Gallium, a metal that easily formslarge crystals.

An apatite crystal sits front andcenter on cherry-red rhodochroiterhombs, purple fluorite cubes, quartzand a dusting of brass-yellow pyritecubes.

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Boules of silicon, like this one, are animportant type of industrially-produced single crystal.

A specimen consisting of a bornite-coated chalcopyrite crystal nestled ina bed of clear quartz crystals andlustrous pyrite crystals. The bornite-coated crystal is up to 1.5 cm across.

See also

Atomic packing factorCocrystalColloidal crystalCrystal oscillatorLiquid crystalAnticrystal

References

1. ^ κρύσταλλος (http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkru%2Fstallos), Henry George Liddell, Robert Scott, AGreek-English Lexicon, on Perseus Digital Library

2. ^ κρύος (http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkru%2Fos), Henry George Liddell, Robert Scott, AGreek-English Lexicon, on Perseus Digital Library

3. ^ "Kreus" (http://www.bartleby.com/61/roots/IE243.html). The American Heritage Dictionary of the EnglishLanguage: Fourth Edition: Appendix I: Indo-European Roots. 2000.

4. ^ The surface science of metal oxides, by Victor E. Henrich, P. A. Cox, page 28, google books link(http://books.google.com/books?id=X6x1MmPisKkC&pg=PA2)

5. ^ One or more of the preceding sentences incorporates text from a publication now in the publicdomain: Chisholm, Hugh, ed. (1911). "Petrology". Encyclopædia Britannica (11th ed.). Cambridge University

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Press.6. ^ National Geographic, 2008. Cavern of Crystal Giants (http://ngm.nationalgeographic.com/2008/11/crystal-

giants/shea-text)7. ^ International Union of Crystallography (1992). "Report of the Executive Committee for 1991". Acta Cryst.

A48 (6): 922. doi:10.1107/S0108767392008328 (http://dx.doi.org/10.1107%2FS0108767392008328).8. ^ Steurer W. (2004). "Twenty years of structure research on quasicrystals. Part I. Pentagonal, octagonal,

decagonal and dodecagonal quasicrystals". Z. Kristallogr. 219 (7–2004): 391–446. Bibcode:2004ZK....219..391S(http://adsabs.harvard.edu/abs/2004ZK....219..391S). doi:10.1524/zkri.219.7.391.35643(http://dx.doi.org/10.1524%2Fzkri.219.7.391.35643).

9. ^ "The Nobel Prize in Chemistry 2011" (http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2011/).Nobelprize.org. Retrieved 2011-12-29.

Further reading

Howard, J. Michael; Darcy Howard (Illustrator) (1998). "Introduction to Crystallography and MineralCrystal Systems" (http://www.rockhounds.com/rockshop/xtal/index.html). Bob's Rock Shop.Retrieved 2008-04-20.Krassmann, Thomas (2005–2008). "The Giant Crystal Project" (http://giantcrystals.strahlen.org).Krassmann. Retrieved 2008-04-20.Various authors (2007). "Teaching Pamphlets" (http://www.iucr.ac.uk/iucr-top/comm/cteach/pamphlets.html). Commission on Crystallographic Teaching. Retrieved 2008-04-20.Various authors (2004). "Crystal Lattice Structures:Index by Space Group" (http://cst-www.nrl.navy.mil/lattice/spcgrp/). U.S. Naval Research Laboratory, Center for ComputationalMaterials Science. Retrieved 2008-04-20.Various authors (2010). "Crystallography" (http://www.xtal.iqfr.csic.es/Cristalografia/index-en.html).Spanish National Research Council, Department of Crystallography. Retrieved 2010-01-08.

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Categories: Crystals Concepts in physics

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