A crystal or crystalline solid is a solid material whose constituents
(such as atoms, molecules, or ions) 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.
The word crystal derives from the
Ancient Greek word
κρύσταλλος (krustallos), meaning both "ice" and "rock
crystal", from κρύος (kruos), "icy cold, frost".
Examples of large crystals include snowflakes, diamonds, and table
salt. Most inorganic solids are not crystals but polycrystals, i.e.
many microscopic crystals fused together into a single solid. Examples
of polycrystals include most metals, rocks, ceramics, and ice. A third
category of solids is amorphous solids, where the atoms have no
periodic structure whatsoever. Examples of amorphous solids include
glass, wax, and many plastics.
Crystals are often used in pseudoscientific practices such as crystal
therapy, and, along with gemstones, are sometimes associated with
spellwork in Wiccan beliefs and related religious movements.
Crystal structure (microscopic)
Crystal faces and shapes
3 Occurrence in nature
3.3 Organigenic crystals
4 Polymorphism and allotropy
6 Defects, impurities, and twinning
7 Chemical bonds
Special properties from anisotropy
11 Image gallery
12 See also
14 Further reading
Crystal structure (microscopic)
Halite (table salt, NaCl): Microscopic and macroscopic
Microscopic structure of a halite crystal. (Purple is sodium ion,
green is chlorine ion.) There is cubic symmetry in the atoms'
Macroscopic (~16cm) halite crystal. The right-angles between crystal
faces are due to the cubic symmetry of the atoms' arrangement.
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.
Quasicrystals 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 polycrystalline structure. In the final
block of ice, each of 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 periodic arrangement of atoms,
because the periodic pattern is broken at the grain boundaries. Most
macroscopic inorganic solids are polycrystalline, including almost all
metals, ceramics, ice, rocks, etc. Solids that are neither crystalline
nor polycrystalline, such as glass, are called amorphous solids, also
called glassy, 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 crystallographic space groups. These are
grouped into 7 crystal systems, 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 ordinary water ice).
Crystal faces and shapes
As a halite crystal is growing, new atoms can very easily attach to
the parts of the surface with rough atomic-scale structure and many
dangling bonds. Therefore, these parts of the crystal grow out very
quickly (yellow arrows). Eventually, the whole surface consists of
smooth, stable faces, where new atoms cannot as easily attach
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.
Euhedral 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 facets) of a euhedral crystal are oriented
in a specific way relative to the underlying atomic arrangement of the
crystal: they are planes of relatively low Miller index. This
occurs because some surface orientations are more stable than others
(lower surface energy). 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 crystallography
consists of measuring the three-dimensional orientations 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 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
Fossil shell with calcite crystals
By volume and weight, the largest concentrations of crystals in the
Earth are part of its solid bedrock. 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.
As of 1999[update], the world's largest known naturally occurring
crystal is a crystal of beryl from Malakialina, Madagascar, 18 m
(59 ft) long and 3.5 m (11 ft) in diameter, and
weighing 380,000 kg (840,000 lb).
Some crystals have formed by magmatic and metamorphic processes,
giving origin to large masses of crystalline rock. The vast majority
of igneous rocks are formed from molten magma and the degree of
crystallization depends primarily on the conditions under which they
solidified. Such rocks as granite, which have cooled very slowly and
under great pressures, have completely crystallized; but many kinds of
lava were poured out at the surface and cooled very rapidly, and in
this latter group a small amount of amorphous or glassy matter is
common. Other crystalline rocks, the metamorphic rocks such as
marbles, mica-schists and quartzites, are recrystallized. This means
that they were at first fragmental rocks like limestone, shale and
sandstone and have never been in a molten condition nor entirely in
solution, but the high temperature and pressure conditions of
metamorphism 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 quartz veins. The evaporites such as
halite, gypsum and some limestones have been deposited from aqueous
solution, mostly owing to evaporation in arid climates.
Water-based ice in the form of snow, sea ice and glaciers is a very
common manifestation of crystalline or polycrystalline matter on
Earth. A single snowflake is a single crystal or a
collection of crystals, while an ice cube is a polycrystal.
Many living organisms are able to produce crystals, for example
calcite and aragonite in the case of most molluscs or hydroxylapatite
in the case of vertebrates.
Polymorphism and allotropy
Polymorphism (materials science)
Polymorphism (materials science) 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 ice is ordinarily found in the
Ice Ih, but can also exist as the cubic
Ice Ic, the
rhombohedral ice II, 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 amorphous 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
For pure chemical elements, polymorphism is known as allotropy. For
example, diamond and graphite are two crystalline forms of carbon,
while amorphous carbon 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.
Polyamorphism is a similar phenomenon where the same atoms can exist
in more than one amorphous solid form.
Vertical cooling crystallizer in a beet sugar factory.
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 single crystal, perhaps with
various possible phases, stoichiometries, impurities, defects, and
habits. Or, it can form a polycrystal, 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
ambient pressure, the temperature, 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
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
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
Main articles: Crystallographic defect, Impurity,
Two types of crystallographic defects. Top right: edge dislocation.
Bottom right: screw dislocation.
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 vacancy defects (an
empty space where an atom should fit), interstitial defects (an extra
atom squeezed in where it does not fit), and dislocations (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 impurity, meaning
that the "wrong" type of atom is present in a crystal. For example, a
perfect crystal of diamond would only contain carbon atoms, but a real
crystal might perhaps contain a few boron atoms as well. These boron
impurities change the diamond's color to slightly blue. Likewise, the
only difference between ruby and sapphire is the type of impurities
present in a corundum crystal.
Twinned pyrite crystal group.
In semiconductors, a special type of impurity, called a dopant,
drastically changes the crystal's electrical properties. Semiconductor
devices, such as transistors, are made possible largely by putting
different semiconductor dopants into different places, in specific
Twinning is a phenomenon somewhere between a crystallographic defect
and a grain boundary. 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,
Mosaicity is a spread of crystal plane orientations. A mosaic crystal
is supposed to consist of smaller crystalline units that are somewhat
misaligned with respect to each other.
In general, solids can be held together by various types of chemical
bonds, such as metallic bonds, ionic bonds, covalent bonds, van der
Waals bonds, and others. None of these are necessarily crystalline or
non-crystalline. However, there are some general trends as follows.
Metals are almost always polycrystalline, though there are exceptions
like amorphous metal 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.) Ionic
compound materials are usually crystalline or polycrystalline. In
practice, large salt crystals can be created by solidification of a
molten fluid, or by crystallization out of a solution. Covalently
bonded solids (sometimes called covalent network solids) are also very
common, notable examples being diamond and quartz. Weak van der Waals
forces also help hold together certain crystals, such as crystalline
molecular solids, as well as the interlayer bonding in graphite.
Polymer materials generally will form crystalline regions, but the
lengths of the molecules usually prevent complete
crystallization—and sometimes polymers are completely amorphous.
The material holmium–magnesium–zinc (Ho–Mg–Zn) forms
quasicrystals, which can take on the macroscopic shape of a
dodecahedron. (Only a quasicrystal, not a normal crystal, can take
this shape.) The edges are 2 mm long.
Main article: Quasicrystal
A quasicrystal 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 diffraction 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
Dan Shechtman for the discovery of
Special properties from anisotropy
Crystals can have certain special electrical, optical, and mechanical
properties that glass and polycrystals normally cannot. These
properties are related to the anisotropy of the crystal, i.e. the lack
of rotational symmetry in its atomic arrangement. One such property is
the piezoelectric effect, where a voltage across the crystal can
shrink or stretch it. Another is birefringence, 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, graphite 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
glasses or polycrystals that have been made anisotropic by working or
stress—for example, stress-induced birefringence.
Main article: 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 known
crystal structures are stored in crystallographic databases.
Insulin crystals grown in earth orbit.
Hoar frost: A type of ice crystal (picture taken from a distance of
about 5 cm).
Gallium, a metal that easily forms large crystals.
An apatite crystal sits front and center on cherry-red rhodochroite
rhombs, purple fluorite cubes, quartz and a dusting of brass-yellow
Boules of silicon, like this one, are an important type of
industrially-produced single crystal.
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.
Atomic packing factor
^ Stephen Lower. "Chem1 online textbook—States of matter". Retrieved
^ Ashcroft and Mermin (1976).
Solid state physics. CS1 maint:
Uses authors parameter (link)
^ κρύσταλλος, Henry George Liddell, Robert Scott, A
Greek-English Lexicon, on Perseus Digital Library
^ κρύος, Henry George Liddell, Robert Scott, A Greek-English
Lexicon, on Perseus Digital Library
^ "The American Heritage Dictionary of the English Language". Kreus.
^ Regal, Brian. (2009). Pseudoscience: A Critical Encyclopedia.
Greenwood. p. 51. ISBN 978-0-313-35507-3
^ Patti Wigington (31 August 2016). "Using Crystals and Gemstones in
Magic". About.com. Retrieved 14 November 2016.
^ "The Magic of Crystals and Gemstones". WitchesLore. 14 December
2011. Retrieved 14 November 2016.
^ The surface science of metal oxides, by Victor E. Henrich, P. A.
Cox, page 28, google books link
^ G. Cressey and I. F. Mercer, (1999) Crystals, London, Natural
History Museum, page 58
^ One or more of the preceding sentences incorporates text
from a publication now in the public domain: Chisholm, Hugh, ed.
(1911). "Petrology". Encyclopædia Britannica (11th ed.). Cambridge
^ Libbrecht, Kenneth; Wing, Rachel (2015-09-01). The Snowflake:
Winter's Frozen Artistry. Voyageur Press.
^ "Cave of
Crystal Giants — National Geographic Magazine".
^ Britain), Science Research Council (Great (1972). Report of the
Council. H.M. Stationery Office.
^ International Union of
Crystallography (1992). "Report of the
Executive Committee for 1991". Acta Crystallogr. A. 48 (6): 922.
doi:10.1107/S0108767392008328. PMC 1826680 .
^ 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.
Nobel Prize in Chemistry
Nobel Prize in Chemistry 2011". Nobelprize.org. Retrieved
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Howard, J. Michael; Darcy Howard (Illustrator) (1998). "Introduction
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Crystal Systems". Bob's Rock Shop.
Krassmann, Thomas (2005–2008). "The Giant
Krassmann. Retrieved 2008-04-20.
Various authors (2007). "Teaching Pamphlets". Commission on
Crystallographic Teaching. Retrieved 2008-04-20.
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Crystal Lattice Structures:Index by Space
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Patterns in nature
D'Arcy Wentworth Thompson
On Growth and Form
The Chemical Basis of Morphogenesis
How Long Is the Coast of Britain? Statistical Self-Similarity and