A crystal or crystalline solid is a solid
material whose constituents (such as atom
s, or ion
s) are arranged in a highly ordered microscopic structure, forming a crystal lattice
that extends in all directions. In addition, macroscopic single crystal
s 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
The word ''crystal'' derives from the Ancient Greek
word (), meaning both "ice
" and "rock crystal
", from (), "icy cold, frost".
Examples of large crystals include snowflake
s, and table salt
. Most inorganic solids are not crystals but polycrystal
s, 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 solid
s, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass
, and many plastic
Despite the name, lead crystal, crystal glass
, and related products are ''not'' crystals, but rather types of glass, i.e. amorphous solids.
Crystals are often used in pseudoscientific
practices such as crystal therapy
, and, along with gemstone
s, are sometimes associated with spellwork
n beliefs and related religious movements.
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. (Quasicrystal
s 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 "crystallite
s" 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 metal
, etc. Solids that are neither crystalline nor polycrystalline, such as glass
, are called ''amorphous solid
s'', also called glass
y, 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 system
s, 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
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 facet
s) 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
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. , the world's largest known naturally occurring crystal is a crystal of beryl
from Malakialina, Madagascar
, long and in diameter, and weighing .
Some crystals have formed by magmatic
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 glass
y matter is common. Other crystalline rocks, the metamorphic rocks such as marble
s and quartzite
s, are recrystallized. This means that they were at first fragmental rocks like limestone
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
s such as Halite (mineral)
and some limestones have been deposited from aqueous solution, mostly owing to evaporation
in arid climates.
in the form of snow
, sea ice
, and glacier
s are common crystalline/polycrystalline structures on Earth and other planets. 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
in the case of most mollusc
s or hydroxylapatite
in the case of vertebrate
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 ice
is ordinarily found in the hexagonal form 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 polycrystals.
For pure chemical elements, polymorphism is known as allotropy
. For example, diamond
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.
is a similar phenomenon where the same atoms can exist in more than one amorphous solid
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
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
, 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
, or simply solvent-based crystallization
Large single crystals can be created by geological processes. For example, selenite
crystals in excess of 10 meter
s 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 protein
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 defect
s, 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 defect
s (an empty space where an atom should fit), interstitial defect
s (an extra atom squeezed in where it does not fit), and dislocation
s (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
is the type of impurities present in a corundum
s, a special type of impurity, called a dopant
, drastically changes the crystal's electrical properties. Semiconductor device
s, such as transistor
s, 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 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, mirror-image way.
is a spread of crystal plane orientations. A mosaic crystal
consists 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 bond
s, such as metallic bond
s, ionic bond
s, covalent bond
s, van der Waals bond
s, and others. None of these are necessarily crystalline or non-crystalline. However, there are some general trends as follows.
s 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
. Weak van der Waals force
s also help hold together certain crystals, such as crystalline molecular solid
s, as well as the interlayer bonding in graphite
materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization—and sometimes polymers are completely amorphous.
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
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 quasicrystals.
Special properties from anisotropy
Crystals can have certain special electrical, optical, and mechanical properties that glass
s 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 glass
es or polycrystal
s that have been made anisotropic
—for example, stress-induced birefringence
'' 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 database
File:Insulincrystals.jpg|Insulin crystals grown in earth orbit.
File:Hoar frost macro2.jpg|Hoar frost: A type of ice crystal (picture taken from a distance of about 5 cm).
File:Gallium crystals.jpg|Gallium, 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 silicon, like this one, are an important type of industrially-produced single crystal.
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 millerite crystals partially encased in calcite crystal and oxidized on their surfaces to zaratite; from the Devonian Milwaukee Formation of Wisconsin
* Atomic packing factor
* Colloidal crystal
* Crystal growth
* Crystal oscillator
* Liquid crystal
* Time crystal