Crystals ofamethystquartzMicroscopically, asingle crystal has atoms in a near-perfectperiodic arrangement; a polycrystal is composed of many microscopic crystals (called "crystallites" or "grains"); and anamorphous solid (such asglass) has no periodic arrangement even microscopically.
Acrystal orcrystalline solid is asolid material whose constituents (such asatoms,molecules, orions) are arranged in a highly ordered microscopic structure, forming acrystal lattice that extends in all directions.[1][2] In addition, macroscopicsingle crystals are usually identifiable by theirgeometrical shape, consisting of flatfaces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known ascrystallography. The process of crystal formation via mechanisms ofcrystal growth is calledcrystallization orsolidification.
The wordcrystal derives from theAncient Greek wordκρύσταλλος (krustallos), meaning both "ice" and "rock crystal",[3] fromκρύος (kruos), "icy cold, frost".[4][5]
Examples of large crystals includesnowflakes,diamonds, andtable salt. Most inorganic solids are not crystals butpolycrystals, i.e. many microscopic crystals fused together into a single solid. Polycrystals include mostmetals, rocks,ceramics, andice. A third category of solids isamorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids includeglass,wax, and manyplastics.
Despite the name,lead crystal, crystal glass, and related products arenot crystals, but rather types of glass, i.e. amorphous solids.
The scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called thecrystal structure. A crystal is a solid where the atoms form a periodic arrangement. (Quasicrystals are an exception, seebelow).
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 apolycrystalline 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 doesnot have a periodic arrangement of atoms, because the periodic pattern is broken at thegrain boundaries. Most macroscopicinorganic solids are polycrystalline, including almost allmetals,ceramics,ice,rocks, etc. Solids that are neither crystalline nor polycrystalline, such asglass, are calledamorphous solids, also calledglassy, 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 thelatent heat of fusion, but forming a crystal does.
A crystal structure (an arrangement of atoms in a crystal) is characterized by itsunit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells arestacked in three-dimensional space to form the crystal.
As ahalite crystal is growing, new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and manydangling 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 themselves.
Crystals are commonly recognized, macroscopically, by their shape, consisting of flat faces with sharp angles. These shape characteristics are notnecessary 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 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 calledfacets) of aeuhedral crystal are oriented in a specific way relative to the underlyingatomic arrangement of the crystal: they areplanes of relatively lowMiller index.[10] This occurs because some surface orientations are more stable than others (lowersurface 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 ofcrystallography consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlyingcrystal symmetry.
A crystal'scrystallographic forms are sets of possible faces of the crystal that are related by one of the symmetries of the crystal. For example, crystals ofgalena 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 theisometric 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.[11]
Acrystal's habit is its visible external shape. This is determined by thecrystal 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.
By volume and weight, the largest concentrations of crystals in the Earth are part of its solidbedrock. 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 ofberyl from Malakialina,Madagascar, 18 m (59 ft) long and 3.5 m (11 ft) in diameter, and weighing 380,000 kg (840,000 lb).[12]
Some crystals have formed bymagmatic andmetamorphic processes, giving origin to large masses of crystallinerock. The vast majority ofigneous rocks are formed from molten magma and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks asgranite, which have cooled very slowly and under great pressures, have completely crystallized; but many kinds oflava were poured out at the surface and cooled very rapidly, and in this latter group a small amount of amorphous orglassy matter is common. Other crystalline rocks, the metamorphic rocks such asmarbles,mica-schists andquartzites, are recrystallized. This means that they were at first fragmental rocks likelimestone,shale andsandstone and have never been in amolten condition nor entirely in solution, but the high temperature and pressure conditions ofmetamorphism have acted on them by erasing their original structures and inducing recrystallization in the solid state.[13]
Other rock crystals have formed out of precipitation from fluids, commonly water, to formdruses orquartz veins.Evaporites such ashalite,gypsum and some limestones have been deposited from aqueous solution, mostly owing toevaporation in arid climates.
Ice
Water-basedice in the form ofsnow,sea ice, andglaciers are common crystalline/polycrystalline structures on Earth and other planets.[14] A singlesnowflake is a single crystal or a collection of crystals,[15] while anice cube is apolycrystal.[16] Ice crystals may form from cooling liquid water below its freezing point, such as ice cubes or a frozen lake.Frost, snowflakes, or small ice crystals suspended in the air (ice fog) more often grow from asupersaturated gaseous-solution of water vapor and air, when the temperature of the air drops below itsdew point, without passing through a liquid state. Another unusual property of water is that it expands rather than contracts when it crystallizes.[17]
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, waterice is ordinarily found in the hexagonal formIce Ih, but can also exist as the cubicIce Ic, therhombohedralice II, and many other forms. The different polymorphs are usually called differentphases.
In addition, the same atoms may be able to form noncrystallinephases. For example, water can also formamorphous ice, while SiO2 can form bothfused silica (an amorphous glass) andquartz (a crystal). Likewise, if a substance can form crystals, it can also form polycrystals.
For pure chemical elements, polymorphism is referred to asallotropy. For example,diamond andgraphite are two crystalline forms ofcarbon, whileamorphous carbon 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.Chocolate can form six different types of crystals, but only one has the suitable hardness and melting point for candy bars and confections. Polymorphism insteel is responsible for its ability to beheat treated, giving it a wide range of properties.
Crystallization is the process of forming a crystalline structure from a fluid or from materials dissolved in a fluid. (More rarely, crystals may bedeposited directly from gas; see:epitaxy andfrost.)
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 asingle crystal, perhaps with various possiblephases,stoichiometries, impurities,defects, andhabits. Or, it can form apolycrystal, 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, theambient pressure, thetemperature, and the speed with which all these parameters are changing.
Large single crystals can be created by geological processes. For example,selenite crystals in excess of 10 m are found in theCave of the Crystals in Naica, Mexico.[18] For more details on geological crystal formation, seeabove.
Crystals can also be formed by biological processes, seeabove. Conversely, some organisms have special techniques toprevent crystallization from occurring, such asantifreeze proteins.
Anideal crystal has every atom in a perfect, exactly repeating pattern.[19] However, in reality, most crystalline materials have a variety ofcrystallographic 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.
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 containcarbon atoms, but a real crystal might perhaps contain a fewboron atoms as well. These boron impurities change thediamond's color to slightly blue. Likewise, the only difference betweenruby andsapphire is the type of impurities present in acorundum crystal.
Insemiconductors, a special type of impurity, called adopant, drastically changes the crystal's electrical properties.Semiconductor devices, such astransistors, are made possible largely by putting different semiconductor dopants into different places, in specific patterns.
Twinning is a phenomenon somewhere between a crystallographic defect and agrain 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.
Mosaicity is a spread of crystal plane orientations. Amosaic crystal consists of smaller crystalline units that are somewhat misaligned with respect to each other.
Metals crystallize rapidly and are almost always polycrystalline, though there are exceptions likeamorphous metal 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 2telluric iron, but larger pieces generally do not unless extremely slow cooling occurs. For example, ironmeteorites 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, calledWidmanstatten patterns.[20]
Ionic compounds 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 (saltpeter), with crystals that are often brittle and cleave relatively easily. Ionic materials are usually crystalline or polycrystalline. In practice, largesalt crystals can be created by solidification of amolten fluid, or by crystallization out of a solution. Some ionic compounds can be very hard, such as oxides likealuminium oxide found in many gemstones such asruby andsynthetic sapphire.
Covalently bonded solids (sometimes calledcovalent 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 beingdiamond andquartz respectively.[21]
Weakvan der Waals forces also help hold together certain crystals, such as crystallinemolecular solids, as well as the interlayer bonding ingraphite. Substances such asfats,lipids andwax 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 anddry ice are examples of other materials with molecular bonding.[22]Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization—and sometimes polymers are completely amorphous.
Aquasicrystal 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 inx-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 (seecrystallographic restriction theorem).
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.[24] The 2011Nobel Prize in Chemistry was awarded toDan Shechtman for the discovery of quasicrystals.[25]
Crystals can have certain special electrical, optical, and mechanical properties thatglass andpolycrystals normally cannot. These properties are related to theanisotropy of the crystal, i.e. the lack of rotational symmetry in its atomic arrangement. One such property is thepiezoelectric effect, where a voltage across the crystal can shrink or stretch it. Another isbirefringence, where a double image appears when looking through a crystal. Moreover, various properties of a crystal, includingelectrical conductivity,electrical permittivity, andYoung'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.
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.
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.
^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).ISBN904812641X, 9789048126415
^Nucleation of Water: From Fundamental Science to Atmospheric and Additional Applications by Ari Laaksonen, Jussi Malila -- Elsevier 2022 Page 239--240
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