This is a list of the allotropes of
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
.
Diamond
Diamond is one of the best known
allotropesAllotropy or allotropism is a behavior exhibited by certain chemical elements: these elements can exist in two or more different forms, known as allotropes of that element. In each allotrope, the element's atoms are bonded together in a different manner...
of carbon, whose hardness and high dispersion of light make it useful for industrial applications and jewellery. Diamond is the hardest known natural
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
, which makes it an excellent abrasive and makes it hold polish and luster extremely well. No known naturally occurring substance can cut (or even scratch) a diamond.
The market for industrial-grade diamonds operates much differently from its gem-grade counterpart. Industrial diamonds are valued mostly for their hardness and heat conductivity, making many of the
gemologicalGemology or gemmology is the science, art and profession of identifying and evaluating gemstones. It is considered a geoscience and a branch of mineralogy. Some jewelers are academically trained gemologists and are qualified to identify and evaluate gems.- Background :With further post-graduate...
characteristics of diamond, including clarity and color, mostly irrelevant. This helps explain why 80% of mined diamonds (equal to about 100 million carats or 20,000 kg annually), unsuitable for use as gemstones and known as
bortBort or boart is a term used in the diamond industry to refer to shards of gem-grade/quality diamonds. In the manufacturing and heavy industries, "bort" is used to describe dark, imperfectly formed/crystallized diamonds of varying levels of opacity. The lowest grade, "crushing bort", is crushed by...
, are destined for industrial use. In addition to mined diamonds,
synthetic diamondSynthetic diamond is diamond produced in a technological process; as opposed to natural diamond, which is created in geological processes. Synthetic diamond is also widely known as HPHT diamond or CVD diamond where HPHT and CVD refer to the production method, namely high-pressure high-temperature...
s found industrial applications almost immediately after their invention in the 1950s; another 400 million carats (80,000 kg) of synthetic diamonds are produced annually for industrial use—nearly four times the mass of natural diamonds mined over the same period.
The dominant industrial use of diamond is in cutting, drilling (drill bits), grinding (diamond edged cutters), and polishing. Most uses of diamonds in these technologies do not require large diamonds; in fact, most diamonds that are gem-quality can find an industrial use. Diamonds are embedded in drill tips or saw blades, or ground into a powder for use in grinding and polishing applications. Specialized applications include use in laboratories as containment for high pressure experiments (see diamond anvil), high-performance
bearingsA bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
, and limited use in specialized
windowA window is a transparent opening in a wall that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material...
s.
With the continuing advances being made in the production of synthetic diamond, future applications are beginning to become feasible. Garnering much excitement is the possible use of diamond as a
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
suitable to build
microchipIn electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin substrate of semiconductor material...
s from, or the use of diamond as a
heat sinkA heat sink is an environment or object that absorbs and dissipates heat from another object using thermal contact...
in
electronicsElectronics is a branch of science and technology that deals with the controlled flow of electrons. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation,...
. Significant research efforts in
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
,
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
, and the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
are under way to capitalize on the potential offered by diamond's unique material properties, combined with increased quality and quantity of supply starting to become available from synthetic diamond manufacturers.
Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedrons together form a 3-dimensional network of six-membered carbon rings (similar to
cyclohexaneCyclohexane is a cycloalkane with the molecular formula C
6H
12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which are intermediates used in the production...
), in the chair conformation, allowing for zero bond angle strain. This stable network of covalent bonds and hexagonal rings, is the reason that diamond is so incredibly strong.
Graphite
Graphite (named by
Abraham Gottlob WernerAbraham Gottlob Werner , was a German geologist who set out a now obsolete theory about the stratification of the Earth's crust and coined the now obsolete word Neptunism.-Life:...
in 1789, from the Greek γράφειν (
graphein, "to draw/write", for its use in pencils) is one of the most common allotropes of carbon. Unlike diamond, graphite is an electrical conductor, and can be used, for instance, as the material in the electrodes of an electrical arc lamp. Graphite holds the distinction of being the most stable form of carbon under standard conditions. Therefore, it is used in thermochemistry as the
standard stateIn chemistry, the standard state of a material is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry recommends a conventional set of standard states...
for defining the heat of formation of carbon compounds.
Graphite is able to
conductConduct may refer to:*Behavior*a personal behavior, a way of acting and showing one's behaviour*using hand gestures to direct*Action , in relation to moral or ethical precepts*Conducting a musical ensemble...
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
, due to delocalization of the
pi bondIn chemistry, pi bonds are covalent chemical bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital...
electrons above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted along the plane of the layers. In diamond all four outer electrons of each carbon atom are 'localised' between the atoms in covalent bonding. The movement of electrons is restricted and diamond does not conduct an electric current. In graphite, each carbon atom uses only 3 of its 4 outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a delocalised system of electrons that is also a part of the chemical bonding. The delocalised electrons are free to move throughout the plane. For this reason, graphite conducts electricity along the planes of carbon atoms, but does not conduct in a direction at right angles to the plane.
Graphite powder is used as a dry
lubricantA lubricant is a substance introduced between two moving surfaces to reduce the friction between them, improving efficiency and reducing wear often used while having sex...
. Although it might be thought that this industrially important property is due entirely to the
loose interlamellar couplingCleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
between sheets in the structure, in fact in a
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty...
environment (such as in technologies for use in
spaceOuter space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
), graphite was found to be a very poor lubricant. This fact led to the discovery that graphite's lubricity is due to adsorbed air and water between the layers, unlike other layered dry lubricants such as
molybdenum disulfideMolybdenum disulfide is the inorganic compound with the formula MoS
2. This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. It is the principal ore from which molybdenum metal is extracted. The natural amorphous form is known as the rarer mineral jordisite...
. Recent studies suggest that an effect called
superlubricitySuperlubricity is a regime of motion in which friction vanishes or very nearly vanishes.Superlubricity may occur when two crystalline surfaces slide over each other in dry incommensurate contact...
can also account for this effect.
When a large number of crystallographic defects bind these planes together, graphite loses its lubrication properties and becomes what is known as
pyrolytic carbonPyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets as a result of imperfections in its production....
, a useful material in blood-contacting implants such as prosthetic
heart valveIn anatomy, the heart valves maintain the unidirectional flow of blood in the heart by opening and closing depending on the difference in pressure on each side...
s.
Natural and crystalline graphites are not often used in pure form as structural materials due to their shear-planes, brittleness and inconsistent mechanical properties.
In its pure glassy (isotropic) synthetic forms, pyrolytic graphite and
carbon fiberCarbon fiber is a material consisting of extremely thin fibers about 0.005–0.010 mm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber...
graphite is an extremely strong, heat-resistant (to 3000 °C) material, used in reentry shields for missile nosecones,
solid rocketA solid rocket or a solid-fuel rocket is a rocket with a motor that uses solid propellants . The earliest rockets were solid-fueled and powered by gunpowder; they were used by the Chinese, Mongols and Arabs in warfare as early as the 13th century...
engines,
high temperature reactorsThe pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of Very high temperature reactor [formally known as the high temperature gas reactor ], one of the six classes of nuclear reactors in the Generation IV initiative...
,
brakeA brake is a device for applying a force against the friction of the road, slowing or stopping the motion of a machine or vehicle, or alternatively a device to restrain it from starting to move again. The kinetic energy lost by the moving part is usually translated to heat by friction...
shoes and
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
brushes.
Intumescent or expandable graphites are used in fire seals, fitted around the perimeter of a fire door. During a fire the graphite intumesces (expands and chars) to resist fire penetration and prevent the spread of fumes. A typical start expansion temperature (SET) is between 150 and 300 °C.
Density: its specific gravity is 2.3 which makes it lighter than diamond.
Effect of heat: it is the most stable allotrope of carbon. At high temperatures and pressures (roughly 2000 °C and 5 GPa), it can be transformed into diamond. At about 700 °C it burns in oxygen forming carbon dioxide.
Chemical activity: it is slightly more reactive than diamond. This is because the reactants are able to penetrate between the hexagonal layers of carbon atoms in graphite. It is unaffected by ordinary solvents, dilute acids, or fused alkalis. However, chromic acid oxidises it to carbon dioxide.
Graphene
A single layer of graphite, once thought to be impossible, is called
grapheneGraphene is a one-atom-thick planar sheet of sp
2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds...
and has extraordinary electrical, thermal, and physical properties. It can be produced by epitaxy (vapor deposition) on an insulating surface or by mechanical exfoliation (repeated peeling). Its applications may include replacing silicon in high-performance electronic devices.
Amorphous carbon
Amorphous carbon is the name used for
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
that does not have any
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
line structure. As with all
glassyAn "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms. . Most classes of solid materials can be found or prepared in an amorphous form...
materials, some short-range order can be observed, but there is no long-range pattern of atomic positions. While entirely amorphous carbon can be produced, most amorphous carbon actually contains microscopic crystals of
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
-like, or even
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
-like carbon.
CoalCoal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
and
sootSoot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
or
carbon blackCarbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface area to volume ratio, although its surface area to...
are informally called amorphous carbon. However, they are products of
pyrolysisPyrolysis is the chemical decomposition of condensed organic substances by heating. The word is coined from the Greek-derived elements pyro "fire" and lysys "decomposition"....
(the process of decomposing a substance by the action of heat), which does not produce true amorphous carbon under normal conditions. The coal industry divides coal up into various grades depending on the amount of carbon present in the sample compared to the amount of impurities. The highest grade,
anthraciteAnthracite is a hard, compact variety of mineral coal that has a high lustre...
, is about 90% carbon and 10% other elements.
Bituminous coalBituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than anthracite coal....
is about 75-90% carbon, and
ligniteLignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat. It is considered the lowest rank of coal; it is mined in Russia, the United States, Australia and many European countries,...
is the name for coal that is around 55% carbon.
Buckminsterfullerenes
The
buckminsterfullerenes, or usually just
fullerenes for short, were discovered in 1985 by a team of scientists from Rice University and the University of Sussex, three of whom were awarded the 1996 Nobel Prize in Chemistry. They are named for the resemblance of their alliotropic structure to the geodesic structures devised by the scientist and architect
Richard Buckminster "Bucky" FullerRichard Buckminster “Bucky” Fuller was an American architect, author, designer, inventor, and futurist.Fuller published more than thirty books, inventing and popularizing terms such as "Spaceship Earth", ephemeralization, and synergetics...
. Fullerenes are molecules of varying sizes composed entirely of carbon, which take the form of a hollow sphere, ellipsoid, or tube.
As of the early twenty-first century, the chemical and physical properties of fullerenes are still under heavy study, in both pure and applied research labs. In April 2003, fullerenes were under study for potential medicinal use — binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma.
Buckyballs
Spherical fullerenes are popularly known as buckyballs, and have the formula C
60.
Carbon nanotubes
Carbon nanotubes, also called buckytubes, are cylindrical
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
molecules with novel properties that make them potentially useful in a wide variety of applications (e.g., nano-electronics,
opticsOptics is the branch of physics which studies the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
, materials applications, etc.). They exhibit extraordinary strength, unique
electricalElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
properties, and are efficient conductors of
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
. Inorganic nanotubes have also been synthesized.
A nanotube is a member of the
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is
cylindricalA cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given straight line, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...
, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
Carbon nanobuds
Carbon NanoBuds are a newly discovered allotrope of
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in which
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
like "buds" are covalently attached to the outer sidewalls of the carbon nanotubes. This hybrid material has useful properties of both fullerenes and carbon nanotubes. In particular, they have been found to be exceptionally good
field emittersField emission is an emission of electrons induced by external electromagnetic fields. FE can happen from solid and liquid surfaces, or individual atoms into vacuum or open air, or result in promotion of electrons from the valence to conduction band of semiconductors...
.
Glassy carbon
Glassy carbon or
vitreous carbon is a class of non-graphitizing
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
which is widely used as an electrode material in
electrochemistryElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
, as well as for high temperature crucibles and as a component of some prosthetic devices. It was first produced by workers at the laboratories of The General Electric Company, UK, in the early 1960s, using cellulose as the starting material. A short time later, Japanese workers produced a similar material from
phenolic resinPhenolic resin can include any of various synthetic thermosetting resins such as Bakelite, obtained by the reaction of phenols with simple aldehydes such as formaldehyde. Phenolics can be used to make molded products including pool balls, laboratory countertops, and as coatings and...
.
It was first produced by Bernard Redfern in the mid 1950's at the laboratories of The Carborundum Company, Trafford Park, Manchester, UK. He set out to develop a polymer matrix to mirror a diamond structure and discovered a resole (phenolic) resin that would, with special preparation, set without a catalyst. Using this resin the first glassy carbon was produced. Patents were filed some of which were withdrawn in the interests of national security. Original research samples of resin and product exist.
The preparation of glassy carbon involves subjecting the organic precursors to a series of heat treatments at temperatures up to 3000
oC. Unlike many non-graphitizing carbons, they are impermeable to gases and are chemically extremely inert, especially those which have been prepared at very high temperatures. It has been demonstrated that the rates of oxidation of certain glassy carbons in oxygen, carbon dioxide or water vapour are lower than those of any other carbon. They are also highly resistant to attack by acids. Thus, while normal
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
is reduced to a powder by a mixture of concentrated sulfuric and nitric acids at room temperature, glassy carbon is unaffected by such treatment, even after several months.
Carbon nanofoam
Carbon nanofoam is the fifth known allotrope of carbon discovered in 1997 by Andrei V. Rode and co-workers at the
Australian National UniversityThe Australian National University, commonly abbreviated to ANU, is a public teaching and research university located in Canberra, Australia, the federal capital city...
in
CanberraCanberra is the capital city of Australia. With a population of over 345,000, it is Australia's largest inland city and the eighth largest Australian city overall. The city is located at the northern end of the Australian Capital Territory, south-west of Sydney, and north-east of Melbourne...
. It consists of a low-density cluster-assembly of carbon atoms strung together in a loose three-dimensional web.
Each cluster is about 6 nanometers wide and consists of about 4000 carbon
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s linked in
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
-like sheets that are given negative curvature by the inclusion of
heptagonIn geometry, a heptagon is a polygon with seven sides and seven angles. In a regular heptagon, in which all sides and all angles are equal, the sides meet at an angle of 5π/7 radians, 128.5714286 degrees. Its Schläfli symbol is {7}...
s among the regular
hexagonIn geometry, a hexagon is a polygon with six edges and six vertices. A regular hexagon has Schläfli symbol {6}.- Regular hexagon :The internal angles of a regular hexagon are all 120° and the hexagon has 720 degrees T. It has 6 rotational symmetries and 6 reflection symmetries, making up the...
al pattern. This is the opposite of what happens in the case of buckminsterfullerenes, in which carbon sheets are given positive curvature by the inclusion of
pentagonIn geometry, a pentagon is any five-sided polygon. A pentagon may be simple or self-intersecting. The internal angles in a simple pentagon total 540°.- Regular pentagons :...
s.
The large-scale structure of carbon nanofoam is similar to that of an
aerogelAerogel is a manufactured material with the lowest bulk density of any known porous solid.
It is derived from a gel in which the liquid component of the gel has been replaced with a gas. The result is an extremely low-density solid with several remarkable...
, but with 1% of the density of previously produced carbon aerogels - only a few times the density of air at
sea levelMean sea level is the average height of the ocean's surface ; used as a standard in reckoning land elevation.- Measurement :...
. Unlike carbon aerogels, carbon nanofoam is a poor
electrical conductorElectrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current...
.
Lonsdaleite (hexagonal diamond)
Lonsdaleite is a hexagonal allotrope of the carbon allotrope
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
, believed to form from
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
present in
meteorMETEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
ites upon their impact to
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
lattice. Hexagonal diamond has also been synthesized in the lab, by compressing and heating graphite either in a static press or using explosives. It can also be produced by the thermal decomposition of a polymer,
poly(hydridocarbyne)Poly is one of a class of carbon-based random network polymers primarily composed of tetrahedrally hybridized carbon atoms, each having one hydride substituent, exhibiting the generic formula [HC]n. PHC is made from bromoform, a liquid halocarbon that is commercially manufactured from methane. At...
, at atmospheric pressure, under inert gas atmosphere (e.g. argon, nitrogen), starting at temperature .
Variability of carbon
The system of carbon allotropes spans an astounding range of extremes, considering that they are all merely structural formations of the same element.
Between diamond and graphite:
- Diamond crystallizes in the cubic system but graphite crystallizes in the hexagonal system.
- Diamond is clear and transparent, but graphite is black and opaque
- Diamond is hardest mineral known (10 on the Mohs scale
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science...
), but graphite is one of the softest (1–2 on Mohs scaleThe Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science...
).
- Diamond is the ultimate abrasive, but graphite is soft and is a very good lubricant.
- Diamond is an excellent electrical insulator, but graphite is a conductor of electricity.
- Diamond is an excellent thermal conductor, but some forms of graphite are used for thermal insulation (for example heat shields and firebreaks)
Despite the hardness of diamonds, the chemical bonds that hold the carbon atoms in diamonds together are actually weaker than those that hold together graphite. The difference is that in diamond, the bonds form an inflexible three-dimensional lattice. In graphite, the atoms are tightly bonded into sheets, but the sheets can slide easily making graphite soft.
Other possible forms
- Chaoite
Chaoite or white carbon is a mineral described as an allotrope of carbon whose existence is disputed. It was discovered in shock-fused graphite gneiss from the Ries crater in Bavaria. It has been described as slightly harder than graphite, with a reflection colour of grey to white. From its...
is a mineral believed to have been formed in meteorite impacts. It has been described as slightly harder than graphite with a reflection colour of grey to white. However, the existence of carbyne phases is disputed – see the entry on chaoiteChaoite or white carbon is a mineral described as an allotrope of carbon whose existence is disputed. It was discovered in shock-fused graphite gneiss from the Ries crater in Bavaria. It has been described as slightly harder than graphite, with a reflection colour of grey to white. From its...
for details.
- Metallic carbon: Theoretical studies have shown that there are regions in the phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically-distinct phases can occur at equilibrium...
, at extremely high pressures, where carbon has metallic character.
- At ultrahigh pressures of above 1000 GPa, diamond is predicted to transform into the so-called C8 structure, a body-centered cubic structure with 8 atoms in the unit cell. This cubic carbon phase might have importance in astrophysics. Its structure is known in one of the metastable phases of silicon and is similar to cubane
Cubane is a synthetic hydrocarbon molecule that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substance, cubane is one of the Platonic hydrocarbons. It was first synthesized in 1964 by Philip Eaton, a...
. Superdense and superhard material resembling this phase has been synthesized recently.
- Prismane C8 is a theoretically-predicted metastable carbon allotrope comprising an atomic cluster
In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
of eight carbon atoms, with the shape of an elongated triangular bipyramid—a six-atom triangular prismIn geometry, an n-sided prism is a polyhedron made of an n-sided polygonal base, a translated copy, and n faces joining corresponding sides. Thus these joining faces are parallelograms. All cross-sections parallel to the base faces are the same...
with two more atoms above and below its bases.
External links
- http://www.dendritics.com/scales/c-allotropes.asp
- http://cst-www.nrl.navy.mil/lattice/struk/carbon.html
- diamond 3D animation