Carbon (from Latin: carbo "coal") 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. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life
of about 5,730 years.[11]
Carbon is one of the few
elements known since antiquity.[12]
There are several allotropes of carbon of which the best known are graphite, diamond, and amorphous carbon.[13]
The physical properties of carbon vary widely with the allotropic form. For
example, diamond is highly transparent, while graphite is opaque and black. Diamond is the hardest naturally-occurring
material known, while graphite is soft enough to form a streak on paper (hence
its name, from the Greek word "γράφω" which means "to
write"). Diamond has a very low electrical conductivity, while graphite is a very good conductor. Under normal conditions, diamond, carbon nanotube
and graphene have the
highest thermal conductivities of all
known materials.
All carbon allotropes are solids under
normal conditions, with graphite being the most thermodynamically
stable form. They are chemically resistant
and require high temperature to react even with oxygen. The most common oxidation state
of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide
and other transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones,
dolomites
and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil and methane clathrates.
Carbon forms a vast number of compounds,
more than any other element, with almost ten million compounds described to
date,[14]
which in turn are a tiny fraction of such compounds that are theoretically
possible under standard conditions.
Carbon is the 15th most
abundant element in the Earth's crust, and
the fourth
most abundant element in the universe by mass
after hydrogen, helium, and oxygen. It is present in all known life forms, and in the human body
carbon is the second most abundant element by mass (about 18.5%) after oxygen.[15]
This abundance, together with the unique diversity of organic compounds
and their unusual polymer-forming ability at the temperatures commonly encountered on
Earth, make
this element the chemical basis of all known life.
On 21 February 2014, NASA announced a greatly upgraded database
for tracking polycyclic
aromatic hydrocarbons (PAHs) in the universe.
According to scientists, more than 20% of the carbon in the universe may be
associated with PAHs, possible starting materials for the formation of life. PAHs
seem to have been formed shortly after the Big Bang, are
widespread throughout the universe, and are associated with new stars
and exoplanets.[16]
Characteristics
The different forms or allotropes
of carbon (see below) include the hardest naturally occurring substance, diamond, and also
one of the softest known substances, graphite.
Moreover, it has an affinity for bonding with
other small atoms,
including other carbon atoms, and is capable of forming multiple stable covalent bonds
with such atoms. As a result, carbon is known to form almost ten million
different compounds; the large majority of all chemical compounds.[14]
Carbon also has the highest sublimation point of all elements. At atmospheric pressure it has no melting point as its triple point
is at 10.8 ± 0.2 MPa and 4,600 ± 300 K (~4,330 °C or 7,820 °F),[2][3]
so it sublimes at about 3,900 K.[17][18]
Carbon sublimes in a carbon arc
which has a temperature of about 5,800 K (5,530 °C; 9,980 °F). Thus,
irrespective of its allotropic form, carbon remains solid at higher
temperatures than the highest melting point metals such as tungsten or rhenium. Although
thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements
such as iron and copper that are weaker reducing agents at room temperature.
Carbon compounds form the basis of
all known life on Earth, and the carbon-nitrogen cycle
provides some of the energy produced by the Sun and other stars. Although it forms an extraordinary variety of compounds,
most forms of carbon are comparatively unreactive under normal conditions. At
standard temperature and pressure, it resists all but the strongest oxidizers.
It does not react with sulfuric acid,
hydrochloric acid, chlorine or any alkalis. At
elevated temperatures carbon reacts with oxygen to form carbon oxides, and will reduce such metal oxides as iron oxide to the metal. This exothermic
reaction is used in the iron and steel industry to control the carbon content
of steel:
Fe
3O
4 + 4 C(s) → 3 Fe(s) + 4 CO(g)
3O
4 + 4 C(s) → 3 Fe(s) + 4 CO(g)
C(s) + H2O(g)
→ CO(g) + H2(g).
Carbon combines with some metals at
high temperatures to form metallic carbides, such as the iron carbide cementite
in steel, and tungsten carbide, widely used as an abrasive and for
making hard tips for cutting tools.
As of 2009, graphene appears
to be the strongest material ever tested.[19]
However, the process of separating it from graphite will
require some technological development before it is economical enough to be
used in industrial processes.[20]
The system of carbon allotropes
spans a range of extremes:
|
Graphite is one of the softest
materials known.
|
|
|
Diamond is the best known
naturally occurring thermal
conductor
|
Some forms of graphite are used
for thermal insulation (i.e. firebreaks and heat shields), but some other forms are good thermal conductors.
|
|
Diamond is highly transparent.
|
|
Allotropes
Atomic carbon is a very short-lived species and, therefore, carbon is
stabilized in various multi-atomic structures with different molecular
configurations called allotropes. The three relatively well-known allotropes of carbon are amorphous carbon,
graphite, and diamond. Once
considered exotic, fullerenes are nowadays commonly synthesized and used in research;
they include buckyballs,[26][27]
carbon nanotubes,[28]
carbon nanobuds[29]
and nanofibers.[30][31]
Several other exotic allotropes have also been discovered, such as lonsdaleite,[32]
glassy carbon,[33]
carbon nanofoam[34]
and linear
acetylenic carbon (carbyne).[35]
The amorphous
form is an assortment of carbon atoms in a non-crystalline, irregular, glassy
state, which is essentially graphite but not
held in a crystalline macrostructure. It is present as a powder, and is the
main constituent of substances such as charcoal, lampblack
(soot) and activated carbon.
At normal pressures carbon takes the form of graphite, in which
each atom is bonded trigonally to three others in a plane composed of fused hexagonal
rings, just like those in aromatic hydrocarbons.[36]
The resulting network is 2-dimensional, and the resulting flat sheets are
stacked and loosely bonded through weak van der Waals forces. This gives graphite its softness and its cleaving properties (the sheets slip easily past one another).
Because of the delocalization of one of the outer electrons of each atom to
form a π-cloud, graphite conducts electricity,
but only in the plane of each covalently bonded
sheet. This results in a lower bulk electrical conductivity for carbon than for most metals. The delocalization also accounts for the energetic
stability of graphite over diamond at room temperature.
At very high pressures carbon forms
the more compact allotrope diamond, having nearly twice the density of graphite. Here, each
atom is bonded tetrahedrally to four others, thus making a 3-dimensional network of
puckered six-membered rings of atoms. Diamond has the same cubic structure as silicon and germanium and because of the strength of the carbon-carbon bonds, it is
the hardest naturally occurring substance in terms of resistance to scratching. Contrary to the popular belief that "diamonds
are forever", they are in fact thermodynamically unstable under normal
conditions and transform into graphite.[13]
However, due to a high activation energy barrier, the transition into graphite
is so extremely slow at room temperature as to be unnoticeable. Under some
conditions, carbon crystallizes as lonsdaleite.
This form has a hexagonal crystal lattice where all atoms are covalently bonded. Therefore,
all properties of lonsdaleite are close to those of diamond.[32]
Fullerenes have a graphite-like structure, but instead of purely hexagonal packing, they also contain pentagons (or even heptagons) of
carbon atoms, which bend the sheet into spheres, ellipses or cylinders. The
properties of fullerenes (split into buckyballs,
buckytubes and nanobuds) have not yet been fully analyzed and represent an intense
area of research in nanomaterials. The names "fullerene" and "buckyball"
are given after Richard Buckminster Fuller, popularizer of geodesic domes,
which resemble the structure of fullerenes. The buckyballs are fairly large
molecules formed completely of carbon bonded trigonally, forming spheroids
(the best-known and simplest is the soccerball-shaped C60 buckminsterfullerene).[26]
Carbon nanotubes are structurally similar to buckyballs, except that each atom
is bonded trigonally in a curved sheet that forms a hollow cylinder.[27][28]
Nanobuds were first reported in 2007 and are hybrid bucky tube/buckyball
materials (buckyballs are covalently bonded to the outer wall of a nanotube)
that combine the properties of both in a single structure.[29]
Of the other discovered allotropes, carbon nanofoam
is a ferromagnetic allotrope discovered in 1997. It consists of a low-density
cluster-assembly of carbon atoms strung together in a loose three-dimensional
web, in which the atoms are bonded trigonally in six- and seven-membered rings.
It is among the lightest known solids, with a density of about 2 kg/m3.[37]
Similarly, glassy carbon contains a high proportion of closed porosity,[33]
but contrary to normal graphite, the graphitic layers are not stacked like
pages in a book, but have a more random arrangement. Linear
acetylenic carbon[35]
has the chemical structure[35]
-(C:::C)n-. Carbon in this modification is linear with sp orbital hybridization, and is a polymer with
alternating single and triple bonds. This type of carbyne is of considerable
interest to nanotechnology as its Young's modulus
is forty times that of the hardest known material – diamond.[38]
Occurrence
Carbon is the fourth
most abundant chemical element
in the universe by mass after hydrogen, helium, and oxygen. Carbon is abundant
in the Sun, stars, comets, and in the atmospheres of most planets.[16]
Some meteorites
contain microscopic diamonds that were formed when the solar system
was still a protoplanetary disk. Microscopic diamonds may also be formed by the intense
pressure and high temperature at the sites of meteorite impacts.[39]
In combination with oxygen in carbon dioxide,
carbon is found in the Earth's atmosphere (approximately 810 gigatonnes of
carbon) and dissolved in all water bodies (approximately 36,000 gigatonnes
of carbon). Around 1,900 gigatonnes of carbon are present in the biosphere.
Hydrocarbons
(such as coal, petroleum,
and natural gas) contain carbon as well. Coal "reserves"
(not "resources") amount to
around 900 gigatonnes with perhaps 18 000 Gt of resources.[40] Oil reserves
are around 150 gigatonnes. Proven sources of natural gas are about
175 1012 cubic metres (representing about 105 gigatonnes
carbon), but it is estimated that there are also about 900 1012
cubic metres of "unconventional" gas such as shale gas,
representing about 540 gigatonnes of carbon.[41]
Carbon is also locked up as methane hydrates
in polar regions and under the seas. Various estimates of the amount of carbon
this represents have been made: 500 to 2500 Gt,[42] or 3000
Gt.[43]
In the past, quantities of hydrocarbons were greater. According to one source,
in the period from 1751 to 2008 about 347 gigatonnes of carbon were released as
carbon dioxide to the atmosphere from burning of fossil fuels.[44]
However, another source puts the amount added to the atmosphere for the period
since 1750 at 879 Gt, and the total going to the atmosphere, sea, and land
(such as peat bogs) at almost 2000 Gt.[45]
Carbon is a major component in very
large masses of carbonate rock (limestone,
dolomite, marble and so on). Coal is the largest commercial source of mineral carbon,
accounting for 4,000 gigatonnes or 80% of fossil carbon fuel.[46]
It is also rich in carbon – for example, anthracite
contains 92–98%.[47]
As for individual carbon allotropes,
graphite is found in large quantities in the United States
(mostly in New York and Texas), Russia, Mexico, Greenland, and India. Natural diamonds occur in the rock kimberlite,
found in ancient volcanic "necks", or "pipes". Most diamond
deposits are in Africa, notably in South Africa,
Namibia, Botswana, the Republic of the Congo, and Sierra Leone.
There are also deposits in Arkansas, Canada, the Russian Arctic, Brazil and in Northern and Western Australia.
Diamonds are now also being recovered from the ocean floor off the Cape of Good Hope.
However, though diamonds are found naturally, about 30% of all industrial
diamonds used in the U.S. are now made synthetically.
Carbon-14 is formed in upper layers
of the troposphere and the stratosphere, at altitudes of 9–15 km, by a
reaction that is precipitated by cosmic rays.[48]
Thermal neutrons are produced that collide with the nuclei of nitrogen-14,
forming carbon-14 and a proton.
Carbon-rich asteroids are relatively
preponderant in the outer parts of the asteroid belt
in our solar system. These asteroids have not yet been directly sampled by
scientists. The asteroids can be used in hypothetical Space-based carbon mining,
which may be possible in the future, but is currently technologically
impossible.[49]
Isotopes
Isotopes of carbon are atomic nuclei
that contain six protons plus a number of neutrons (varying
from 2 to 16). Carbon has two stable, naturally occurring isotopes.[11]
The isotope carbon-12 (12C) forms 98.93% of the carbon on Earth, while
carbon-13
(13C) forms the remaining 1.07%.[11]
The concentration of 12C is further increased in biological
materials because biochemical reactions discriminate against 13C.[50]
In 1961, the sotope carbon-12 was adopted
as the basis for atomic weights.[51]
Identification of carbon in NMR experiments is done with the isotope 13C.
Carbon-14 (14C) is a naturally occurring radioisotope
which occurs in trace amounts on Earth of up to 1 part per trillion
(0.0000000001%), mostly confined to the atmosphere and superficial deposits,
particularly of peat and other
organic materials.[52]
This isotope decays by 0.158 MeV β− emission.
Because of its relatively short half-life
of 5730 years, 14C is virtually absent in ancient rocks, but is
created in the upper atmosphere (lower stratosphere
and upper troposphere) by interaction of nitrogen with cosmic rays.[53]
The abundance of 14C in the atmosphere
and in living organisms is almost constant, but decreases predictably in their
bodies after death. This principle is used in radiocarbon dating, invented in 1949, which has been used extensively to
determine the age of carbonaceous materials with ages up to about
40,000 years.[54][55]
There are 15 known isotopes of
carbon and the shortest-lived of these is 8C which decays through proton emission
and alpha decay and has a half-life of 1.98739x10−21 s.[56]
The exotic 19C exhibits a nuclear halo,
which means its radius is appreciably larger than would be expected if the nucleus were a sphere of constant density.[57]
Formation
in stars
Formation of the carbon atomic
nucleus requires a nearly simultaneous triple collision of alpha particles
(helium nuclei)
within the core of a giant or supergiant star which is known as the triple-alpha process, as the products of further nuclear fusion reactions of
helium with hydrogen or another helium nucleus produce lithium-5 and beryllium-8 respectively, both of which are highly unstable and decay
almost instantly back into smaller nuclei.[58]
This happens in conditions of temperatures over 100 megakelvin and helium
concentration that the rapid expansion and cooling of the early universe
prohibited, and therefore no significant carbon was created during the Big Bang. Instead,
the interiors of stars in the horizontal branch
transform three helium nuclei into carbon by means of this triple-alpha process.[59]
In order to be available for formation of life as we know it, this carbon must
then later be scattered into space as dust, in supernova
explosions, as part of the material which later forms second, third-generation
star systems which have planets accreted from such dust.[60][16]
The Solar System is one such third-generation star
system. Another of the fusion mechanisms powering stars is the CNO cycle,
in which carbon acts as a catalyst to allow the reaction to proceed.
Rotational transitions of various
isotopic forms of carbon monoxide (for example, 12CO, 13CO,
and C18O) are detectable in the submillimeter wavelength range, and are used in the study of newly forming stars
in molecular clouds.[61]
Carbon
cycle
Under terrestrial conditions,
conversion of one element to another is very rare. Therefore, the amount of
carbon on Earth is effectively constant. Thus, processes that use carbon must
obtain it somewhere and dispose of it somewhere else. The paths that carbon
follows in the environment make up the carbon cycle.
For example, plants draw carbon dioxide
out of their environment and use it to build biomass, as in carbon respiration or the Calvin cycle,
a process of carbon fixation. Some of this biomass is eaten by animals, whereas some
carbon is exhaled by animals as carbon dioxide. The carbon cycle is
considerably more complicated than this short loop; for example, some carbon
dioxide is dissolved in the oceans; dead plant or animal matter may become petroleum
or coal, which
can burn with the release of carbon, should bacteria not consume it.[62][63]
Compounds
Organic
compounds
Carbon has the ability to form very
long chains of interconnecting C-C bonds. This property is called catenation.
Carbon-carbon bonds are strong, and stable. This property allows carbon to form
an almost infinite number of compounds; in fact, there are more known
carbon-containing compounds than all the compounds of the other chemical
elements combined except those of hydrogen (because almost all organic
compounds contain hydrogen as well).
The simplest form of an organic
molecule is the hydrocarbon—a large family of organic molecules
that are composed of hydrogen atoms bonded to a chain of carbon atoms. Chain length, side
chains and functional groups all affect the properties of organic molecules.
Carbon occurs in all known organic life and
is the basis of organic chemistry. When united with hydrogen, it forms
various hydrocarbons which are important to industry as refrigerants,
lubricants,
solvents, as
chemical feedstock for the manufacture of plastics and petrochemicals
and as fossil fuels.
When combined with oxygen and
hydrogen, carbon can form many groups of important biological compounds
including sugars, lignans, chitins, alcohols, fats, and aromatic esters, carotenoids and terpenes. With nitrogen it forms alkaloids,
and with the addition of sulfur also it forms antibiotics,
amino acids,
and rubber products.
With the addition of phosphorus to these other elements, it forms DNA and RNA, the chemical-code carriers of life, and adenosine triphosphate (ATP), the most important energy-transfer molecule in all
living cells.
Inorganic
compounds
Commonly carbon-containing compounds
which are associated with minerals or which do not contain hydrogen or
fluorine, are treated separately from classical organic compounds;
however the definition is not rigid (see reference articles above). Among these
are the simple oxides of carbon. The most prominent oxide is carbon dioxide
(CO
2). This was once the principal constituent of the paleoatmosphere, but is a minor component of the Earth's atmosphere today.[64] Dissolved in water, it forms carbonic acid (H
2CO
3), but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable.[65] Through this intermediate, though, resonance-stabilized carbonate ions are produced. Some important minerals are carbonates, notably calcite. Carbon disulfide (CS
2) is similar.
2). This was once the principal constituent of the paleoatmosphere, but is a minor component of the Earth's atmosphere today.[64] Dissolved in water, it forms carbonic acid (H
2CO
3), but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable.[65] Through this intermediate, though, resonance-stabilized carbonate ions are produced. Some important minerals are carbonates, notably calcite. Carbon disulfide (CS
2) is similar.
The other common oxide is carbon monoxide
(CO). It is formed by incomplete combustion, and is a colorless, odorless gas.
The molecules each contain a triple bond and are fairly polar,
resulting in a tendency to bind permanently to hemoglobin molecules, displacing
oxygen, which has a lower binding affinity.[66][67]
Cyanide (CN–),
has a similar structure, but behaves much like a halide ion (pseudohalogen). For example it can form the nitride cyanogen molecule
((CN)2), similar to diatomic halides. Other uncommon oxides are carbon suboxide
(C
3O
2),[68] the unstable dicarbon monoxide (C2O),[69][70] carbon trioxide (CO3),[71][72] cyclopentanepentone (C5O5)[73] cyclohexanehexone (C6O6),[73] and mellitic anhydride (C12O9).
3O
2),[68] the unstable dicarbon monoxide (C2O),[69][70] carbon trioxide (CO3),[71][72] cyclopentanepentone (C5O5)[73] cyclohexanehexone (C6O6),[73] and mellitic anhydride (C12O9).
With reactive metals, such as tungsten, carbon
forms either carbides (C4–), or acetylides
(C2−
2) to form alloys with high melting points. These anions are also associated with methane and acetylene, both very weak acids. With an electronegativity of 2.5,[74] carbon prefers to form covalent bonds. A few carbides are covalent lattices, like carborundum (SiC), which resembles diamond.
2) to form alloys with high melting points. These anions are also associated with methane and acetylene, both very weak acids. With an electronegativity of 2.5,[74] carbon prefers to form covalent bonds. A few carbides are covalent lattices, like carborundum (SiC), which resembles diamond.
Organometallic
compounds
Organometallic compounds by
definition contain at least one carbon-metal bond. A wide range of such
compounds exist; major classes include simple alkyl-metal compounds (for
example, tetraethyllead), η2-alkene compounds (for example, Zeise's salt),
and η3-allyl compounds (for example, allylpalladium
chloride dimer); metallocenes
containing cyclopentadienyl ligands (for example, ferrocene);
and transition
metal carbene complexes. Many metal carbonyls
exist (for example, tetracarbonylnickel); some workers consider the carbon monoxide
ligand to be purely inorganic, and not organometallic.
While carbon is understood to
exclusively form four bonds, an interesting compound containing an octahedral
hexacoordinated carbon atom has been reported. The cation of the compound is
[(Ph3PAu)6C]2+. This phenomenon has been
attributed to the aurophilicity of the gold ligands.[75]
History
and etymology
The English name carbon
comes from the Latin carbo
for coal and charcoal,[76] whence
also comes the French charbon, meaning charcoal. In German, Dutch and Danish, the
names for carbon are Kohlenstoff, koolstof and kulstof
respectively, all literally meaning coal-substance.
Carbon was discovered in prehistory
and was known in the forms of soot and charcoal to the earliest human civilizations. Diamonds were known probably as early as 2500 BCE in
China, while carbon in the form of charcoal was made
around Roman times by the same chemistry as it is today, by heating wood in a pyramid covered
with clay to
exclude air.[77][78]
In 1722, René
Antoine Ferchault de Réaumur
demonstrated that iron was transformed into steel through the absorption of
some substance, now known to be carbon.[79]
In 1772, Antoine Lavoisier showed that diamonds are a form of carbon; when he burned
samples of charcoal and diamond and found that neither produced any water and
that both released the same amount of carbon dioxide
per gram. In 1779,[80]
Carl Wilhelm Scheele showed that graphite, which had been thought of as a form
of lead, was
instead identical with charcoal but with a small admixture of iron, and that it
gave "aerial acid" (his name for carbon dioxide) when oxidized with
nitric acid.[81]
In 1786, the French scientists Claude Louis Berthollet, Gaspard Monge and C. A. Vandermonde confirmed that graphite was mostly
carbon by oxidizing it in oxygen in much the same way Lavoisier had done with
diamond.[82]
Some iron again was left, which the French scientists thought was necessary to
the graphite structure. However, in their publication they proposed the name carbone
(Latin carbonum) for the element in graphite which was given off as a
gas upon burning graphite. Antoine Lavoisier then listed carbon as an element in his
1789 textbook.[83]
A new allotrope
of carbon, fullerene, that was discovered in 1985[84]
includes nanostructured forms such as buckyballs
and nanotubes.[26]
Their discoverers – Robert Curl,
Harold Kroto
and Richard Smalley – received the Nobel Prize
in Chemistry in 1996.[85]
The resulting renewed interest in new forms lead to the discovery of further
exotic allotropes, including glassy carbon,
and the realization that "amorphous carbon"
is not strictly amorphous.[33]
Production
Graphite
Commercially viable natural deposits
of graphite occur in many parts of the world, but the most important sources
economically are in China, India, Brazil and North Korea.
Graphite deposits are of metamorphic
origin, found in association with quartz, mica and feldspars in schists, gneisses and metamorphosed sandstones
and limestone
as lenses or veins,
sometimes of a meter or more in thickness. Deposits of graphite in Borrowdale,
Cumberland,
England were at
first of sufficient size and purity that, until the 19th century, pencils were made simply by sawing blocks of natural graphite into
strips before encasing the strips in wood. Today, smaller deposits of graphite
are obtained by crushing the parent rock and floating the lighter graphite out
on water.[86]
There are three types of natural
graphite—amorphous, flake or crystalline flake, and vein or lump. Amorphous
graphite is the lowest quality and most abundant. Contrary to science, in
industry "amorphous" refers to very small crystal size rather than
complete lack of crystal structure. Amorphous is used for lower value graphite
products and is the lowest priced graphite. Large amorphous graphite deposits
are found in China, Europe, Mexico and the United States. Flake graphite is
less common and of higher quality than amorphous; it occurs as separate plates
that crystallized in metamorphic rock. Flake graphite can be four times the
price of amorphous. Good quality flakes can be processed into expandable
graphite for many uses, such as flame retardants.
The foremost deposits are found in Austria, Brazil, Canada, China, Germany and
Madagascar. Vein or lump graphite is the rarest, most valuable, and highest
quality type of natural graphite. It occurs in veins along intrusive contacts
in solid lumps, and it is only commercially mined in Sri Lanka.[86]
According to the USGS, world production of natural graphite was 1.1 million
tonnes in 2010, to which China contributed 800,000 t, India 130,000 t,
Brazil 76,000 t, North Korea 30,000 t and Canada 25,000 t. No natural
graphite was reported mined in the United States, but 118,000 t of synthetic
graphite with an estimated value of $998 million was produced in 2009.[86]
Diamond
The diamond supply chain is
controlled by a limited number of powerful businesses, and is also highly
concentrated in a small number of locations around the world (see figure).
Only a very small fraction of the
diamond ore consists of actual diamonds. The ore is crushed, during which care
has to be taken in order to prevent larger diamonds from being destroyed in
this process and subsequently the particles are sorted by density. Today,
diamonds are located in the diamond-rich density fraction with the help of X-ray fluorescence, after which the final sorting steps are done by hand.
Before the use of X-rays became commonplace, the separation was done with grease
belts; diamonds have a stronger tendency to stick to grease than the other
minerals in the ore.[87]
Historically diamonds were known to
be found only in alluvial deposits in southern India.[88]
India led the world in diamond production from the time of their discovery in
approximately the 9th century BCE[89] to
the mid-18th century AD, but the commercial potential of these sources had been
exhausted by the late 18th century and at that time India was eclipsed by
Brazil where the first non-Indian diamonds were found in 1725.[90]
Diamond production of primary
deposits (kimberlites and lamproites) only started in the 1870s after the
discovery of the Diamond fields in South Africa. Production has increased over
time and now an accumulated total of 4.5 billion carats have been mined
since that date.[91]
About 20% of that amount has been mined in the last 5 years alone, and during
the last ten years 9 new mines have started production while 4 more are waiting
to be opened soon. Most of these mines are located in Canada, Zimbabwe, Angola,
and one in Russia.[91]
In the United States, diamonds have
been found in Arkansas, Colorado and Montana.[92][93]
In 2004, a startling discovery of a microscopic diamond in the United States[94]
led to the January 2008 bulk-sampling of kimberlite pipes
in a remote part of Montana.[95]
Today, most commercially viable
diamond deposits are in Russia, Botswana, Australia and the Democratic
Republic of Congo.[96]
In 2005, Russia produced almost one-fifth of the global diamond output, reports
the. Australia has the richest diamantiferous pipe with production reaching
peak levels of 42 metric tons (41 long tons; 46 short tons) per year in the
1990s.[92]
There are also commercial deposits being actively mined in the Northwest Territories of Canada, Siberia (mostly in Yakutia territory;
for example, Mir pipe and Udachnaya pipe),
Brazil, and in Northern and Western Australia.
Applications
Carbon is essential to all known living
systems, and without it life as we know it could not exist (see alternative
biochemistry). The major economic use of carbon
other than food and wood is in the form of hydrocarbons, most notably the fossil fuel
methane gas and crude oil
(petroleum). Crude oil is used by the petrochemical industry to produce, amongst other things, gasoline and kerosene, through
a distillation process, in refineries.
Cellulose
is a natural, carbon-containing polymer produced by plants in the form of cotton, linen, and hemp. Cellulose is mainly used for maintaining structure in plants.
Commercially valuable carbon polymers of animal origin include wool, cashmere and silk. Plastics are made from synthetic carbon polymers, often with oxygen
and nitrogen atoms included at regular intervals in the main polymer chain. The
raw materials for many of these synthetic substances come from crude oil.
The uses of carbon and its compounds
are extremely varied. It can form alloys with iron, of which the most common is carbon steel.
Graphite is combined
with clays to form
the 'lead' used in pencils used for writing and drawing. It is
also used as a lubricant and a pigment, as a molding material in glass manufacture, in electrodes
for dry batteries and in electroplating
and electroforming, in brushes for electric motors
and as a neutron moderator in nuclear reactors.
Charcoal is used as a drawing material in artwork, for grilling, and in many other uses including iron smelting. Wood, coal
and oil are used as fuel for production of energy and space heating. Gem quality diamond is used
in jewelry, and industrial diamonds are used in drilling, cutting and polishing tools for
machining metals and stone. Plastics are made from fossil hydrocarbons, and carbon fiber,
made by pyrolysis of synthetic polyester
fibers is used
to reinforce plastics to form advanced, lightweight composite materials. Carbon fiber is made by pyrolysis of extruded and stretched filaments of
polyacrylonitrile (PAN) and other organic substances. The crystallographic
structure and mechanical properties of the fiber depend on the type of starting
material, and on the subsequent processing. Carbon fibers made from PAN have
structure resembling narrow filaments of graphite, but thermal processing may
re-order the structure into a continuous rolled sheet. The result is fibers
with higher specific tensile strength than steel.[97]
Carbon black is used as the black pigment in printing ink, artist's oil paint and water colours, carbon paper,
automotive finishes, India ink and laser printer
toner. Carbon black
is also used as a filler in rubber products such as tyres and in plastic
compounds. Activated charcoal is used as an absorbent and adsorbent in filter material in applications as diverse as gas masks,
water purification and kitchen extractor hoods and in medicine to absorb toxins, poisons, or gases from the digestive system. Carbon is used in chemical
reduction at high temperatures. Coke is used
to reduce iron ore into iron. Case hardening
of steel is achieved by heating finished steel components in carbon powder. Carbides of silicon, tungsten, boron and titanium, are
among the hardest known materials, and are used as abrasives
in cutting and grinding tools. Carbon compounds make up most of the materials
used in clothing, such as natural and synthetic textiles and leather, and
almost all of the interior surfaces in the built environment
other than glass, stone and metal.
Diamonds
The diamond industry
can be broadly separated into two basically distinct categories: one dealing
with gem-grade diamonds and another for industrial-grade diamonds. While a
large trade in both types of diamonds exists, the two markets act in
dramatically different ways.
A large trade in gem-grade
diamonds exists. Unlike precious metals
such as gold or platinum, gem
diamonds do not trade as a commodity:
there is a substantial mark-up in the sale of diamonds, and there is not a very
active market for resale of diamonds.
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 gemological 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 tonnes annually), unsuitable for use as
gemstones and known as bort, are destined for industrial use.[98]
In addition to mined diamonds, synthetic diamonds
found industrial applications almost immediately after their invention in the
1950s; another 3 billion carats (600 tonnes) of synthetic
diamond is produced annually for industrial use.[99]
The dominant industrial use of diamond is in cutting, drilling, grinding, and
polishing. Most uses of diamonds in these technologies do not require large
diamonds; in fact, most diamonds that are gem-quality except for their small
size, 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.[100]
Specialized applications include use in laboratories as containment for high pressure experiments (see diamond anvil cell), high-performance bearings, and limited use in specialized windows.[101][102]
With the continuing advances being made in the production of synthetic
diamonds, future applications are beginning to become feasible. Garnering much
excitement is the possible use of diamond as a semiconductor
suitable to build microchips from, or the use of diamond as a heat sink
in electronics.[103]
Precautions
Pure carbon has extremely low toxicity to humans
and can be handled and even ingested safely in the form of graphite or
charcoal. It is resistant to dissolution or chemical attack, even in the acidic
contents of the digestive tract, for example. Consequently once it enters into
the body's tissues it is likely to remain there indefinitely. Carbon black
was probably one of the first pigments to be used for tattooing, and Ötzi the Iceman
was found to have carbon tattoos that survived during his life and for
5200 years after his death.[104]
However, inhalation of coal dust or soot (carbon black)
in large quantities can be dangerous, irritating lung tissues and causing the
congestive lung disease coalworker's
pneumoconiosis. Similarly, diamond dust used as an
abrasive can do harm if ingested or inhaled. Microparticles of carbon are
produced in diesel engine exhaust fumes, and may accumulate in the lungs.[105]
In these examples, the harmful effects may result from contamination of the
carbon particles, with organic chemicals or heavy metals for example, rather
than from the carbon itself.
Carbon generally has low toxicity to
almost all life on Earth; however, to some creatures it can still be toxic. For
instance, carbon nanoparticles are deadly to Drosophila.[106]
Carbon may also burn vigorously and
brightly in the presence of air at high temperatures. Large accumulations of
coal, which have remained inert for hundreds of millions of years in the
absence of oxygen, may spontaneously combust when exposed to air, for example in coal mine waste tips.
In nuclear applications
where graphite is used as a neutron moderator,
accumulation of Wigner energy followed by a sudden, spontaneous release may occur. Annealing to at least 250°C can release the energy safely, although
in the Windscale fire the procedure went wrong, causing other reactor materials
to combust.
The great variety of carbon
compounds include such lethal poisons as tetrodotoxin,
the lectin ricin from seeds of the castor oil plant
Ricinus communis, cyanide (CN−) and carbon
monoxide; and such essentials to life as glucose and protein.
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