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Ilmu
|
Appearance |
Clear (diamond), black (graphite)
|
General properties |
Name, symbol, number | carbon, C, 6 |
Pronunciation | /ˈkɑrbən/ |
Element category | nonmetal |
Group, period, block | 14, 2, p |
Standard atomic weight | 12.0107 g·mol−1 |
Electron configuration | 1s2 2s2 2p2 or [He] 2s2 2p2 |
Electrons per shell | 2,4 (Image) |
Physical properties |
Phase | Solid |
Density (near r.t.) | amorphous:[1] 1.8 - 2.1 g·cm−3 |
Density (near r.t.) | graphite: 2.267 g·cm−3 |
Density (near r.t.) | diamond: 3.515 g·cm−3 |
Sublimation point | 3915 K, 3642 °C, 6588 °F |
Triple point | 4600 K (4327°C), 10800[2][3] kPa |
Heat of fusion | 117 (graphite) kJ·mol−1 |
Specific heat capacity | (25 °C) 8.517(graphite),
6.155(diamond) J·mol−1·K−1 |
Atomic properties |
Oxidation states | 4, 3 [4], 2, 1 [5], 0, -1, -2, -3, -4[6] |
Electronegativity | 2.55 (Pauling scale) |
Ionization energies
(more) | 1st: 1086.5 kJ·mol−1 |
2nd: 2352.6 kJ·mol−1 |
3rd: 4620.5 kJ·mol−1 |
Covalent radius | 77(sp³), 73(sp²), 69(sp) pm |
Van der Waals radius | 170 pm |
Miscellanea |
Magnetic ordering | diamagnetic[7] |
Thermal conductivity | (300 K) 119-165 (graphite)
900-2300 (diamond) W·m−1·K−1 |
Thermal expansion | (25 °C) 0.8 (diamond) [8] µm·m−1·K−1 |
Speed of sound (thin rod) | (20 °C) 18350 (diamond) m/s |
Young's modulus | 1050 (diamond) [8] GPa |
Shear modulus | 478 (diamond) [8] GPa |
Bulk modulus | 442 (diamond) [8] GPa |
Poisson ratio | 0.1 (diamond) [8] |
Mohs hardness | 1-2 (Graphite)
10 (Diamond) |
CAS registry number | 7440-44-0 |
Most stable isotopes |
Main article: Isotopes of carbon |
|
|
Carbon 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 5730 years.
[9] Carbon is one of the
few elements known since antiquity.
[10][11] The name "carbon" comes from
Latin language carbo,
coal, and, in some
Romance and
Slavic languages, the word carbon can refer both to the element and to coal.
There are several
allotropes of carbon of which the best known are
graphite,
diamond, and
amorphous carbon.
[12] 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 among the hardest materials known, while graphite is soft enough to form a streak on paper (hence its name, from the Greek word "to write"). Diamond has a very low
electrical conductivity, while graphite is a very good
conductor. Under normal conditions, diamond has the highest
thermal conductivity of
all known materials. All the allotropic forms are solids under normal conditions but graphite is the most
thermodynamically stable.
All forms of carbon are highly stable, requiring 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 more
compounds than any other element, with almost ten million pure
organic compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions.
[13]
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
lifeforms, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen.
[14] 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.
Characteristics
Theoretically predicted phase diagram of carbon
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.
[13] Carbon also has the highest
melting and
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 4600 ± 300 K,
[2][3] so it sublimates at about 3900 K.
[15][16].
Carbon sublimes in a carbon arc which has a temperature of about 5800 K. 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:
- Fe3O4 + 4 C(s) → 3 Fe(s) + 4 CO(g)
with
sulfur to form
carbon disulfide and with steam in the coal-gas reaction:
- 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.
[17] However, the process of separating it from
graphite will require some technological development before it is economical enough to be used in industrial processes.
[18]
The system of carbon allotropes spans a range of extremes:
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,
[19][20] carbon nanotubes,
[21] carbon nanobuds[22] and
nanofibers.
[23][24] Several other exotic allotropes have also been discovered, such as
lonsdaleite,
[25] glassy carbon,
[26] carbon nanofoam[27] and
linear acetylenic carbon.
[28]
- 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. 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, thanks to the strength of the carbon-carbon bonds, 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.[12] But 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.[25]
- 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 structure C60 buckminsterfullerene).[19] Carbon nanotubes are structurally similar to buckyballs, except that each atom is bonded trigonally in a curved sheet that forms a hollow cylinder.[20][21] Nanobuds were first published 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.[22]
- 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.[29] Similarly, glassy carbon contains a high proportion of closed porosity.[26] But unlike normal graphite, the graphitic layers are not stacked like pages in a book, but have a more random arrangement. Linear acetylenic carbon[28] has the chemical structure[28] -(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.[30]
Occurrence
An estimate of the global carbon budget:[citation needed]
Biosphere, oceans, atmosphere |
0.45 × 1018 kilograms |
Crust |
Organic carbon | 13.2 × 1018 kg |
Carbonates | 62.4 × 1018 kg |
Mantle |
1200 × 1018 kg |
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. 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.
[31]
In combination with
oxygen in
carbon dioxide, carbon is found in the Earth's atmosphere (in quantities of approximately 810
gigatonnes) and dissolved in all water bodies (approximately 36,000 gigatonnes). Around 1,900 gigatonnes 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, and
oil reserves around 150 gigatonnes. With smaller amounts of
calcium,
magnesium, and
iron, carbon is a major component in very large masses of
carbonate rock (
limestone,
dolomite,
marble etc.).
Coal is a significant commercial source of mineral carbon;
anthracite containing 92–98% carbon
[32] and the largest source (4,000 Gt, or 80% of coal, gas and oil reserves) of carbon in a form suitable for use as fuel.
[33]
Graphite is found in large quantities in
New York and
Texas, the
United States,
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.
Thermal neutrons are produced that collide with the nuclei of nitrogen-14, forming carbon-14 and a proton.
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.
[9] The isotope
carbon-12 (
12C) forms 98.93% of the carbon on Earth, while
carbon-13 (
13C) forms the remaining 1.07%.
[9] The concentration of
12C is further increased in biological materials because biochemical reactions discriminate against
13C.
[34] In 1961 the
International Union of Pure and Applied Chemistry (IUPAC) adopted the isotope
carbon-12 as the basis for
atomic weights.
[35] 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.
[36] 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.
[37] 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.
[38][39]
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.
[40] 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.
[41]
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. This happens in conditions of temperature 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. 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. The
Solar System is one such
third-generation star system.
One of the fusion mechanisms powering stars is the
carbon-nitrogen cycle.
Rotational transitions of various isotopic forms of carbon monoxide (e.g.
12CO,
13CO, and C
18O) are detectable in the
submillimeter regime, and are used in the study of
newly forming stars in
molecular clouds.
Carbon cycle
Main article:
Carbon cycle Diagram of the carbon cycle. The black numbers indicate how much carbon is stored in various reservoirs, in billions of tons ("GtC" stands for gigatons of carbon; figures are circa 2004). The purple numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ~70 million GtC of carbonate rock and kerogen.
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.
[42]
Compounds
Organic compounds
Structural formula of
methane, the simplest possible organic compound.
Correlation between the
carbon cycle and formation of organic compounds. In plants, carbon dioxide formed by carbon fixation can join with water in
photosynthesis (
green) to form organic compounds, which can be used and further converted by both plants and animals.
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 too).
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. By
IUPAC's definition, all the other organic compounds are functionalized compounds of hydrocarbons.
[citation needed]
Carbon occurs in all known
organic life and is the basis of
organic chemistry. When united with
hydrogen, it forms various flammable compounds called
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.
[43] Dissolved in
water, it forms
carbonic acid (
H2CO3), but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable.
[44] Through this intermediate, though, resonance-stabilized
carbonate ions are produced. Some important minerals are carbonates, notably
calcite.
Carbon disulfide (
CS2) 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.
[45][46] 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 (
C3O2),
[47] the unstable
dicarbon monoxide (C
2O),
[48][49] carbon trioxide (CO
3),
[50][51] cyclopentanepentone (C
5O
5),
[52] cyclohexanehexone (C
6O
6)
[52], and
mellitic anhydride (C
12O
9).
With reactive
metals, such as
tungsten, carbon forms either carbides (C
4–), 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,
[53] 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 (e.g.
tetraethyl lead), η
2-alkene compounds (e.g.
Zeise's salt, and η
3-allyl compounds (e.g.
allylpalladium chloride dimer;
metallocenes containing cyclopentadienyl ligands (e.g.
ferrocene); and
transition metal carbene complexes. Many
metal carbonyls exist (e.g.
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 [(Ph
3PAu)
6C]
2+. This phenomenon has been attributed to the
aurophilicity of the gold ligands.
[54]
History and etymology
Antoine Lavoisier in his youth
The
English name
carbon comes from the
Latin carbo for coal and charcoal,
[55] and from hence 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.
[56][57]
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.
[58] In 1772,
Antoine Lavoisier showed that diamonds are a form of carbon, when he burned samples of carbon and diamond then showed that neither produced any water and that both released the same amount of
carbon dioxide per
gram.
Carl Wilhelm Scheele showed that graphite, which had been thought of as a form of
lead, was instead a type of carbon.
[59] In 1786, the French scientists
Claude Louis Berthollet,
Gaspard Monge and C. A. Vandermonde then showed that this substance was carbon.
[60] In their publication they proposed the name carbone (Latin carbonum) for this element. Antoine Lavoisier listed carbon as an
element in his 1789 textbook.
[61]
A new
allotrope of carbon,
fullerene, that was discovered in 1985
[62] includes
nanostructured forms such as
buckyballs and
nanotubes.
[19] Their discoverers (Curl, Kroto, and Smalley) received the
Nobel Prize in Chemistry in 1996.
[63] 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.
[26]
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.
[64] 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 1800s,
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.
According to the
USGS, world production of natural graphite in 2006 was 1.03 million tons and in 2005 was 1.04 million tons (revised), of which the following major exporters produced: China produced 720,000 tons in both 2006 and 2005, Brazil 75,600 tons in 2006 and 75,515 tons in 2005 (revised), Canada 28,000 tons in both years, and Mexico (amorphous) 12,500 tons in 2006 and 12,357 tons in 2005 (revised). In addition, there are two specialist producers: Sri Lanka produced 3,200 tons in 2006 and 3,000 tons in 2005 of lump or vein graphite, and Madagascar produced 15,000 tons in both years, a large portion of it "crucible grade" or very large flake graphite. Some other producers produce very small amounts of "crucible grade".
According to the
USGS, U.S. (synthetic) graphite electrode production in 2006 was 132,000 tons valued at $495 million and in 2005 was 146,000 tons valued at $391 million, and high-modulus graphite (carbon) fiber production in 2006 was 8,160 tons valued at $172 million and in 2005 was 7,020 tons valued at $134 million.
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.
[65]
Historically diamonds were known to be found only in alluvial deposits in
southern India.
[66] India led the world in diamond production from the time of their discovery in approximately the 9th century BCE
[67] 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.
[68]
Diamond production of primary deposits (kimberlites and lamproites) only started in the 1870s after the discovery of the Diamond fields in South Africa balls. Production has increased over time and now an accumulated total of 4.5 billion carats have been mined since that date.
[69] Interestingly 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.
[69]
In the United States, diamonds have been found in
Arkansas,
Colorado, and
Montana.
[70][71] In 2004, a startling discovery of a microscopic diamond in the United States
[72] led to the January 2008 bulk-sampling of
kimberlite pipes in a remote part of
Montana.
[73]
Today, most commercially viable diamond deposits are in
Russia,
Botswana,
Australia and the
Democratic Republic of Congo.
[74] In 2005, Russia produced almost one-fifth of the global diamond output, reports the
British Geological Survey. Australia boasts the richest diamantiferous pipe with production reaching peak levels of 42 metric tons (41 LT; 46 ST) per year in the 1990s.
[70]
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. Diamond prospectors continue to search the globe for diamond-bearing
kimberlite and
lamproite pipes.
Applications
Pencil leads for mechanical pencils are made of graphite (often mixed with a clay or synthetic binder).
Sticks of vine and compressed charcoal.
A cloth of woven carbon filaments
The
C60 fullerene in crystalline form
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 others,
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.
[75]
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 20,000 kg annually), unsuitable for use as gemstones and known as
bort, are destined for industrial use.
[76] In addition to mined diamonds,
synthetic diamonds found industrial applications almost immediately after their invention in the 1950s; another 3 billion carats (600
metric tons) of synthetic diamond is produced annually for industrial use.
[77] 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.
[78] 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.
[79][80] 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.
[81]
Precautions
Pure carbon has extremely low toxicity 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.
[82] 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.
[83] 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 may also burn vigorously and brightly in the presence of air at high temperatures, as in the
Windscale fire, which was caused by sudden release of stored
Wigner energy in the graphite core. 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.
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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