Graphite (mineral)
Graphite is a naturally occurring form of carbon known for its distinct layered crystal structure, which gives it unique properties such as high electrical and thermal conductivity, lubricating ability, and softness at room temperature. It exists in two primary forms: alpha (hexagonal) and beta (rhombohedral), with similar physical characteristics but differing crystal arrangements. Graphite is widely distributed across the globe, with significant deposits found in countries like Sri Lanka, India, North Korea, and Brazil. While the United States largely imports natural graphite, most of its usage involves synthetic graphite, derived from carbon-rich materials like coal and petroleum coke, due to its superior density and purity.
Graphite's most recognizable application is in "lead" pencils, but it is also extensively used in various industrial applications, including lubricants, brake linings, and as a key material in the production of batteries and high-temperature equipment. Its unique properties make it valuable in fields ranging from aerospace to nuclear energy, where it serves as a neutron moderator. Additionally, graphite can be transformed into diamonds under extreme pressure and temperature conditions. Overall, graphite plays a significant role in both everyday and advanced technological applications.
Graphite (mineral)
Where Found
Natural graphite is distributed widely in the world. Major deposits are found in Sri Lanka, North and South Korea, India, Austria, Germany, Norway, Canada, Mexico, China, Brazil, and Madagascar. The United States imports virtually all of the natural graphite it needs from the latter four countries. However, most of the graphite used in the United States is synthesized from a wide variety of carbon-containing materials—for example, anthracite coal and petroleum coke. Synthetic graphite is denser, purer, and more expensive than the natural form.
Primary Uses
Graphite is used in “lead” pencils. To a lesser extent, it is also used in brake linings, steelmaking, and lubricants.
Technical Definition
Graphite is composed of parallel planes of fused hexagonal rings of carbon atoms. It exists in two forms, alpha (also called hexagonal) and beta (also called rhombohedral), which have apparently identical physical properties but differ in their crystal structure. In the alpha form, the carbon atoms in alternate layers are directly above each other, while in the beta form, the carbons do not line up again until every fourth layer. In both forms, the distance between neighboring carbon atoms within the layers is 142 picometers, which is intermediate between the length of typical single and double C—C bonds. The distance between the layers is 335 picometers. The larger distance between the layers reflects the weaker forces holding the planes together compared with the forces holding neighboring atoms together within the planes.
Because of graphite’s weak interplanar forces, the planes can readily slip past each other, causing graphite to cleave easily and preferentially parallel to its planes. This process accounts for its flaky appearance and excellent lubricating ability even when dry. The planar structure also causes several of its physical properties to be highly anisotropic (exhibiting different properties when measured in different directions). For example, its thermal conductivity is several hundred times larger, and its electrical conductivity is several thousand times larger, when measured parallel to the planes than perpendicular to them.
The density of synthetic graphite is 2.26 grams per cubic centimeter, but that of natural graphite is usually lower, varying from 2.23 to 1.48 grams per cubic centimeter, due to the presence of pore spaces and impurities.
Description, Distribution, and Forms
Graphite anddiamond are the two predominant forms in which free carbon is found in nature. Graphite is a greasy, opaque, highly reflective black or gray solid.
Although graphite can be found throughout the world, much of it is of little economic importance. Large crystals, called flake, occur in metamorphosed sedimentarysilicate rocks such as quartz, schists, and gneisses and have an average crystal size of about four millimeters (ranging from fractions of a millimeter to about six millimeters). Deposits have also been found in the form of lenses up to 30 meters thick and stretching several kilometers, with average carbon content of 25 percent (reaching 60 percent in Madagascar). The graphite in these cases was probably formed from the carbon in organic materials. Deposits containing microcrystalline graphite (sometimes referred to as “amorphous carbon”) can contain up to 95 percent carbon. In Mexico such amorphous carbon occurs in metamorphosed coal beds. The graphite deposit in New York occurs in a hydrothermalvein and was probably formed from carbon-bearing rocks during metamorphism in the region. Graphite occurs occasionally as an original constituent of igneous rocks (for example in India), and it has been observed in meteorites. Graphite has the unusual property that it is very soft at room temperature (with a hardness between 0.5 and 1 on the Mohs scale, which is similar to talc) but has increasing strength at high temperatures. At about 2,000° Celsius, its crushing strength is increased by 20 percent, and at about 3,000° Celsius, its tensile strength is increased by 50 to 100 percent. Other important properties of graphite that are exploited in its many uses listed previously are its stability at high temperatures and in the presence of corrosive and reactive chemicals.
History
Carbon was known in prehistory in the forms of charcoal and soot, but it was not recognized as a chemical element until the second half of the eighteenth century. In 1779, graphite was shown to be carbon by Carl Wilhelm Scheele, a Swedish chemist; ten years later the name “graphite” was proposed by Abraham Gottlob Werner, a German geologist, and D. L. G. Harsten, from the Greek graphein (to write). Commercially, “lead” pencils were first manufactured in about 1564 in England during Queen Elizabeth’s reign, using Cumberland graphite. In 1896, Edward Goodrich Acheson, an American chemist, was granted a patent for his process whereby graphite is made from coke, and within one or two years, production began on a large scale. Diamond was first synthesized from graphite between 1953 and 1955.
Obtaining Graphite
Graphite can be made to sublime directly to carbon vapor or to melt to liquid carbon at temperatures above approximately 3,500° Celsius, depending on the pressure and other conditions. It can also be transformed into diamond at extremely high pressures and temperatures (for example, 100,000 atmospheres and 1,000°-2,000° Celsius). The rate of conversion of diamond back to graphite at atmospheric pressure is not significant below temperatures of about 4,000° Celsius.
The mining and purification process of natural graphite includes flotation followed by treatment with acids and then heating in a vacuum to temperatures on the order of 1,500° Celsius.
Uses of Graphite
The most familiar use of graphite is in the manufacture of “lead” pencils, where it is mixed with clay and other materials and baked at high temperatures. The “lead” increases in softness as the ratio of graphite to clay increases. Graphite has much more extensive use in the manufacture of lubricants and oilless bearings; electrodes in batteries and industrial electrolysis; high-temperature rocket casings, chemical process equipment, furnaces, and crucibles for holding molten metals; tanks for holding corrosive chemicals; and strong and lightweight composite materials that are used, for example, in airplanes and high-quality sports equipment such as tennis rackets and golf clubs. Graphite is also a component in the cores of some nuclear reactors as the moderator to slow down the neutrons, and it is the major raw material for synthetic diamonds.
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