Graphite

Graphite, mineral form of carbon. Carbon is allotropic (that is, it exists in more than one form); other forms include diamond and buckminsterfullerene. Although graphite is chemically the same as diamond, it differs greatly from that mineral in most of its physical properties. Graphite is black, opaque, and metallic in luster and has a density of 2.09 to 2.2 g/cm³. Graphite is extremely soft—its hardness rates 1 to 2 on a scale of 1 to 10. It smudges anything with which it comes in contact; it feels greasy or slippery to the touch. It crystallizes in flakes or large irregular masses rather than as well-developed crystals. A good conductor of electricity, graphite is a poor conductor of heat. It occurs in nature as a mineral that invariably contains impurities.

Graphite is widely distributed over the world; important deposits are found in China, India, North Korea, Mexico, Brazil, the Czech Republic, and Turkey. Graphite is made artificially by baking a mixture of petroleum coke and coal tar pitch at 950°C (1740°F) for 11 to 13 weeks, then transferring the baked product to electric graphitizing furnaces and heating it to about 2800°C (5600°F) for 4 or 5 weeks. Most of the graphitizing takes place because of the temperature, but the process is also a refining step, as virtually all the metal oxide impurities in the raw carbon are reduced to metal and vaporized.

The cores of so-called lead pencils actually contain no lead but are made of graphite mixed with clay. Graphite is used as electrodes in electrochemical industries where corrosive gases are given off, and for electric furnaces that reach extremely high temperatures. It is used as a lubricant either by itself or mixed with grease, oil, or water. It is also used in crucibles that must withstand extremely high temperatures and in certain paints. Graphite of the highest purity has been used as a moderator in nuclear reactors, where it slows down neutrons without capturing them.