Graphite (named by Abraham Gottlob Werner in 1789,
from the Greek "to draw/write", for its use in pencils)
is one of the allotropes of carbon. Unlike diamond, graphite is
a conductor, and can be used, for instance, as the material in the
electrodes of an electrical arc lamp.
Associated minerals include: quartz, calcite, micas, iron meteorites
and tourmalines. Notable occurrences include New York and Texas
in the USA, Russia, Mexico, Greenland, and India.
Other characteristics: thin flakes are flexible
but inelastic, mineral can leave black marks on hands and paper,
conducts electricity, and displays superlubricity. Best field indicators
are softness, luster, density and streak.
Detailed properties and uses
The unit cell dimensions are a = b = 245.6 picometres, c = 669.4
pm. The carbon-carbon bond length in the bulk form is 141.8 pm,
and the interlayer spacing is c/2 = 334.7 pm.
Each carbon atom possesses an sp2 orbital hybridisation.
The pi orbital electrons delocalized across the hexagonal atomic
sheets of carbon contribute the graphite's conductivity. In an oriented
piece of graphite, conductivity parallel to these sheets is greater
than that perpendicular to these sheets.
The acoustic and thermal properties of graphite
are also highly anisotropic, since phonons propagate very quickly
along the tightly-bound planes, but are slower to travel from one
plane to another.
The loose coupling among the sheets in graphite
contributes to another industrially important property -- graphite
powder is used as a dry lubricant. Recent studies suggest that an
effect called superlubricity can also account for this effect. When
a large number of crystallographic defects bind these planes together,
graphite loses this property and becomes known as pyrolytic carbon,
a useful material in blood-contacting implants such as prosthetic
heart valves. Graphite is an excellent computer fan lubricant. Two
or three drops into the ball-bearings of the fan will greatly improve
the efficiency of the fan and quieten it.
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 fiber graphite is an extremely strong,
heat-resistant (to 3000C) material, used in reentry shields for
missile nosecones, solid rocket engines, high temperature reactors,
brake shoes and electric motor brushes.
Pyrolytic graphite has excellent biocompatibility,
and is used in medical equipment to prevent adhesion of blood clots.
Carbon fiber and carbon nanotubes are also used
to reinforce plastics, and in heat-resistant composites such as
reinforced carbon-carbon (RCC)). They have also successfully reinforced
The mechanical properties of carbon fiber composites
and grey cast iron are strongly influenced by the role of graphite
in these materials.
Graphite also finds use as a matrix and moderator
within nuclear reactors. Its low neutron cross section also recommends
it for use in proposed fusion reactors. Care must be taken that
reactor-grade graphite is free of neutron absorbing materials such
as boron, widely used as the seed electrode in commercial graphite
deposition systems-- this caused the failure of the German's World
War II graphite-based nuclear reactors. Since they could not isolate
the difficulty they were forced to use far more expensive heavy