graphite

A mineral form of carbon, found in metamorphic rocks; black, soft, and greasy to the touch. It is a very good electrical conductor and dry lubricant. Mixed with clay, it is used in pencil ‘leads’.

Graphite
General
Category	Native mineral
Chemical formula	Carbon, C
Identification
Color	Steel black, to gray.
Crystal habit	Tabular, six-sided foliated masses, granular to compacted masses.
Crystal system	Hexagonal (6/m 2/m 2/m)
Cleavage	Perfect in one direction.
Fracture	Flaky, otherwise rough when not on cleavage
Mohs Scale hardness	1 - 2
Lustre	Dull metallic, earthy
Refractive index	Opaque
Pleochroism	None
Streak	Black
Density	2.09–2.23 g/cm³
Fusibility	?
Solubility	Molten Ni

Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: "to draw/write", for its use in pencils) is one of the allotropes of carbon.

The bond between the atoms within a layer is strong but the force between two layers of graphite is weak.

Graphite can conduct electricity due to the vast electron delocalization within the carbon layers.

Graphite powder is used as a dry lubricant, although it might be thought that this industrially important property is due entirely to the loose interlamellar coupling between sheets in the structure, in fact in a vacuum environment (such as in technologies for use in space), graphite was found to be a very poor lubricant, leading to the discovery that in fact lubrication is due to adsorbed air and water between the layers, unlike other layered dry lubricants such as molybdenum disulfide. A structural analog of graphite, hexagonal boron nitride, is used as a high-temperature lubricant as well, and due to its similarity to graphite is sometimes called white graphite.

When a large number of crystallographic defects bind these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic carbon, a useful material in blood-contacting implants such as prosthetic heart valves.

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 3000 °C) material, used in reentry shields for missile nosecones, solid rocket engines, high temperature reactors, brake shoes, electric motor brushes and as electrodes in EDM electrical discharge machines.

Carbon fiber and carbon nanotubes are also used in graphite reinforced plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC)). 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 Germans' World War II graphite-based nuclear reactors. Graphite used for nuclear reactors is often referred to as Nuclear Graphite.