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Graphite
*** Shopping-Tip: Graphite
{{otheruses}}
{| border=1 cellspacing=0 align=right cellpadding=0 width=250 valign=top style="margin-left:1em"
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!colspan=2 align=center|Graphite
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!colspan=2|
Image:GraphiteUSGOV.jpg thumb|center
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!colspan=2|General
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|Category|| Native
mineral
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Chemical formula||
Carbon, C
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!colspan=2|Identification
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| Color || Steel black, to gray.
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Crystal habit .html">Foliation (geology)
foliated masses, granular to compacted masses.
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Crystal structure Crystal system ||
Hexagonal (crystal system) Hexagonal (6/m 2/m 2/m)
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Cleavage (crystal) Cleavage|| Perfect in one direction.
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Fracture|| Flaky, otherwise rough when not on clevage
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Mohs Scale hardness || 1 - 2
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Lustre|| Dull metallic, earthy
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Refractive index|| Opaque
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Pleochroism|| None
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Mineral#Streak Streak|| Black
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Density|| 2.09–2.23 g/cm³
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Fusibility|| ?
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Solubility|| Molten Ni
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Image:Graphit_gitter.png thumb|Crystal structure of graphite
'''Graphite''' (named by Abraham Gottlob Werner in
1789, from the
Greek language Greek γÏ?αφειν: "to draw/write", for its use in
pencils) is one of the
allotropes of carbon. Unlike
diamond, graphite is a
Conductor (material) conductor, and can be used, for instance, as the material in the electrodes of an electrical
arc lamp. Graphite holds the distinction of being the most stable form of solid carbon ever discovered.
Occurrence
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.
Image:GraphiteOreUSGOV.jpg thumb|Graphite ore
Structure
Each
carbon atom is
covalent covalently Chemical bond bonded to three other surrounding carbon atoms. The flat sheets of carbon atoms are bonded into
Hexagonal (crystal system) hexagonal structures. These exist in layers, which are not covalently connected to the surrounding layers.
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 sp
2 orbital hybridisation. The
electron configuration 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 bond between the atoms within a layer is strong but the force between two layers of graphite is weak. Therefore, layers of it can slip over each other making it soft.
Detailed properties and uses
The
Acoustics acoustic and
thermal properties of graphite are highly anisotropic, since
phonons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another.
Graphite is able to
conduct electricity due to the unpaired fourth
electron in each carbon atom. This unpaired 4th electron forms
delocalised planes above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.
Graphite powder is used as a dry
lubricant, although it might be thought that this industrially important property is due entirely to the
cleavage (crystal) loose interlamellar coupling between sheets in the structure, in fact in a
vacuum environment (such as in technologies for use in
Outer space 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. 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 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,
Pebble bed reactor high temperature reactors,
brake shoes,
electric motor brushes and as electrodes in
EDM electrical discharge machines.
Intumescent or expandable graphites are used in fire seals, fitted around the perimeter of a fire door. During a fire the graphite intumesces (expands and chars) to resist fire penetration and prevent the spread of fumes. A typical start expansion temperature (SET) is between 150 and 300 degrees Celsius.
Carbon fiber and
carbon nanotubes are also used to
graphite reinforced plastics, and in heat-resistant composites such as
reinforced carbon-carbon (RCC)). They have also successfully
reinforced concrete. The mechanical properties of carbon fiber graphite-reinforced plastic composites and grey
cast iron are strongly influenced by the role of graphite in these materials.
Graphite also finds use as a matrix and
Neutron moderator moderator within
nuclear reactors. Its low
neutron Cross section (physics) cross section also recommends it for use in proposed
thermonuclear 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 Germans'
World War II graphite-based nuclear reactors. Since they could not isolate the difficulty they were forced to use far more expensive
heavy water moderators.
Media
{{multi-video start}}
{{multi-video item |
filename = graphite stereo animation.gif |
title = Graphite animation |
description = Rotating graphite
stereogram. (2.79
Megabyte MB,
animated GIF format). |
format =
animated GIF
}}
{{multi-video end}}
See also
*
Carbon fiber
*
Pyrolytic graphite
*
Diamond
*
Lonsdaleite
*
Graphene
*
Carbon nanotube
*
Pencil lead
Reference
* Klein, Cornelis and Cornelius S. Hurlbut, Jr. (1985) ''Manual of Mineralogy: after Dana'' 20
th ed. ISBN 0-471-80580-7
External links
{{Commons|Graphite}}
-
The Graphite Page
-
Mineral galleries
-
Webmineral
-
Mindat w/ locations
-
Intumescent graphite for fireproofing
Category:Chemical elements Carbon, Graphite
Category:Native element minerals
Category:Lubricants
Category:Art materials
Category:Carbon forms
Category:Refractory materials
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ca:Cristal·lografia
da:Grafit
de:Graphit
et:Grafiit
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eo:Grafito
fa:گراÙ?یت
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lt:Grafitas
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