University of Manitoba, Department of Chemistry

Crystal structures of diamond and graphite

Diamond and graphite provide a vivid demonstration of the importance of crystal structure to mineral properties. Both are composed of carbon, but differ in the bonding and arrangement of the atoms. Diamond is used as an abrasive because of its great hardness, whereas graphite is used as a lubricant. Diamond is the high pressure polymorph, and hence has the denser structure (specific gravity = 3.52 versus 2.23 for graphite).  For comparison, the crystal structure of silicon carbide (sold as an abrasive under the name carborundum) is also shown.

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Diamond

Diamond has a face-centered cubic structure, but the lattice points are defined by pairs of carbon atoms rather than single atoms. Compare it with silicon carbide (below), which has the same arrangement but with two types of atoms. 
SiC

Silicon carbide

Si atoms are bonded to 4 C atoms.
C atoms are bonded to 4 Si atoms.

SiC has the same structure as diamond (above), made up of  two different elements.  There are some similarities to the ionic fluorite structure as well, but note that SiC is a covalent network.
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Graphite

Graphite is also a covalent network form of pure carbon. Notice that the network is planar rather than three-dimensional.  Graphite is soft and can act as a lubricant because the planes are relatively weakly held together by dispersion forces.  This allows them to slide past each other. 
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This page was modified for Chemistry 2.130 by Dr. P.G. Hultin from an original
created by Dr. Thomas Menard, U. of Calgary