spinning ball & stick model of diamond unit cell spinning polyhedral model of diamond unit cell

The unit cell (red) of diamond is a cube.  Groups of five carbon atoms (blue spheres) form tetrahedra at the cube corners, at the centers of each of the cube faces, and at four sites within the cube.   This is shown best in the view on the right, a polyhedral model showing the tetrahedra rather than the atoms.  The carbon atoms are located at the center and four apices of each tetrahedron.  The carbon tetrahedra are covalently bonded, accounting for diamond's extreme hardness. The toughness of diamond is compromised somewhat by the presence of  4 planar directions of weakness, its octahedral cleavage.  These cleavage directions are at 45o to the corners of the cube.

diamond ball & stick showing cleavage

diamond polyhedral model showing cleavage

Diamond oriented so that a cleavage plane (pink line) is horizontal.

Same view but with carbon atoms shown as tetrahedra to better show this plane of weakness. Note the prominent sheets of carbon atoms parallel to the cleavage.

cubes stacked to form an octahedron

A diamond crystal is composed of an enormous number of cubic unit cells that are stacked to produce crystal faces.  Stacking of cubes to produce an octahedron, a common diamond crystal shape, is shown above.

Illustrations constructed with XTALDRAW

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Updated 08/19/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin