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Posted by Ryan On November - 13 - 2008 1 Comment

Even though graphite and diamonds are both made of carbon, the properties of these two forms of carbon (these are two allotropes of carbon) are very different. Diamonds are very hard, clear, reflective, and non-conductive. Graphite is soft, black, and conducts electricity. How could this be?

If we take a look at the electron configuration and geometry of the carbon atoms in diamonds and graphite, we find different hybridization of the carbon atoms. In diamonds we find that carbon is sp3 hybridized, forming a tetrahedral geometry within the network solid. The graphite is sp2 hybridized, forming planar sheets of carbon with the p orbitals providing weak bonding perpendicular to the plane. The three dimensional tetrahedral bonds within a diamond crystal explains its hardness, with equal strength in all directions. The graphite has a weak bond between the planes, forming slippery sheets of carbon. This explains why carbon “slides” off when we write with a pencil.

So why is graphite black and diamonds are clear? The electrons are tightly held in diamonds within bonds, allowing no where for electrons to absorb photons and enter an excited state. Instead, photons are reflected from diamonds providing a clear crystal. Graphite, on the other hand has electrons not bound within sigma bonds. These electrons are free to absorb photons freely, giving it its black color – nothing is reflected.

Similar to the color, the conductivity of graphite has to do with the ability for electrons to flow. These free flowing electrons are those not bound by a sigma bond. Within diamonds, the electrons are all locked into place within a bond, allowing for no electron flow – non-conductive.

Formation and Mining of Diamonds:

Diamonds are formed at least 160km below the surface. At these depths the pressure is about 5 times greater than surface pressure and the temperature is near iron’s melting point. They stay below the surface until they are elevated through the earth’s crust by a volcanic eruption, a kimberlite eruption. Most stay below the surface in the kimberlite pipe, the vertical chamber that once was the volcanic core. These kimberlite pipes are mined for their diamonds.

One Response

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