The answer lies in the way the carbon atoms form bonds with each other. Note about 3D molecules -- Our files on this page now use Jsmol instead of Jmol. These files make use of Javascript which permits viewing of 3D molecules on tablets and phones and is also easier to use on Macs. Try this:. Rotate the Graphite molecule. Hold the left mouse button down over the image and move the mouse to rotate the graphite molecule.
Notice that graphite is layered. While there are strong covalent bonds between carbon atoms in each layer, there are only weak forces between layers. This allows layers of carbon to slide over each other in graphite. On the other hand, in diamond each carbon atom is the same distance to each of its neighboring carbon atoms.
In this rigid network atoms cannot move. This explains why diamonds are so hard and have such a high melting point. Unfortunately w-BN is extremely rare in nature and difficult to produce in sufficient quantities to properly test this claim by experiment. Synthetic diamond has also been around since the s and is often reported to be harder than natural diamond because of its different crystal structure.
It can be produced by applying high pressure and temperature to graphite to force its structure to rearrange into the tetrahedral diamond, but this is slow and expensive. Another method is to effectively build it up with carbon atoms taken from heated hydrocarbon gases but the types of substrate material you can use are limited.
This contrasts with the large monocrystals of most natural diamonds used for jewellery. The smaller the grain size, the more grain boundaries and the harder the material. Recent research on some synthetic diamond has shown it to have a Vickers hardness of up to GPa. More recently, researchers at North Carolina State University created what they described as a new form of carbon, distinct from other allotropes, and reported to be harder than diamond. This has led them to expect Q-carbon to be harder than diamond itself, although this still remains to be proven experimentally.
Q-carbon also has the unusual properties of being magnetic and glowing when exposed to light. But so far its main use has been as an intermediate step in producing tiny synthetic diamond particles at room temperature and pressure. It's a very special kind of eruption, thought to be quite violent, that occurred a long time ago in the Earth's history.
We haven't seen such eruptions in recent times. They were probably at a time when the earth was hotter, and that's probably why those eruptions were more deeply rooted. These eruptions then carried the already-formed diamonds from the upper mantle to the surface of the Earth. When the eruption reached the surface it built up a mound of volcanic material that eventually cooled, and the diamonds are contained within that.
These are the so-called Kimberlites that are typically the sources of many of the world's mined diamonds. One of the things we know, therefore, about any diamonds that were brought to the surface is that the process of the Kimberlite eruption bringing the diamonds from the upper mantle to the surface of the Earth had to happen very quickly, because if they were traveling too long and too slowly they would have literally turned into graphite along the way.
And so by moving quickly they essentially got locked into place into the diamond structure. Once the diamonds have been brought from high temperature to low temperature very quickly—and by quickly, we mean in a matter of hours—these eruptions, these Kimberlite pipes moving to the surface, may have been traveling at rates of 20 to 30 miles per hour.
Once the diamonds are brought to the surface and cooled relatively quickly, those carbon atoms are locked into place and there's just not enough energy to now start rearranging them into graphite. Diamonds are made of carbon so they form as carbon atoms under a high temperature and pressure; they bond together to start growing crystals. Because of the temperature and pressure, under these conditions, carbon atoms will bond to each other in this very strong type of bonding where each carbon atom is bonded to four other carbon atoms.
That's why a diamond is such a hard material because you have each carbon atom participating in four of these very strong covalent bonds that form between carbon atoms. So as a result you get this hard material. Again where the carbon is coming from, how quickly they're growing, those are all still open questions, but obviously the conditions are such that you've got some group of carbon atoms that are in close enough proximity that they start to bond.
As other carbon atoms move into the vicinity they will attach on. That's the way any crystal grows. It's the process of atoms locking into place that produces this repeating network, this structure of carbon atoms, that eventually grows large enough that it produces crystals that we can see.
Each of these crystals, each diamond, one carat diamond, represents literally billions and billions of carbon atoms that all had to lock into place to form this very orderly crystalline structure.
You mentioned that scientists don't know where the carbon comes from. What are some possible sources?
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