Diamonds

I’m sure this is a simple minded question, but if the mantle is homogenous why aren’t diamonds found everywhere?

When I was a child, this wasn’t a question. I was told in fourth grade that all the ferns in the swamps got covered by debris and turned into coal in the Pennsylvanian period. I don’t know if I was also told that all you had to do to create a diamond was add more pressure, or if I make the connection later when I heard a diamond is simply a form of carbon.

I was wrong about coal and diamonds, but it’s a common enough conclusion. All you have to do is enter the two words in a Google search to see how many others have the same idea. Indeed some of the first men to try to make synthetic diamonds began with charcoal. The ones who succeeded used graphite in a belt press able to producing pressures beyond 10 gigapascals at temperatures above 2000 degrees Celsius.

Once scientists at General Electric had produced that first synthetic stone it became possible to know, fairly precisely, the conditions required to make a diamond. But first, it was necessary to understand more about carbon.

The carbon atom is created from helium in a giant or supergiant star which is then scattered as dust in a supernova explosion. That dust is then coalesced into planets in third generation stars like our solar system.

The number of carbon atoms on Earth was set at creation, although some have since been introduced by meteorites. The carbon atom is particularly promiscuous, able to join with other elements like oxygen and hydrogen in long chained molecules. Perhaps ten million have been identified so far, and more are possible.

However, carbon atoms can also combine with themselves into crystalline structures. Graphite has a hexagonal structure, diamond a cubic one. These are the two main allotropes of carbon. Moving between them, means converting the crystalline pattern of one into the structure of the other.

I suppose it made little sense to believe coal could somehow become a diamond. Coal is mined. The first diamonds were found in alluvial expanses in India, then in Brazil. It wasn’t until the late nineteenth century that men in South Africa found diamonds beneath the surface deposits they were exhausting.

Then it became possible to know something about their origins. They had come up to the surface through volcanic pipes made of kimberlite. The ones in India and Brazil had eroded away, leaving a false impression of their provenance. The ones in South Africa still existed.

Diamonds are believed to be formed at pressures between 4.5 and 6 gigapascals and at temperatures between 900 and 1300 degrees Celsius, both lower than conditions in the first belt press.

The only parts of the mantle that meet the conditions for a diamond lie between 140 and 190 kilometers beneath the thickest, oldest parts of continental crust. Ocean bottom crust is thinner so temperatures rise more quickly with depth. The requisite condition never materializes.

Diamonds today are found in India, Brazil, South Africa, Siberia, and parts of Canada and Australia. These lie under the Indian, Amazonian, African, Angaran, Canadian and Australian shields. The last two are near areas where the oldest known rocks have been found.

Neither a view of a homogenous mantle nor one with two reservoirs explains the appearance of diamonds. The first view would have to be amended to suggest the chemistry was the same throughout the mantle, but varied by location from weight from above. The other looks for the reservoirs near the upper and lower heat sources, not somewhere above the boundary between the upper and lower mantles.

Note: Most of the information came from Wikipedia entries on carbon, diamond, kimberlite and synthetic diamond.