How Are Diamonds Formed? Answered

How Are Diamonds Formed?

Photo by wu yi on Unsplash
Diamonds form within narrow ranges of extreme temperature and pressure, at depths between 90 and 110 miles below the Earth's crust. These areas of formation, or diamond stability zones, exist in the upper mantle. Here, temperatures of 2,200 deg F and pressures of 725,000 psi act on suitable carbon material to create diamonds. The precise type of carbon material used is not known, but most of it originated in the Earth's mantle during the planet's formation. Sometimes tectonic plate movement subducts carbon from the crust to the mantle - this provides another source.

Why Diamonds Are Not Formed By Coal

There's a common myth that diamonds come from coal. This is disproved by comparing the ages of these two pure carbon materials. Coal occurs in horizontal planes of sedimentary rock no deeper than a couple of miles below the Earth's surface. It's created from rotting vegetation and animal remains buried no earlier than 450 million years ago. While the molecular lattice structure of diamond itself can't be dated, mineral impurities such as potassium can be. Using radioactive techniques, an approximate sense of the diamond’s age can be ascertained. Most diamonds found to date are between one and three billion years old. Outside that range, some diamonds were formed at about the same time of the Earth, while others are a little more than half a billion years old. This means the youngest diamonds are still at least a hundred million years older than the first coal deposits.

How Diamonds Reach The Surface

Carbon, subjected to intense temperature and pressure, forms diamonds 100 miles below the surface. But even the Earth's upper mantle is far too deep to drill down; without a means of delivering these diamonds to the surface they would remain buried. (In fact, this isn’t completely true as some diamonds form as a result of meteor strikes. The impact causes the high temperatures and pressures needed for diamond formation; the meteor crater is the crucible.)

We know of diamonds only because of their bizarre transportation system. Diamonds eruct from the Earth’s surface in large, violent volcanic eruptions. The magnitude of these eruptions means that no diamonds would have escaped the Earth’s mantle in recent times.

Once the intense heat and pressure has crushed the atoms of pure carbon material into a diamond lattice, it begins its journey to the top. It does so through so-called kimberlite (or, rarely, lamproite) pipes. These are channels of igneous rock leading from the mantle to the surface. Molten kimberlite surges up through the pipe, carrying with it mantle fragments and solid rocks (xenoliths). The kimberlite bursts through the mantle and deposits itself in mounds of cooled volcanic rock. The xenoliths trapped within the kimberlite often contain clusters of diamonds. But not all kimberlite pipes produce diamonds, while others do but not in the quantities which are economically viable to mine.

For diamonds to retain their crystalline lattice, their journey up the kimberlite pipe cannot take longer than a few hours. If their progress to the top is too slow, the diamond turns to graphite, a far less popular material to set into wedding rings.

Diamonds Unearthed, by Cate Lineberry, December 2006

How Diamonds are Formed: In Nature & In the Lab -, September 15, 2016


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