Conditions necessary for the formation of diamonds

Carbon source

The primary element needed for diamond formation is carbon. This carbon can come from various sources. The prime source of carbon is inorganic carbon which comes from the earths mantle. A second source of carbon is derived from the earths subduction process, where carbon rich rocks are thought to have been subducted deep into the mantle by plate tectonic processes – then returned to the surface.

High temperature and pressure

Diamonds are formed at depths of about 140 to 190 kilometers (87 to 118 miles) within the Earth’s mantle. At these depths, temperatures can reach over 2,200 degrees Celsius (3,992 degrees Fahrenheit), and pressures can exceed 725,000 pounds per square inch (50 gigapascals). The combination of high pressure and high temperature causes the carbon atoms to arrange themselves into a crystalline structure, forming diamond. This process is known as “crystallization.” The carbon atoms bond together in a tetrahedral lattice structure, creating the extremely hard and rigid structure that gives diamonds their unique properties.

Time

It’s important to note that the entire process of diamond formation, from the initial carbon source to the final diamond crystals being mined, takes millions to billions of years. Natural diamonds are incredibly rare and precious due to the unique and complex conditions required for their formation.

The journey to the surface

Most commercial diamond deposits are thought to have formed when volcanic eruptions from deep within the earth’s mantle delivered diamonds to the surface. In these eruptions, magma travels rapidly from deep within the mantle, often passing through a diamond stability zone (where temperature and pressure conditions are optimal for the formation of diamonds) on its route to the surface.

As the magma passes through the diamond stability zone, it picks up some of the diamonds on its route to the surface. The magma erupts on the surface and forms a number of volcanoes, some of which contain diamonds. These volcanic pipes are called kimberlites.

Most known kimberlites formed in the period between 70 and 150 million years ago, but some are over 1200 million years old.

Generally speaking, kimberlites are found only in cratons, the oldest surviving areas of continental crust, which are found in the central part of continental landmasses and have remained virtually unchanged since their formation millions of years ago.

Kimberlitic magmas form about 150 km below Earth’s surface, i.e. at much greater depths than any other volcanic rocks. The temperatures and pressures at such depths are so high that carbon can crystallize in the form of diamonds.

The volcanic kimberlite pipes are the primary source of the majority of diamonds found Worldwide. However, over millions of years these kimberlite pipes are eroded and their embedded diamonds are scattered over alluvial plains and transported by rivers towards the sea. These secondary diamond deposits often contain smaller but higher quality diamonds and have been mined in areas such as the west coast of South Africa for many years.