High Pressure Origin Of Diamond

As early as 1694, Florentine academicians suspected that diamond was made of carbon when they found that the precious stone could be burned completely in air. In 1772, this speculation was confirmed by the French chemist Antoine Lavoisier who discovered that the gas released from a burnt diamond was indeed carbon dioxide. In 1797, Smithson Tennant proved beyond any doubt that diamond is made of carbon by burning a diamond in pure oxygen. He measured the amount of carbon dioxide released and found that the carbon content matched exactly the original weight of gasified diamond (Mellor, 1924).

As soon as this precious gem was known to be no more than ordinary carbon, the quest for synthesizing diamond began. But success did not come until one and a half century later. Since the density of diamond is 3.52 g/cm3 and 56% higher than the next densest form of carbon (graphite), it was soon realized that high pressure is essential in order to squeeze carbon into diamond.

Another clue that diamond could be formed under high pressure comes from the occurrence of natural diamond. The very first natural diamond was found on a riverbed in India, but its source was eventually traced back to Brazil. South Africa confirmed that a special volcanic pipe called kimberlite was found on this continent, one that contained natural diamonds. Kimberlite contains a mineral assemblage that includes garnet and biotite, both of which are formed under high pressure.

Volcanic eruptions are typically derived from shallow origins. According to plate tectonics, the surface of earth is covered by several large plates that can move horizontally. These plates are driven by the convection flow of mantle that supports these plates.

When the plates rub against one another along the borderline, rocks from these plates may melt to form a magma that may erupt as volcanoes. However, magma with shallow roots will solidify to form mostly granite and basalt that contain no diamond.

On the other hand, with stable continental shields that are far away from tectonic active areas, kimberlite from deep within the Earth may extrude to the surface. A geotherm underneath a continental shield is milder than those found underneath tectonic active regions. The colder geotherm may intersect the phase boundary of graphite and diamond at a depth greater than 130 km (Fig. 4.1).

Kimberlites are derived from ascending mantle plumes that rise to release heat. While they move upward, they may melt due to the reduction of pressure. The volume of magma will rapidly increase so it may extrude to the surface. If the rising kimberlite flows across the region where diamonds are located, it will carry them up to the surface (Fig. 4.2).

Kimberlites contain a very low abundance of diamond, typically about 0.05 PPM. Less than 10% of these diamonds can be polished to become gems.

Most kimberlite pipes were extruded about 100 million years ago, but the diamonds that they carry could have been formed even 30 billion years earlier.

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Figure 4.1. The continental geotherm intersects the phase boundary of graphite and diamond at a depth of 130 km.
Figure 4.2. The occurrence of a kimberlite pipe in South Africa. Mining shafts were used to recover the diamond from these pipes. The funnel shape of the pipe was formed by the expansion of magma as it decompresses near the surface.
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