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How much pressure is needed to make a diamond from coal?

Diamond is the hardest mineral on Earth. The scope of its application is incredibly wide, but reserves in the subsurface are limited, and open deposits are being depleted. All this leads to an increase in prices for the mineral, but natural reserves do not increase. That’s why scientists and engineers have spent centuries developing methods to create artificially grown gemstones.

History of artificial diamonds

Today, two technologies for creating grown diamonds are predominantly used: HPHT (High Pressure High Temperature) and CVD (Chemical Vapor Deposition). The first is the effect of high temperature and pressure on carbon, and the second is the layer-by-layer deposition of diamond. The first goes back to the 18th century, while the second arose only in the last century. We’ll tell you how scientists and engineers developed these methods and what their features are.

First studies

The history of growing diamonds in the laboratory is closely connected with breakthroughs in chemical science. The French naturalist Antoine Laurent Lavoisier (1743−1794), the founder of modern chemistry, made a major discovery in 1772: he established that the crystal lattice of diamond is similar to the crystal structure of carbon. To do this, he heated the mineral to combustion temperature and analyzed the composition of the released gas. After Lavoisier’s discovery, experiments began on cultivating the gemstone. In 1879, Scottish chemist James Ballantyne Hannay claimed to have synthesized diamond by heating charcoal to 6330 °F (3500 °C). However, modern tests of the diamonds that he passed off as artificially created showed that they were natural stones. Yet Hannay’s efforts advanced science: he correctly established one of the principles – temperature effects. But temperature alone is not enough, and this can be observed in nature. Gemstone deposits are often found near volcanoes. When hot lava covers a surface, it not only creates a very hot environment, but also increases pressure. In 1892, French chemist Henri Moissan applied high pressure and high temperature to carbon simultaneously. He stated that he was able to accurately simulate the conditions under which the mineral is formed in nature. Moissan’s experiments could not be repeated by other scientists, and over time they came to the conclusion that the chemist presented unreliable results. Nevertheless, work was carried out in the right direction.

Successful experiments

The first truly successful crystal synthesis was performed by the Swedish company ASEA in 1953. Its specialists have been working since 1942 as part of a secret project. The engineers used a bulky split-sphere apparatus. The temperature inside was about 2400 °C and the pressure was 8,4 GPa. The experiment lasted an hour. The result was several crystals. But their quality and size were not suitable for creating jewelry diamonds, and the samples themselves were not presented to the public. Further research was continued by the American company General Electric, which demonstrated the first grown crystal in 1955. The diamond was created in a belt press that could maintain temperatures above 2000 °C and pressures above 10 GPa. The press used a container where graphite was dissolved in molten iron, nickel or cobalt. The metals acted as catalysts and accelerated the process of turning graphite into diamond. Using this method, it was possible to create a grown diamond with a diameter of 0,15 mm. However, it was still impossible to make a diamond out of it.

Contemporaneity

General Electric first synthesized gem-quality crystal in 1970. To achieve this result, the scientists used a pyrophyllite tube that was seeded with thin pieces of diamond at each end. To this were added a metal solvent, high pressure (5,5 GPa) and a temperature of more than 2000 °C. The grown minerals were not transparent: the hue varied from yellow to brown due to nitrogen contamination. Such samples could not produce very valuable diamonds, even by natural standards. In addition, the stones were almost always interspersed, especially if nickel was used as a catalyst. Later, it was possible to almost completely remove nitrogen by adding aluminum, and then the diamonds turned out transparent. Further experiments showed that the addition of boron gives the stones a blue color. In modern laboratories it is possible to grow diamonds weighing up to 25 carats (5 g) and even more, but such samples are used only for research purposes. For economic reasons, the growth process is stopped when the stones reach 1-1,5 carats (200-300 mg).

Other methods

In the 1950s, the United States and Soviet Union began to explore a way to create a diamond film by pyrolyzing hydrocarbon gases at a relatively low temperature of 800 °C. This does not require high pressure. Microwave radiation creates a carbon plasma above the substrate – carbon atoms are deposited onto it, forming a diamond. The experiments of American and Soviet scientists were replicated in many laboratories and led to the development of low-cost diamond coating technology for industrial and scientific purposes. There are other, less common methods of growing diamonds: explosive (formation of detonation nanodiamonds) and ultrasonic (ultrasonic treatment of a graphite solution). The explosive method is used mainly in China: it is used to produce polishing powder. Ultrasound technology is still not fully optimized, so it has not spread widely.

Grown diamonds in jewelry

  • No inclusions. Modern methods allow you to create perfect diamonds – clean, without inclusions or chips.
  • Price. Lab-grown stones are produced on an industrial scale, cost less, and are not as rare as natural diamonds. That’s why they cost less.
  • Big size. In nature, large diamonds are found very rarely, and due to defects they can noticeably shrink during cutting. Therefore, almost every large diamond is a collector’s item. Grown diamonds allow you to create large, but not too expensive diamonds.

Despite these advantages, grown diamonds cannot replace natural ones, because the latter are attractive due to their centuries-old history and uniqueness: it is impossible to find two identical gems.

Other applications

Grown stones are a complete replacement for natural ones. They are most in demand in four areas:

  • Machine tools and other cutting instruments in medicine and industry (diamond coating). This is the widest area of ​​application of crystals where their hardness is used.
  • Semiconductor technologies. They use two properties of the material – high thermal conductivity with very low electrical conductivity.
  • Optics for transmitting infrared and microwave radiation. This application is possible due to its hardness, chemical inertness and thermal conductivity.
  • Protection of fragile materials and mechanisms. Grown minerals are used in cameras, watch mechanisms, and optical instruments.

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How are diamonds grown?

How artificial diamonds are grown Diamond is the hardest mineral on Earth. The scope of its application is incredibly wide, but reserves in the subsurface are limited, and open deposits are being depleted. All this leads to an increase in prices for the mineral, but natural reserves are gone.

A group of scientists from Germany and Canada has determined exactly how diamonds formed at enormous depths end up in kimberlite pipes. Until recently, this important detail of the formation of the most important gemstones remained unclear. Now scientists hope that their discovery will help to better understand the dynamics of diamond formation processes and, of course, will help to look for new deposits in the future.

Exotica

Pure carbon occurs in nature in several basic forms. The most familiar to everyone is graphite. In this material, the carbon atoms are organized into layers. In each layer, C atoms are located at the vertices of a hexagonal (hexagonal) lattice. The layers are rather loosely connected to each other. Thanks to this (that is, a weak connection), Konstantin Novoselov and Andrey Geim in 2004 were able to obtain graphene – exactly one layer of graphite, using ordinary tape, although this is a completely different story.

It must be said that diamond is not the hardest allotropic modification of carbon. This title currently belongs to specially processed lonsdaleite. The structure of its crystal lattice resembles the lattice structure of diamond, for which this material even received the name hexagonal diamond. As computer modeling has shown, the processed lonsdaleite sample is destroyed at a pressure of 152 gigapascals. Similar materials are formed when meteorites fall.

Diamond – by the way, in Greek “adamas”, which means “indestructible” – is a direct relative of graphite and coal, or, as scientists say, an allotropic modification of carbon (as a result, for example, at a temperature of 2000 degrees Celsius in a stream of oxygen, diamond burns almost without a trace, turning into carbon dioxide). The carbon atoms in it are arranged differently than in graphite. The atoms are arranged in a cubic face-centered lattice – each carbon atom is located in the center of a tetrahedron, the vertices of which are four neighbors. Among other things, it is precisely this arrangement of atoms that explains the extraordinary hardness of diamond – the sample is destroyed at a pressure of 97 gigapascals.

It must be said that this modification of carbon has attracted people since ancient times with its unusual optical properties. The fact is that diamond has high refractive indices and dispersion. As a result, in the case of the correct cut (that is, when we are essentially talking about a diamond), it sparkles very beautifully, decomposing, among other things, light into spectral components. Thanks to this generally interesting, but trivial, from a scientific point of view, feature, diamonds are classified as precious stones. Nowadays, diamonds are widely used in industry due to their hardness.

How are diamonds created? From a geological point of view, there are several ways. Since scientists from Germany and Canada, who were discussed at the beginning of the article, were interested in the most common – magmatic – method, let’s start with the least probable. Scientists know that diamonds are formed, on the one hand, under colossal pressure – 50000 atmospheres – and a relatively low temperature – 900 -1300 degrees Celsius. According to researchers, such conditions can arise, for example, when meteorites fall. Such diamonds include, for example, those discovered in the Popigai crater in Siberia.

Another method, extremely rare, is the transformation of graphite into diamond. Despite the fact that these two materials are related and a similar method of obtaining diamonds was described in “DuckTales” (Scrooge McDuck used peanuts to attract elephants, who, with their stomping, turned coal in a depleted mine into diamonds), there is only one deposit in the world that diamonds in which appeared precisely as a result of such a process. This is the Kumdykul field, and it is located in Northern Kazakhstan, 25 kilometers southwest of the city of Kokshetau. Diamonds were formed here as a result of the sinking of carbon-bearing sedimentary rocks into the mantle. Such diamonds are called metamorphogenic (that is, transformation under the influence of temperature and pressure) type diamonds.

This also includes the so-called carbonados – black diamonds, regarding which there is still no consensus among scientists. According to one opinion, they were formed as a result of a meteorite fall, according to another – they appeared from organic carbon. This is indicated, in particular, by the ratio of different isotopes of this element in diamond.

Kimberlite is not the only material associated with diamonds. In the 70s of the last century, a rich deposit of mainly industrial diamonds associated with lamproites was discovered in Australia. It is also a volcanic rock. It is noteworthy that diamonds found in different rocks are almost the same in properties.

At the same time, ordinary transparent diamonds are formed, from a geological point of view, quite simply. First, a volcanic eruption occurs. If everything went well (in particular, the right magma was found), then a conical kimberlite pipe will form in the place where it broke through to the surface. The rock is named after the city of Kimberley in South Africa, where this rock was first discovered at the end of the 19th century – until that moment, diamonds were found in river beds (the so-called secondary deposits), where they ended up as a result of the erosion of the same kimberlite pipes.

The formation of a kimberlite pipe can only occur if magma rises from a significant depth – approximately 150 kilometers, which is at least three times deeper than the occurrence of “ordinary” magma for volcanoes. The physical conditions mentioned above exist only where cratons—the cores of continents—are located. It is this special magma that rises from the depths and, breaking free, produces diamonds.

Girl’s Best Friend

It must be said that there is a weak point in this theory. As mentioned above, diamonds burn. Of course, there is no pure oxygen in the mantle, but long-term exposure of diamonds to the hot mass should still lead to their destruction. It follows from this that the very special magma mentioned above rises to the surface very, very quickly. Geologists previously avoided this detail (it rises and rises, what can you do), so the exact reasons for this process were unclear.

As part of the new work, scientists used a special smelter to obtain a substance resembling magma from the depths of the earth. In particular, the melt contained a large number of carbonates – salts of carbonic acid. Scientists have suggested that during its life, such magma encounters magma with large amounts of pyroxenes (a group of minerals, often rock-forming, containing silicon). Because of this, the ability of the melt to dissolve various types of substances – for example, carbon dioxide – is reduced several times.

To test their hypothesis, the researchers added pyroxenes to the melt and waited. According to one of the scientists, Kelly Russell, he was shocked when in just 20 minutes the hot substance essentially turned into foam. From this, scientists concluded that such foam pockets may well form at a depth of about 150 kilometers.

the end

What happens when such a pocket forms? At great speed he begins to float up. At the same time, the ascent speed can reach 40 kilometers per hour. In this case, the pocket accelerates as it ascends. According to scientists, this could have significant implications for the theory of diamond formation. Perhaps it will even help in finding new deposits. Be that as it may, the new work allows us to clarify the details of the formation of diamonds. And the devil, as we know, is in these details.

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