What is a Diamond and where are they found?
What is a Diamond and where are they found?
Diamonds are the hardest known mineral, comprising of crystalline carbon, formed under intense heat and pressure. Diamonds are found in ancient volcanic pipes, mainly in South Africa and Siberia, and in deposits off the coast of Namibia. Over 80% of diamonds mined are used in industry mainly for cutting and grinding tools. Others are used in jewellery and in industry.
Diamond Structure and Bonding
* In diamond, each c-atom is covalently bonded to four other c-atoms to give a tetrahedral unit.
* In diamond c-c-c bond angle is 109.5 o (degrees). These basic tetrahedral units unite with one another and produce a cubic unit cell. A diamond is a giant molecular structure containing many of these tetrahedral units making it very rigid.
* On a scale of ten for hardness diamond is 10. This is called Moh's scale. To compare this to other materials gold has 2.5, pearls have 3.5 and sapphires and rubies have 8.
* c-c bond length in diamond is 1.54 Ao (Angstroms, this is 10 to the power of -10 m or 15.4 nano metres).
* c-c bond energy is 347 kj/mole.
* In a diamond crystal basic units join to form a octahedral shape of diamond crystal. This means it has eight sides.
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* What are the properties of Diamond?
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* Diamond is transparent over a larger range of wavelengths (from the ultraviolet into the far infrared) than is any other substance.
* Diamond is a good thermal conductor, better than any other- five times better than the second best element, Silver.
* Diamond has the highest melting point (3820 degrees Kelvin)
* The lattice arrangement of atoms in Diamond are packed closer together than in the atoms of any other substance.
* Diamond fractures along a specific plane when hit but it cannot be compressed.
Carbon exists in two allotropic forms:(1) CRYSTALLINE FORM
(2) AMORPHOUS FORM
There are three allotrope forms of carbon which are the crystalline forms, these are:
(1) Diamond
(2) Graphite
(3) Buckminsterfullerines, commonly known as Bucky ball
Graphite and diamond are well known polymorphs which have the same composition but different crystal structure.
There are various amorphous forms of carbon such as
(1) coal
(2) coke
(3) charcoal
(4) lampblack
(5) gas carbon etc.
The largest Diamond
The largest diamond yet discovered is the Cullinan diamond (3106 carats), found in 1905 at Pretoria. It was cut into two very large stones which are both a part of the British crown jewels. The Cullinan diamond was discovered in 1905 in South Africa and brought to England by Thomas Cullinan; it was named after him, the chairman of the diamond company which excavated the stone. When discovered it weighed 3,106 carats. A carat in diamond terms represents 200 milligrams of diamonds - a diamond of one carat is almost exactly 1/4 inch in diameter.
How and when were Diamonds made?
The ancient Romans believed diamonds were splinters from fallen stars. The Ancient Greeks thought they were tears from the Gods.
Most civilisations have had a fascination for them, if not for their beauty and rarity then for their useful properties - they are the hardest substance known.
Today we know that diamonds form at great pressures (typically 50,000 atmospheric pressures) and at great depths (about 200 km) from carbon deposits compressed so much that the carbon atoms form a crystalline lattice. By a kind of volcanic eruption the diamonds ...
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How and when were Diamonds made?
The ancient Romans believed diamonds were splinters from fallen stars. The Ancient Greeks thought they were tears from the Gods.
Most civilisations have had a fascination for them, if not for their beauty and rarity then for their useful properties - they are the hardest substance known.
Today we know that diamonds form at great pressures (typically 50,000 atmospheric pressures) and at great depths (about 200 km) from carbon deposits compressed so much that the carbon atoms form a crystalline lattice. By a kind of volcanic eruption the diamonds are pushed up through volcanic pipes of diamond bearing rock such as kimberlite. With gradual erosion kimberlite chimneys are exposed at the earths surface revealing diamonds.
But much about the formation of diamonds remains mysterious. From the data they obtained, geologists have now identified three generations of diamonds.
The first generation was formed about 3.3 billion years ago, Steve Shirey, and expert in this field said. "The oldest diamonds are survivors from the Earth's earliest geological times and the oldest known rocks. We were able to identify where they came from under southern Africa."
The second generation of diamonds were formed slightly later, about 2.9 billion years ago. These appear to be more widely scattered throughout a bigger region of the Earth's mantle. These diamonds may have been formed in a slightly different way from their first-generation cousins.
From an analysis of impurities in them scientists believe they formed inside rocks that were originally laid down in an ancient shallow sea. Somehow these rocks were pulled down to great depths where carbon deposits, possibly from living organisms, were heated and compressed to form diamonds.
The third generation of diamonds are termed Proteozic by some scientists. They formed about 1.2 billion years ago and provide a particular insight into the conditions on Earth at a geologically significant time.
Some younger diamonds - about 100 million years old - are known but they are a minority and not fully understood. However these youngsters were formed, it seems that the era of diamond formation is over.
Dr Shirey said in a private interview for the BBC, and I quote, "We believe that the Earth is not forming as many or as big diamonds as it did billions of years ago. Perhaps the planet was hotter on the inside, or the composition of the rocks was subtly different. Whatever it was it has changed now. Diamond formation was chiefly a feature of the Earth's youth." This information is published in the journal Science.
Do planets make diamonds from methane?
October 1999
The high temperatures and pressures found in the middle layers of Neptune and Uranus might be capable of converting methane into diamond. Physicists in the US have recreated the high temperatures and pressures inside the planets in a laser-heated diamond anvil cell. They found that the methane molecules fall apart under these conditions to form complex hydrocarbons and diamond. The results have implications for theories of planetary formation and evolution (Science 286 100).
Scientists have long suspected that the core of the outer planets could consist of diamond, but this is the first evidence for diamond formation in the middle layers of the planets. Both Neptune and Uranus consist of 10-15% methane and, according to Benedetti and colleagues, large quantities of diamond could affect both the luminosities and magnetic fields of the planets.
Total Internal Reflection in Diamonds
If the angle of incidence is increased beyond the critical angle, the light rays will be totally reflected back into the incident medium. This effect is called total internal reflection. But note that total internal reflection cannot occur if light is travelling from a less dense medium to a denser one.
Diamonds achieve their brilliance partially from total internal reflection. Because diamonds have a high index of refraction (about 2.3), the critical angle for the total internal reflection is only about 25 degrees. Incident light therefore strikes many of the internal surfaces before it strikes one less than 25 degrees and emerges. After many such reflections, the colours in the light are separated, and seen individually.
At the wavelengths that humans can see, light does not have enough energy to excite the tightly held electrons in diamond, so diamond appears transparent to us. Impurities in the diamond, contribute electrons that can absorb light at characteristic wavelengths, to give the diamond varying hues of colour. For example: pure diamond appears transparent, nitrogen in diamond causes a yellow to orange tint, and boron causes a blue coloration.
What causes colour change in Diamonds?
Nearly all diamonds mined are coloured and range from yellow, brown, and black to blue, green, pink, and mauve fancies. The majority are the near colourless yellow variety of the Cape Series.
The causes of these colours lies at atomic level. Diamond comprises covalently bonded carbon atoms. In pure diamond visible light does not possess enough energy to excite electrons in the bonds and consequently no light is absorbed in the visible band and all the light falling on the diamond is transmitted and refracted back to the eye causing the diamond to appear completely colourless.
However, most diamonds are not perfect. The diamond lattice contains impurities such as nitrogen and the lattice is sometimes defective with a missing atom. This presence of impurities, or more specifically the electrons associated with the impurities, is the cause of colour in the majority of diamonds.
Yellow: In a yellow diamond nitrogen is the cause of colour.
Brown: Plastic deformation of the diamond lattice whilst in the earth caused it to be a brown colour.
Pink & Mauve: Plastic deformation of the diamond lattice in the upper mantle region of the earth makes a pink/mauve colour.
Black: Black is caused by a vast quantity of dark opaque inclusions.
Blue: This is due to boron impurity in the lattice.
Green: Caused by alpha particle radiation.
What are Diamonds used for?
Most diamonds are used in drill bits and diamond tools. A small number are used for glasscutters and surgical instruments. Only the finest are used as gems. The task of the highly skilled diamond cutter is to place the facets so that the most light rays will reflect through the top facets.
In even the best gem-producing areas only about 25 % of the diamonds mined are of gem quality. The rest, of poor gem quality because of colour or faults, are used in industry.
Diamonds cutters are employed to shape and polish diamonds and other gems. Tiny holes drilled in diamonds are used to draw ductile metals like copper into extremely fine wire for the electronics industry. The stones are also used to true the surfaces of precision grinding wheels. In machine shops, tools tipped with diamonds perform precision-cutting tasks. Geologists and engineers use diamond-tipped hollow steel bits for drilling deep rock samples.
Another use for the diamond crystals is to put a layer of diamond on a carbide substrate by again subjecting this to the high temperature high-pressure process. This yields a product called polycrystalline diamond compacts which are used for oil well drills and cutters for drilling and milling machines.
What are Synthetic diamonds and how are they made?
Since 1960, companies in a number of countries have produced synthetic diamonds on a commercial scale. In 1990 a new synthetic process was developed that makes diamonds that are harder, heavier, and better heat conductors than natural diamonds.
It is important to remember that a synthetic gemstone is one that carries all of the chemical, structural, and colour properties of the natural, but it is synthetically grown in a laboratory. And will therefore not cost near as much as a natural gemstone because it is not as rare and not as valuable.
Modern manufacture of synthetic diamonds utilizes these same methods discovered by Mr. Hall. A mixture of graphite and a catalyst (typically nickel) is subjected to a pressure of approximately 1,000,000 pounds per square inch and a temperature of 1,800 °C for a period of approximately 1 hour. During this time diamond crystals nucleate at many sites in the mixture. The mixture is then cooled and the pressure reduced. The diamond crystals are then separated from the remaining graphite and nickel using an acid wash.
The separated crystals are sorted by shape, size, and impurities. This process is called grading. A typical production cycle will yield approximately 300 carat of synthetic industrial diamond of various grades. The larger diamonds are used for sawing concrete, granite, and marble. Smaller diamonds are used in grinding wheels.
Most synthetic diamonds are not sold as gemstones for jewellery but are used industrially. This is because those diamonds made are generally of lower quality than the naturally created ones. This means that the trade in synthetic diamonds does not really affect the global prices of diamonds in the market place.
