The word ‘diamond’ derives from Greek adamao, meaning ‘I tame’ or ‘I subdue’ or the related word adamas, which means ‘hardest steel’ or ‘hardest substance’. Everyone knows diamonds are hard and beautiful, but did you know a diamond could be the oldest material you might own? While the rock in which diamonds are found may be 50 to 1,600 million years old, the diamonds themselves are approximately 3.3 billion years old. This discrepancy is because the volcanic magma that solidifies into rock where diamonds are found did not create them, but only transported the diamonds from the Earth’s mantle to the surface. Diamonds also may be formed under the high pressures and temperatures at the site of meteorite impacts. The diamonds formed during an impact may be relatively ‘young’, but some meteorites contain star dust, debris from the death of a star, which may include diamond crystals. One such meteorite is known to contain tiny diamonds over 5 billion years old. These diamonds are older than our solar system!
Start with Carbon
Understanding the chemistry of a diamond requires a basic knowledge of the elementcarbon. A neutral carbon atom has 6 protons and 6 neutrons in its nucleus, balanced by 6 electrons. The electron shell configuration of carbon is 1s22s22p2. Carbon has a valence of 4, since 4 electrons can be accepted to fill the 2p orbital. Diamond is made up of repeating units of carbon atoms joined to four other carbon atoms via the strongest chemical linkage, covalent bonds. Each carbon atom is in a rigid tetrahedral network where it is equidistant from its neighboring carbon atoms. The structural unit of diamond consists of 8 atoms, fundamentally arranged in a cube. This network is very stable and rigid, which is why diamonds are so very hard and have a high melting point.
A Diamond is one of the two natural minerals that are produced from carbon. The other mineral is Graphite. Even though both of these minerals are produced from the same element, carbon, they have totally different characteristics. One of the most obvious difference is that Diamond is hard and Graphite is soft. The Diamond is considered to be the most hardest substance found in nature. It scores a ...
Virtually all carbon on Earth comes from the stars. Studying the isotopic ratio of the carbon in a diamond makes it possible to trace the history of the carbon. For example, at the earth’s surface the ratio of isotopes carbon-12 and carbon-13 is slightly different from that of star dust. Also, certain biological processes actively sort carbon isotopes according to mass, so the isotopic ratio of carbon that has been in living things is different from that of the Earth or the stars. Thus it is known that the carbon for most natural diamonds comes most recently from the mantle, but the carbon for a few diamonds is recycled carbon of microorganisms, formed into diamonds by the earth’s crust via plate tectonics. Some minute diamonds that are generated by meteorites are from carbon available at the site of impact; some diamond crystals within meteorites are still fresh from the stars.
The crystal structure of a diamond is a face-centered cubic or FCC lattice. Each carbon atom joins four other carbon atoms in regular tetrahedrons (triangular prisms).
Based on the cubic form and its highly symmetrical arrangement of atoms, diamond crystals can develop into several different shapes, known as ‘crystal habits’. The most common crystal habit is the eight-sided octahedron or diamond shape. Diamond crystals can also form cubes, dodecahedra, and combinations of these shapes. Except for two shape classes, these structures are manifestations of the cubic crystal system. One exception is the flat form called a macle, which is really a composite crystal, and the other exception is the class of etched crystals, which have rounded surfaces and may have elongated shapes. Real diamond crystals don’t have completely smooth faces, but may have raised or indented triangular growths called ‘trigons’. Diamonds have perfect cleavage in four different directions, meaning a diamond will separate neatly along these directions rather than break in a jagged manner. The lines of cleavage result from the diamond crystal having fewer chemical bonds along the plane of its octahedral face than in other directions. Diamond cutters take advantage of lines of cleavage to facet gemstones.
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Graphite is only a few electron volts more stable than diamond, but the activation barrier for conversion requires almost as much energy as destroying the entire lattice and rebuilding it. Therefore, once diamond is formed, it will not reconvert back to graphite because the barrier is too high. Diamonds are said to be metastable, since they are kinetically rather than thermodynamically stable. Under the high pressure and temperature conditions needed to form diamond its form is actually more stable than graphite, and so over millions of years carbonaceous deposits may slowly crystallize into diamond.
TYPES OF DIAMONDS
Type of Diamonds
Natural diamonds are classified by the type and quantity of impurities found within them.
Type Ia – This is the most common type of natural diamond, containing up to 0.3% nitrogen.
Type Ib – Very few natural diamonds are this type (~0.1%), but nearly all synthetic industrial diamonds are. Type Ib diamonds contain up to 500 ppm nitrogen.
Type IIa – This type is very rare in nature. Type IIa diamonds contain so little nitrogen that it isn’t readily detected using infrared or ultraviolet absorption methods.
Type IIb – This type is also very rare in nature. Type IIb diamonds contain so little nitrogen (even lower than type IIa) that the crystal is a p-type semiconductor.
Synthetic Industrial Diamonds
Synthetic industrial diamonds are produced the process of High Pressure High Temperature Synthesis (HPHT).
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In HPHT synthesis, graphite and a metallic catalyst are placed in a hydraulic press under high temperatures and pressures. Over the period of a few hours the graphite converts to diamond. The resulting diamonds are usually a few millimeters in size and too flawed for use as gemstones, but they are extremely useful as edges on cutting tools and drill-bits and for being compressed to generate very high pressures. (Interesting side note: Although used to cut, grind, and polish many materials, diamonds aren’t used to machine alloys of iron because the diamond abrades very quickly, due to a high-temperature reaction between iron and carbon.)
Thin Film Diamonds
A process called Chemical Vapor Deposition (CVD) may be used to deposit thin films of polycrystalline diamond. CVD technology makes it possible to put ‘zero-wear’ coatings on machine parts, use diamond coatings to draw the heat away from electronic components, fashion windows that are transparent over a broad wavelength range, and take advantage of other properties of diamonds.