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Diamond In India

>> Wednesday, 24 August 2011

In mineralogydiamond (from the ancient Greek αδάμας – adámas "unbreakable") is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions. Diamond is renowned as a material with superlative physical qualities, most of which originate from the strong covalent bonding between its atoms. In particular, diamond has the highest hardness and thermal conductivity of any bulk material. Those properties determine the major industrial application of diamond in cutting and polishing tools.
Diamond has remarkable optical characteristics. Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such asboron and nitrogen. Combined with wide transparency, this results in the clear, colorless appearance of most natural diamonds. Small amounts of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (lattice defects), green (radiation exposure), purple, pink, orange or red. Diamond also has relatively high optical dispersion (ability to disperse light of different colors), which results in its characteristic luster. Excellent optical and mechanical properties, combined with efficient marketing, make diamond the most popular gemstone.

The first way, first achieved by Swedish company ASEA in 1953, and then by GEC in the US involves using large pressures and temperatures. Synthesis takes place within a cylindrical capsule containing a source of carbon, a solvent catalyst made from cobalt, nickel and iron, and a seed crystal. The capsule is placed between an anvil and die made from tungsten carbide at a pressure between 5 GPa and 7.1 GPa (50 000 to 70 000 atmospheres) and temperature between 1200 and 1500 oC. A temperature gradient of a few tens of degrees can help growth from the seed diamonds. These high pressure high temperature (HPHT) techniques can grow diamonds from a few hundredths to a few tens of carats. With the largest commercially available diamond being 3 carots (0.6 g).
Most natural diamonds are formed at high-pressure high-temperature conditions existing at depths of 140 to 190 kilometers (87 to 120 mi) in the Earth mantle. Carbon-containing minerals provide the carbon source, and the growth occurs over periods from 1 billion to 3.3 billion years (25% to 75% of theage of the Earth). Diamonds are brought close to the Earth surface through deep volcanic eruptions by a magma, which cools into igneous rocks known as kimberlites and lamproites. Diamonds can also be produced synthetically in a high-pressure high-temperature process which approximately simulates the conditions in the Earth mantle. An alternative, and completely different growth technique is chemical vapor deposition (CVD). Several non-diamond materials, which include cubic zirconia and silicon carbide and are often called diamond simulants, resemble diamond in appearance and many properties. Special gemological techniques have been developed to distinguish natural and synthetic diamonds and diamond simulants.
Uncut HPHT diamonds differ from natural diamonds in there external shape. HPHT diamonds have a cubo-octahedral morphology as opposed to the octahedral morphology of natural diamonds. After cutting the original shapes can be distinguished because the uptake of impurities is different in different growth directions. Individaul faces exhibit different colours or fluoresence intensities. Some HPHT diamonds will exhibit strong fluorescence due to electronic transitions at atomic impurities containing nickel or cobalt from the solvent during growth. Inclusions of titanium or zirconium metal can also be trapped in the diamond when they are added to limit discolouration of the diamond by the prescence of nitrogen.
Chemical vapour deposition (CVD)
Another way of making diamond is to form a plasma from carbon-containing gas which is then deposited onto wafers of synthetic diamond. Carbon is initially deposited as graphite, but hydrogen etches away any graphite that is formed during deposition leaving only diamond structures behind. The ability to produce extermely high-purity diamonds with well-controlled doping makes this the preferred route to explore future technical applications such as electronic devices.



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