Forming of Diamond - Research Paper Example

? Diamond, like Graphite is a Carbon material but is very unlike it in other terms. This research paper attempts to understand the various aspects of Diamond as a mineral, how different it is from other Carbon components, what unique properties of this Carbon mineral make it exhibit the hardness, strength and shine that are so much sought by industrialists as well as ornamentalists among others. The process of diamond forming is studied which is found to be a natural process under the Earth’s crust and brought forward to the surface through violent volcanic activity. Man-made Diamonds are also available and are being increasingly manufactured for large-scale industrial use, and the different equipment used through the life-cycle of its manufacture will be studied. Finally, the different types of the mineral and their identification, inspection and quality control procedures will also be studied as part of the research. The current scenario of Diamond production or manufacture will be understood to gain better insight into the availability of this precious mineral. Table of Contents Introduction Formation of diamond Processing of diamond Manufacturing equipment Life cycle of diamond manufacturing Conclusion References Introduction Properties: Diamond is the hardest mineral on earth. It is a natural mineral and is the hardest on Mohs hardness scale that is used to determine the hardness of solids, particularly minerals. This scale that lists the softest to the hardest solids has diamond as its hardest material with a ranking of 10. Although diamond is just one level above corundum, it is still more than ten times harder than corundum. Table 1 represents the Mohs hardness scale as below (Bonsor and Keener, 2001): Table 1- Mohs hardness scale. Source: (Bonsor and Keener, 2001): 1 Talc Easily scratched by the fingernail 2 Gypsum Just scratched by the fingernail 3 Calcite Scratches and is scratched by a copper coin 4 Fluorite Not scratched by a copper coin and does not scratch glass 5 Apatite Just scratches glass and is easily scratched by a knife 6 Orthoclase Easily scratches glass and is just scratched by a file 7 Quartz (amethyst, citrine, tiger's-eye, aventurine) Not scratched by a file 8 Topaz Scratched only by corundum and diamond 9 Corundum (sapphires and rubies) Scratched only by a diamond 10 Diamond Scratched only by another diamond Although, graphite is also formed of carbon atoms, it is not as strong as diamond. However, graphite is being used in man-made diamonds due to its similarities in composition, where the ring-like structure of carbon atoms of graphite is altered to form a crystalline structure (Bonsor and Keener, 2001). It is a carbon component found in the form of an ore that is processed and used. The unique molecular structure of the material is what gives it its strength, as five carbon atoms forming a tetrahedron by each atom forming covalent bonds with four other carbon atoms gives diamond its unique stability which is hard to break. It is a crystalline structure with billions of carbon atoms bonding together under very high pressure and temperatures inside the Earth’s mantle at about 100 miles below its crust. These crystals are carried to the surface of the Earth by violent volcanic eruptions (Lineberry, 2006). Diamond is chemically inert to most of the acids and alkalis, has low thermal expansion, has a negative electron affinity, is a good thermal conductor, is least compressible, has high specific gravity, and has high refractive index and reflection (Helmenstine, 2012). Common morphologies or or isometric crystal structures of diamond are: cubical, octahedral and do-decahedron (Composition and properties of diamond, 2008). Advantages: Diamonds are usually transparent or pale blue but some coloured diamonds are also found due to the presence of impurities in the lattice structure. Traces of Boron, Nitrogen and other gases result in the blue, yellow and other colours of diamonds. Diamonds are found in Kimberlite and Lamproite rocks that are brought to the Earth’s surface through the volcanic eruptions. They are used on cutting tools and drill-bits, coatings of diamond are used to coat machine parts for zero-wear or to draw heat away, and also to fashion windows (Helmenstine, 2012). As Diamond has the highest thermal conductivity, it is being used in the manufacture of microchips as they need to run at very speeds causing high temperature that are likely to liquefy other microprocessor materials (Davis, 2003). High dispersion and refractive index of Diamond give it the luster that is used to grade the mineral for its quality. Higher the luster, more is the quality and price. Its isometric structure gives the advantage of cutting Diamond along its crystal face while its sides still remain smooth (The Unique Properties of Diamond, 2012). The unique properties of Diamond make it useful in science and engineering for various purposes like surgery, telecommunications, heat spreaders, health care, windows, etc. (Properties of diamonds, 2012). Diamond windows made from thin diamond coatings cover lasers, X-ray machines, etc. and are transparent and resistant to heat and abrasion. Its stiffness makes it useful in speaker domes of high performance speakers without degrading the sound quality. Heat sinks made of diamond conduct heat away from sensitive, high performance microelectronics. The semiconductor nature of diamond can be used to either conduct electricity or block current (Diamond- Mineral uses and properties, 2011). Despite the many advantages of diamond in the industrial sector and other areas, there are some limitations in its use, which are discussed below. Limitations: Geologists identified three generations of diamonds that were formed 3.3 billion years, 2.9 billion years and 1.2 billion years ago; after which there is very less activity under the Earth’s crust that favours the formation of diamonds (Dr. Whitehouse, 2002). The largest and most flawless diamonds are a rarity (A few famous diamonds, 2008). It is very difficult to reach the depths where diamonds are formed as there are no ways of drilling or any other means. Further, these minerals are carried from the upper mantle to the surface of the Earth by Kimberlite eruptions and the mould needs to cool very quickly, within hours, instead of rearranging into graphite (Lineberry, 2006). Formation of diamond Carbon is the constituent atom of both natural and man-made diamond. It is thought that Carbon from carbonate sediments of plants, animals, etc. must have travelled to the upper mantle of Earth’s crust due to techtonic activity and got trapped and locked into strong covalent bonds under high temperature and pressure, to form a network of uniform lattice structures. Each of these crystal lattices is one carat diamond (Lineberry, 2006). However, diamonds are basically formed through four different processes under the Earth’s crust (Figure 1). Figure 1: Diamonds formed through four processes. Source: (How do diamonds form, 2011). Diamonds are formed at a depth of 125-200 km below the Earth’s surface in ‘diamond stability zones’, at temperatures between 900°C and 1,300°C and at pressures of 45-60 kbars, which is about fifty thousand times the atmospheric pressure at the surface of the Earth. Figure 2 represents diamond formation in Earth’s mantle: Figure 2: Deep source eruptions. Source: (How do diamonds form, 2011). Oceanic plates with high density are also a source of diamond formation as they have been subducted deep into the mantle, at about 80 km below the surface with a temperature of 200 degree centrigrade due to techtonic processes (Figure 3). Figure 3: Subduction zone diamonds. Source: (How do diamonds form, 2011). Another process of diamond formation is by asteroids striking the Earth’s crust at high temperatures and pressures to form small diamonds under the impact as the asteroids travel at hypervelocities of 15-20 km/s (Figure 4). Figure 4: Diamond formation due to asteroid impact. Source: (How do diamonds form, 2011). The fourth process through which diamonds are formed are from meteorites in space as Carbon in the meteorites is responsible in the formation of nano-diamonds that are a few nanometers in size (Figure 5). Figure 5: Diamond formation in space. Source: (How do diamonds form, 2011). While Carbon is the main ingredient for the formation of diamonds, the source of this element is debatable. It is seen that Uranium rich carbonaceous mineral assemblages of Precambrian age irradiated graphite at ambient temperatures through radioactive decay to form nanometer-grain polycrystalline diamonds. This has been established in the laboratory when fine grain polycrystalline graphite sheets were irradiated with 400 MeV Kr ions that resulted in the formation of ppm concentrations of nano-diamonds as the solid-state transformation of graphite to diamond takes place (Daulton et al, 1998). Diamonds are also seen to be formed by the crystallization of C-O-H fluids in the thermodynamically stable region of diamond also called the diamond stability zone. As graphitic carbon reacts or dissolves in C-O-H fluids, it precipitates into diamond at high temperatures and high pressures. This reaction has been experimentally confirmed by analyzing the solid-fluid reaction between graphite and water, graphite and oxalic acid dehydrate and graphite and malonic acid at 7.7 GPa and over 1600 degree centigrade using a molybdenum double capsule. It is observed that euhedral diamond crystals with octahedral morphology are formed during this solid-liquid reaction and the time taken to form the diamond crystals is more when the temperature is less than 1600 degree centigrade (Kumar et al, 2000). The diamond-graphite equilibrium curve represents the temperatures and pressures appropriate for thermodynamic stability of diamond (Figure 6) (Bovenkerk et al, 1959): Figure 6: Carbon phase diagram. Source: (Bovenkerk et al, 1959). Further, it is observed that the transformation from graphite to diamond occurs along a thin film of 0.1 mm when the temperature and pressure are at thermodynamic equilibrium of diamond. It is observed that five types of chemical reactions are associated with the formation of diamond. These are: direct transition of graphite to diamond, reactions involving carbon and oxygen, reactions involving carbon present as carbides, reactions of carbon dissolved in molten metals and other chemical reductions. Also, there are lists of conditions that have been observed to aid the formation of diamond: 1. The pressure and temperature of the system is that for which the diamond is thermodynamically stable, 2. In the presence of a catalyst, the temperature and pressure of the diamond forming system is lower for the intersection of the melting line for metal-carbon eutectic and graphite-diamond equilibrium line, 3. Metallic catalysts like platinum, rhodium, iron, etc. can be used to induce formation of diamond, 4. Temperature and pressure of the diamond forming systems falling in the diamond stable region and away from the graphite-diamond equilibrium can increase the rate of nucleation and the growth of diamond but decreases the size of the diamond formed, 5. Diamond synthesis rates can be as high as 0.1 mm per minute in a typical laboratory setting, 6. Diamond formation occurs along a thin line of interface between graphite and diamond formed, 7. Temperature gradients can accelerate formation of diamond as temperature depends on the solubility of carbon, 8. Carbon source determines the kind and number of diamonds formed, 9. Some of the catalyst metals can be trapped inside a diamond crystal as they are formed at higher rates around a mixture of other particles, 10. The crystal structure formed depends on the temperature of formation and cubes, mixed cubes (Figure 7), cubo-octahedra and dodecahedra (Figure 8) and octahedral (Figure 9) crystals are formed at lower, intermediate and higher temperatures respectively and 11. Colour of the diamond crystal depends on the temperature of formation and varies from black through dark green, light green and yellow, to white at lower to higher temperatures (Bovenkerk et al, 1959). Figure 7: Cubes of diamond. Source: (Formation of diamonds, 2009). Figure 8: Dodecahedron diamonds. Source: (Formation of diamonds, 2009). Figure 9: Octahedron diamond. Source: (Formation of diamonds, 2009). Two methods of diamond formation that more or less involve the above discussed criteria are: natural and man-made. While the natural method of diamond formation involves the above discussed criteria with or without a known catalyst, man-made diamonds require special apparatus and usually a catalyst under similar thermodynamic conditions. Explosive shock is also said to yield diamond as graphite is subject to instantaneous stress (Giardini and Tydings, 1962; Daulton et al, 1998; Bonsor and Keener, 2001; DeCarli and Jamieson, 1961). Explosive shock can be used to understand the formation of natural diamonds, brute force involved in the formation of man-made diamonds can help understand the formation of diamond in the laboratory where carbon is under 58,000 atmospheres of pressure at 2,300 degrees Fahrenheit to yield yellow diamond crystals. Chemical Vapour Deposition, or CVD is another method of producing diamond in the laboratory in which carbon gases are vaporized and deposited on a diamond chip to form the same structure as that of the diamond, atom by atom (Maney, 2005; Reginald et al, 2005). Processing of diamond Diamonds are processed after they are found in kimberlites (volcanic pipes and channels) and placer deposits (stream sand). Magma containing dissolved carbon dioxide reaches the Earth’s surface through violent explosions forming a pipe called kimberlite. The magma rises and cools down quickly and there is no time for diamond (molten magma) to convert to graphite. The primary deposits are present in the mineral-rich kimberlites, lamproites and komatites in the volcanic pipes and streams on the surface transported by water (Origin and formation of diamonds , 2008) which are then processed for the 4Cs: cut, carat, colour and clarity based on which diamonds are usually classified and valued (The 4Cs, 2008). While the carat, colour and clarity are formed, the diamond can be manufactured through controlling the cut characteristic to obtain different shapes and sizes. Different types of cuts are used for cutting the diamond: point cut, single cut, table cut, old mine cut, old European cut, and the modern brilliant cut. While the simplistic or point cut is a basic four-sided cut resembling a rough octahedral crystal structure, the modern brilliant cut is a round outline with symmetrical triangular and kite-shaped faces, a table and with or without a culet (Figure 10). Four steps are involved in cutting or processing of the diamond: planning, cleaving or sawing, bruiting and polishing (Learn about diamonds, 2010). Figure 10: Modern round brilliant cut diamond. Source: (Learn about diamonds, 2010). During the planning phase of manufacturing the diamond, the size and value of the cut diamond rough will be determined by considering the size, clarity and crystal direction after which the cleave or sawed at the determined position using a diamond-coated rotary saw or a laser. Later, bruting prepares the diamond rough for faceting as it forms a basic face-up outline of the round diamond using a diamond on a rotating lathe. Finally, the basic symmetry of the diamond is obtained by a process called blocking that gives the diamond rough its finished proportions. This stage of polishing also establishes the performance of the finished diamond crystal where 17 to 18 facets of the crystal are cut to make it a round single cut crystal. While the process ends with blocking for smaller diamonds, for larger diamonds, there is an additional stage called brillianteering involving final polishing of the facets which ultimately determines the fire and the brilliance of the diamond (Learn about diamonds, 2010). Diamonds are classified based on the 4Cs and the quality of a diamond depends on these four characteristics. Colourless gems of diamonds are the most rare and popular quality diamonds. Colour intensity is another characteristic that determines the quality of a diamond. Further, the clarity of the gem which has no internal flaws allows more light to pass through, making it a quality gem in terms of clarity. As diamonds of large size are a rarity, they are costlier when compared to smaller or lesser carat gems of the same quality. However, cut is the most important characteristic that is under human control and determines the quality of the gem as it influences the geometric appearance, lustre and fire of the gem by making it highly reflective (Diamond: geology.com, 2011). Manufacturing equipment Daulton et al. (1999) used Argonne Tandem Linear Accelerator System (ATLAS) in the large scattering facility at Argonne National Laboratory (ANL) to irradiate fine polycrystalline graphite sheets with 99.95% carbon purity and 125 pm thickness by 400 MeV Kr ions to result in the formation of nanocrystals of diamonds in the acid residue of graphite yielding roughly 0.08 nanodiamonds per ion. Kumar et al. (2000) used a belt-type high-pressure apparatus with a bore of 32 mm in diameter in which the experiment was carried out at 7.7 GPa and 1300-1500oC within which about 10 percent of graphite transformed into euhedral octahedral faced small diamond crystals in 240-24 hrs based on the temperature. Bovenkerk et al. (1959) too used a similar but modified type of tapered piston belt assembly at ultra high pressure and high temperature to attain the thermodynamic stability of diamond. Other instruments used in the manufacture of diamond include the ‘supported stepped piston cylinder’ used by the US Army Electronic Research and Development Laboratory, capable of generating sustained pressures of 100 kbars and temperatures of 2500oC, H.T.Hall’s ‘multi anvil’ pressure apparatus functioning on the extruded compressible gasket used for obtaining tetrahedral configurations and used by Allmiinna Svenska Elektriska Aktiebolaget for obtaining cubic configurations, the ‘girdle’ chamber using multiple steel binding elastic rings and developed by Battelle Memorial Institute, and the explosive shock apparatus within which 400-500kbars of pressures and 1000-1500 oC temperatures are generated for one microsecond to directly convert graphite into diamond; are all used as basic experimental setups for the manufacture of diamonds (Giardini and Tydings, 1962). Life cycle of diamond manufacturing Natural diamond formation has a life cycle of 900 million years and in some cases even to 4.25 billion years (Origin and formation of diamonds, 2008). However, scientists believe that not as many or as big diamonds are being formed under the Earth’s crust of late due to the lack of the suitable conditions for diamond forming (Dr. Whitehouse, 2002). Cratonic areas where diamond deposits are found are isolated and restricted locations. The mines of South Africa (DeBeers) controls the world market for diamonds and there are large deposits in Siberia and Western Australia too (Snelling, 1993). But man-made diamonds are being manufactured in a week in the laboratories predominantly through brute force and CVD to make single diamond crystals and other multiple carat diamonds too (Maney, 2005). Conclusion Diamonds are the most precious of metals for various reasons. However, the most clear and large size diamonds are a rarity as their formation under the Earth’s crust is a long process under unique conditions of temperatures and pressures. Naturally found diamonds have been formed about 900 million to 4.25 billion years ago and usually found in kimberlite pipes and lamproite rocks and other secondary rocks or placer deposits. Diamonds are normally formed at high pressures and high temperatures of about 100-500 kbars and 1000-2500oC, respectively by the transformation of carbon containing graphite into diamond crystals. The crystalline structure of diamond is what differentiates diamond from amorphous graphite with its quality being valued based primarily on its carat, cut, clarity and colour, 4Cs. Both natural and man-made diamonds are classified based on the 4Cs but the manufacturing process differs slightly although the pressure and temperatures are similarly high. Various equipment have been used for manufacturing diamond in the laboratory at high pressures and temperatures, with the feasibility of producing large amounts and in bigger sizes as compared to naturally available diamonds. However, natural diamond formation has become scarce due to unsuitable conditions under the Earth’s crust. References 1. Bonsor, Kevin and Keener, Candace, 2001, How Diamonds Work, http://science.howstuffworks.com/environmental/earth/geology/diamond2.htm 2. Lineberry, Cate, 2006, Diamonds unearthed, Science and nature, Smithsonian magazine. 3. How do diamonds form? 2011, geology.com. 4. Helmenstine, M. Anne, 2012, Chemistry of diamond, About.com. 5. Composition and properties of diamond, 2008, DeBeers. 6. Davis, Joshua, 2003, The New Age Diamond, Wired. Issue 11.09. 7. The Unique Properties of Diamond, 2012, Dentritics Inc. 8. Properties of diamonds, 2012, The Natural History Museum. 9. Diamond- Mineral uses and properties, 2011, geology.com. 10. Dr. Whitehouse, David, 2002, Earth’s diamond days are over, BBC News. 11. A few famous diamonds, 2008, DeBeers Group. 12. Daulton, L.T, Rehn, E.L, Kirk, A.M, and Lewis, S.R, 1998, Heavy-ion irradiation induced diamond formation in carbonaceous materials, ANL and Uninversity of Chicago. 13. Kumar, M.D. Shaji, Akaishi, Minoru and Yamaoka, Shinobu, 2000, Formation of diamond from supercritical H2O-CO2 fluid at high pressure and high temperature, Journal of crystal growth, Elsevier Science, Vol. 213. 14. Bovenkerk, P.H, Bundy, P.F, Hall, T.H, Strong, M. H and Wentorf, H.R, 1959, Preparation of diamond, Nature, Vol. 184. 15. Formation of diamonds, 2009, DTC Botswana. 16. Giardini, A.A and Tydings, E.J, 1962, Diamond synthesis: observations on the mechanism of formation, The American Mineralogist, Vol. 47. 17. DeCarli, S. Paul and Jamieson, C. John, 1961, Formation of Diamond by Explosive Shock, Science, Vol. 133, No. 3467. 18. Maney, Kevin, 2005, Man-made diamonds sparkle with potential, USAToday.com. 19. Reginald, B. Little, Eric, Lochner and Robert, Godard, 2005, Magnetically orchestrated formation of diamond at lower temperatures and pressures, Physica Scripta, Royal Swedish Academy of Sciences, Vol. 71, No. 4. 20. Origin and formation of diamonds, 2008, DeBeers. 21. The 4Cs, 2008, DeBeers. 22. Learn about diamonds, 2010, Diamond Ideals. 23. Diamond, 2011, geology.com. 24. Snelling, A. Andrew, 1993, Diamonds—Evidence of Explosive Geological Processes, Answers in Genesis. Read More