Diamond

DIAMOND (from the Turkish "almas", from the Greek "adamas", which means "unbreakable" * EN: diamond; DE: Diamant; FR: diamant; ES: diamante; RU: алмаз) is the mineral, the crystalline cubic modification of the native carbon.

The structure of the diamond. The unit cell of the spatial crystal lattice of the diamond is the face-centered cube with the 4 additional atoms, which are located inside the cube (Figure).

The size of the edge of the unit cell a0 is 0.357 nanometres (at the temperature of 25 degrees Celsius and pressure of 1 atmosphere). The shortest distance between the two neighbouring atoms C is 0.154 nanometres. The carbon atoms in the structure of the diamond form the strong covalent bonds, which are directed at the angle of 109 degrees 28 minutes relative to each other, thanks to which the diamond is the hardest of the substances, which are known in the nature. In the electronic band structure of the diamond, the width of the band gap for the non-vertical transitions is 5.5 eV, the width of the band gap for the vertical transitions is 7.3 eV, the width of the valence band is 20 eV. The mobility of the electrons mn is 0.18 square metres per Volt-second, the mobility of the holes mr is 0.15 square metres per Volt-second.

The cell of the crystal lattice of the diamond

The morphology of the diamond. The crystals of the diamond have the form of the octahedron, rhombic dodecahedron, cube, and tetrahedron, with smooth and plate-stepped faces, or with the rounded surfaces, on which the various accessory minerals are developed. There are characteristic the flattened, elongated, and complexly-distorted crystals of the simple and combined forms, the twins of the contact and penetration according to the spinel law, parallel and arbitrarily oriented intergrowths. The varieties of the diamond represent themselves as the polycrystalline formations: the bort is the intergrowths of the numerous small faceted crystals and grains of the irregular shape, of the gray and black colour; the ballas is the spherulites of the radiating structure; the carbonado is the cryptocrystalline, dense, with the enamel-like surface or the slag-like porous formations, which are consisting mainly of the submicroscopic (approximately 20 micrometres) grains of the diamond, which are tightly intergrown one with another. The size of the natural diamonds ranges from the microscopic grains to the very large crystals with the mass of the hundreds and thousands of carats (1 carat = 0.2 grams). The mass of the diamonds, which are extracted, is usually 0.1-1.0 carat; large crystals (more than 100 carats) are found rarely. There are shown in the table the largest diamonds in the world, which have been extracted from the deep of the earth.

The chemical composition. There exist in the diamond the Si, Al, Mg, Ca, Na, Ba, Mn, Fe, Cr, Ti, B, and other admixtures. Using the alpha particles, radioisotopes of the H, N, O, Ar, and other elements. Nitrogen is the main impurity, which provides the great influence on the physical properties of the diamond. The diamond crystals, which are opaque to the ultraviolet radiation, are named the diamond of the I type; all other belong to the II type. The content of the nitrogen in the predominant majority of the crystals of the diamond, which belong to the I type, constitutes approximately 0.25%. There are found less often the nitrogen-free diamonds, which belong to the II type, in which the admixture of the nitrogen does not exceed 0.001%. The nitrogen is incorporated isomorphically into the structure of the diamond, and forms itself or in the conjunction with the structural defects (vacancies, dislocations) the centres, which are responsible for the colour, luminescence, absorption in the ultraviolet, optical, infrared, and microwave regions, the character of the scattering of the X-rays, and other properties.

The physical properties. The diamonds may be colourless or with the barely noticeable colour tint, as well as clearly coloured in the varying degrees in the yellow, brown, mauve, green, cyan, blue, milky white, and gray (to black) colour. The diamond acquires the green or blue colour during the irradiation by the charged particles. They could not perform yet the reverse process, namely, the metamorphosis of the coloured diamond into the colorless one. There are characteristic for the diamond the strong lustre, the high refractive index (n = 2.417), and the strongly expressed effect of the dispersion (0.063), which causes the multi-colour play of light in the faceted diamond gemstones. As a rule, there manifests itself in the crystals of the diamond the anomalous birefringence because of the stresses, which are emerging in association with the structural defects and inclusions. The crystals of the diamond are transparent, translucent, or opaque, depending on the saturation with the microscopic inclusions of the graphite, other minerals, and gas-liquid vacuoles. During the illumination by the ultraviolet light rays, the significant quantity of the transparent and translucent crystals of the diamond fluoresce with the blue, cyan, and less often with the yellow, yellow-green, orange, pink, and red colours. The crystals of the diamond (with rare exceptions) fluoresce under the influence of the X-rays. The glow of the diamond is excited by the cathode rays and during the bombardment by the fast particles. There is often observed after removal of the excitation the afterglow of the various duration (phosphorescence). There also manifest themselves in the diamond the electroluminescence, triboluminescence, and thermoluminescence.

The diamond, as the hardest substance in the nature, is used in the various instruments for the sawing, drilling, and machining of all the other materials. The relative hardness is 10 on the Mohs scale, the absolute maximal microhardness, which is measured by the indenter on the (111) face, is 0.1 terapascals. The hardness of the diamond on the different crystallographic faces is not the same, the octahedral (111) face is the most hard one. The diamond is very fragile, it has the very perfect cleavage along the (111) face. The Young's modulus is 0.9 terapascals. The density of the transparent crystals of the diamond is 3515 kilograms per cubic metre, the density of the translucent and opaque diamonds is 3500 kilograms per cubic metre, the density of the certain Australian diamonds is 3560 kilograms per cubic metre; the density of the bort diamonds and carbonado (black) diamonds may decrease to 3000 kilograms per cubic metre because of their porosity. The clean surface of the crystals of the diamond has high hydrophobicity (the contact angle is 104-105 degrees). There form on the surface of the natural diamonds, especially of the diamonds from the alluvial placer deposits, the thinnest films, which increase the wettability of the surface of the diamonds.

Diamond is the dielectric. The resistivity r of all the nitrogen crystals of the diamond of the I type is 10^12-10^14 Ohm-metres. There are found sometimes among the nitrogen-free diamonds of the II type the crystals, in which the resistivity r is less than 10^6 Ohm-metres, sometimes down to 10-10^-2 Ohm-metres. Such diamonds have the conductivity of the r-type and photoconductivity, and under the same conditions, the photocurrent in the diamond of the II type is larger, than the photocurrent, which is excited in the diamond of the I type, by the order of magnitude. Diamond is the diamagnetic. The magnetic susceptibility per the unit of the mass is 1.57 • 10^-6 of the SI units at the 18 degrees Celsius. The diamond is resistant to all the acids even at the high temperature. The oxidative dissolution of the diamond proceeds in the molten KOH, NaOH alkali and other substances in the presence of O, OH, CO, CO2, H2O. The ions of the certain elements (Ni, Co, Cr, Mg, Ca, and others) have the catalytic activity and accelerate this process. Diamond has the high thermal conductivity (especially the nitrogen-free diamond of the II type). The thermal conductivity of the diamond at the room temperature is 5 times higher than than of the copper, while the coefficient of the thermal conductivity decreases from 6 to 0.8 kilojoules per metre-Kelvin withing the increase of the temperature in the 100-400 degrees Kelvin range. The polymorphic transition of the diamond to graphite at the atmospheric pressure occurs at 1885±5 degrees Celsius throughout all the volume of the crystal. The formation of the films of the graphite on the surface of the (111) faces of the crystals of the diamond under the influence of the oxygen may proceed starting from the 650 degrees Celsius. Diamond burns in the air at the 850 degrees Celsius temperature.

The distribution and origin. The diamonds have been found in the meteorites, impact rocks, which are associated with the meteorite craters (astroblemes), in the kimberlites and the small xenoliths of the deep mantle rocks of the peridotite and eclogite compositions, and also in the secondary sources, namely, the placer deposits of the various age and genesis (alluvial, diluvial, eluvial, littoral, proluvial, and others). There is no uniform opinion on the origin of the diamonds. Certain scientists believe, that diamonds crystallize themselves in the kimberlite pipes proper during their formation, or in the intermediate chambers, which are originating at the small (3-4 kilometres) depths (the subvolcanic chambers). Other scientists believe, that diamonds form themselves at the great depth in the parental kimberlite melt, and continue to crystallize during its lifting into the upper part of the earth crust. There develop most substantially the notions, that diamonds are genetically related to various peridotite and eclogite rocks of the upper mantle, and are moved out of them together with other xenogenic material, which is located in the kimberlites. There also exist other ideas about the genesis of the diamond (for example, the crystallization at the low pressures with the usage of the carbon from the methane of the deep origin, and of the carbonates of the host rocks).

The largest diamonds
The name of the diamond Mass, carats The place, where the diamond has been found Year The quantity of the faceted diamonds, which have been obtained from the specimen The mass of the largest faceted diamonds, carats
Cullinan 3106.0 Southern Africa, Transvaal 1905 105 530.2; 317.4
Excelsior 971.50 Southern Africa, South Africa 1893 21 69.8; 47.15
Star of Sierra Leone 968.90 Western Africa, Sierra Leone 1972 17 153.96
Wye river 770.00 - 1945 30 31.35
President Vargas 726.60 Brazil, Minas Gerais state 1938 29 48.26
Jonker 726.00 Southern Africa, Transvaal 1934 12 125.65
Jubilee 650.80 Southern Africa, South Africa 1895 2 245.35
Unnamed 616.00 Southern Africa 1974 not faceted -
Baumgold 609.25 Southern Africa 1922 14 50.00
Lesotho 601.25 Southern Africa, Lesotho 1967 18 71.73
Star of Yakutia 232.10 USSR, Yakutia 1973 not faceted -
Komsomol 60 years 200.74 USSR 1979 not faceted -

The deposits of the diamonds. There have the industrial importance the diamondiferous kimberlite rocks, and the alluvial placer deposits, which are forming because of the erosion of the kimberlite rocks. The kimberlites may be found mainly on the ancient shields and platforms; there are characteristic for them mainly the bodies of the tubular shape, and also the veins, dikes, and sills. The dimensions of the kimberlite pipes are from one to several thousand metres in the transverse cross section (for example, the Mwadui pipe in Tanzania with the parameters of 1525x1068 metres). There are known on all the platforms more than 1,500 kimberlite bodies, but only few of them have the industrial content of the diamonds. The diamonds are distributed in the kimberlites extremely unevenly. There are considered industrial the pipes with the content of the diamonds from 0.4 carats per cubic metre and more. In exceptional cases, when the pipes contain the increased percentage of the high-quality diamonds, there may be also cost-effective the operation with the lesser content, for example 0.08-0.10 carats per cubic metre (Jagersfontein in the South Africa). There predominate in kimberlites the crystals with the size of 0.5-4.0 millimetres (0.0025-1.0 carats). The mass fraction of them usually constitutes 60-80% of the total mass of the diamonds, which are extracted. The reserves of the individual deposits account for the tens of the million carats. The largest primary deposits of the diamonds have been explored in Zaire, South Africa, Botswana, Tanzania, Lesotho, Angola, Sierra Leone, and other countries.

Major extraction of the diamonds is performed from the placer deposits (80-85%) of the various genetic types, which are operated with the content of 0.25-0.50 carat per cubic metre. They distinguish among the placer deposits the eluvial-diluvial (Zaire, South Africa, Ghana, Ivory Coast) ones, alluvial (Zaire, Angola, Central African Republic, Sierra Leone, Venezuela, and others) ones, littoral and marine (Namaqualand in South Africa, Namibia and Angola) ones. Littoral and marine placer deposits distinguish themselves by the good grade composition, by the relatively uniform content, and by the high quality of the diamonds. The reserves in the large extended placer deposits account for the tens of the million carats (for example, in the basin of the Bushimae river, Zaire, the initial reserves are estimated at 109 carats). Main deposits of the diamonds are located in Africa; besides this, the industrial deposits are known and are developed in the South America, Asia (India and Indonesia). Approximate evaluation of the reserves of the diamonds in the industrialized capitalistic and developing countries is 1.2 billion carats, of which the technical diamonds account for approximately 75%. Major reserves of the technical diamonds are concentrated in the Zaire (approximately half of all the foreign reserves of the diamonds), Botswana, South Africa, Ghana. Major resources of the jewelry raw materials are concentrated in the South Africa, Namibia, Angola, Zaire, and Sierra Leone (see also the "Diamond industry" article).

There are known in the USSR both primary and placer deposits of the diamonds (for example, in the Western Yakutia, at the Urals).

The extraction of the diamonds. The upper horizons of the kimberlite pipes are developed by the open pit method, the bottom horizons are developed by the underground method (the unsealing is performed by the vertical shaft and crosscuts); the development is performed with the shrinkage of the kimberlite rock, and with the output of the rock through the ore chutes to the transport horizons. The placer deposits are developed by the open pit method with the usage of the excavators, scrapers, or dredgers.

Benefication. The rock at the placer deposits is first washed in the rocker boxes for the removal of the binding clayish mass and for the separation of the large clastic material; the isolated loose material is separated into the four classes, namely, -16+8, -8+4, -4+2, -2+0.5 millimetres. The benefication is performed by gravity methods (wet and air jigging, benefication in the heavy suspensions, in the concentration bowls). There are used for the extraction of the tiny diamonds and diamond grit the film and froth flotation with the preliminary cleaning of the surface. The reagents are the amines, aeroflots, fatty acids, kerosene, cresyl acid. The fat process (for the grains with the size of 2-0.2 millimetres), which is based on the selective ability of the diamonds to adhere to the fat surfaces, has got the widest distribution for the extraction of the diamonds. They use as the fatty coating the Vaseline, petroleum, motor oil and its mixture with paraffin, oleic acid, gearbox oil, and others. They use along with the fatty process (for the grains with the size of 3-0.1 millimetres) the electrostatic separation, which is based on the different conductivity of the minerals (diamond is the bad conductor of electricity). There is used the X-ray luminescent method of the extraction of the relatively large diamonds, which is based on the ability of the crystals of the diamond to luminesce (the X-ray luminescent machines).

Usage. The diamonds are divided into the jewellery and technical ones. The first ones have the high transparency. The most valuable diamonds are colourless ("pure") or with good coloration. There belong to the technical ones all other extracted diamonds, regardless of their quality and size. The sorting of the diamonds in the USSR is performed according to the technical specifications, which are supplemented according to the the expansion of the fields of the usage of the diamonds. The diamond raw materials are classified into the categories depending on the types and on the assignment; there are allocated in each category the groups and subgroups, which determine the size, shape, specific conditions of the assignment of the crystals of the diamonds. Approximately 25% of the diamonds, which are extracted in the world, are used in the jewellery industry for the production of the faceted diamonds.

Diamonds, which have the extremely high hardness, are indispensable for the production of the various tools and instruments (drill bits and chisels; indenters for the measurement of the hardness of the materials; dies; needles for the profilometers, profilographs, pantographs; drills, cutters, bearing crystals for the marine chronometers; glaziers; and others). Diamonds are widely used for the production of the abrasive powders and pastes, for the filling of the diamond saws. There are machined by the diamond tools certain metals, semiconductor materials, ceramics, reinforced concrete building materials, lead crystal glass, and others. Because of the combination of the series of the unique properties, the diamonds may be used for the creation of the electronic devices, which are assigned for the work in the strong electric fields, at high temperatures, in the conditions of the increased radiation level, in the aggressive chemical environments. There have been created on the basis of the diamonds the nuclear radiation detectors, heat sinks in the electronic devices, thermistors and transistors. The transparency of the diamonds for the infrared radiation, and the negligible absorption of the X-rays, permit to use the diamonds in the infrared receivers, in the chambers for the study of the phase transitions at the high temperatures and pressures.

The synthetic diamonds. There has started in the middle of the 50s the development of the industrial synthesis of the technical diamonds. There are synthesized mainly the small single crystals and the larger polycrystalline formations like the ballas and carbonado. The major methods of the synthesis are: the static one, in the metal-graphite system at the high pressures and temperatures; the dynamic one, polymorphic transition of the graphite into the diamonds under the influence of the shock wave; the epitaxial one, the grouth of the diamond films on the diamond seeds from the gaseous hydrocarbons at the low pressures and temperature of approximately 1000 degrees Celsius. Synthetic diamonds are used in the same manner as the natural technical ones. The total volume of the production of the synthetic diamonds significantly exceeds the volume of the extraction of the natural ones.