Introduction
The name diamond owes its origin to Greece. The term is a corruption of the Greek term Adamas, which means the invincible or the unbreakable. In mineralogy, diamond refers to an allotrope of carbons, which is capable of transiting from one state to another. The carbon atoms within the mineral are arranged in a cubic-centered crystal known as the diamond lattice. The diamond mineral has the formula C and exists in Isometric-Hexoctahedral (cubic) crystal system. It is also essential to note that diamond exists in the space group C63/mmc.
Diamond exists in different varieties, as distinguished by their colors. Typically, there are pink diamonds, white diamonds, champagne diamonds, pink champagne diamonds, yellow diamonds, blue diamonds and green diamonds. The different colors re conferred by the impurities found within the diamond structure. The cause of coloration is structural defects and impurities within the diamond structure.
The countries in which diamond is found in plenty include Brazil, Angola, South Africa, Russia, Democratic Republic of Congo, Ghana, Namibia, Canada, Australia and Botswana. Naturally, diamond is found in two types of rocks. These are harzburgite and eclogite rock types. These two types of rocks are relatively rare, and indigenous in nature. Harzburgite rocks are made of bronzite, pyrope, and olvine. Therefore, diamonds found within the harzburgite rocks contain a mixture of those three items (bronzite, pyrope, and olvine). On the other hand, eclogite rocks are composed of garnet and omphacite.
Although diamonds were quite rare in the past, they are not rare anymore. They are not rare because, in economic sense, the supply of diamond is more than demand. It has been argued that a cartel by the name De Beers hoards the mineral and sells it in small amounts in order to create an artificial scarcity. Consequently, the artificial scarcity maintains the high prices. Nevertheless, gem-quality diamonds are not found in plenty.
Structure
Diamond exists as a face-centered lattice. Every carbon atom within the diamond structure is joined to other four carbon atoms in regular tetrahedrons. The form of the cubes as well as the symmetrical arrangement of the carbon atoms can differ from one diamond to another. This makes it possible for diamond crystals to form into different shapes. This ability is known as the crystal habits. The most conventional crystal habit is the octahedron. Other crystal habits include the dodecahedra, and other combinations of this particular crystal shape. However, there are other crystal systems that are an exceptional to this cubic crystals system rule. For example, there are flat shaped diamond crystals called the macle that are composite in nature. The other exception is the etched diamond crystals that have elongated shapes and round surfaces.
Figure 1 Showing the Giant Covalent Structure of Diamond
Source: http://www.chemguide.co.uk
Figure 2 Showing the Diamond Crystallography
Naturally occurring diamond crystals lack a smooth face. However, they posses raised and triangular growths called ‘trigons’. Another unique characteristic of the diamond structure is the perfect cleavage. The perfect cleavage of diamonds means that diamond can separate neatly along the four different directions without breaking in a jagged manner. Diamond cutters use the perfect cleavage lines to plane the mineral. The perfect cleavage lines result from the diamond crystals having fewer chemical bonds along the octahedral face as compared to other directions.
Although graphite is more stable than diamond by a few electron volts, the energy that is required to convert diamond into graphite has the capacity to destroy its lattice and rebuild it. As a result, after diamond is formed, it is difficult to convert it into graphite because of the high activation barrier.
Composition
Diamond is strictly made up of carbon. The repeating units of carbon atoms are joined together through covalent bonds. A single unit of diamond comprises of eight carbon atoms arranged in a cube. Every carbon atom within the diamond structure is found within a tetrahedral network, and it is equidistant from the other neighboring atoms. The network formed by the carbon atoms is rigid, and this explains why diamonds are very hard and why they have a high melting point.
Apart from carbon, other small substances such as nitrogen and sulphur become trapped within the diamond crystal. These impurities give the distinctive color of diamond.
Since it is possible to study the ratio of the isotropic carbons within diamonds, it is also possible to tell whether the carbons forming the diamond are from organic or inorganic sources. There are biological processes that are able to sort out the carbon isotopes depending on their mass hence the isotropic ratio of carbon found in living things is different from that of the earth or the stars. From these differences, it has been shown that the carbon found in most natural diamonds comes from the earth’s mantle, while the carbon for the rare diamonds comes from recycled microorganisms, and, then, formed into diamond by the crust of the earth through plate tectonics.
Although diamond is the hardest naturally occurring substance on earth, it also has some structural weaknesses that make it fairly tough. Toughness in this case refers to the ability to resist breakage after falling or after colliding with other materials. The perfect cleavage makes diamond prone to breakage. Although the toughness of diamond is higher compared to other gemstones, it ranks poorly when compared to other engineering materials.
Physical and Optical Properties
Diamond is the hardest naturally occurring substance on earth. The hardness is attributed to the strong covalent bonds that join the carbon atoms within the lattice structure. In terms of color, there are many variations depending on the impurities trapped within the diamond mantel. For example, some pieces of diamond are yellow, others are white in color, and others are pink. Others common colors for the mineral include blue and green.
In terms of optical capacity, pure diamond is the most transparent naturally occurring substance on earth. When in pure form, the mineral reflects visible light, infrared light as well as ultra-violet light.
The most common crystal form of diamond is octahedral. It has an octahedral cleavage of 111 that is perfect in four directions. The octahedral geometry means that diamond has the shape of an octahedron, which refers to a polyhedron characterized by eight faces. This explains the platonic solid base of diamond and the eight equilateral triangles. Four of those triangles meet at the centre. In terms if luster, diamond has admantine luster. In mineralogy cleavage refers to the capacity of a crystalline material to split along specific structural planes. Diamond has an octahedral cleavage that occurs on the 111 crystal planes. The octahedral cleavage is consistent with the octahedral geometry of diamond crystals.
Luster refers to the means through which a mineral, a rock or surface interacts with light. Minerals in the adamantine category have a high superlative luster. This explains the reason why pure diamond is the most transparent substance known, and the reason why it has a high refractive index. However, minerals with true adamantine luster are very few, with the exception of cerussite and zircon.
Other unique properties of diamond include the ability to conduct heat at a relatively faster rate. This explains why diamond is able to conduct heat five times faster than copper. However, in terms of conducting electricity, diamond can either be a good conductor of heat or an insulator. It has the unique capacity to allow electricity through it or block it. When it comes to the ability to survive physical, chemical and radioactive forces, diamond can remain intact in environments that would ordinarily destroy other minerals.
Optical Ability
Pleochroism
Pleochroism refers to the capacity of a gem to convey different colors when viewed from different directions. Diamond does not show pleochroism due to its crystalline nature. This is also true for other gems such as spinel and garnet. The many colors seen after rotating a diamond is not as a result of pleochroism but due to dispersion.
Biaxial and Uniaxial
Biaxial minerals refer to minerals with crystals that have two optic axes. Uniaxial, on the other hand, refers to the minerals with crystals having a single optic axis. However, diamond does not exhibit uniaxial or biaxial indicatrix.
Birefringence
Birefringence refers to the capacity of material to exhibit double refraction. This happens when a ray is split into two parallel rays that are polarized perpendicularly. From its morphology and its etching figures, diamond is not expected to show birefringence except when strained. However, this is not entirely true because many diamonds show double refraction. Nevertheless, the degree of birefringence in diamonds is weak.
Optical sign
Diamond is an isometric mineral. This is contrary to graphite which is uniaxial.
Dispersion
Diamond has a dispersion of 0.044. Natural diamond has the capacity to disperse white light into a spectrum of colors. These colors include blue, green, yellow, purple and orange. The dispersion takes place within the diamond, and what the human eye can see is known as fire.
Refraction
Refraction refers to the ability to bend or curve light. Diamond has a refractive index of 2.417. The refractive index is calculated through comparing the speed of light in an ir medium to the speed of light in diamond.
Refractive index = Speed of light in air = 300000 = 2417
of diamond speed of light in diamond 124120
Scintillation
Scintillation refers to the brilliance that is seen when diamond is moved away from light or when passed through a fixed source of light. When diamond is fixed through a fixed source of light, the diamond emits small particles of colorful light. Again, when light is moved
Paragenesis
The formation of diamond occurs when carbon atoms are exposed to high pressure and temperatures for long periods of time. Naturally, the earth’s crust has some regions that are at high temperatures and pressure. When temperatures of the earth’s crust reach 9001/4 C to 1400 ¼ C and pressure reaches 5 to 6 GPa, carbon deposits are liquefied into diamonds. Generally, diamond is formed at depths of 60 miles to 120 miles under the continental crust. At these depths, the pressure of the earth’s crust is approximately 5 GPa, while the temperatures are around 1200 0c. This happens in the diamond-stable conditions, which are dependent on the graphite-diamond equilibrium boundary. Interestingly, diamond has the same chemical structure as graphite – though both minerals have quite different physical properties.
The formation of diamond on the oceanic crust differs from the formation of diamond on the earth’s crust. For example, the formation takes place at depths that are very deep because of the low surface temperatures. This is compared to the earth’s crust whereby depths of 60-120 miles are enough to allow for the formation of diamond because of the high surface temperatures and pressure.
In most cases, the correct temperature-pressure combinations that form diamond are found in thick, stable parts of continental plates where parts of lithosphere called cratons are known to exist. After residing in the cratonic lithosphere for a long period, the diamond crystals grow larger. The carbon isotopes found within diamond show that carbon atoms come from both organic and inorganic sources. For example, some diamonds found within the harzburgite rocks show that such diamonds are formed from inorganic carbons found deep within the earth’s mantle. On the other hand, ecologitic diamonds are formed from organic carbons that have been pushed down the earth’s surface through subduction. They are then transformed into diamond later. It is important to note that diamonds that have been excavated from the earth’s surface are generally old, with their ages ranging from 1 billion to 3.3 billion years.
Most of the diamonds that have been discovered here on earth are delivered from the earth’s crust through volcanic eruptions. The volcanic eruptions erupt at the mantle tear out pieces of the rock and convey them surface of earth without having to melt. The blocks that come from the mantle are christened xenoliths. The xenoliths contain diamonds formed that were originally formed at the high pressure-temperature combination within the earth’s mantle. In order to produce the diamonds, people have to mine the rock containing the xenoliths or through mining the sediments formed from the diamond rock withering away.
Uses
As one of the rare naturally-occurring minerals on earth, diamond has many uses. For example, diamond is widely used in jewelry industry for beauty and ornamental purposes. There are many rings, necklaces and other clothing accessories that are made of diamond. Diamonds that are well cut out are very expensive and make excellent jewelry. The reason why they are used is because they are beautiful and sparkle in the light. The purer the diamond, the more expensive it becomes. Apart from being used in the jewelry industry, diamonds are used in the manufacture of windows. Some windows have diamonds within the vacuum chambers and other types have diamond covers for the lasers and x-rays. Nowadays, diamonds are finding wide application as microchips and microprocessors that are used in many electronic devices. They are used in the electronic devices due to their unique ability to act as semi-conductors.
As one of the hardest naturally-occurring substances on earth, diamond finds wide application as a cutting tool. It is also effective in making drilling equipment due to this ability. On a scale of 1-10, diamond has the highest possible degree of hardness (10), and this could explain the reason why 75-80 % of the diamonds are used in this manner. The rest of the diamonds (20-25%) are used as jewelry. Polishing hard metals, videodisc needles and phonographs also use diamond as one of their ingredients due to the hardness quality.
Conclusion
Diamond is a naturally occurring mineral coined from a Greek word. It exists in octahedral crystals with a perfect octahedral cleavage. Although pure diamond is colorless and transparent, impurities and structural defects confer the colors found in most of the diamonds. This explains the several colors found in diamonds such as pink, yellow, blue and green. The process of forming diamond takes place within the earth’s crust, and at deep depths. A combination of high pressure and temperatures, over a long period of time, results into volcanic eruptions that deposit the diamond rock on the earth’s surface. The mineral has a lot of uses including the use in jewelry and as a cutting material due to its hardness.
Works Cited
Barnard, Amanda S. The Diamond Formula: Diamond Synthesis--a Gemmological Perspective .
Oxford : Butterworth-Heinemann, 2000. Print.
Berman, R. Physical properties of diamond. Oxford : Clarendon Press , 1965. Print.
Deer, William Alexander, Robert A. Howie and Jack Zussman. Rock-forming minerals, volume
2A . New York, NY : John Wiley & Sons , 1978. Print.
Li, Xin, et al. "Detection and analysis of diamond fingerprinting feature and its application."
Neves, A J and Maria Helena Nazaré. Properties, growth and applications of diamond. London :
INSPEC, 2001. Print.
Wang, Wuyi. "Formation of diamond with mineral inclusions of "mixed" eclogite and peridotite
paragenesis." Earth and planetary science letters (1998): 160 (3-4), 831 . Print.
Zaitsev, A M. Optical properties of diamond : a data handbook. New York, NY: Springer ,
2001. Print.
