Allotropes of Carbon: Properties of Diamond

There are many allotropes of carbon, and one of them is diamond. It is a crystalline allotropic form of carbon and is considered a precious metal. The diamonds are solid, solid, inorganic and naturally occurring elements. The carbon atoms in a diamond are arranged in a crystal structure, forming a diamond cubic. The major properties of diamond are it is the hardest natural material with excellent thermal conductivity, and on the Mohs Hardness scale is ranked 10. The natural diamond is extracted from the upper mantle part of the Earth’s surface and is formed under high pressure. The topic gives a comprehensive overview of the properties of diamonds.

Structure of Diamond  

The molecule and its symmetry make diamond the hardest element. It has a three-dimensional network of carbon atoms joining together with the help of strong covalent bonds. Each of the carbon atoms in the diamond is in the sp3 hybridisation linked to the four neighbouring carbon atoms tetrahedrally. The C-C bonds are equal, whereas each C-C-C bond angle is 109°28. The carbon atoms are arranged in a tight bond lattice arrangement, and hence there are no free electrons.

How are diamonds formed?

The crystallised and naturally occurring diamonds are formed beneath the surface of Earth over millions of years under conditions of pressure and heat. It is a rare and precious mineral, and most of it reaches the upper mantle surface from where it is mined through geological processes like deep source volcanic eruptions. The eruptions originate in Earth’s mantle area and reach the surface of Earth without melting. Such blocks from which diamonds are mined are called xenoliths.

Properties of Diamond Mineral

There are many properties of diamond obtained due to its structure and geometry. The hardness of a diamond is due to the molecular structure of the diamond as the carbon atoms are linked to each other forming a lattice structure. Here are some of the important properties of diamonds.

• One of the significant properties of diamond is it is the hardest element, but its hardness depends on purity, crystalline structure, and orientation
• The diamond’s atomic arrangement in the crystal structure is densely packed, and covalent bonds between each carbon atom make it the hardest
• Diamond is one of the purest and densest carbon varieties, with a density of 3.51 g cm
• The diamond has a very high melting point of 3843K and is insoluble in solvents
• It has a compressive yield strength in the range of 130 – 140 GPa
• Diamond is a brittle structure, and hence attempting to bend diamond can lead to its fracture
• The diamond is elastically stretched with 9 to 10% tensile strain and maximum local tensile stress of 89 to 98 GPa
• Diamond is a bad conductor of electricity as there is no free carbon atom
• All the valence electrons of each carbon form carbon-carbon sigma bonds, and there are no unpaired electrons
• Diamond is resistant to salts, alkalis and acids as it is inert
• Diamond is slowly oxidised to form CO2 when heated with a mixture of sulphuric acid and potassium dichromate to 475K
• Water-repelling is one of the properties of diamond due to negative electron affinity, but it easily accepts hydrocarbons like wax or grease
• Diamond has a low coefficient of friction and low thermal expansion coefficient
• It is resistant to chemical corrosion and is biologically compatible
• Nature diamond has a broad range of optical transparency from ultraviolet to infrared and consists of nearly 99.95% carbon

Optical Properties of Diamond

The four optical characteristics of diamond include lustre, dispersion of light, transparency, and colour. Diamond is isotropic and a crystal element that is translucent to opaque. Here are the optical properties of a diamond mineral.

• Diamonds display fluorescence and, hence, emit light of different colours under long-wave ultraviolet light of 365nm
• The diamond’s value depends on its colour and size
• The most expensive and precious diamonds are bluish-white, while the black diamonds are the cheapest
• The pure diamond transmits visible light and appears as a clear colourless crystal
• The diamonds are available in different colours like brown, black, white, blue, grey, orange, red and others

Depending on transparency and colour, there are two classes of the diamond which are-

• Type I diamond consists of nearly 1% of nitrogen (N) atoms. If the N atoms are large in large pairs, it does not impact the colour of the diamond
• Type II diamonds have fewer nitrogen impurities than Type I diamonds
• The diamonds are lustre in nature, and it has a refractive index of 2.417. It has a high dispersion of 0.044

Conclusion

Graphite is the hardest element, and its natural crystals are formed in a million years under high pressure and temperature from its existence under the Earth’s surface. The optical properties of diamond include lustre and consist of a range of colours like brown, grey, blue, yellow, white, orange, red etc. Diamond is largely used in making jewellery, cutting instruments, medical tools necessary for precision operations, and even thin metal wiring. The strong structure of diamond is due to covalent bonds that exist between each of the carbon atoms of the diamond with other carbon atoms. The lack of free electrons makes it a poor conductor of electricity.