Diamond and Graphite
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| Diamond and Graphite Structure |
Diamond and graphite are both crystalline forms of the element carbon. Based on this fact, one would think they would be similar in many ways. But in fact they are as different as could be, and this highlights some important truths about crystals.
Both diamond and graphite have a very simple chemical composition: they are both pure carbon. But whereas diamond is the hardest mineral known to man (10 on the Mohs scale), graphite is one of the softest (less than 1 on the Mohs scale). As a result, diamond is the ultimate abrasive whereas graphite is an excellent lubricant. Diamond is an electrical insulator while graphite is a good conductor of electricity. Diamond is usually transparent, while graphite is opaque.
Diamond is obviously far more valuable than graphite. Graphite is so cheap it is used to make pencil lead. But graphite is as interesting a material as diamond in many ways. Graphite is the high-strength component in composites used to build automobiles, aircraft, and high-tech golf club shafts and tennis racquets.
So why is diamond so hard and graphite so soft? And if graphite is so soft, how can it be used as a high-tech material for golf club shafts and aircraft?
The answer lies in the different structures of diamond and graphite molecules. Graphite forms in layers or sheets, where the carbon atoms have strong bonds on the same plane or layer, but only weak bonds to the layer above or below. The carbon atoms in diamond, on the other hand, have strong bonds in three dimensions. In diamond, the atoms are very closely packed, and each atom is connected to four other carbon atoms, giving it a very strong and rigid three dimensional structure.
Only the bonds between graphite layers are weak, however. So when the layers or sheets are rolled up into fibers, and those fibers twisted into threads, the true strength of the bonds becomes apparent. The threads are molded into shape, and held in place by a binder such as an epoxy resin. The resulting composite materials have some of the highest strength-to-weight ratios of any materials.
The differences between diamond and graphite highlight how important crystal structure is to the properties of a gemstone. For example, all the gems which have a cubic crystal habit, where the crystal structure is completely symmetrical, have an important property in common. Cubic solids refract light in all directions at the same velocity. So even though diamond, garnet and spinel have quite different chemical composition, they are all singly refractive because of their similar crystal structure. Other less symmetrical crystal groups are doubly refractive, including orthorhombic, monoclinic, and triclinic, all of which have specific internal symmetries and axes that dictate how light bends as it enters the crystal.
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