Crystals are basically solids that are formed by the orderly and repeated arrangement of the constituent elements like the atoms, ions, or molecules. The word 'crystal' is derived from the Greek word, krustallos, which means both ice and rock crystal or quartz.
Types of Crystals
They can be classified into different types depending on their shape and properties. Based on their shape, they are divided into seven types - cubic or isometric, tetragonal, orthorhombic, hexagonal, rhombohedral, monoclinic, and triclinic crystals. On the basis of their physical and chemical properties, they are classified into the following four types - covalent, metallic, molecular, and ionic.
The Process of Crystallization
Crystallization refers to the process of crystal formation from a solution, a molten substance, or a gas. The entire process of crystallization can be divided into two main stages - nucleation and crystal growth. Nucleation refers to the accumulation of the solute into clusters. However, the clusters should be stable enough to ensure the formation of crystals. Otherwise, they can dissolve again in the solution.
The stable clusters form nuclei, and for this, the clusters have to attain a critical size determined by the operating conditions, like temperature and supersaturation (refers to a solution containing more of the dissolved material than what can be dissolved under normal conditions). At this stage of nucleation, atoms get arranged in geometrical shapes in a periodic or repeated manner, which determines the structure of the crystal.
The next stage of crystallization is crystal growth, which refers to the growth of the stable nuclei. This helps the crystals attain the critical cluster size, after which it can no longer dissolve in the solution. Nucleation and crystal growth take place simultaneously as long as supersaturation exists. So, the most important condition for crystallization is the existence of supersaturation, as it determines the rate of nucleation and crystal growth. When supersaturation ceases to exist, the solid-liquid system attains equilibrium and the process of crystallization comes to an end.
Crystals are generally formed when magma or molten rock cools and solidifies. The rapid cooling of the molten rock results in the formation of small crystals. However, if it cools slowly, then large crystals are formed. Some crystals like diamonds are formed deep in the Earth from the carbon atoms present in the molten rock. The high pressure and intense heat cause the carbon atoms to come together, and form small diamond crystals.
Crystals can be formed due to evaporation as well. When you dissolve a soluble substance or solute in a solvent, the crystal structure of the substance breaks down into individual atoms, ions, or molecules, which get dissolved in the solution. When evaporation takes place, the amount of solvent gets reduced. This in turn, causes the excess solute to gather into clusters and crystallize.
Crystallization can be augmented by changing the temperature of the solvent. Generally, solubility can be reduced by lowering the temperature of the solvent. The rate of crystallization can also be increased by changing the nature of the solvent. This can be done by adding a non-solvent to the solution, which can reduce solubility and ensure rapid crystallization.
The shining, glittering crystals have a wide range of applications. Diamonds, emeralds, rubies, and other gemstones are known for their dazzling beauty, while crystals like sugar and salt are an indispensable part of the human diet. Crystals can also be used for healing purposes, which is known as crystal healing. They are believed to be capable of enhancing the energy fields of the body by emitting uniform vibrations. Quartz is another important crystal that is used in computers, watches, and radio stations, for its constant energy field.