Properties of Large Crystals

The word crystal is derived from the ancient Greek word for ice. The ancient Greeks believed that quartz (SiOj) was water so deeply frozen it would never melt. Although quartz was found much later to contain silicon and oxygen rather than hydrogen and oxygen, the name crystal is still used as a general term today. 

Some properties of large crystals found in museums or grown in the laboratory are given below  

Although the angles between faces remain constant, the relative sizes of faces may vary from crystal to crystal. This behaviour is described as crystal habit, and the growth of the same substance from different solutions can result in different shapes. For example, sodium chloride crystals grown in water are cubic, but if urea is added to the water they become octahedral (a regular octahedron and a cube have the same overall symmetry). 

Polymorphism occurs when different structures can occur for the same chemical formula. The atoms are the same but their atomic arrangement differs between the structures. Although ultimately thermodynamics (the minimum Gibbs free energy) dictates which of the probable structures is formed at a particular temperature and pressure, other factors such as electrostatic interaction mean that a variety of different structures is possible. Polymorphism applies not only to elements, e.g. black and red phosphorus, but also to compounds such as calcium carbonate, which can exist in a number of forms including calcite, aragonite and vaterite. 

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