Polymorphism in minerals
For some minerals, the manifestation of such a property as polymorphism is characteristic. It consists in the fact that a mineral having the same chemical composition, under different external conditions, can crystallize in different structures, thus forming a different mineral. Probably the most common example of polymorphism is diamond-graphite-lonstdaleite. All three of these minerals are formed under different conditions and have different properties, but their chemical composition is unchanged - C (carbon). The resulting minerals are called polymorphic modifications, and usually in a mineral that is characterized by polymorphism, there are two or more of them. Conversions from one modification to another can be carried out reversibly and irreversibly. So native sulfur, crystallizing in the rhombic system, when heated to 95.5 degrees, transforms into the monoclinic modification, and when the temperature decreases, the transformation goes in the opposite direction. The same happens with the alpha and beta modifications of quartz only with a critical temperature of 573 degrees. An example of an irreversible transition is the transformation from diamond to graphite.