2.3: Metamorphic rocks

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gemology
Gemology Project

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Basic

The metamorphic cycle is the third largest cycle in mineral and rock formation. Metamorphism is the alteration of mineral paragenesis (the order of formation) after their deposition, by external action such as contact with magmetic rocks, regional changes in the pressure and temperature (e.g. contact metamorphosed limestones, crystalline schists, etc.). The consolidated rocks are altered in composition, texture or internal structure through pressure, heat and new chemical substances.

There are two kinds of metamorphism: Regional and Contact.

Regional metamorphism is caused due to a rise in temperature and directed pressure, affecting the earth's crust.
Contact metamorphism is caused when magma is intruded into a preexisting rock mass. The heat and pressure of this magma cause a metamorphic change in the rock it intrudes.

Both igneous and sedimentary rocks can change in texture or chemical composition as the result of either contact or regional metamorphism. Thus existing rocks change into new types of rocks. They are usually harder and denser than the original material.

For example, shale may alter into slate and further metamorphose into schist. Limestone is converted into marble. Sometimes schists contain gem minerals like garnet, emerald, and corundum.

Some minerals that grow in metamorphic rocks are:

Beryl
Corundum
Danburite
Garnets
Iolite
Lapis lazuli
Jadeite
Nephrite
Spinel
Alexandrite
Andalusite

Figure \(\PageIndex{1}\)