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Classes of Granite

Granite is a type of igneous rock (rock formed from cooling and solidification of magma) that is formed under extreme pressure beneath the earth's surface. In the 2000s, it is commonly used in households for countertop surfaces because of its beauty and durability. The first classification system for granite was proposed by B. W. Chappell and A. J. R. White in 1974 and subsequently adopted. The Chappell and White classification system designated granite as S-type, I-type, M-type, or A-type. This letter-based system classifies granite according to its molecular structure and origins.

S-Type
- S-type stands for sedimentary protolith granite. This granite is formed from the partial melting of metasedimentary rocks in the earth's crust. S-type granite is affected by sedimentary fractionation, which is a type of surface weathering in which sodium is removed from the rock by sea water. For this reason, S-type granite typically contains low levels of sodium, but it typically has high levels of carbon, silicon oxide and sulphur.
I-Type
- I-type is igneous protolith granite. The primary difference between I-type and S-type granite is that I-type does not endure sedimentary fractionation, so it retains higher levels of sodium. I-type granite typically has xenoliths, or small rock formations that get trapped within a larger rock during its final stages of hardening. Like S-type, I-type granite is also formed within the earth's crust.
M-Type
- M-type granite is formed from magma stored in the earth's mantle. It is created from basalt through the process of fractional crystallization. This is a very difficult process, which makes M-type granite much rarer than other types.
A-Type
- A-type, or anorogenic granite, is formed in the lower part of the earth's crust, typically around volcanic hot spots. This type of granite is created when the crust melts in extremely dry conditions. It can be found in volcanic areas such as Yellowstone Park.

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