File:Pegmatitic granite (Precambrian; Black Hills, South Dakota, USA) 1 (32139287362).jpg

Pegmatitic granite from the Precambrian of South Dakota, USA. (~10.6 cm across at its widest)

Igneous rocks form by the cooling and crystallization of hot, molten rock (magma and lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Granite is a common intrusive igneous rock. Garden-variety granites are composed of quartz, potassium feldspar (K-feldspar), sodic plagioclase feldspar, hornblende amphibole, and mica. Granites have a felsic chemistry. Felsic igneous rocks are generally light-colored, have >65% silica (“silica” = SiO2 chemistry) (felsic has also been defined as >70% silica), are rich in potassium (K) and sodium (Na), and are dominated by the minerals quartz and K-feldspar.

The granite specimen shown above has a pegmatitic texture (many granites have a phaneritic texture, with crystals between 1 mm and 1 cm in size each). All or almost all of the crystals in a pegmatitic rock are >1 cm in size each. Pegmatitic granite usually forms by cooling of a relatively water-rich magma. Very slow cooling of magma can also result in a pegmatitic texture. This sample has potassium feldspar, quartz, and muscovite mica (e.g., large sheet-like mass at right).