Pegmatitic granite from the Precambrian of Colorado, USA. (~13.2 centimeters across along the base)

Grayish = quartz (SiO2) Salmon-colored = potassium feldspar (KAlSi3O8)

Pegmatitic texture refers to intrusive igneous rocks that have very large crystals - all or almost all the crystals are >1 centimeter in size. Compositionally, most pegmatitic-textured rocks are granites. Pegmatitic granite can be bimineralic, with quartz and potassium feldspar only - such varieties are called alaskites. Many pegmatites also have biotite or muscovite mica. Incompatible elements are often concentrated in pegmatites, resulting in the presence of unusual minerals, including tourmaline and beryl and uraninite.

Prima facie, pegmatites form by very slow cooling - the crystals had lots of time to grow large. In reality, most pegmatites form by cooling of water-rich magma. The water allows the atoms to move around fairly quickly, resulting in the growth of large crystals. The largest single crystal from a pegmatite that I've ever heard reported was a 59 feet long beryl in a mine in northwestern Russia.

The Colorado pegmatite sample seen here is from an igneous dike that intruded Precambrian metamorphic rocks. The locality consists of gneiss/lineated granite, injection gneisses, and apparent amphibolite. Those rocks are intruded by pegmatitic granite dikes and aplite dikes. Graphic granite was also observed at the site.

Age: early Mesoproterozoic, 1.36 Ga