File:"Patagonia Granite" (pegmatitic granite; Brazil) 3.jpg

English: "Patagonia Granite" - pegmatitic granite from Brazil.

Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & 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 greater than 65% silica (“silica” = SiO2 chemistry), are rich in potassium (K) & sodium (Na), and are dominated by the minerals quartz and K-feldspar.

The cut-and-polished decorative stone sample seen here is a granite with a pegmatitic texture (many granites have a phaneritic texture, with crystals between 1 millimeter and 1 centimeter in size each). All or almost all of the crystals in a pegmatitic rock are larger than 1 centimeter 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.

Four minerals are present in this sample. The gray material is quartz (SiO2 - silica/silicon dioxide) - it's part of one crystal. The whitish material at left is potassium feldspar (KAlSi3O8 - potassium aluminosilicate) - it's also part of one crystal. The black mass at right is a single crystal of tourmaline. The two relatively small, dark brown, elongated crystals at ~center are biotite mica.

Age: unknown/undetermined/undisclosed

Locality: attributed to a commercial quarry in Brazil