"Patagonia Granite" - pegmatitic granite from Brazil. (~30.0 cm across)

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 >65% silica (“silica” = SiO2 chemistry) (felsic has also been defined as >70% silica), are rich in potassium (K) & sodium (Na), and are dominated by the minerals quartz and K-feldspar.

The cut-and-polished 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.

Only two minerals are present in this sample. The dark gray glassy material is quartz (SiO2 - silica/silicon dioxide). The whitish material is potassium feldspar (KAlSi3O8 - potassium aluminosilicate). There are only two very large crystals present - the gray quartz is part of one crystal and the whitish feldspar is part of one crystal.

"Patagonia Granite" is a relatively new variety of decorative stone in the retail granite/marble trade.

Age: unknown/undetermined/undisclosed