"Patagonia Granite" - pegmatitic granite from Brazil. (~27.5 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.

Three or four minerals are present in this sample. The dark gray glassy material at left and right is quartz (SiO2 - silica/silicon dioxide). The whitish material is potassium feldspar (KAlSi3O8 - potassium aluminosilicate). The black crystal near the upper left is schorl tourmaline. Tourmaline is a group of complex silicate minerals. It is often called a "garbage-can mineral" because it has a mix of many elements. A more or less ideal chemical formula for tourmaline is (Na,Ca)(Li,Mg,Al)(Fe,Mn,Al)6(BO3)3(Si6O18)(OH,F)4 - sodium calcium lithium magnesium iron manganese hydroxy-fluoro-boro-aluminosilicate. Tourmaline forms small to medium-sized, elongated, stick-like crystals or large, more squat, 15-sided crystals. Well-formed tourmaline crystals have a rounded triangular cross-section. They are quite hard (H = 7 to 7.5 on the Mohs Hardness Scale) and are sometimes used as gemstones. The most common variety is black tourmaline, which is called schorl. Two smaller black tourmaline crystals also appear to be present in this sample. The black mass along the lower margin, approaching the lower right corner, may be biotite mica. The white feldspar is one crystal. The dark gray, glassy quartz masses represent parts of single large crystals.

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

Age: unknown/undetermined/undisclosed