Granite

Granite is a light-colored plutonic rock found throughout the continental crust, most commonly in mountainous areas. It consists of coarse grains of quartz (10-50%), potassium feldspar, and sodium feldspar. These minerals make up more than 80% of the rock. Other common minerals include mica (muscovite and biotite) and hornblende (see amphibole). The chemical composition of granite is typically 70-77% silica, 11-13% alumina, 3-5% potassium oxide, 3-5% soda, 1% lime, 2-3% total iron, and less than 1% magnesia and titania. Volcanic rock of equivalent chemical composition and mineralogy is called rhyolite. Granites are the most abundant plutonic rocks of mountain belts and continental shield areas. They occur in great batholiths that may occupy thousands of square kilometers and are usually closely associated with quartz monzonite, granodiorite, diorite, and gabbro.

Debate has long centered on whether granite is igneous or metamorphic in origin. Originally granite was thought to form mainly from magmatic differentiation of basaltic magma, but geologists now believe there is simply too much of it for it to have formed this way, except locally. Most granite seems to have formed either by melting, partial melting, or metamorphism of deeply buried shale and sandstone. Granite dikes are clearly igneous, and granite emplaced in the upper few kilometers of the Earth's crust also often shows evidence of forceful intrusion into surrounding rocks, whereas some granites that formed deeper within the crust seem not to have been forcefully emplaced. Evidence of intrusion or great mobility is considered to indicate an igneous origin that stems from melting of sediments; but where no good evidence of either a magma chamber or of fluidity is observed, a metamorphic origin must be considered.

Granite is used as a building and ornamental stone. Many ore deposits (copper, lead, zinc, gold, and silver, for example) were produced by hydrothermal solutions created during late stages of cooling of granite bodies. These may be emplaced around the peripheries or related to fissures and fractures within bodies of granite. approx. 27% quartz + mica, amphibole, pyroxene, albite feldspar, a building stone.