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Granite is a light-colored plutonic rock found throughout the
continental crust, most commonly in mountainous areas. It consists
of coarse grains of quartz (****0%), potassium feldspar, and sodium
feldspar. These minerals make up more than *0% of the rock. Other
common minerals include mica (muscovite and biotite) and hornblende
(see amphibole). The chemical composition of granite is typically
****7% silica, ****3% alumina, **5% potassium oxide, **5% soda, 1%
lime, **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. *7% quartz + mica, amphibole, pyroxene, albite
feldspar, a building stone.
