Granite plan: granite granitic rocks are classified according to the qapf diagram for coarse

GRANITE
PLAN:

1. 
   GRANITE
   
2. 
   GRANITIC ROCKS ARE CLASSIFIED ACCORDING TO THE QAPF DIAGRAM FOR COARSE
   
3. 
   ALPHABET CLASSIFICATION SYSTEM
   

Granite (/ˈɡrænət/) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.
Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals.

Thin section of granite
Granite is nearly always massive (lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history
The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a completely crystalline rock.^[1] Granitic rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy.^[2]
The alkali feldspar in granites is typically orthoclase or microcline and is often perthitic. The plagioclase is typically sodium-rich oligoclase. Phenocrysts are usually alkali feldspar.^[3]
Granitic rocks are classified according to the QAPF diagram for coarse grained plutonic rocks and are named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half of the diagram. True granite (according to modern petrologic convention) contains between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of alkali feldspar. Granitic rocks poorer in quartz are classified as syenites or monzonites, while granitic rocks dominated by plagioclase are classified as granodiorites or tonalites. Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites. Granitic rock with more than 60% quartz, which is uncommon, is classified simply as quartz-rich granitoid or, if composed almost entirely of quartz, as quartzolite.^[4][5][6]
True granites are further classified by the percentage of their total feldspar that is alkali feldspar. Granites whose feldspar is 65% to 90% alkali feldspar are syenogranites, while the feldspar in monzogranite is 35% to 65% alkali feldspar.^[5][6] A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites, as described below.^[7][8]
Another aspect of granite classification is the ratios of metals that potentially form feldspars. Most granites have a composition such that almost all their aluminum and alkali metals (sodium and potassium) are combined as feldspar. This is the case when K₂O + Na₂O + CaO > Al₂O₃ > K₂O + Na₂O. Such granites are described as normal or metaluminous. Granites in which there is not enough aluminum to combine with all the alkali oxides as feldspar (Al₂O₃ < K₂O + Na₂O) are described as peralkaline, and they contain unusual sodium amphiboles such as riebeckite. Granites in which there is an excess of aluminum beyond what can be taken up in feldspars (Al₂O₃ > CaO + K₂O + Na₂O) are described as peraluminous, and they contain aluminum-rich minerals such as muscovite
Granitic rock is widely distributed throughout the continental crust.^[17] Much of it was intruded during the Precambrian age; it is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents. Outcrops of granite tend to form tors, domes or bornhardts, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations, very coarse-grained pegmatite masses occur with granite.^[18]
Granite forms from silica-rich (felsic) magmas. Felsic magmas are thought to form by addition of heat or water vapor to rock of the lower crust, rather than by decompression of mantle rock, as is the case with basaltic magmas.^[19] It has also been suggested that some granites found at convergent boundaries between tectonic plates, where oceanic crust subducts below continental crust, were formed from sediments subducted with the oceanic plate. The melted sediments would have produced magma intermediate in its silica content, which became further enriched in silica as it rose through the overlying crust.^[20]
Early fractional crystallisation serves to reduce a melt in magnesium and chromium, and enrich the melt in iron, sodium, potassium, aluminum, and silicon.^[21] Further fractionation reduces the content of iron, calcium, and titanium.^[22] This is reflected in the high content of alkali feldspar and quartz in granite.
The presence of granitic rock in island arcs shows that fractional crystallization alone can convert a basaltic magma to a granitic magma, but the quantities produced are small.^[23] For example, granitic rock makes up just 4% of the exposures in the South Sandwich Islands.^[24] In continental arc settings, granitic rocks are the most common plutonic rocks, and batholiths composed of these rock types extend the entire length of the arc. There are no indication of magma chambers where basaltic magmas differentiate into granites, or of cumulates produced by mafic crystals settling out of the magma. Other processes must produce these great volumes of felsic magma. One such process is injection of basaltic magma into the lower crust, followed by differentiation, which leaves any cumulates in the mantle. Another is heating of the lower crust by underplating basaltic magma, which produces felsic magma directly from crustal rock. The two processes produce different kinds of granites, which may be reflected in the division between S-type (produced by underplating) and I-type (produced by injection and differentiation) granites, discussed below.^[23]