Granite is a coarse-grained, crystalline, plutonic, felsic, and acidic igneous rock. It contains more than 66% of silica “SiO2”, and about 14.42% of alumina “Al2O3”. The quartz makes up 10% : 50% of the whole rock. The alkali feldspars range from 65% to 90% of the entire rock. It’s leucocratic “light-colored”. The equivalent volcanic rock of granite is rhyolite. The melting point of most dry granites ranges between 1215 – 1260 degrees Celcius. The average density of granite is 2.75 gm/cm3. Granite makes up collectively the most batholiths in the world. Also, granite may form some other stocks, dikes, sills, lopoliths, and laccoliths. In addition, granite is the main composition of the continental crust.

Mineralogy of granite:

1. Feldspars make up the greater part of granitic rocks.
2. Two types of peralkaline and alkali granite may be distinguished according to the nature of the alkali feldspar present:
   * Hyper-solve types have a single alkali feldspar, normally a microperthite.
   * Sub-solve types have two alkali feldspars, microperthite and independent albite.
3. The alkali feldspar found in granitic rocks is a potash-soda or potash variety, microperthite, orthoclase, or microcline.
4. The lime-bearing plagioclase “of lamellar twinning” present in granites, granodiorites, and tonalites is commonly subhedral and frequently zoned.
5. The quartz is found filling up these spaces between the other constituents as small plates or aggregates of grain. Few granitic rocks have more than 40％ of quartz and the average content for granites is 30％ for granodiorites and about 25％ for tonalites.
6. The micas of granitic rocks include both biotite and muscovite in characteristic plates and flakes. The biotite of granitic rocks is rich in iron, brown, and strongly pleochroic in thin sections.
7. Amphibole, when present, is a strongly pleochroic sodic variety in the peralkaline granites, commonly riebeckite or arfvedsonite, while ferrohastingsite may occur in alkali granites and in certain granites and granodiorites.
8. Pyroxene is uncommon in granitic rocks, except in peralkaline granites, where aegirine or aegirine-augite may be present. Some alkali granites and certain granites and granodiorite have hedenbergite, and a few granodiorites and tonalites carry augite and orthopyroxene.
9. Olivine is normally absent, but, some granitic rocks that are very low in magnesia have some fayalite.
10. Among the accessory constituents, apatite in little prisms and needles, small crystals of zircon, and grains of opaque ore are almost ubiquitous.
11. The main ore is magnetite but, ilmenite is present in some rocks, also pyrrhotite and chalcopyrite can be found in small amounts.
12. Crystals of light brown pleochroic sphene are often seen and may be relatively abundant, but this mineral is usually absent in rocks having muscovite as a primary constituent.
13. Sporadic crystals of orthite are widely distributed, and both monazite and xenotime may be found.
14. Fluorite appears in some granites.

 

Textures of granite:

1. Subhedral granular texture: all crystals are approximately equal in size.
2. Porphyritic texture: large crystals “phenocrysts” are embedded in a matrix of smaller crystals “groundmass” commonly with phenocrysts of potash feldspar.
3. Rapakivi texture: ovoid crystals of potash feldspar are mantled by lime-bearing plagioclase.
4. Ophitic texture: all pyroxene crystals that appear in thin sections enclose lath-shaped euhedral plagioclase crystals.
5. Sub-ophitic texture: a part of pyroxene crystals encloses plagioclase crystals.
6. Poikilitic texture: small crystals are surrounded and enclosed by large crystals in thin sections of granitic rocks.
7. Some granites have a banded or layered structure in the field.
8. Few granitic rocks show orbicular structure.
9. High-level granitic rocks may develop a drusy structure.

Main varieties of granite:

Granites have been classified into five main types, according to the QAFP Diagram of their mineral composition, where: (Q) is for quartz, (A) is for alkali-feldspar, (F) is for feldspathoids, and (P) is for plagioclase, as the following:

1. Alkali-feldspar Granites: they have more than 90% of alkali-feldspar, such as orthoclase, anorthoclase, microperthite, microcline, or barium-feldspar that may be either hyalophane or celsian. Without or with but rarely plagioclase-feldspars.
2. Syenogranites: they have 65% of alkali-feldspar minerals, with 40% of plagioclase and quartz.
3. Monzogranites: they have 50% of alkali-feldspar and also 50% of plagioclase, with quartz.
4. Granodiorites: they have 65% of plagioclase, without or with but rarely alkali-feldspar and quartz is about 40%.
5. Tonalites: they have more than 90% of plagioclase, with less amount of quartz and, without alkali-feldspars.

 

The geochemical classification of granites:

1. Peralkaline Granites: they have crystallized from a magma undersaturated with alumina. The main varieties include riebeckite, aegirine, or arfvedsonite as their mafic constituents. The ratio (Al2O3 / Na2O + K2O + CaO) is much lower than (1). The ratio of Al2O3 is less than the ratio of Na2O + K2O + CaO. They have essentially Quartz, K-Feldspars, and Felspathoids.
2. Alkali Granites: the more important varieties of alkali granite are ferrohastingsite alkali granite, with or without biotite, biotite alkali granite, and muscovite-biotite alkali granite.
3. Sub-aluminous Granites: The ratio (Al2O3 / Na2O + K2O + CaO) is less than (1). The ratio of Al2O3 is less than the ratio of Na2O + K2O + CaO. But the ratio of Al2O3 is equal to the ratio of Na2O + K2O. They have hornblende.
4. Meta-aluminous Granites: The ratio (Al2O3 / Na2O + K2O + CaO) is greater than (1). The ratio of Al2O3 is less than the ratio of Na2O + K2O + CaO. But the ratio of Al2O3 is greater than Na2O + K2O. They have hornblende.
5. Peraluminous Granites: The ratio (Al2O3 / Na2O + K2O + CaO) is greater than (1.1). The ratio of Al2O3 is greater than the ratio of Na2O + K2O + CaO. They have muscovite and garnet.

S-type & I-type granites:

Granites have been also divided into two main types; I-type and S-type, according to their magmatic origin. The prefix letter “S” refers to its origin as metasedimentary source rock. S-type is formed by partial melting of these metasedimentary rocks, a process called anataxis or ultra metamorphism. And the prefix letter “I” refers to that they are derived from the igneous source rock.

The following table shows the utmost differences between I-type and S-type:

The differences between G1-, G2-, and G3-granites can be summarized in the following table:

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