Granite

Quarrying granite for the Mormon Temple, Utah Territory. The ground is strewn with boulders and detached masses of granite, which have fallen from the walls of Little Cottonwood Canyon. The quarrying consists of splitting up the blocks.
Quarrying granite for the Mormon Temple, Utah Territory. The ground is strewn with boulders and detached masses of granite, which have fallen from the walls of Little Cottonwood Canyon. The quarrying consists of splitting up the blocks.

Granite is a common and widely-occurring type of intrusive felsic igneous rock.

Granites are usually a white or buff colour and are medium to coarse grained, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark grey or even black, depending on their chemistry and mineralogy.

Outcrops of granite tend to form tors, rounded massifs, and terrains of rounded boulders cropping out of flat, sandy soils. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive, hard and tough, and it is for this reason it has gained widespread use as a construction stone.

The average density of granite is 2.75 g/cm3; with a range of 1.74 to 2.80.

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Mineralogy

Granite primarily consists of orthoclase and plagioclase feldspars, quartz, hornblende, biotite, muscovite and minor accessory minerals such as magnetite, garnet, zircon and apatite. Rarely, a pyroxene is present.

Granite is classified according to the QAPF diagram for coarse grained plutonic rocks (granitoids) and is named according to the percentage of Quartz, Alkali feldspar (orthoclase) and Plagioclase Feldspar on the A-Q-P half of the diagram. Highly peralkaline forms of granite which are silica undersaturated may have a feldspathoid such as nepheline, and are classified on the A-F-P half of the diagram. See Figure 1, below.

Figure 1. QAPF diagram of granitoids and phaneritic foidolites (plutonic rocks).
Figure 1. QAPF diagram of granitoids and phaneritic foidolites (plutonic rocks).

True granite according to modern petrology contains both plagioclase and orthoclase feldspars. When a granitoid is devoid of orthoclase the rock is referred to as alkali granite or adamellite. When a granitoid contains <5% orthoclase it is known as a granodiorite, or tonalite when pyroxene is present.

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.

The extrusive equivalent of plutonic granite rock is called Rhyolite.

Occurrence

Granite occurs as relatively small, less than 100 km2 stock-like masses and as large batholiths often associated with orogenic mountain ranges and is frequently of great extent. Small dikes of granitic composition called aplites are associated with granite margins. In some locations very coarse-grained pegmatite masses occur with granite.

Granite has been intruded into the crust of the Earth during all geologic periods; much of it is of Precambrian age. Granite is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.

Origin

Granite is an igneous rock, and is formed from magma. Granite magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50km depth within thick continental crust.

The origin of granite is contentious and has led to varied schemes of classification. The 'alphabet soup' scheme of Chappel & White was proposed initially to divide granites into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite. Both of these types of granite are formed by melting of high grade metamorphic rocks, either other granite or depleted mantle, or buried and subducted sediment, respectively.
M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallised mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basalt into granite via fractional crystallisation.
A-type or anorogenic granites are formed above hot spot activity and have peculiar mineralogy and geochemistry. These granites are formed by melting of the lower crust under conditions which are usually too dry. The granite caldera of Yellowstone National Park is an example of an A-type granite.

Classification schemes are regional; there is a French scheme, a British scheme and an American scheme. This confusion arises because the classification schemes define granite by different means. Generally the 'alphabet-soup' classification is used because it classifies based on genesis or origin of the melt.

The granitization theory states that granite is formed in place by extreme metamorphism. The production of granite by metamorphic heat is difficult, but is observed to occur in certain amphibolite and granulite terrains. In-situ granitisation or melting by metamorphism is difficult to recognise except where leucosome and melanosome textures are present in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as a transitional between the two, but are not technically granite as they do not actually intrude into other rocks. In all cases, melting of solid rock requires temperature, and also water which acts like a catalyst by lowering the solidus temperature of the rock.

Emplacement mechanisms

The problem of emplacing large volumes of molten rock within the solid Earth has faced geologists for over a century, and is not entirely resolved. Granite magma must make room for itself or be intruded into other rocks in order to form an intrusion, and several mechanisms have been proposed to explain how large batholiths have been emplaced.

  • Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust
  • Diapirism where the density of the lighter granite causes relative buoyancy and the granite pushes upwards, warping and folding the rock above it
  • Assimilation, where the granite melts its way up into the crust and removes overlying material in this way
  • Inflation, where the granite body inflates under pressure and is injected into position

Most geologists today accept that a combination of these phenomenon can be used to explain granite intrusions, and that not all granites can be explained by one or another mechanism.

Uses

Antiquity

The Red Pyramid of Ancient Egypt (c. 26th century BC), named for the light crimson hue of its exposed granite surfaces, is the third largest of Egyptian pyramids. Menkaure's Pyramid, likely dating to the same era, was constructed of limestone and granite blocks. The Great Pyramid of Giza (c. 2580 BC) contains a huge granite sarcophagus fashioned of "Red Aswan Granite." The mostly ruined Black Pyramid dating from the reign of Amenemhat III once had a polished granite pyramidion or capstone, now on display in the main hall of the Egyptian Museum in Cairo (see Dahshur). Other uses in Ancient Egypt, [1] include columns, door lintels, sills, jambs, and wall and floor veneer.

How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt [2] has postulated that the Egyptians used emery shown to have higher hardness on the Mohs scale.

Modern

Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Polished granite has been a popular choice for kitchen countertops due to its high durability and aesthetic qualities.

In the world of sports, curling rocks are traditionally fashioned of granite.

Azul Noce (Spain)
Giallo Veneziano (Brazil)
Gran Violet (Brazil)
Lavanda Blue (Brazil)