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Granite

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

   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.
   Enlarge
   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 ( IPA: /ˈgranɪt/) is a common and widely occurring type of
   intrusive, felsic, igneous rock.

   Granites are usually a white, black 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 gray 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·cm^−3 with a range of
   1.74 g·cm^−3 to 2.80 g·cm^−3.

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

Mineralogy

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

   Granite primarily consists of orthoclase and plagioclase feldspars,
   quartz, hornblende, muscovite and/or biotite micas, and minor accessory
   minerals such as magnetite, garnet, zircon and apatite. Rarely, a
   pyroxene is present. Very rarely, iron-rich olivine, fayalite, occurs.

   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, sanidine, or microcline) and
   plagioclase feldspar on the A-Q-P half of the diagram. Granite-like
   rocks which are silica-undersaturated may have a feldspathoid such as
   nepheline, and are classified on the A-F-P half of the diagram (Figure
   1).

   True granite according to modern petrologic convention contains both
   plagioclase and alkali feldspars. When a granitoid is devoid or nearly
   devoid of plagioclase the rock is referred to as alkali granite. When a
   granitoid contains <10% orthoclase it is called tonalite; pyroxene and
   amphibole are common in tonalite.

   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 volcanic equivalent of plutonic granite is rhyolite.

Chemical Composition

   A worldwide average of the average proportion of the different chemical
   components in granites, in descending order by weight percent, is ::
     * SiO[2] — 72.04%
     * Al[2]O[3] — 14.42%
     * K[2]O — 4.12%
     * Na[2]O — 3.69%
     * CaO — 1.82%
     * FeO — 1.68%
     * Fe[2]O[3] — 1.22%
     * MgO — 0.71%
     * TiO[2] — 0.30%
     * P[2]O[5] — 0.12%
     * MnO — 0.05%
          + Based on 2485 analyses

Occurrence

   The Stawamus Chief is a granite monolith in British Columbia
   Enlarge
   The Stawamus Chief is a granite monolith in British Columbia

   Granite is currently known only on Earth where it forms a major part of
   continental crust. 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.

   Despite being fairly common throughout the world, the areas with the
   most commercial granite quarries are located in the Scandinavian
   Peninsula (mostly in Finland and Norway), Spain (mostly Galicia and
   Asturias), Brazil, India and several countries in the South end of the
   African continent, namely Angola, Namibia, Zimbabwe and South Africa.

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  km and up to 50 km depth within thick continental crust.

   The origin of granite is contentious and has led to varied schemes of
   classification. 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 magma.

Geochemical origins

   Granitoids are a ubiquitous component of the crust. They have
   crystallized from magmas that have compositions at or near a eutectic
   point (or a temperature minimum on a cotectic curve). Magmas will
   evolve to the eutectic because of igneous differentiation, or because
   they represent low degrees of partial melting. Fractional
   crystallisation serves to reduce a melt in iron, magnesium, titanium,
   calcium and sodium, and enrich the melt in potassium and silicon -
   alkali feldspar (rich in potassium) and quartz (SiO[2]), are two of the
   defining constituents of granite.

   This process operates regardless of the origin of the parental magma to
   the granite, and regardless of its chemistry. However, the composition
   and origin of the magma which differentiates into granite, leaves
   certain geochemical and mineralogical evidence as to what the granite's
   parental rock was. The final mineralogy, texture and chemical
   composition of a granite is often distinctive as to its origin.

   For instance, a granite which is formed from melted sediments may have
   more alkali feldspar, whereas a granite derived from melted basalt may
   be richer in plagioclase feldspar. It is on this basis that the modern
   "alphabet" classification schemes are based.

Alphabet Soup Classification

   The 'alphabet soup' scheme of Chappell & 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 intrusive mafic rocks, or buried 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 volcanic "hot spot"
   activity and have peculiar mineralogy and geochemistry. These granites
   are formed by melting of the lower crust under conditions that are
   usually extremely dry. The granite caldera of Yellowstone National Park
   is an example of an A-type granite.

Granitization

   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 high temperature, and also water or volatiles which
   act as 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 phenomena 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 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, 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 has postulated that the Egyptians used emery shown to
   have higher hardness on the Mohs scale.

   Many large Hindu temples in southern India, particularly those built by
   the 11th century king Rajaraja Chola I, were made of granite. In fact,
   the amount of granite in them is comparable to the Great Pyramid of
   Giza.

Modern

   Granite has been extensively used as a dimension stone and as flooring
   tiles in public and commercial buildings and monuments. With increasing
   amounts of acid rain in parts of the world, granite has begun to
   supplant marble as a monument material, since it is much more durable.
   Polished granite has been a popular choice for kitchen countertops due
   to its high durability and aesthetic qualities.

   Engineers have traditionally used polished granite surfaces to
   establish a plane of reference, since they are relatively impervious
   and inflexible.

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

   Sandblasted concrete with a heavy aggregate content has an appearance
   similar to rough granite, and is often used as a substitute when use of
   real granite is impractical.

                              Azul Noce (Spain)

                          Giallo Veneziano (Brazil)

                            Gran Violet (Brazil)

                            Lavanda Blue (Brazil)

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