Granite
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Granite_for_temple.jpg
Granite is a common and widely-occurring group of intrusive felsic igneous rocks that form at great depths and pressures under continents. Granite 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. Ordinary granite always carries a small amount of plagioclase, but when this is absent the rock is referred to as alkali granite. An increasing proportion of plagioclase feldspar causes granite to pass into granodiorite. A rock consisting of equal proportions of orthoclase and plagioclase plus quartz may be considered a quartz monzonite. A granite containing both muscovite and biotite micas is called a binary granite. Depending upon the proportions of feldspar and quartz, the Mohs hardness of granite ranges between 5.5 and 7 [1] (http://www.findstone.com/daniel1.htm). The average density 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.
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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, except perhaps the most recent; 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 rock veneer of the continents.
Origin
There are two theories for the origin of granite. The magmatic theory states that granite is derived by the crystal fractionation of magma. Thus granite bodies are the result of intrusion of liquid magma into the existing rocks. The granitization theory states that granite is formed in place by extreme metamorphism. There is evidence to support both theories, and both are useful to explain different observed features. The two may actually merge: as metamorphic conditions increase to the melting point of the metamorphosed granite, it will melt and become a liquid magma, and then harden into igneous granite.
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, [2] (http://www.eeescience.utoledo.edu/Faculty/Harrell/Egypt/Mosques/CAIRO_Rocks_1.htm) 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 [3] (http://hebsed.home.comcast.net/hunt.htm) 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. It is also gaining fashion for use as kitchen countertops.
In the world of sports, curling rocks are traditionally fashioned of granite.
See also
External link
- The Emplacement and Origin of Granite (http://www.geologynet.com/granite1.htm)bg:Гранит
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