thumb|Diorite
thumb|Orbicular diorite from Corsica ([[corsite)]]
thumb|upright=1.7|[[QAPF diagram with dioritoid fields highlighted in yellow and diorite in red]]
thumb|upright=1.4|Mineral assemblage of igneous rocks
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silica (mafic) gabbro and high-silica (felsic) granite.
Diorite is found in mountain-building belts (orogens) on the margins of continents. It has the same composition as the fine-grained volcanic rock, andesite, which is also common in orogens.
Diorite has been used since prehistoric times as decorative stone. It was used by the Akkadian Empire of Sargon of Akkad for funerary sculptures, and by many later civilizations for sculptures and building stone.
Description
Diorite is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and sometimes pyroxene. The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. It is distinguished from gabbro on the basis of the composition of the plagioclase species; the plagioclase in diorite is richer in sodium and poorer in calcium.
Geologists use rigorous quantitative definitions to classify coarse-grained igneous rocks, based on the mineral content of the rock. For igneous rocks composed mostly of silicate minerals, and in which at least 10% of the mineral content consists of quartz, feldspar, or feldspathoid minerals, classification begins with the QAPF diagram. The relative abundances of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoid (F), are used to plot the position of the rock on the diagram. on account of its characteristic, easily identifiable large crystals of hornblende. A ferrodiorite is a dioritoid enriched in iron and titanium. Ferrodiorites are common in the lower oceanic crust.
Coarse-grained (phaneritic) dioritoids are produced by slow crystallization of magma having the same composition as the lava that solidifies rapidly to form fine-grained (aphanitic) andesite. Diorite is occasionally porphyritic. It usually contains enough mafic minerals to be dark in appearance. Orbicular diorite shows alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus, within a diorite porphyry matrix.
Occurrence
thumb|Hornblende diorite from the Henry Mountains, Utah, US
Diorite results from the partial melting of a mafic rock above a subduction zone. It is found in volcanic arcs, and in cordilleran mountain building, such as in the Andes Mountains. However, while its extrusive volcanic equivalent, andesite, is common in these settings, diorite is a minor component of the plutonic rocks, which are mostly granodiorite or granite. Diorite also makes up some stocks intruded beneath large calderas.
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Diorite source localities include Leicestershire and Aberdeenshire, UK; Thuringia and Saxony in Germany; Finland; Romania; central Sweden; southern Vancouver Island around Victoria, Canada; the Darran Range of New Zealand; the Andes Mountains; and Concordia in South Africa. Hornblende diorite is a common rock type in the Henry, Abajo, and La Sal Mountains of Utah, US, where it was emplaced as laccoliths.
An orbicular variety found in Corsica was formerly called corsite. An obsolete name for microdiorite, markfieldite, was given by Frederick Henry Hatch in 1909 to exposures near the village of Markfield, England. Esterellite is a local name for microdiorite given by Auguste Michel-Lévy to exposures in the Esterel Massif in France.
Use
Human use of diorite dates at least to the Middle Neolithic, when it was used in a passage grave at Le Dolmen du Mont Ubé, Jersey. The use of stone of contrasting colour suggests that diorite was deliberately selected for its appearance.
The first great Mesopotamian empire, the Akkadian Empire of Sargon of Akkad, began using diorite for sculpture after sources of the rock came under Akkadian control. Diorite was used to depict rulers or high officials in ceremonial poses or attitudes of prayer, and the sculptures may have been designed to receive funerary offerings. Diorite was also used for stone vases by Bronze Age craftspeople, who developed considerable skill at polishing diorite and other stones. The Egyptians had become skilled at shaping diorite and other hard stones by 4000 BCE. A large diorite stela in the Louvre Museum dating to 1700 BCE is inscribed with the Code of Hammurabi.
Diorite was used by the Inca civilization as structural stone. It was used by medieval Islamic builders to construct water fountains in the Crimea. In later times, diorite was commonly used as cobblestone; today many diorite cobblestone streets can be found in England and Guernsey. Guernsey diorite was used in the steps of St Paul's Cathedral, London.
Today, diorite is uncommon in construction, although it shares similar physical properties with granite. Diorite is often sold commercially as "black granite". Diorite's modern uses include construction aggregate, curbing, usage as dimension stones, cobblestone, and facing stones.
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File:Jar with Lug Handles LACMA M.71.73.54.jpg|Naqada II jar with lug handles; ; height: ; Los Angeles County Museum of Art (US)
File:Gudea of Lagash Girsu.jpg|Statue of Gudea I, dedicated to the god Ningishzida; 2120 BC (the Neo-Sumerian period); height: , width: , depth: ; Louvre
File:Weight Shulgi Louvre AO22187.jpg|Weight dedicated by King Shulgi of Ur with a crescent moon on it; 2094-2047 BC; weight: 248 g; Louvre
File:Head of a bearded god wearing a cap with horns dedicated by Puzur-Eshtar of Mari.jpg|Assyrian head of a bearded god wearing a cap with horns dedicated by Puzur-Eshtar of Mari; middle Bronze Age; height: ; (Berlin, Germany)
File:Head of a Cow Goddess (Hathor or Mehetweret) MET DP311571.jpg|Head of a cow goddess (Hathor or Mehet-Weret); 1390-1352 BC; height: , width: , depth: ; Metropolitan Museum of Art (New York City)
File:Paris - Toutânkhamon, le Trésor du Pharaon - Amon protégeant Toutânkhamon - 005.jpg|Statue of Amun; 1336-1327 BC; height: , width: 44, length: ; Louvre
File:Block Statue of the God's Father Pameniuwedja, son of Nesmin and Nestefnut MET 07.228.25 EGDP023155.jpg|Block statue of the god's father Pameniuwedja, son of Nesmin and Nestefnut; 4th century BC; height: , width: , depth: ; Metropolitan Museum of Art
File:Pair of vases MET DT4387.jpg|Vase with gilt bronze ornaments; ; ; Metropolitan Museum of Art
File:Palazzo delle Poste di Napoli - ingresso.jpg|, Naples, Italy, Gino Franzi, 1936. Modernism, constructed with marble and diorite.
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See also
- List of rock types