Diorite ( ) is an intrusive rock 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 . It is intermediate in composition between low-silica (mafic) gabbro and high-silica (felsic) granite.
Diorite is found in mountain-building belts ( ) 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.
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. The rock will be classified as either a dioritoid or a gabbroid if quartz makes up less than 20% of the QAPF content, feldspathoid makes up less than 10% of the QAPF content, and plagioclase makes up more than 65% of the total feldspar content. Dioritoids are distinguished from gabbroids by an anorthite (calcium plagioclase) fraction of their total plagioclase of less than 50%.
The composition of the plagioclase cannot easily be determined Field work, and then a preliminary distinction is made between dioritoid and gabbroid based on the content of mafic minerals. A dioritoid typically has less than 35% mafic minerals, typically including hornblende, while a gabbroid typically has over 35% mafic minerals, mostly pyroxenes or olivine. The name diorite (from Ancient Greek italic=no, "to distinguish") was first applied to the rock by René Just Haüy on account of its characteristic, easily identifiable large crystals of hornblende.
Dioritoids form a family of rock types similar to diorite, such as monzodiorite, quartz diorite, or nepheline-bearing diorite. Diorite itself is more narrowly defined, as a dioritoid in which quartz makes up less than 5% of the QAPF content, feldspathoids are not present, and plagioclase makes up more than 90% of the feldspar content.
Diorite may contain small amounts of quartz, microcline, and olivine. Zircon, apatite, titanite, magnetite, ilmenite, and sulfide mineral occur as accessory minerals. Varieties deficient in hornblende and other dark minerals are called leucodiorite. A ferrodiorite is a dioritoid enriched in iron and titanium. Ferrodiorites are common in the lower oceanic crust.
Coarse-grained (phanerite) dioritoids are produced by slow crystallization of magma having the same composition as the lava that solidifies rapidly to form fine-grained (aphanitic) andesite. Rock of similar composition to diorite or andesite but with an intermediate texture is sometimes called microdiorite.
Diorite source localities include Leicestershire and Aberdeenshire, UK; Thuringia and Saxony in Germany; Finland; Romania; central Sweden; southern Vancouver Island around Victoria, Canada;
Muller, J.E. (1980). Geology Victoria Map 1553A. Ottawa: Geological Survey of Canada the Darran Range of New Zealand; the Andes Mountains;
An orbicular variety found in Corsica was formerly called napoleonite. 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.
The first great 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 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 , cobblestone, and facing stones.
Occurrence
Use
See also
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