Diorite (// or //) is a grey to dark-grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. It is distinguished from gabbro on the basis of the composition of the plagioclase species, where average An-content of plagioclase in a diorite is less than An50. It may contain small amounts of quartz, microcline, and olivine. Zircon, apatite, titanite, magnetite, ilmenite, and sulfides occur as accessory minerals. It can also be black or bluish-grey, and frequently has a greenish cast. Varieties deficient in hornblende and other dark minerals are called leucodiorite. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makes the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than ten percent, the rock type grades into monzodiorite or granodiorite. Diorite has a medium grain size texture, occasionally with porphyry.
The phaneritic salt and pepper texture common to diorite is due to random distribution of somewhat medium-sized grains of the white albite and black or nearly black color amphibole and pyroxene crystals, with minor glints of biotite. The common silicate mineral assemblage mainly includes sodium rich plagioclase with calcium plagioclase, amphibole, pyroxene, and biotite. Minor amounts of muscovite and quartz, and oxides, such as magnetite and ilmenite, may also be present. A dioritic rock containing feldspathoid mineral/s while absent of quartz is termed foid-bearing or foid diorite according to content. Diorite may also be porphyritic. Orbicular diorite shows several alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus within a diorite porphyry matrix.
Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from the partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building, such as in the Andes Mountains, as large batholiths. The extrusive volcanic equivalent rock type is andesite.
Diorite is a relatively rare rock; source localities include Leicestershire (one name for microdiorite—markfieldite—exists due to the rock's being found in the village of Markfield) and Aberdeenshire, UK; Guernsey; Sondrio, Italy; Thuringia and Saxony in Germany; Finland; Romania; Northeastern Turkey; central Sweden; the Darrans range of New Zealand; the Andes Mountains; Basin and Range province; Minnesota in the USA, and Idahet in Egypt.
Diorite is an extremely hard rock, making it difficult to carve grand work with. It is so hard that ancient civilizations (such as Ancient Egypt) used diorite balls to work granite. Its hardness, however, also allows it to be worked finely and take a high polish, and to provide a durable finished work.
One comparatively frequent use of diorite was for inscription, as it is easier to carve in relief than in three-dimensional statuary. Perhaps the most famous diorite work extant is the Code of Hammurabi, inscribed upon a 2.23 m (7 ft 4 in) pillar of black diorite. The original can be seen today in Paris' Musée du Louvre. The use of diorite in art was most important among very early Middle Eastern civilizations such as Ancient Egypt, Babylonia, Assyria, and Sumer. It was so valued in early times that the first great Mesopotamian empire—the Empire of Sargon of Akkad—listed the taking of diorite as a purpose of military expeditions.
Although one can find diorite art from later periods, it became more popular as a structural stone and was frequently used as pavement due to its durability. Diorite was used by both the Inca and Mayan civilizations, but mostly for fortress walls, weaponry, etc. It was especially popular with medieval Islamic builders. In later times, diorite was commonly used as cobblestone; today many diorite cobblestone streets can be found in England, Guernsey and Scotland, and scattered throughout the world in such places as Ecuador and China. Although diorite is rough-textured in nature, its ability to take a polish can be seen in the diorite steps of St. Paul's Cathedral, London, where centuries of foot traffic have polished the steps to a sheen.
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