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Diorite (IPA: /ˈdʌɪərʌɪt/) is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), hornblende, and/or pyroxene. 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. It is often described as "salt and pepper" when composed largely of light-colored minerals randomly interspersed with dark minerals. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro. The presence of quartz makes the rock type quartz-diorite or tonalite, and if orthoclase (potassium feldspar) is present the rock type grades into granodiorite. Wikipedia:http://en.wikipedia.org/wiki/Diorite
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An intermediate igneous rock from the middle of Bowen's Reaction Series. Typically intermediate colored with a subequal mixture of light colored sodium plagioclase and dark colored amphibole (although biotite may also be present). Appearance is often described as "salt and pepper" because of the mix. The mafic/plagioclase mix can vary widely, however. This specimen is low on the mafic side, but the next specimen runs high. Quartz is sometime present between 5% and 20%, but if quartz exceeds 20% the rock is called a "grano-diorite." What keeps this from then being a monzonite is the high mafic content; monzonites have less than 5% mafics. Orthoclase is universally absent. Diorite is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas
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