Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine ), hornblende, and/or pyroxene. 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 .
Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic 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 appears in thousands of square miles of large batholiths (mass of intrusive igneous rock believed to have solidified deep within the earth). The extrusive volcanic equivalent is andesite.
Diorite is a relatively rare rock; source localities include Sondrio, Italy; Thuringia and Sassonia in Germany; Finland; Romania; central Sweden; Scotland; the Andes Mountains; and the Basin and Range province and Minnesota in the USA.
An orbicular variety found in Corsica is called corsite.
Last updated: 10-29-2005 02:13:46