Diorite
Diorite is a plutonic igneous rock composed of coarse
grains of plagioclase feldspar and less than 40 percent
hornblende (see amphibole) and biotite (see mica), or, more
rarely, pyroxene or olivine. Small amounts of potassium
feldspar and quartz may also occur, along with traces of
magnetite, apatite, sphene, and zircon. Diorite is the
plutonic equivalent of the volcanic rock andesite and is
intermediate between gabbro and granite. Diorite occurs
around margins of granitic batholiths, in separate plutons,
and in dikes. It forms by the melting of rocks in the
lower crust, by the assimilation of crustal rocks in
basaltic magma, or as by metamorphic processes.