GRANODIORITE

medium- to coarse-grained rock that is among the most abundant intrusive igneous rocks. It contains quartz and is distinguished from granite by its having more plagioclase feldspar than orthoclase feldspar; its other mineral constituents include hornblende, biotite, and augite. The plagioclase (andesine) usually forms twinned crystals, sometimes wholly encased by orthoclase. The mode of formation and occurrence, physical appearance, and mineralic composition and texture of granodiorite are much like those of granite (q.v.); granodiorite is darker in colour, however, because of its greater plagioclase content.