July 1, 2022

Granodiorite (from « grain » and from « diorite ») is a plutonic igneous rock with a phaneritic texture similar to granite. It is mainly made up of quartz (>20%) and feldspars, but unlike granite, it contains more plagioclase than orthoclase. Secondary minerals are biotite, amphibole and pyroxene. Granodiorite deriving from granites (or from monzonites) due to an increase in plagioclase; rare black minerals[1]. Granodiorite derived from diorites or gabbros, due to an increase in the amount of quartz; black minerals up to 40%. The famous Rosetta Stone is granodiorite.


According to the QAPF diagram, the granodiorite is more than 20% quartz, and 65-90% of the feldspar is plagioclase. A higher proportion of plagioclase would correspond to tonalite. Granodiorite has a felsic to intermediate composition. It is the intrusive igneous equivalent of dacite (extrusive igneous). It contains a large amount of plagioclase rich in sodium (Na) and calcium (Ca), potassium feldspar, quartz, and lesser amounts of muscovite mica as lighter colored mineral components. Biotite and amphiboles, often in the form of hornblende, are more abundant in the granodiorite than in the granite, giving it a more obvious bicolor appearance or an overall darker hue. Mica can occur in well-formed hexagonal crystals, and hornblende can occur as acicular (needle-shaped) crystals. Minor amounts of minerals from the oxide group, such as magnetite, ilmenite, and ulvite, as well as some from the sulfide group, may also be present.


External Links

Wikimedia Commons has a media category for Granodiorite. (in English) Definition in the United States Geological Survey.