Gabbro is a coarse-grained and usually dark-colored igneous rock. Gabbro is an intrusive rock. It means that it formed as magma cooled slowly in the crust. Igneous rocks with similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead).
The most important mineral groups that make up gabbro are plagioclase and pyroxene. Plagioclase usually predominates over pyroxene. Plagioclase is sodium-calcium feldspar. It contains more calcium than sodium in gabbro. If there is more sodium in the plagioclase, then the rock type is named diorite. Diorite is usually lighter in color and contains more amphiboles than pyroxenes.
Gabbro with large augite phenocrysts embedded in white plagioclase. These two are the principal minerals in gabbro. Tangen, Norway. Width of sample 11 cm.
“Gabbro” in the strict sense of the term is an intrusive rock that is chiefly composed of monoclinic pyroxene (clinopyroxene, abbreviated Cpx) and plagioclase. If more than 5% of Cpx is replaced with orthorhombic pyroxene (orthopyroxene, abbreviated Opx), the rock is named gabbronorite. If more than 95% of the pyroxene is Opx, then we have norite. These rocks are all collectively called gabbroic rocks. Sometimes the term “gabbro” is used loosely to include them all. Rocks that contain more than 90% pyroxene is pyroxenite. Gabbroic rocks may contain small amounts of quartz (up to 5%). If there is more quartz present, the rock must be named quartz gabbro. If quartz makes up more than 20%, then it is not gabbro anymore. Such rocks are called granitoids1.
The classification scheme for gabbroic rocks without taking into account olivine and feldspathoids. Cpx is clinopyroxene, Opx is orthopyroxene, Plag is plagioclase. Rocks that contain less than 10% plagioclase are ultramafic (pyroxenite if pyroxenes are the dominant minerals). Pyroxene in gabbro is almost exclusively clinopyroxene and norite is conversely rich in orthopyroxene. Anorthosite is almost monomineralic plagioclase-rock.
The classification scheme of olivine-bearing (>10%) gabbroic rocks. “Olivine” should be added to the rock name if it contains more than 10% of it. Gabbroic rock that contains almost no pyroxene is troctolite.
There are even more varieties possible. Olivine can be present or feldspathoids just like in basalt. Hence, rock names like nepheline-bearing gabbro or olivine norite are possible. Anorthosite and troctolite are similar rocks also. Anorthosite is almost monomineralic rock — more than 90% of the plagioclase-pyroxene pair is plagioclase. Troctolite contains mostly olivine and plagioclase, pyroxene again forms no more than 10% of the composition.
It is often mentioned that the term “gabbro” was brought into geological terminology by a German geologist Leopold von Buch. It is probably not correct. The term was used by Italian geologist Tozzetti in 1768. The term “gabbro” itself comes from Italy. Von Buch redefined the term in 1810 as a rock composed of diallage and feldspar or saussurite. Saussurite is a mixture of epidote, albite, and other alteration minerals. Saussuritized gabbro can be named metagabbro nowadays because its original composition has clearly been altered by metamorphism. Diallage is not defined as a mineral now but it is monoclinic pyroxene anyway (either diopside or augite). So we can say that von Buch used the term roughly as we use it today.
Gabbros form in the crust. This is the magma that did not break to the surface to cool as a basalt. Gabbros are not as widespread as granitoids but they are definitely not rare rocks. Gabbroic rocks are usually equigranular (composed of similarly sized grains) mixtures of black, brown or greenish pyroxene and white, gray, or greenish plagioclase. Pyroxenite and other ultramafic rocks are darker- and diorite is lighter-colored.
1. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.