Gabbroids are distinguished from dioritoids by an anorthite (calcium plagioclase) fraction of their total plagioclase of greater than 50%. The rock will be classified as either a gabbroid or a dioritoid if quartz makes up less than 20% of the QAPF content, feldspathoid makes up less than 10% of the QAPF content, and plagioclase makes up more than 65% of the total feldspar content. The relative abundances of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoid (F), are used to plot the position of the rock on the diagram. For igneous rocks composed mostly of silicate minerals, and in which at least 10% of the mineral content consists of quartz, feldspar, or feldspathoid minerals, classification begins with the QAPF diagram. Geologists use rigorous quantitative definitions to classify coarse-grained igneous rocks, based on the mineral content of the rock. Gabbro is distinguished from diorite by an anorthosite content of greater than 50% of its plagioclase and from anorthite by a mafic mineral content greater than 10%. QAPF diagram with the gabbro field highlighted in yellow. Gabbroids are distinguished from dioritoids by an anorthosite content of greater than 50% of their plagioclase. QAPF diagram with the gabbroid/dioritoid fields highlighted in yellow. When present, hornblende is typically found as a rim around augite crystals or as large grains enclosing smaller grains of other minerals ( poikilitic grains).

Gabbro is composed of pyroxene and calcium-rich plagioclase, with minor amounts of hornblende, olivine, and accessory minerals. Gabbro is a coarse-grained ( phaneritic) igneous rock that is relatively low in silica and rich in iron, magnesium, and calcium.

Petrology Mineral assemblage of igneous rocks He assigned the name 'gabbro' to rocks that geologists nowadays would more strictly call 'metagabbro' ( metamorphosed gabbro). Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his description of these Italian ophiolitic rocks. It was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany. The term 'gabbro' was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. By rough analogy, gabbro is to basalt as granite is to rhyolite. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely 'gabbroic'. Gabbro is also found as plutons associated with continental volcanism. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. r oʊ/) is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Coarse-grained mafic intrusive rock Gabbro specimen Photomicrograph of a thin section of gabbro

Black indigo gabbro