Gabbro

Gabbro is a coarse‑grained, mafic intrusive igneous rock that crystallizes slowly underground from magma. It is the plutonic counterpart of basalt and shares a similar basaltic chemical composition, rich in magnesium and iron.

The mineral content of gabbro is dominated by calcium-rich plagioclase feldspar and pyroxene, with minor amounts of olivine and amphibole in some varieties. Accessory minerals such as magnetite, ilmenite, and apatite are common, while quartz is rare.

Texture and formation: Gabbro has a phaneritic, or coarse-grained, texture, in which large crystals are visible to the naked eye. It forms when mafic magma cools at depth, often in magma chambers or cumulate bodies within layered intrusions. It can occur as large plutons, as well as concordant intrusions like sills and discordant bodies such as dikes.

Occurrence and varieties: Gabbro is widespread in continental crust and constitutes part of the lower oceanic crust in ophiolites. It may appear in various forms, including hornblende-bearing varieties, and it can grade into related rocks such as norite or troctolite depending on mineral proportions.

Naming and uses: The name gabbro derives from a locality called Gabbro in Tuscany, Italy, where the rock was first described. Commercially, gabbro is quarried for crushed stone and dimension stone, and certain varieties are used as decorative or architectural stone.