gabro

Gabro, also spelled gabbro in many contexts, is a coarse-grained, dark-colored intrusive igneous rock. It belongs to the mafic family and is the plutonic (intrusive) counterpart of basalt. The typical mineral assemblage consists mainly of plagioclase feldspar (often calcium-rich varieties) with pyroxene (clinopyroxene) and sometimes amphibole; olivine may be present in cumulate varieties. Accessory minerals such as magnetite, ilmenite, and apatite are common. The texture is phaneritic, with crystals large enough to identify with the naked eye, usually ranging from about 1 millimeter to several millimeters.

Formation and occurrence: Gabro forms when magma crystallizes slowly deep within the crust, in plutons, sills, and dikes, allowing large crystals to develop. It is a major component of the lower continental crust and of many ophiolites that represent slices of oceanic crust and upper mantle. In exposure, gabro can occur as extensive intrusions or as part of layered mafic complexes, where cumulate textures record fractional crystallization processes.

Relation to other rocks: As the intrusive equivalent of basalt, gabro shares a similar chemical composition (basaltic to dioritic in broad terms) but differs in texture due to slower cooling. When exposed at the surface, weathering can produce a durable rock suitable for construction and decorative stone, while variably enriched variants may feature hornblende instead of pyroxene.

Etymology and use: The name is derived from a European toponym (the Italian toponym Gabbro). In practical terms, gabro is widely used as crushed stone and aggregate in construction and, where aesthetically suitable, as dimension or decorative stone.