gabbro

A coarse-grained basic (low in silica) igneous rock composed of calcic plagioclase feldspar, pyroxene, and sometimes olivine.

The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges.

Petrology

Gabbro is dense, greenish or dark-colored and contains varied percentages of pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in large quantities) The pyroxene is mostly clinopyroxene, small amounts of orthopyroxene may be present.

Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Gabbro may be extremely coarse grained to pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro, although these may exhibit acicular crystal habits.

Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase has grown earlier than the groundmass minerals.

Distribution

Gabbro can be formed as a massive uniform intrusion or as part of a layered ultramafic intrusions as a cumulate rock formed by settling of pyroxene and plagioclase.

Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes as parts of zones III and IV (sheeted dyke zone to massive gabbro zone).

Uses

Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.

Ocellar varieties of gabbro are often used as ornamental facing stones, paving stones and it is also known by the trade name of 'black granite', which is a popular type of headstone used in funerary rites.

Etymology

Gabbro was named by the German geologist Christian Leopold von Buch after a town in the Italian Tuscany region.