Gabbro



  Gabbro (magma is trapped beneath the Earth's surface and cools into a crystalline mass.

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 olivine (olivine gabbro when olivine is present in large quantities).

The pyroxene is mostly ulvospinel.

Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called 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 oikocrysts have grown earlier than the groundmass minerals.

Distribution

Gabbro can be formed as a massive uniform intrusion or as part of a layered cumulate formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase cumulate.

Gabbro is an essential part of the oceanic crust, and can be found in many basalt volcanism.

Uses

Gabbro often contains valuable amounts of 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 graveyard headstone used in funerary rites. It is even more popular now for use in kitchen and their countertops, also under the misnomer of 'black granite'. Note that the term "granite" is widely but wrongly applied to almost any polished surfacing stone.

Etymology

Gabbro was named by the German geologist Christian Leopold von Buch after a town in the Italian Tuscany region. Essexite is named after the type locality in Essex County, MA, USA.

See also

 
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