Gabbro
Gabbro is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt. The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust. It is a plutonic rock, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass. German geologist Christian Leopold von Buch named gabbro after a town in the Italian Tuscany region.
This rock often contains valuable amounts of sulfides of various metals, including sulfides of chromium, nickel, cobalt, gold, silver, platinum, and copper. In addition, some varieties of the rock, known as "black granite," are often used as ornamental stones, paving stones, and graveyard headstones.
Composition and grain
Gabbro is dense, greenish or dark-colored and contains varied proportions of different minerals, such as pyroxene, plagioclase, amphibole, and olivine. Gabbros contain minor amounts (typically a few percent) of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.
The pyroxene is mostly clinopyroxene, and small amounts of orthopyroxene may be present.[1] Quartz gabbros are also known to occur and are probably derived from magma that was oversaturated with silica. On the other hand, essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of nepheline. (Silica saturation of a rock can be evaluated by normative mineralogy).
Gabbro is generally coarse grained, with crystals in the size range of 1 millimeter (mm) or greater. Finer grained equivalents of gabbro are called diabase, although the vernacular term microgabbro is often used when extra descriptiveness is desired. 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.
Formation and 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. Cumulate gabbros are more properly termed pyroxene-plagioclase cumulate.
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). Long belts of gabbroic intrusions are typically formed at proto-rift zones and around ancient rift zone margins, intruding into the rift flanks. Mantle plume hypotheses may rely on identifying similar massive gabbro intrusions and coeval basalt volcanism.
Uses
Gabbro often contains valuable quantities of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.
Ocellar (spotted) varieties of gabbro are often used as ornamental facing stones and paving stones. It is also known by the trade name of "black granite," which is a popular type of graveyard headstone.
See also
Notes
- ↑ If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then called a norite.
ReferencesISBN links support NWE through referral fees
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412 and ISBN 978-0754815419.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060 and ISBN 978-0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321 and ISBN 978-1582381329.
External links
- ODP/TAMU Science Operator. 2006. Igneous Petrology. Integrated Ocean Drilling Program, Texas A&M University. Retrieved March 24, 2007.
- Carey, Bjorn. 2006. Scientists Find the Elusive Gabbro. LiveScience.com, Forces of Nature. Retrieved March 24, 2007.
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