Difference between revisions of "Gabbro" - New World Encyclopedia
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| − | [[Image:Gabro.jpg|thumb|right|250px| | + | [[Image:Gabro.jpg|thumb|right|250px|'''Gabbro''']] |
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| − | '''Gabbro''' | ||
| + | '''Gabbro''' is a dark, coarse-grained, intrusive [[igneous rock]] chemically equivalent to [[basalt]] because it shares a common origin, [[magma]], with basalt. As magma emerges from the [[mid-ocean ridge]]s the upper layers harden to form basalt while the lower layers trapped beneath the basalt layer cool into the crystalline mass that is the [[Intrusive|plutonic]] rock, gabbro. Vast expanses of the [[Earth]]'s surface are underlain by gabbro within the [[oceanic crust]]. German geologist [[Christian Leopold von Buch]] named gabbro after a town in the [[Italy|Italian]] [[Tuscany]] region. | ||
| + | {{toc}} | ||
This rock often contains valuable amounts of [[sulfide]]s of various [[metal]]s, 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. | This rock often contains valuable amounts of [[sulfide]]s of various [[metal]]s, 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 == | == Composition and grain == | ||
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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]]. | 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. | + | The pyroxene in gabbro is mostly [[clinopyroxene]], and small amounts of [[orthopyroxene]] may be present. (If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then called a [[norite]].) [[Quartz]] gabbros are also known to occur and are probably derived from magma that was oversaturated with [[silica]]. On the other hand, [[essexite]]s 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 | + | Gabbro is generally coarse grained, with crystals in the size range of one millimeter 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 [[cumulate rocks|cumulates]] are essentially coarse grained gabbro, although these may exhibit acicular crystal habits. |
==Formation and distribution== | ==Formation and distribution== | ||
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Gabbro can be formed as a massive uniform intrusion or as part of a layered [[ultramafic intrusions]] as a [[cumulate rocks|cumulate]] rock formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase cumulate. | Gabbro can be formed as a massive uniform intrusion or as part of a layered [[ultramafic intrusions]] as a [[cumulate rocks|cumulate]] rock formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase cumulate. | ||
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==Uses== | ==Uses== | ||
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Gabbro often contains valuable quantities of [[chromium]], [[nickel]], [[cobalt]], [[gold]], [[silver]], [[platinum]], and [[copper]] [[sulfide]]s. | Gabbro often contains valuable quantities of [[chromium]], [[nickel]], [[cobalt]], [[gold]], [[silver]], [[platinum]], and [[copper]] [[sulfide]]s. | ||
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== See also == | == See also == | ||
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* [[Amphibole]] | * [[Amphibole]] | ||
* [[Olivine]] | * [[Olivine]] | ||
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* [[Rock (geology)]] | * [[Rock (geology)]] | ||
* [[Sulfide]] | * [[Sulfide]] | ||
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== References == | == References == | ||
| + | * 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 978-0754815419. | ||
| + | * Pellant, Chris. 2002. ''Rocks and Minerals''. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 978-0789491060. | ||
| + | * Shaffer, Paul R., Herbert S. Zim and Raymond Perlman. 2001. ''Rocks, Gems and Minerals''. New York: St. Martin's Press. ISBN 978-1582381329. | ||
| − | + | ==External links== | |
| + | All links retrieved April 15, 2024. | ||
| − | * | + | * ODP/TAMU Science Operator. 2006. [http://www-odp.tamu.edu/publications/prelim/176_PREL/176IGNE.HTML Igneous Petrology.] Integrated Ocean Drilling Program, Texas A&M University. |
| − | + | * Carey, Bjorn. 2006. [http://www.livescience.com/4095-scientists-find-elusive-gabbro.html Scientists Find the Elusive Gabbro.] LiveScience.com, Forces of Nature. | |
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[[Category:Physical sciences]] | [[Category:Physical sciences]] | ||
Latest revision as of 07:32, 15 April 2024
Gabbro is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt because it shares a common origin, magma, with basalt. As magma emerges from the mid-ocean ridges the upper layers harden to form basalt while the lower layers trapped beneath the basalt layer cool into the crystalline mass that is the plutonic rock, gabbro. Vast expanses of the Earth's surface are underlain by gabbro within the oceanic crust. 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 in gabbro is mostly clinopyroxene, and small amounts of orthopyroxene may be present. (If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then called a norite.) 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 one millimeter 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
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 978-0754815419.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 978-0789491060.
- Shaffer, Paul R., Herbert S. Zim and Raymond Perlman. 2001. Rocks, Gems and Minerals. New York: St. Martin's Press. ISBN 978-1582381329.
External links
All links retrieved April 15, 2024.
- ODP/TAMU Science Operator. 2006. Igneous Petrology. Integrated Ocean Drilling Program, Texas A&M University.
- Carey, Bjorn. 2006. Scientists Find the Elusive Gabbro. LiveScience.com, Forces of Nature.
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