Editing Metamorphic rock (section)

==Description==
[[File:Staurolite-Almandine-36948.jpg|thumb|Metamorphic rock containing staurolite and almandine garnet]]
Metamorphic rocks are characterized by their distinctive mineral composition and texture.

=== Metamorphic minerals ===
Because every mineral is stable only within certain limits, the presence of certain minerals in metamorphic rocks indicates the approximate temperatures and pressures at which the rock underwent metamorphism. These minerals are known as [[index mineral]]s. Examples include [[sillimanite]], [[kyanite]], [[staurolite]], [[andalusite]], and some [[garnet]].{{sfn|Yardley|1989|pages=8-10}}

Other minerals, such as [[olivine]]s, [[pyroxene]]s, [[hornblende]], [[mica]]s, [[feldspar]]s, and [[quartz]], may be found in metamorphic rocks but are not necessarily the result of the process of metamorphism. These minerals can also form during the [[crystallization]] of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process.<ref>{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut | first2=Cornelius S. Jr. |title=Manual of mineralogy : (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=047157452X |pages=449, 480, 483, 497, 516, 518, 529, 539, 543 |edition=21st}}</ref>

===Texture===
[[Image:Mylonite Strona.jpg|thumb|A mylonite (through a [[petrographic microscope]])]]
Metamorphic rocks are typically more coarsely crystalline than the protolith from which they formed. Atoms in the interior of a crystal are surrounded by a stable arrangement of neighboring atoms. This is partially missing at the surface of the crystal, producing a ''[[surface energy]]'' that makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy.{{sfn|Yardley|1989|p=148-158}}

Although grain coarsening is a common result of metamorphism, rock that is intensely deformed may eliminate [[strain energy]] by recrystallizing as a fine-grained rock called ''[[mylonite]]''. Certain kinds of rock, such as those rich in quartz, [[carbonate mineral]]s, or olivine, are particularly prone to form mylonites, while feldspar and garnet are resistant to mylonitization.{{sfn|Yardley|1989|p=158}}

=== Foliation ===
[[Image:Migma ss 2006.jpg|thumb|250px|Folded foliation in a metamorphic rock from near [[Geirangerfjord]], Norway]]
{{main|Foliation (geology)}}
Many kinds of metamorphic rocks show a distinctive layering called ''[[Foliation (geology)|foliation]]'' (derived from the [[Latin]] word ''folia'', meaning "leaves"). Foliation develops when a rock is being shortened along one axis during recrystallization. This causes crystals of platy minerals, such as [[mica]] and [[Chlorite group|chlorite]], to become rotated such that their short axes are parallel to the direction of shortening. This results in a banded, or foliated, rock, with the bands showing the colors of the minerals that formed them. Foliated rock often develops planes of [[Cleavage (geology)|cleavage]].  [[Slate]] is an example of a foliated metamorphic rock, originating from [[shale]], and it typically shows well-developed cleavage that allows slate to be split into thin plates.{{sfn|Yardley|1989|p=22, 168-170}}

The type of foliation that develops depends on the metamorphic grade.<ref>{{cite web |url=https://geologyglasgow.org.uk/minerals-rocks-fossils/metamorphic-rocks/ |title=Metamorphic Rocks |website=Geological Society of Glasgow |access-date=27 December 2024}}</ref> For instance, starting with a [[mudstone]], the following sequence develops with increasing temperature: The mudstone is first converted to slate, which is a very fine-grained, foliated metamorphic rock, characteristic of very low grade metamorphism. Slate in turn is converted to [[phyllite]], which is fine-grained and found in areas of low grade metamorphism. [[Schist]] is medium to coarse-grained and found in areas of medium grade metamorphism. High-grade metamorphism transforms the rock to [[gneiss]], which is coarse to very coarse-grained.<ref name="Wicander & Munroe">{{cite book | url=https://books.google.com/books?id=7l7FfgRr1TwC&q=shale+phyllite+slate+schist+mudrock+metamorphism&pg=PA175 | title=Essentials of Geology | publisher=Cengage Learning | author=Wicander R. & Munroe J. | year=2005 | pages=174–77 | isbn=978-0495013655}}</ref>

Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated. Marble lacks platy minerals and is generally not foliated, which allows its use as a material for sculpture and architecture.

=== Classification ===
[[Image:MississippianMarbleUT.JPG|thumb|right|[[Mississippian age|Mississippian]] marble in Big Cottonwood Canyon, [[Wasatch Mountains]], Utah]]
Metamorphic rocks are one of the three great divisions of all rock types, and so there is a great variety of metamorphic rock types. In general, if the protolith of a metamorphic rock can be determined, the rock is described by adding the prefix ''meta-'' to the protolith rock name. For example, if the protolith is known to be [[basalt]], the rock will be described as a metabasalt. Likewise, a metamorphic rock whose protolith is known to be a [[Conglomerate (geology)|conglomerate]] will be described as a [[metaconglomerate]]. For a metamorphic rock to be classified in this manner, the protolith should be identifiable from the characteristics of the metamorphic rock itself, and not inferred from other information.{{sfn|Yardley|1989|pp=21-27}}<ref name="BGS">{{cite journal |last1=Robertson |first1=S. |title=BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks |journal=British Geological Survey Research Report |date=1999 |volume=RR 99-02 |url=http://nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf |access-date=27 February 2021}}</ref><ref name="schid-etal=2007">{{cite book |last1=Schmid |first1=R. |last2=Fettes |first2=D. |last3=Harte |first3=B. |last4=Davis |first4=E. |last5=Desmons |first5=J. |year=2007 |chapter=How to name a metamorphic rock. |title=Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks |publisher=Cambridge University Press |location=Cambridge |pages=3–15 |url=https://stuff.mit.edu/afs/athena.mit.edu/course/12/12.115/www/12.114%20Papers/BGS/Metamorphic/DetailedMetamorphic/1%20How%20to%20name%20a%20metamorphic%20rock.pdf |access-date=28 February 2021}}</ref>

Under the [[British Geological Survey|British Geological Survey's]] classification system, if all that can be determined about the protolith is its general type, such as sedimentary or volcanic, the classification is based on the mineral mode (the volume percentages of different minerals in the rock). Metasedimentary rocks are divided into carbonate-rich rock (metacarbonates or calcsilicate-rocks) or carbonate-poor rocks, and the latter are further classified by the relative abundance of mica in their composition. This ranges from low-mica [[psammite]] through semipelite to high-mica [[pelite]]. Psammites composed mostly of quartz are classified as quartzite. Metaigneous rocks are classified similarly to igneous rocks, by [[silica]] content, from meta-ultramafic-rock (which is very low in silica) to metafelsic-rock (with a high silica content).<ref name="BGS"/>

Where the mineral mode cannot be determined, as is often the case when rock is first examined in the [[Field work|field]], then classification must be based on texture. The textural types are:
* [[Schist]]s, which are medium-grained strongly foliated rocks.<ref name="BGS"/> These show the most well-developed schistosity, defined as the extent to which platy minerals are present and are aligned in a single direction, so that the rock easily splits into plates less than a centimeter (0.4 inches) thick.<ref name="schid-etal=2007"/>
* [[Gneiss]]es, which are more coarse grained and show thicker foliation than schists, with layers over 5mm thick.<ref name="BGS"/> These show less well-developed schistosity.<ref name="schid-etal=2007"/>
* [[Granofels]], which show no obvious foliation<ref name="BGS"/> or schistosity.<ref name="schid-etal=2007"/>
A [[hornfels]] is a granofels that is known to result from contact metamorphism. A [[slate]] is a fine-grained metamorphic rock that easily splits into thin plates but shows no obvious compositional layering. The term is used only when very little else is known about the rock that would allow a more definite classification. Textural classifications may be prefixed to indicate a sedimentary protolith (''para-'', such as paraschist) or igneous protolith (''ortho-'', such as orthogneiss). When nothing is known about the protolith, the textural name is used without a prefix. For example, a schist is a rock with schistose texture whose protolith is uncertain.<ref name="BGS"/>

Special classifications exist for metamorphic rocks with a [[volcaniclastic]] protolith or formed along a [[Fault (geology)|fault]] or through [[hydrothermal circulation]]. A few special names are used for rocks of unknown protolith but known modal composition, such as marble, [[eclogite]], or [[amphibolite]].<ref name="BGS"/> Special names may also be applied more generally to rocks dominated by a single mineral, or with a distinctive composition or mode or origin. Special names still in wide use include amphibolite, [[greenschist]], phyllite, marble, [[serpentinite]], eclogite, [[migmatite]], [[skarn]], [[granulite]], mylonite, and slate.<ref name="schid-etal=2007"/>

The basic classification can be supplemented by terms describing mineral content or texture. For example, a metabasalt showing weak schistosity might be described as a gneissic metabasalt, and a pelite containing abundant staurolite might be described as a staurolite pelite.<ref name="BGS"/><ref name="schid-etal=2007"/>

====Metamorphic facies====
{|style="float: right; margin: 10px; border: 1px #CCCCCC solid; background:#F9F9F9"
|{{metamorphic facies to click}}
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|<small>Figure 1. Diagram showing metamorphic facies in [[pressure]]-[[temperature]] space. The domain of the<br> graph corresponds to conditions within the Earth's crust and [[upper mantle (Earth)|upper mantle]].</small> 
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{{Main|Metamorphic facies}}
A metamorphic facies is a set of distinctive assemblages of minerals that are found in metamorphic rock that formed under a specific combination of pressure and temperature. The particular assemblage is somewhat dependent on the composition of that protolith, so that (for example) the amphibolite facies of a marble will not be identical with the amphibolite facies of a pelite. However, the facies are defined such that metamorphic rock with as broad a range of compositions as is practical can be assigned to a particular facies. The present definition of metamorphic facies is largely based on the work of the Finnish geologist, [[Pentti Eskola]], with refinements based on subsequent experimental work. Eskola drew upon the zonal schemes, based on index minerals, that were pioneered by the British geologist, [[George Barrow (geologist)|George Barrow]].{{sfn|Yardley|1989|pp=49-51}}

The metamorphic facies is not usually considered when classifying metamorphic rock based on protolith, mineral mode, or texture. However, a few metamorphic facies produce rock of such distinctive character that the facies name is used for the rock when more precise classification is not possible. The chief examples are [[amphibolite]] and [[eclogite]]. The British Geological Survey strongly discourages the use of ''[[granulite]]'' as a classification for rock metamorphosed to the granulite facies. Instead, such rock will often be classified as a granofels.<ref name="BGS"/> However, this approach is not universally accepted.<ref name="schid-etal=2007"/>
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