Editing Metamorphic rock (section)

=== 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.