Editing Gneiss (section)

===Gneissic banding===
[[File:Pure shear.png|thumb|Pure shear deformation of rock producing gneissic banding. The undeformed rock is shown at upper left, and the result of pure shear deformation at upper right. At lower left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as shown by the arrows. The rock is simultaneously rotated to produce the final configuration, repeated at lower right.]]
The minerals in gneiss are arranged into layers that appear as bands in cross section. This is called gneissic banding.<ref name="Marshak 2009">{{Cite book | last = Marshak | first = Stephen | year = 2013 | title = Essentials of Geology | edition = 4th | publisher = W.W. Norton | isbn = 978-0-393-91939-4 |pages=194–95; Figs. 7.6a–c}}</ref> The darker bands have relatively more [[mafic]] minerals (those containing more [[magnesium]] and [[iron]]). The lighter bands contain relatively more [[felsic]] minerals (minerals such as feldspar or [[quartz]], which contain more of the lighter elements, such as [[aluminium]], [[sodium]], and [[potassium]]).{{sfn|Yardley|1989|p=22}}

The banding is developed at high temperature when the rock is more strongly compressed in one direction than in other directions (''nonhydrostatic stress''). The bands develop perpendicular to the direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers.{{sfn|Blatt|Tracy|1996|p=359}}

A common cause of nonhydrodynamic stress is the subjection of the [[protolith]] (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction.<ref name="Marshak 2009"/> These forces stretch out the rock like a [[Plasticity (physics)|plastic]], and the original material is spread out into sheets. Per the [[polar decomposition theorem]], the deformation produced by such shearing force is equivalent to rotation of the rock combined with shortening in one direction and extension in another.<ref>{{cite book |last1=Fossen |first1=Haakon |title=Structural geology |date=2016 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=9781107057647 |page=38 |edition=Second}}</ref>

Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of [[sandstone]] (lighter) and [[shale]] (darker), which is metamorphosed into bands of [[quartzite]] and mica.<ref name="Marshak 2009"/>

Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.<ref name="Marshak 2009"/>