Foliation in Metamorphic Rock

Foliation makes a rock appear striped or streaked, and/or make them able to split into sheets.



Foliation is caused by mineral crystals that are flattened and/or lay parallel to each other, and/or by alterating dark and light-coloured layers. It is caused by differential stress during metamorphism.

Such rocks can be distinguished from each other by their grain size, composition, and the nature of foliation.

 

Slate, for example, has a slaty cleavage, which is caused by the similar orientation of mineral grains. It is formed by differential stress.

Phyllite also has a foliation caused by orientation of minerals. It has a silky luster, called phyllitic luster.

  

Metaconglomerate has a foliation formed by flattened clasts, created by pressure solution and plastic deformation.

Schist also has a foliation, "shistosity", formed by parallel mica flakes caused by differential stress during metamorphism.

Gneiss has a different foliation. It is due to alterating dark and light coloured layers. It is a compositional layering called gneissic banding.

Metamorphic rocks that are sometimes or never foliated include hornfels, amphibolite, marble and quartzite.