Foliation in metamorphic rock
has a reprtition of layers.
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.
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