Gneiss

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Gneiss

Gneiss (IPA: /ˈnʌɪs/) is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks are coarsely foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. However, depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.

Gneiss resembles granite, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes. If a rock shows minerals occurring in distinct bands, it is probably gneiss.

The word "gneiss" is from an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.

The Acasta Gneiss is the oldest known crustal rock in the world.

[edit] Augen gneiss

Augen gneiss from Rio de Janeiro, Brazil

Augen gneiss is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.

Etymology: from the German augen, meaning "eyes".