1.a laminated metamorphic rock similar to granite
GneissGneiss (nīs), n. [G.] (Geol.) A crystalline rock, consisting, like granite, of quartz, feldspar, and mica, but having these materials, especially the mica, arranged in planes, so that it breaks rather easily into coarse slabs or flags. Hornblende sometimes takes the place of the mica, and it is then called hornblendic gneiss or syenitic gneiss. Similar varieties of related rocks are also called gneiss.
The etymology of the word "gneiss" is disputed. Some sources say it comes from the Middle High German verb gneist (to spark; so called because the rock glitters). It has occurred in English since at least 1757.
Other sources claim the root to be an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.
Gneissic rocks are usually medium- to coarse-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. 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.
Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. These are amongst the oldest rocks in Europe and some of the oldest in the world, having been formed in the Precambrian "super-eon", up to 3 billion years ago. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma. The gneiss's delicate pink colours are exposed throughout the islands and it is sometimes referred to by geologists as "The Old Boy".
Augen gneiss, from the German Augen [ˈaʊɡən], meaning "eye", 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.
Media related to Gneiss at Wikimedia Commons
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