This page was last edited on 7 June 2020, at 00:13. Augen gneiss is not a pure substance but a mixture of rocks. Gneiss is a metamorphic rock form characterized by banding caused by segregation of different types of rock, typically light and dark silicates.Rather than an indication of specific mineral composition, the term is an indication of texture. A schist containing porphyroblasts of K-feldspar would be called a K-spar porphyroblastic schist. Gneisses of Archean and Proterozoic age occur in the Baltic Shield. Gneiss. [2] The banding is developed under high temperature and pressure conditions. See also: A variety of rocks with a banded or coarsely foliated structure formed by regional metamorphism. Orthogneisses are formed from igneous rocks and paragneisses from sedimentary rocks. Metamorphic rock - Metamorphic rock - Pressure: The pressure experienced by a rock during metamorphism is due primarily to the weight of the overlying rocks (i.e., lithostatic pressure) and is generally reported in units of bars or kilobars. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose rocks have properties similar to gneiss. It is an igneous rock that is a mixture of others. A coarse-grained, dark metamorphic rock; composed mainly of quartz, feldspar, mica, and other minerals corresponding in composition to granite, in which the minerals are arranged in layers. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. [2] The appearance of layers, called 'compositional banding', occurs because the layers, or bands, are of different composition. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation. The standard scientific notation for pressure is expressed in pascals or megapascals (1 pascal is equivalent to 10 bars). a metamorphic rock, foliated, rich in feldspar, and containing smaller quantities of such minerals as quartz, bio-tite, muscovite, amphibole, pyroxene, garnet, and sillimanite. McKirdy, Alan, Roger Crofts and John Gordon (2007). A common cause of the banding 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. Paragneiss is gneiss derived from sedimentary rock (such as sandstone). Paragneiss is gneiss derived from sedimentary rock (such as sandstone). You can change your choices at any time by visiting Your Privacy Controls. [2] These forces stretch out the rock like a plastic, and the original material is spread out into sheets. [5] These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma.[6]. https://en.wikipedia.org/w/index.php?title=Gneiss&oldid=961169389, Short description is different from Wikidata, Articles containing Middle High German (ca. All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. "Kinematics of Late Paleozoic continental collision between Laurentia and Gondwana". Minerals have specific chemical comppositions, that is, minerals are compounds, but rocks are mixtures of two or more minerals and have variable compositions. Gneiss (/ˈnaɪs/) is a common and widely distributed type of metamorphic rock. Augen gneiss, from the German: Augen [ˈaʊɡən], meaning "eyes", is a coarse-grained gneiss 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. Only a small part of the gneiss shows up on the geologic map shown in this website. Gneiss forms at higher temperatures and pressures than schist. The foliation of gneiss also varies. it has been subjected to higher temperatures and pressures than schist. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. As this occurred, fault lines in the Earth's crust allowed magma to force its way to the surface, altering the chemical composition of surrounding mudstone rocks. Chemical - If the general chemical composition can be determined from the mineral assemblage, then a chemical name can be employed. It has deformed into two sequential forms. Blatt, Harvey and Robert J. Tracy (1996). metamorphic rock: Gneiss. CHEMICAL COMPOSITION OF GRANITIC GNEISSES, ETC., N. NORWAY 361 V Baseness In A Sentence,
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