A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called a hornblende-pyroxene gneiss. Gneiss is formed from sedimentary or igneous rock exposed to temperatures greater than 320 °C and relatively high pressure. Sample of gneiss exhibiting "gneissic banding". Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica. Phyllite Chemical Composition. metamorphic rock: Gneiss. Rocks are naturally occurring solid masses or aggregates of minerals.There are three major types of rocks as sedimentary rocks, … Blatt, Harvey and Robert J. Tracy (1996). The second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest. Phyllite is mainly composed of fine grains of mica minerals that are muscovite, or sericite. 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. Its chemical composition will change depending on where it comes from. In some cases it is a remnant of the initial layering of sedimentary rocks, while in other cases it is a remnant of layered intrusions of igneous material. [2], Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.[2]. The word gneiss has been used in English since at least 1757. The light bands of gneiss are generally composed of quartz quartz, one of the commonest of all rock-forming minerals and … A schist containing porphyroblasts of K-feldspar would be called a K-spar porphyroblastic schist. It is formed by the metamorphosis of Gneiss forms from volcanic rock, shale or granitie. Gneissose rocks have properties similar to gneiss. Gneisses of Archean and Proterozoic age occur in the Baltic Shield. 1050-1500)-language text, Wikipedia articles incorporating a citation from the 1911 Encyclopaedia Britannica with Wikisource reference, Wikipedia articles incorporating a citation from the New International Encyclopedia, Creative Commons Attribution-ShareAlike License. Paragneiss is gneiss derived from sedimentary rock (such as sandstone). Gneiss is a high grade metamorphic rock i.e. 1.2 Composition 1.2.1 Mineral Content It is an igneous rock that is a mixture of others. A coarse-grained metamorphic rock having discontinuous foliation; usually dark; composed mainly of quartz, feldspar, mica, and ferromagnesian minerals. The foliation of gneiss also varies. The standard scientific notation for pressure is expressed in pascals or megapascals (1 pascal is equivalent to 10 bars). It has deformed into two sequential forms. Gneisses which can be metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, and so on. The "gneissic texture" refers to … [2] These forces stretch out the rock like a plastic, and the original material is spread out into sheets. Chemical Composition of Gneiss Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do no longer deliver large quantities of micas, chlorite or different platy minerals. Orthogneiss is gneiss derived from igneous rock (such as granite). We and our partners will store and/or access information on your device through the use of cookies and similar technologies, to display personalised ads and content, for ad and content measurement, audience insights and product development. Gneiss appears to be striped in bands like parallel lines in shape, called gneissic banding. [3], Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. Gneiss. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Augen gneiss is not a pure substance but a mixture of rocks. a metamorphic rock, foliated, rich in feldspar, and containing smaller quantities of such minerals as quartz, bio-tite, muscovite, amphibole, pyroxene, garnet, and sillimanite. Find out more about how we use your information in our Privacy Policy and Cookie Policy. 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. On the other hand Crystals of other metamorphic minerals such as andalusite, biotite, cordierite, garnet, and staurolite might also form within phyllite. Fig, L Simplified map of The Glomfjord Region showing general relation of the Meloy Group to the gra.nite gneisses a.nd … Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Not all gneiss rocks have detectable banding. Gneiss is used to make gravel, pavement slabs, and facing materials. 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. Also fine grained feldspar and quartz are frequently major in phyllite. Only a small part of the gneiss shows up on the geologic map shown in this website. Gneiss (/ˈnaɪs/) is a common and widely distributed type of metamorphic rock. To enable Verizon Media and our partners to process your personal data select 'I agree', or select 'Manage settings' for more information and to manage your choices. 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. A gneiss is produced by intense metamorphism, at high temperature and pressure. 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. Sacks, Paul E. and Donald T. Secor (1990). So far, the evidence for the origin and age of the Swakane terrane is inconclusive. The formation of gneiss is associated with the deep metamorphism of various rocks and sometimes with fusion because of easily fused granite material in these rocks. https://en.wikipedia.org/w/index.php?title=Gneiss&oldid=961169389, Short description is different from Wikidata, Articles containing Middle High German (ca. This page was last edited on 7 June 2020, at 00:13. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. Gneiss forms at higher temperatures and pressures than schist. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. 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. In addition, foliation may be the result of the viscous flow of fused material during metamorphism. All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. You can change your choices at any time by visiting Your Privacy Controls. 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. Yahoo is part of Verizon Media. McKirdy, Alan, Roger Crofts and John Gordon (2007). it has been subjected to higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation. As a result of the flow of material, certain granites can acquire the features of gneiss. It is borrowed from the German word Gneis, formerly also spelled Gneiss, which is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters).[1].
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