Metamorphic minerals in metapelites, metacarbonates, metabasites, metagranites, etc., are all different because of differences in rock chemistry. Because they are hard to study and frequently do not represent chemical equilibrium, many metamorphic petrologists prefer to study higher-grade rocks. The photos below show some additional examples of high-pressure minerals. Many mantle xenoliths, carried up as nodules within magma, are eclogites. 8.53 Phlogopite in marble, earthphysicsteaching.homestead.com These minerals may dehydrate to produce new metamorphic minerals at medium and high grade. They progress through the zeolite, prehnite-pumpellyite, greenschist, amphibolite, and granulite facies. Metamorphic grade is important, not just because different kinds of rocks and minerals form at different temperatures, but because temperature affects chemical reaction rates. For example, the foliation in kyanite gneiss may come from alignment of light-colored kyanite crystals in an otherwise quartz- and muscovite-rich rock. 8.55 Forsterite marble, modified from gimpf, flickr 8.46 Blue kyanite in a schist, James St. John, Wikimedia Commons • At low temperature, reactions are very sluggish; they may not have time to reach equilibrium. Medium-grade metamorphism, forming at temperatures between 400 and about 600 °C, often produces rocks containing conspicuous metamorphic minerals we can easily see and study. 8.79 Eclogite from Almenning, Norway. The laws of thermodynamics say that rocks will change mineralogy in response to increasing temperature (prograde metamorphism), so why don’t they undergo opposite (retrograde metamorphism) changes when temperature decreases as the rock reaches Earth’s surface? With continued increase in metamorphic grade the sequence is phyllite, then schist and finally gneiss. The degree or intensity of metamorphism that has affected a rock, is called grade of metamorphism Tilley has described as four types of grades 1.Very low grade 2.Low grade 3.Medium grade 4.High grade Increasing the grade of metamorphism is also … However, many granites contain mafic minerals, most commonly biotite and hornblende. Metamorphic rock minerals which indicate the pressure and temperature range that a rock formed at are referred to as. Metamorphic petrologists study metamorphic rocks to interpret those histories. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. Most of the examples of metamorphosed ultramafic rocks that we see are in ophiolites, slivers of Earth’s oceanic crust and mantle uplifted and accreted onto continents. Conductive heat transfer occurs when heat flows naturally from a place of high temperature to one of low temperature with no associated movement of matter. It forms because the albite component in plagioclase changes by solid-solid reaction into Na-pyroxene. In effect, such rocks originated as high-grade metamorphic rocks. 8.4 Outcrop of schist, Green Mountains, Vermont, 8.5 Outcrop of gneiss near Sudbury, Ontario. slate, phyllite, schist, gneiss. Missed the LibreFest? If andalusite accompanies the kaolinite and quartz, pressure is restricted to less than 1 kbar. The pyrite in this photo is somewhat tarnished. Slate. The next type of foliation is only seen in the metamorphic rock called slate, which forms by the low temperature and pressure alteration of a shale protolith. metamorphic definition: 1. The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". Geologists use index minerals that They may not go to completion and frequently do not reach stable equilibrium. In subduction zones (right column, Figure 8.17), generally cooler temperatures are present. Schists are primarily composed of silicate minerals such as mica (muscovite and biotite), quartz, and feldspar . Typically, these rocks contain serpentine that developed by hydration of olivine. At medium- or high-grade, Barrovian metamorphism often yields rocks containing kyanite or sillimanite with garnet. Lithostatic pressure is the same in all directions (Figure 8.18), and thus can cause an object to become smaller without altering its overall shape. Figure 8.36, earlier in this chapter, showed a marble consisting only of blue calcite. Regional metamorphism of clay-rich parent rock results in an increase in grain size and preferred planar alignment of large visible platy grains. Eskola originally identified eight facies. Metamorphism, which may affect any kind of rock, occurs over a wide range of pressure and temperature conditions. 8.21 Mylonite, anonymous, Wikimedia Commons The light-colored crystals in Figure 8.80 are lawsonite. Schist exhibits schistosity, which is formed by the alignment of platy medium- to coarse-grained minerals formed under moderate- ... Phyllite is a low- to moderate-grade metamorphic rock that contains aligned platy mica minerals and has slaty cleavage. TEXTURE GRAIN medium Medium Fine Fine Fine coarse Scheme for Metamorphic Rock Identification MAP SYMBOL TYPE OF COMPOSITION METAMORPHISM COMMENTS Low-g rade metamorphism of shale Foliation surfaces … It occurs because of sudden pressure exerted by faults or meteorite impacts. Hydration reactions and carbonation reactions consume H2O and CO2, respectively. Figure 8.34 shows a 9-cm wide sample of greenstone from Ely, Minnesota. Topics similar to or like Schist. If a pluton intrudes a limestone or dolostone, contact metamorphism may cause CO2 to flow out of the carbonate and combine with H2O that comes from the pluton. It is this luster – which is absent from slate and schist – that really defines a phyllite. Schist is a foliated metamorphic rock made up of plate-shaped mineral grains that are large enough to see with an unaided eye. Many granulites are foliated, but this one is not. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. It can, for example, create ore deposits by concentrating minerals (most commonly copper, iron, or lead sulfides) in host rocks where they did not exist previously. The laws of thermodynamics allow us to predict which minerals form under particular conditions. An augen gneiss, such as the gneiss shown in Figure 8.32, contains large feldspar crystals – “eyes” (augen is German for eyes) – stretched in one direction. Wikipedia. Greenstones are fine-grained, very low-grade metabasites that have a conspicuous light- to dark-gray or green color. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. Compositions range from pelitic to mafic. Schist is one of the most widespread rock types in the continental crust. • More complex, low-grade minerals often have difficulty nucleating and growing. 8.29 A biotite-quartz gneiss. At the highest grades, all amphiboles become unstable and dehydrate to produce pyroxenes. What is Schist? Then the clays begin to revert to the mica minerals from … The left column in Figure 8.17 shows temperature-depth relationships for a normal geotherm typical of regions where all heat transfer is by conduction. With even more metamorphism, mafic rocks may become mafic gneisses. As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. At higher grades, greenalite, minnesotaite, and glauconite (all iron silicates) may form. When we stretch a rubber band, we are also applying directed stress. 8.5 Outcrop of gneiss, James St. John, Wikimedia Commons The other asbestos minerals are amphiboles. We find these rocks, typically, as blocks in fault contact with greenschist facies rocks. For example, hornfels are dark colored fine-grained rocks lacking both lineation and foliation. Quartzite, also a common nonfoliated metamorphic rock, forms by metamorphism of sandstone. Other articles where Metamorphic grade is discussed: metamorphic rock: Regional metamorphism: (Metamorphic grades refer to the degree and intensity of the metamorphism: they are determined by the pressure and temperatures to which the rock has been subjected.) Figure 8.47 contains centimeter-sized crystals of blue cordierite. It contains serpentine and chlorite, both hydrous minerals, that formed during metamorphism of a mafic protolith. And, the absence of fluids means that some low-grade minerals cannot form. During progressive metamorphism, a series of reactions occur as the degree of metamorphism increases. The sample is from Norway’s Western Gneiss Region. 6.5: Metamorphic … The table below lists the most common minerals in metamorphosed granites (also called metagranites). Metamorphic foliations range from gneissic to schistose; locally, foliation is lacking and crystallization of metamorphic minerals appears to be static (Till and others, 1986; Lieberman, 1988; Calvert and others, 1999). xxxandalusite = sillimanite For example, pelitic or calcareous rocks do not form greenschists (green mafic schists) or amphibolites (mafic rocks dominated by amphibole and plagioclase) even when metamorphosed at conditions within the greenschist or amphibolite facies. Chlorite schist includes the low-grade index mineral chlorite. Mg-silicates such as forsterite (Mg2SiO4), and enstatite (Mg2Si2O6), for example, may react to form talc or serpentine (both hydrated Mg-silicates), brucite (Mg hydroxide), or magnesite (Mg carbonate), at low temperature. Ultramafic rocks come from Earth’s mantle. The CO2-H2O fluid can have profound effects on the carbonate nearby, and fluid composition controls the formation of many minerals. The fluids act as catalysts and fluxes that promote reactions and large crystal growth. A mineral assemblage is at chemical equilibrium if no such changes are occurring. Cordierite can be difficult to identify unless it has this diagnostic blue bottle glass appearance. At lower pressures, Buchan metamorphism may produce rocks with andalusite, and often cordierite instead of garnet. 8.31 Deformed granitic gneiss, Chmee2, Wikimedia Commons 8.82 Glaucophane with fuchsite, Didier Descouens, Wikimedia Commons The table below summarizes key mineral assemblages for each facies. Topic. Metamorphism often involves fluids, most commonly water-rich but sometimes dominated by carbon dioxide, sulfur, or other components. The answer to the second is that the rocks are of a composition that does not melt easily when water is absent. This produces a quartzite, a hard, nonfoliated metamorphic rock. The garnets in this granulite are only a few millimeters wide at most. Finally, some metamorphic rocks form by retrograde reactions (metamorphism in response to temperature decrease). which of the following sequences describes the metamorphic changes in a shale with increasing metamorphic grade? Click on image to see enlarged photo. The metamorphism of limestone or dolostone composed only of carbonate minerals produces few mineralogical changes. They were never unmetamorphosed rocks at low pressure and temperature. Quick NavTop About High-grade metamorphic rock Classification Sub-divisions References Internet Links Mineral and/or Locality Mindat.org is an outreach project of the Hudson Institute of Mineralogy , a 501(c)(3) not-for-profit organization. Al2SiO5 = Al2SiO5. When metamorphosed, dehydration reactions change clay minerals into new minerals containing less H2O. The drawing in Figure 8.19 shows greater stress being applied horizontally than vertically, causing compression in one dimension. grade Increasing temperature Slate Schist Hornfels Gnei Blueschist rphic grade . Whether they reach it depends on many things, including temperature, grain size, and reaction kinetics. The answer to the first question is not known. Low-grade metamorphic rocks form at low temperatures, generally between 150 and 450 °C. Gray glassy quartz, white plagioclase, and black biotite are also present. It is derived from shale by regional metamorphism. Besides talc and kyanite, they contain pyrope (white to pink Mg-garnet), phengite (a white mica related to muscovite), and quartz as major minerals. The photo below in Figure 8.65 shows a typical greenstone outcrop in northern Minnesota. Slates, which form during low-grade metamorphism of shales, comprise primarily microscopic clay grains, perhaps with some minor mica. 8.50 Quartzite, Gabriel Haute Maurienne, Wikimedia Commons Talc schist The word schist is derived from the Greek word schízein meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie. xxxMg2SiO4 + 2 CO2 = 2 MgCO3 + SiO2. Figure 8.5 shows another example of a deformed gneiss. Further confusion arises because petrologists use some facies names in a more restricted sense, referring to particular rock types with important tectonic significance. The crystals are up to 2 cm long. Within Earth, convection occurs mostly because of flowing water and flowing magmas. There are few places where rocks metamorphosed at that depth are found at Earth’s surface – especially if their protoliths originally came from the shallow crust. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Thus the rade of metamorphism in these areas is generally low except in the vicinity of more highly deformed areas or close to the igneous intrusions, where minerals of a higher metamorphic grade are locally developed. 8.65 Greenstone, James St. John, Wikimedia Commons The pink color in this sample comes from hematite that may have been part of the cement that held the sandstone together. Lawsonite has about the same composition as anorthite. Porphyroblasts are one kind of metamorphic fabric, but there are others. The more common minerals found in such rocks include … At still higher grade, chlorite, epidote, and actinolite break down by dehydration reactions, producing a specific kind of rock called an amphibolite. The different fields are the ranges of pressure and temperature where each polymorph is stable. Low-grade metamorphism or alteration of olivine-bearing rocks often produces a brownish, highly weathered, appearance, such as seen in this outcrop photo (Figure 8.71). The second photo (Figure 8.54) shows gray blades of tremolite in a marble that also contains small (hard to see) specs of graphite. We call such rocks gneisses. The greenish color is due to chlorite or epidote that grew during metamorphism. Glaucophane, the inky blue mineral in the lower left photo (Figure 8.82) is an Na-rich amphibole. He observed that the equilibrium mineral assemblage and texture of metabasites vary with pressure and temperature.
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