schist metamorphic grade

8.1 Zoisite, corundum, and hornblende, James St. John, Wikimedia Commons The felsic light-colored layers typically contain quartz and feldspars, and the more mafic darker layers typically contain biotite, hornblende, or pyroxene. The table seen here lists the most common minerals in metabasites. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. So, the rate of temperature increase with depth is less than normal. 6.5: Metamorphic … Some investigators believe that a magmatic heat must have been involved. Figure 8.57 photo shows an example of iron formation from western Australia. At higher grades, greenalite, minnesotaite, and glauconite (all iron silicates) may form. What about four minerals together? Epidote, garnet, zoisite, quartz, and other accessory minerals may also be present. 8.61 Greenalite, erocks.com Whether they reach it depends on many things, including temperature, grain size, and reaction kinetics. Serpentine has three polymorphs: antigorite, lizardite, and chrysotile. Metamorphic rocks, and the processes that create them, are key parts of the rock cycle that also includes igneous and sedimentary rocks and processes. Share. Figure 8.66, below, shows a greenschist from the Homestake Gold Mine in Lead, South Dakota. Minor minerals at all grades include many that are present in mafic igneous rocks. Thus, we see that the more minerals present, the more restricted the conditions of formation. How it Forms: Schist is a metamorphic rock, which forms through recrystallization of pre-existing rocks under fairly high temperature and pressure (medium grade metamorphism). The table above lists key mineral assemblages in mafic rocks, but the assemblages will never be present in rocks of other compositions. The reaction lines separating the fields show the conditions at which chemical reactions occur. At still higher grade, chlorite, epidote, and actinolite break down by dehydration reactions, producing a specific kind of rock called an amphibolite. Some metamorphic rocks are named based on the highest grade of index mineral present. Unlike lithostatic pressure, high levels of directed stresses are not sustained for long because rocks deform to reduce the stress. For example, the greenish quartzite seen below (Figure 8.50) contains disseminated green chlorite. Sometimes gneissic banding is deformed, as seen in Figure 8.31. Examples are shown earlier in this chapter in Figures 8.23 and 8.26 through 8.30. 8.49 Quartzite, James St. John, Wikimedia Commons Hydrothermal metamorphism occurs when warm fluids significantly alter protolith rocks. 8.7 Metamorphism along a fault zone. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. When conditions change, metamorphic reactions may create a new paragenesis as some minerals disappear and others grow. Topic. 8.45 Staurolite-muscovite schist, James St. John, Wikimedia Commons This leads to some confusion. We call this kind of metamorphism Buchan metamorphism, named after the Buchan region where it is found. (of rock…. 8.14 Talc-kyanite schist, pinterest.com If the metamorphism is gradual and predictable, we call it progressive metamorphism. They are secondary minerals in many igneous rocks, and form by hydration of feldspars when water flows through the protolith. During metamorphism, K-feldspar recrystallized to form very large pink crystals. The pyrite in this photo is somewhat tarnished. Al2SiO5 = Al2SiO5, andalusite = sillimanite (reaction 2) 8.80 Lawsonite crystals from Marin County, California. Schists, which form under medium-grade metamorphic conditions, contain medium-to-coarse flakes of aligned mica that we can easily see. The effects of contact metamorphism may be profound, because of the high temperature contrast between magmas and upper crustal rocks. All the minerals considered have the same composition and are related by three reactions: kyanite = andalusite (reaction 1) The laws of thermodynamics allow us to predict which minerals form under particular conditions. This diagram says it could have formed in any of the fields 1 through 5. This is because, geologists once called basalts basic rocks. The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are slate, phyllite, schist, and gneiss (Figure 7.8). 8.51 Kyanite quartzite, Siim Sepp Schist exhibits schistosity, which is formed by the alignment of platy medium- to coarse-grained minerals formed under moderate-to high-grade metamorphic conditions. They come from deep in Earth, and special conditions are required to create them and bring them to Earth’s surface. Exceptions, however, do exist (see Box 8.1, below). The most significant causes of metamorphism are mountain building processes (tectonism) that bury, while heating and squeezing, rocks. The photos below show talc (hydrated Mg-silicate), brucite (Mg-hydroxide), and magnesite (Mg-carbonate). The photo above (Figure 8.64) shows a metagranite from the Western Gneiss Region of Norway. 8.78 Blueschist, James St. John, Wikimedia Commons The last photo (Figure 8.56) shows a marble that contains green diopside. It lists all the different 3-mineral assemblages that are stable within fields between reactions. Figures 8.34 and 8.35, earlier in this chapter, showed other examples. They contain the essential minerals pyrope (Mg-rich garnet) and the green Na-rich clinopyroxene called omphacite. Heat transfer by convecting magmas can be much more significant and can warm huge regions of the crust. The characteristic green color comes from fine-grained chlorite and epidote in the rocks. When we stretch a rubber band, we are also applying directed stress. Consider a rock that contains only diaspore. The drawing in Figure 8.19 shows greater stress being applied horizontally than vertically, causing compression in one dimension. Dynamic metamorphism is in a distant third place. Metamorphic rocks in these areas may contain cordierite and andalusite, two low-pressure minerals commonly absent from Barrovian terranes. Typically, these rocks contain serpentine that developed by hydration of olivine. 8.65 Greenstone from Ely, Greenstone Belt, Minnesota, 8.66 Greenschist from the Homestake Mine, South Dakota, 8.67 Amphibolite from the Geopark Prague, Czech Republic. Schists are primarily composed of silicate minerals such as mica (muscovite and biotite), quartz, and feldspar . Sometimes, however, flowing fluids and metasomatism can be the dominant forces controlling metamorphism. Low-grade minerals are at the top of the table and high-grade minerals at the bottom. 8.36 Blue calcite marble, James St. John, Wikimedia Commons Suppose a rock contains diaspore with pyrophyllite. In Field 6, diaspore is not stable and breaks down to corundum and H2O. Schists, which form under medium-grade metamorphic conditions, contain medium-to-coarse flakes of aligned mica that we can easily see. However, most limestones contain some quartz and other minerals besides carbonates. Burial causes rocks to experience lithostatic pressure, also called confining pressure. 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. These minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in. Metamorphism often involves fluids, most commonly water-rich but sometimes dominated by carbon dioxide, sulfur, or other components. Such a low-grade metamorphic rock exhibiting a foliation, but with minerals too small to be seen with the naked eye is called slate. In some deformed rocks, mineral grains assume a distinctive arrangement that gives metamorphic rocks a lineation, long mineral grains all pointing in the same direction, or a foliation, minerals lining up to give a planar fabric. No matter their compositions, they contain conspicuous mineralogy. Slate Properties Specimen of slate. These rocks are know as schist. which of the following statements about the metamorphism of shale is false? 8.53 Phlogopite in marble, earthphysicsteaching.homestead.com A general coarsening of grain size is typical as small mineral grains recrystallize to form larger ones. Thus, for example, heat is always flowing from Earth’s hot interior to the cooler surface by conduction. The high-grade rock shown in Figure 8.10 contains conspicuous cm-sized red garnet, black hornblende, and white plagioclase feldspar. It is characterized by an abundance of platy or elongated minerals (micas, chlorite, talc, graphite, amphiboles) in a preferred orientation. Sometimes they follow a Buchan facies series (lower pressure) and sometimes they follow a Barrovian facies series (higher pressure). The formation of greenstones is said by many to be the beginning of metamorphism. 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. Porphyroblasts are one kind of metamorphic fabric, but there are others. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Sometimes small micas and other dark minerals may be scattered evenly throughout. In some cases gneisses are produced by higher grade metamorphism than schists. The defining characteristics of most gneisses, such as the gneisses seen in Figure 8.29 and Figure 8.30, are that the rocks are medium- to coarse-grained and contain alternating layers of light and dark-colored minerals that give the rock foliation called gneissic banding. Figure 8.9, near the beginning of this chapter showed another example. • Prograde metamorphism liberates fluids not present when retrogression occurs. Rocks that form at high temperatures generally do not have the same problems. 8.30 Garnet gneiss, James St. John, Wikimedia Commons The chart in Figure 8.42 shows typical minerals in metapelites at different grades. In this specimen, a silvery and greenish chrome mica, fuchsite, accompanies the glaucophane. Figure 8.47 contains centimeter-sized crystals of blue cordierite. What is Schist? It has schistose foliation, medium grained, and is abundant in micas (looks Super glittery). But plagioclase (including anorthite and albite components) becomes unstable at high pressure, so the anorthite part hydrates and we get lawsonite instead. 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. Prograde metamorphism occurs when low-grade or unmetamorphosed rocks change mineralogy or texture in response to a temperature increase. However, many granites contain mafic minerals, most commonly biotite and hornblende. So, some rocks related to subduction zones follow the high P/T facies series, experiencing conditions in the zeolite, prehnite – pumpellyite, blueschist, and possibly eclogite facies. The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade. Aureoles and may be anywhere from a few centimeters to many kilometers thick. Within Earth, convection occurs mostly because of flowing water and flowing magmas. (Mg,Fe)3Al2Si3O12. Slate is an extremely dense, fine-grained metamorphic rock form under low-grade regional metamorphism emerged from pelitic sedimentary rocks such as shales and fine-grained tuffs ( Table 6.1 ). Additionally, although not seen in Figure 8.27, layering in some phyllites is deformed, giving the rocks a sort of wavy or crinkly appearance. Gneisses, the highest temperature-pressure kinds of foliated metamorphic rock, typify many regions that have undergone high-temperature metamorphism. See more ideas about Metamorphic rocks, Metamorphic, Rocks and minerals. 8.76 Brucite, Andrew Silver, Wikimedia Commons The diagram on the right shows the same information, but the reactions are labeled, not the stability fields. Figure 8.48 shows pressure-temperature stability fields for common pelitic mineral assemblages. (This sometimes leads to confusion because builders and others use the same word to describe any polished slab of rock.). Metamorphic petrologists study metamorphic rocks to interpret those histories. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions. Hydration reactions and carbonation reactions consume H2O and CO2, respectively. Then the clays begin to revert to the mica minerals from … Schist. The table above lists the most common minerals in metamorphosed ultramafic rocks. 8.77 Magnesite, Prof.lumacorno, Wikimedia Commons 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. Glaucophane, the inky blue mineral in the lower left photo (Figure 8.82) is an Na-rich amphibole. Schist is faliated medium grade metamorphic rock. Mineral grains may rotate, align, become distorted, or disintegrate. It forms because the albite component in plagioclase changes by solid-solid reaction into Na-pyroxene. During shearing, mineral grains can become elongated in one direction, and fractures can develop that give a rock a planar texture. The original rock is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change.The protolith may be a sedimentary, igneous, or existing metamorphic rock. Grade… Metagranites usually contain the minerals that igneous granites contain. Schist is a medium-grained strongly-foliated crystalline metamorphic rock, formed by dynamic metamorphism, that can be readily split into thin flakes or slabs due to the well-developed parallelism of more than 50% of the minerals present, particularly those of lamellar or elongate prismatic habit, e.g., mica and amphiboles. Heat transfer by water can have a significant, although generally quite local, effect. As shown in Figure 8.17, pressure of around 12 kbar is reached at 40 kilometers depth, although pressure depends, in part, on the density of overlying rocks. The two photos above (Figures 8.7 and 8.8) show examples: metamorphism of rocks along a fault zone, and shatter cones created by a meteorite impact in Quebec. These lamellar (flat, planar) minerals include micas, chlorite, talc, hornblende, graphite, and others. Coesite (a high-pressure polymorph of SiO2) and ellenbergite (another high-pressure mineral) are found as small inclusions in some of the pyrope crystals. This property sets it apart from slate. 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. The photo in Figure 8.52 shows a marble that contains only course crystals of white calcite. 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. For example, the foliation in kyanite gneiss may come from alignment of light-colored kyanite crystals in an otherwise quartz- and muscovite-rich rock. Like slates, phyllites exhibit fissility. It is defined by having more than 50% platy and elongated minerals (such as micas or talc), [2] often finely interleaved with quartz and feldspar. Sometimes key minerals are often included in rock names. a.gneiss. Phlogopite is typically one of the first minerals to form during carbonate metamorphism. Magnesium oxides and hydroxides may also form. But petrologists use the same names when talking about rocks of other compositions. The photo on the right above (Figure 8.25) shows foliation (vertical fracture traces) that cuts across a bedding plane separating rock of different compositions. Gneissic banding most commonly forms in response to directed stress. Uncredited graphics/photos came from the authors and other primary contributors to this book. Large and aligned flaky minerals, easily seen with the naked eye, define schists. If you enlarge the photo you can see the gold. Schist is one of the most widespread rock types in the continental crust. When ironstones and iron formations are metamorphosed, they quickly lose any original hydrous minerals. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally gneiss. Kaolinite, pyrophyllite and chloritoid may also be present at low grade but are less common and are omitted from this figure. As temperature rises, the zeolite facies gives way to the prehnite-pumpellyite facies, the greenschist facies, the amphibolite facies, and the granulite facies. 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. Muscovite surrounds them. Metamorphism, which may affect any kind of rock, occurs over a wide range of pressure and temperature conditions. Greenalite (Figure 8.61) is an iron-rich variety of serpentine. A blue amphibole, called glaucophane, is responsible for the name of the facies. So, when a rock is heated or squeezed, chemical reactions occur that may consume old minerals and create new ones. Some are present in low-pressure rocks but not in high-pressure rocks. Less commonly, however, schist may form by metamorphism of fine-grained igneous rocks, such as tuff or basalt. Actinolite, seen in Figure 8.59, is a calcium-iron amphibole. 8.20 Gneiss, slideserve.com 8.24 Aligned crystals of black hornblende give this rock lineation. 8.75 Talc, Siim Sepp 8.22 Fluid inclusions, Kazantseva Mary, Wikimedia Commons Schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. 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. For example, the diagram in this box is modified from Figure 8.40. The garnet porphyroblasts in Figure 8.10 are 1-2 cm wide. The garnets in Figure 8.23 are good examples of porphyroblasts surrounded by groundmass. 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. Shatter cones are akin to the damage that a pebble does when it strikes the windshield of your car. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. 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. They mostly form at low pressures, too. Although hard to see, the specimen contains native gold near the bottom of the sample. In mountain belts and other places where volcanic activity occurs, convective heat flow due to rising magmas contributes much more heat than normal conduction. The banding in the garnet gneiss (Figure 8.30) is not particularly well-developed but is present. 8.52 Marble, James St. John, Wikimedia Commons The diagram also allows us to make predictions: for example, if a rock containing andalusite is metamorphosed at high temperature, the andalusite will change into sillimanite. 8.78 Blueschist from Marin County, California. Schist Schist is medium grade metamorphic rock, formed by the metamorphosis of mudstone / shale, or some types of igneous rock, to a higher degree than slate, i.e. Medium-grade metamorphic rock formed from mudstone or shale. The bounding white lines are really diffuse boundaries and the reactions that relate one assemblage to another are complex. This is Ostwald ripening in action (refer to the discussion in Chapter 4). xxxMg2SiO4 + 2 CO2 = 2 MgCO3 + SiO2. Because of their tectonic significance, petrologists group high-pressure metamorphic rocks into a class unrelated to rock composition. These rocks include mainly blueschists and eclogites, both quite rare. Low-grade metamorphic rocks are often fine grained. Protoliths can be igneous, sedimentary, or metamorphic rock of all sorts. Metabasites are generally more massive and less foliated than pelitic rocks, but at higher grades they do form schist and gneiss. The crystals are up to 2 cm long. Pentti Eskola, a geology professor at the University of Helsinki, introduced the idea of metamorphic facies in 1920. 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. Schist is a medium-grade foliated metamorphic rock. 8.21 Mylonite, anonymous, Wikimedia Commons Figure 8.10 shows a different granulite; it contains hornblende and plagioclase besides large garnets. Whiteschists are unusual rocks that have been reported from multiple different places. Metamorphism often begins with the formation of zeolites, or of prehnite. The photos below show examples of each. In either case, the metamorphism involves hot, generally water-rich fluids that flow through cracks and along grain boundaries. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Gray glassy quartz, white plagioclase, and black biotite are also present. 8.57 Banded iron formation, Graeme Churchard, Wikimedia Commons The Dora Maira rocks were carried to great depth and returned to the surface during the Alpine Orogeny in southern Europe. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. The figure shows a phase diagram of three index minerals—sillimanite, kyanite, and andalusite—with the same chemical formula (Al2SiO5) but having different crystal structures (polymorphism) created by different pressure and temperature conditions. Many schists are medium-grade rocks. Blueschist chemistry is variable. This reaction may occur when a metamorphosed shale is heated to high temperature. Figure 8.36, earlier in this chapter, showed a marble consisting only of blue calcite. xxxAl2Si2O5(OH)4 + 2 SiO2 = Al2Si4O10(OH)2 + H2O, xxxenstatite + 2 H2O = 2 brucite + 2 quartz Schist is a medium-grade metamorphic rock with medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). Eskola based his facies names on minerals and textures of mafic rocks. The need for stability may cause the structure of minerals to rearra… It seems that conditions of low T and high P exist only in special places. Oct 1, 2019 - Explore monica de leon's board "Metamorphic rocks" on Pinterest. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions. 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. 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. Chemical reactions of many sorts occur during metamorphism. 8.8 Shatter cones, jmgas, Wikimedia Commons 8.37 Quartzite, Kurt Hollocher Other petrologists have divided some to more precisely represent pressure and temperature ranges. Varieties of this rock type share similarities in appearance (schistosity) but may be highly variable in composition. Antigorite, the most common serpentine mineral that forms during metamorphism of ultramafic rocks, is stable over a wide range of metamorphic conditions. 8.29 A biotite-quartz gneiss. This foliation, slaty cleavage, gives slates a property called fissility – an ability to break into thin sheets of rock with flat smooth surfaces. The width of an aureole mainly depends on the size of the intrusion and how much fluid (mostly H2O and CO2) it gives off. An intermediate grade metamorphic rock produced from the metamorphism of shale. 8.73 Lizardite with stichtite, James St. John, Wikimedia Commons This occurs when magma that intrudes the crust rises close to, or all the way to, the surface. Like omphacite, it incorporates its sodium component from albite. Petrologists have described blueschists from many places, but the two classic examples of the blueschist facies series are rocks of the Sanbagawa metamorphic belt of Japan and of the Franciscan Complex of California. Pyrophyllite itself breaks down at higher temperatures – around 400 ̊C, so pyrophyllite is only be stable over a limited range of temperature. Some of the reactions give off water vapor, labeled as H2O. The table below lists the most important of these minerals, roughly in order of their appearance in response to increasing metamorphic grade. Magmatic intrusions can affect a large area. Figure \(\PageIndex{1}\): Garnet schist. 8.31 Deformed granitic gneiss, Chmee2, Wikimedia Commons The 1-euro coin is 2.3 cm across, for scale. The table below lists the most common and important minerals in these rocks. The first photo in the block above (Figure 8.53) shows large, somewhat hexagonal, flakes of phlogopite with calcite behind. At high grade, granitic rocks sometimes develop gneissic banding, even if mineralogy has not significantly changed. Retrograde metamorphism is, in many ways, just the opposite of prograde metamorphism. Garnet, biotite, and light-colored amphiboles such as anthophyllite or cummingtonite may also be present. Sometimes, directed stress causes shearing, which means that different parts of a rock slide past each other. Thin sheets of slate have historically been used for paving or roofing stone. The photos in Figures 8.4 and 8.5 below show two outcrops of regional metamorphic rocks. zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Solid-solid reactions involve no H2O, CO2, or other vapor phase. Many metapelitic rocks contain an Al2SiO5 polymorph (andalusite, kyanite, or sillimanite) besides the mineral listed. 8.54 Tremolite and graphite in marble, Ogdensburg-Mineralogical-Society The main and most widely spread metamorphic rocks from the group of low-grade schist metamorphism are argillaceous rocks namely slate, phyllites and schists as shown in Table 6.1. The most classic example of regionally metamorphosed pelites is in the Scottish Highlands where, in the late 19th and early 20th centuries, George Barrow mapped a large region of variable metamorphic grade. Chapter 5: Metamorphic Rocks 5-E1 | 3 than those for schist and represents a low to intermediate grade of metamorphism. Some common examples include the following: biotite schist, garnet-staurolite schist, sillimanite schist, granitic gneiss, mafic gneiss, etc. Geologists generally call metamorphosed carbonate rocks marbles, although this term is used in different ways by building contractors and others. These rocks may contain no visible layering or fractures and appear as a homogeneous mass. In principle, all rocks tend toward stable equilibrium. The greenish color is due to chlorite or epidote that grew during metamorphism. Metamorphic grades describe rocks on a relative scale from less altered to more altered. Learn more. It usually has better crystallisation of mica minerals. Besides the minerals listed, quartz and plagioclase are present under all conditions. The preexisting, or parent rocks, are called protoliths.

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