Most metapelites in Scotland experienced Barrovian metamorphism, but in an area just north of Aberdeen, metamorphism occurred at slightly lower pressures than classic Barrovian metamorphism. Metamorphic Grade The extent to which the pre-existing rocks have been changed in form/alteredShale-Country Rock Slate-Low Schist-Medium Gneiss-High Low Grade – slight alteration Medium Grade – significant alteration High Grade – extensive/total alteration About 7.5 cm across. 8.45 Staurolite-muscovite schist, James St. John, Wikimedia Commons The pressure on a swimmer’s ears accrues because of the weight of water pushing down from above. Metamorphic grade is a general term we use to describe the temperature at which metamorphism occurs. Metamorphic petrologists study metamorphic rocks to interpret those histories. The specimen is 6.5 cm across, 8.83 Green omphacite and red pyrope from Nordfjord, Norway. Facies diagrams, such as the one in Figure 8.68, are similar to phase diagrams because they divide P-T space into small areas associated with specific minerals or mineral assemblages. metamorphic definition: 1. 8.47 Blue cordierite with quartz, from Brazil. 8.29 Biotite-quartz gneiss , James St. John, Wikimedia Commons The photo in Figure 8.67 is an example. Therefore, gneiss represents the highest-grade metamorphic rock in the sequence of shale, slate, phyllite, schist, and … The bottom left photo (Figure 8.55) show a marble that contains green forsterite (olivine). Further confusion arises because petrologists use some facies names in a more restricted sense, referring to particular rock types with important tectonic significance. Dehydration reactions and decarbonation reactions, such as the examples in this table, liberate H2O and CO2, respectively. As temperature rises, the zeolite facies gives way to the prehnite-pumpellyite facies, the greenschist facies, the amphibolite facies, and the granulite facies. Muscovite surrounds them. In some places, next to large igneous intrusions, contact metamorphism occurs and extremely high temperatures may persist for short times before the intrusions cool. Chemical reactions of many sorts occur during metamorphism. Schist is a metamorphic rock that comes in almost infinite variety, but its main characteristic is hinted at in its name: Schist comes from the ancient Greek for "split," through Latin and French. 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. The garnet porphyroblasts in Figure 8.10 are 1-2 cm wide. The bands consist of alternating iron- and chert-rich layers. 8.59 Actinolite, Wisconsin Geological Survey Figure 8.47 contains centimeter-sized crystals of blue cordierite. The photos below show some additional examples of high-pressure minerals. A quarter for scale. Eclogites are also found as layers or bands associated with some peridotites. For example, many metamorphic rocks are deep in Earth where pressure and temperature are great. Glaucophane, the inky blue mineral in the lower left photo (Figure 8.82) is an Na-rich amphibole. Because of their tectonic significance, petrologists group high-pressure metamorphic rocks into a class unrelated to rock composition. We saw photos of several examples in Chapter 6 (Figures 6.118, 6.119, and 6.120). 8.58 Hematite from near Marquette, Michigan, 8.60 Grunerite from near Marquette Michigan. Lawsonite has about the same composition as anorthite. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. (Mg,Fe)3Al2Si3O12. Topic. Burial causes rocks to experience lithostatic pressure, also called confining pressure. Garnet schist includes the high-grade index mineral garnet and indicating it has experienced much higher pressures and temperatures than chlorite. In quartzites, the once separate quartz crystals become massive quartz with no visible grain boundaries. They pass through the zeolite, prehnite-pumpellyite, low-pressure greenschist, pyroxene hornfels and sanidinite facies with increasing temperature. Occasionally metamorphism occurs without significant tectonism or magmatism. Despite these problems, the facies concept provides a convenient way to discuss general ranges of pressure and temperature, and it receives wide use. Schists are primarily composed of silicate minerals such as mica (muscovite and biotite), quartz, and feldspar . 8.55 Forsterite marble, modified from gimpf, flickr And, if the protolith contained some clay, micas and other aluminous minerals may be present. The answer to the first question is not known. And in Earth’s mantle, the slow creep of solid rock due to plate tectonics also moves heat by convection. Figures 8.23 and 8.28, earlier in this chapter, also show examples of typical medium-grade metapelites. Typical metamorphic rocks form at pressures of 0 to 10 kbars, but we find higher pressure rocks in some places. This gneiss, from the Czech Republic, contains pink K-feldspar rich layers alternating with darker layers that contain biotite. A schist containing porphyroblasts of K-feldspar would be called a K-spar porphyroblastic schist. 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. Sometimes retrogression only affects parts of a rock or parts of some grains in a rock. So, many metasandstones have unexciting mineralogy, but if the original sandstone contained some clay, any of the minerals that can be in metapelites may be present. Metamorphosed basalts and other rocks of similar composition are commonly called metabasites. The table below summarizes these relationships. Metamorphism may obliterate the original bedding as foliation develops perpendicular to the direction of maximum stress. The banding in the garnet gneiss (Figure 8.30) is not particularly well-developed but is present. For example, the foliation in kyanite gneiss may come from alignment of light-colored kyanite crystals in an otherwise quartz- and muscovite-rich rock. Figure 8.11 compares paths of prograde and retrograde metamorphism. Prograde metamorphism involves the breakdown of minerals stable at lower temperature to form minerals stable at higher temperature. • More complex, low-grade minerals often have difficulty nucleating and growing. 8.51 Kyanite quartzite from Kapteeninautio, Finland. They range in metamorphic grade from slaty shales to phyllitic schists, sand-stones to quartzites, and semi-to medium- crystalline limestones. 8.54 Tremolite and graphite in marble, Ogdensburg-Mineralogical-Society Other examples are found in Precambrian shields, relatively flat-lying areas that may be thousands of kilometers across, that are the exposed roots of ancient mountains. This kind of metamorphism can affect large areas and be part of regional metamorphism, or it can be localized and part of contact metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith. 8.4 Green Mountains schist, James St. John, Wikimedia Commons The characteristic green color comes from fine-grained chlorite and epidote in the rocks. For example, rocks containing kaolinite and quartz are constrained to have formed at temperatures below about 300 °C. For example, when we squeeze a lemon, we are applying directed stress. Schist has medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). At the highest grades, all amphiboles become unstable and dehydrate to produce pyroxenes. The table below lists the most common minerals in metamorphosed granites (also called metagranites). This photo (Figure 8.28) shows a typical schist. Metamorphic rock minerals which indicate the pressure and temperature range that a rock formed at are referred to as. 8.63 Pyrite, James St. John, Wikimedia Commons He observed that the equilibrium mineral assemblage and texture of metabasites vary with pressure and temperature. Temperature and pressure are the most important causes of metamorphism. This rock had a limestone protolith. Individual mineral grains are discernible by the naked eye. Amphibolites contain large grains of black hornblende and whitish plagioclase in subequal proportions. The rocks contain distinctive high-temperature mineral assemblages, which include the relatively rare minerals pigeonite, osumilite, and sapphirine. Gneissic banding most commonly forms in response to directed stress. Figure 8.9, near the beginning of this chapter showed another example. 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. For convenience, we divide the most common rock types into general compositional classes. 8.47 Cordierite with quartz, Density, Wikimedia Commons grade Increasing temperature Slate Schist Hornfels Gnei Blueschist rphic grade . The last photo (Figure 8.56) shows a marble that contains green diopside. 8.28 muscovite schist, anonymous, Wikimedia Commons They are biotite, chlorite and muscovite so this called schistosity texture. Siderite (the brown mineral in Figure 8.62) is an iron carbonate, and pyrite (Figure 8.63) is iron sulfide. It usually has better crystallisation of mica minerals. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. The reaction lines separating the fields show the conditions at which chemical reactions occur. Schist is a medium grade metamorphic rock with medium to large, flat, sheet like grains in a preferred orientation. Shale, siltstone, and some sandstones can provide the parent rock for schist. 8.73 Lizardite with stichtite, James St. John, Wikimedia Commons 8.82 Glaucophane with fuchsite, Didier Descouens, Wikimedia Commons Such reactions may be prograde or retrograde. Other kinds of rocks, especially those that contain little H2O, may remain completely solid to temperatures as great as 1100 °C. This diagram says it could have formed in any of the fields 1 through 5. Serpentine has three polymorphs: antigorite, lizardite, and chrysotile. In such cases, heat from the magma can cause contact metamorphism that affects shallow or surface rocks. It is this luster – which is absent from slate and schist – that really defines a phyllite. For example, a garnet gneiss is a gneiss that contains conspicuous garnet crystals. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. Subsequently, getting the rocks back to the surface so we can see them is even more problematic. 8.24 Aligned crystals of hornblende, Kurt Hollocher At higher grades, metamorphic rocks may develop compositional layering because different minerals concentrate in layers of contrasting colors. Other common blueschist minerals include a colorless to green Na-pyroxene called jadeite, green or white lawsonite, and pale aragonite (the high-pressure polymorph of calcite). High-grade metamorphic rocks tend to be coarse-grained. Compared with metasandstones and metapelites, metabasites are relatively poor in Al and Si and rich in Ca, Mg, and Fe. Iron formations generally contain abundant chert and are often well banded with bands ranging from centimeters to meters thick. Knapsack for scale. The foliation is due to parallel alignment of very small – mostly microscopic – muscovite, chlorite, or other micas, sometimes with graphite. Thus, foliation of phyllites is different from the foliation in slates that stems from clay mineral alignment, and different from foliation in schists because schists always contain visible mica grains. Watch the recordings here on Youtube! What about four minerals together? Gray glassy quartz, white plagioclase, and black biotite are also present. ranges in metamorphic grade from weakly cleaved prehnite-pumpellyite facies greywackes and argillites to thoroughly re- crystallized and multiply deformed upper greenschist facies psammitic and pelitic schists (Brown 1967; Bishop 1972). Oct 1, 2019 - Explore monica de leon's board "Metamorphic rocks" on Pinterest. schist - the size of mineral crystals tends to grow larger with increasing metamorphic grade. They progress through the zeolite, prehnite-pumpellyite, greenschist, amphibolite, and granulite facies. They are secondary minerals in many igneous rocks, and form by hydration of feldspars when water flows through the protolith. Pyrophyllite itself breaks down at higher temperatures – around 400 ̊C, so pyrophyllite is only be stable over a limited range of temperature. Each facies name comes from its most characteristic metabasite minerals or rock types. In some cases gneisses are produced by higher grade metamorphism than schists. High-pressure rocks are rare because to get to very high pressure requires that rocks are buried to great depth – an uncommon occurrence. 8.29 A biotite-quartz gneiss. Quartz and Na-rich plagioclase are in rocks of all grades. Figure 8.20 shows how directed stress can change granite (igneous rock) into gneiss (metamorphic rock). Metamorphic rocks may contain all the minerals common in sedimentary and igneous rocks, plus many minerals exclusive to metamorphic rocks. Schist is faliated medium grade metamorphic rock. The garnets are up to 5 mm across. 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. Schist is a medium-grade metamorphic rock with medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). 8.79 Eclogite from Almenning, Norway. Sometimes, during tectonism, they make it to the surface so we can study them. For example, an amphibole-rich rock that exhibits a gneissic texture is called amphibolite. 8.31 Deformed granitic gneiss, Chmee2, Wikimedia Commons The color is an artifact of the way the photo was taken. They contain the essential minerals pyrope (Mg-rich garnet) and the green Na-rich clinopyroxene called omphacite. It lists all the different 3-mineral assemblages that are stable within fields between reactions. Ultramafic rocks come from Earth’s mantle. 8.65 Greenstone, James St. John, Wikimedia Commons At temperatures over about 450 °C, the only stable minerals are corundum, quartz, and the three Al2SiO5 polymorphs (andalusite, kyanite, sillimanite), but corundum and quartz cannot be found together. Earth’s geothermal gradient, the rate at which temperature increases with depth, averages about 25 to 35̊ C/km near the surface in most places. In addition, for some rock compositions, several different mineral assemblages may be stable within a single facies. The light-colored crystals in Figure 8.80 are lawsonite. It is stable in Field 3 only. The diagram on the right shows the same information, but the reactions are labeled, not the stability fields. H2O- and CO2-free minerals react with fluids to produce hydrous or carbonate minerals. Most metamorphic rocks form when heat, pressure, or chemically reactive fluids cause changes in preexisting rocks. The focus is mostly on prograde minerals, but rocks of any composition may undergo retrograde metamorphism or alteration that produce a variety of low-temperature minerals. This photo (Figure 8.28) shows a typical schist. Rock Type: Metamorphic. 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. Metamorphic minerals and rocks form when rocks undergo changes in chemistry, texture, or composition. The minerals that develop in a metamorphic rock depend upon: 1) the grade of metamorphism 2) the composition of the parent material Schist and gneiss are produced by medium to high grade metamorphism. 8.72 Serpentinite, James St. John, Wikimedia Commons 6.5: Metamorphic … Slates, which form during low-grade metamorphism of shales, comprise primarily microscopic clay grains, perhaps with some minor mica. Low-grade metamorphism takes place at temperatures between about 200 to 320 o C, and relatively low pressure. In this region, metamorphic grade increases from southeast to northwest. If you enlarge the photo you can see the gold. The table below summarizes key mineral assemblages for each facies. 8.61 Greenalite, erocks.com 6.5: Metamorphic Environments As with igneous processes, metamorphic rocks form at different zones of pressure (depth) and temperature as shown on the pressure-temperature (P-T) diagram. 8.1 Zoisite, corundum, and hornblende, James St. John, Wikimedia Commons Muscovite schist contains a slightly higher grade muscovite, indicating a greater degree of metamorphism. The rock also has a strong slaty foliation, which is horizontal in this view, and has developed because the rock was being squeezed during metamorphism. (of rock…. At still higher grades, metamorphism may produce actinolite, grunerite, hedenbergite, or fayalite. Chapter 5: Metamorphic Rocks 5-E1 | 3 than those for schist and represents a low to intermediate grade of metamorphism. Metamorphic Type: Regional: Metamorphic Grade: Middle Grade (Middle P - Middle T) Parent Rock: Shale, Mudstone, or Felsic Igneous rocks: Metamorphic Environment: Middle grade regional metamorphism along a convergent plate boundary 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. a.gneiss b.phyllite c.schist d.slate. En savoir plus. Large and aligned flaky minerals, easily seen with the naked eye, define schists. The specimen is 5.5 cm across, 8.82 Glaucophane with fuchsite from Brittany, France. Aureoles often develop concentric zones or layers, each containing a distinct mineral assemblage that reflects the maximum metamorphic temperature attained and the amount of metasomatism. Metagranites usually contain the minerals that igneous granites contain. At slightly higher grades, metabasites become greenschists, obtaining schistosity from parallel arrangements of the green amphibole actinolite and chlorite. All the minerals considered have the same composition and are related by three reactions: kyanite = andalusite (reaction 1) Many granulites are foliated, but this one is not. Compared with metamorphosed pelites, metamorphosed sandstones, also called metasandstones or metapsammites, are often nondescript. For example, we have samples of diamond-bearing kimberlite, like the specimen seen in this photo, that are unstable and should break down at Earth’s surface. Al2SiO5 = Al2SiO5, kyanite = sillimanite (reaction 3) Figure 8.31, earlier in this chapter, showed an example of a granitic granulite. They come from deep in Earth, and special conditions are required to create them and bring them to Earth’s surface. Low-grade metamorphic rocks tend to be fine-grained (the newly formed metamorphic mineral grains that is). As seen in Figure 8.6, contact metamorphism leads to the development of metamorphic zones called contact aureoles, or skarns, that wrap around an intrusion. These rocks are composed of many distinct minerals. 8.46 Blue kyanite in a schist, James St. John, Wikimedia Commons xxxCa3Al2Si3O12 + SiO2 = CaAl2Si2O8 + 2 CaSiO3, xxxmuscovite + quartz = K-feldspar + sillimanite + vapor Grade: medium; Parent Rock: clay-rich mudstone or shale; Nature of Metamorphism: regional with directed stress ; Impt. 8.36 Blue calcite marble, James St. John, Wikimedia Commons Red lines and text in this phase diagram show the stability fields for the different polymorphs: kyanite at high pressure, sillimanite at high temperature, and andalusite at low pressure. Orthopyroxene may also be present in significant quantities. At higher grades metasandstone may recrystallize with quartz grains growing together and becoming coarser. At still higher grade, chlorite, epidote, and actinolite break down by dehydration reactions, producing a specific kind of rock called an amphibolite. But grain size is also dependent on the grain size of the protolith. Besides talc and kyanite, they contain pyrope (white to pink Mg-garnet), phengite (a white mica related to muscovite), and quartz as major minerals. Sometimes, however, flowing fluids and metasomatism can be the dominant forces controlling metamorphism. These rocks are dark brown and sometimes have a slight sheen due to microscopic grains of biotite. While this was occurring, metamorphism produced wine-red garnet crystals – a single large one is near the left side of the photo and many small ones are scattered throughout. Fluids may also cause significant metasomatism and a significant change in rock chemistry. At low grades, metasandstones typically appear massive and homogeneous, containing light-colored quartz and feldspar grains. In subduction zones (right column, Figure 8.17), generally cooler temperatures are present. The photo above (Figure 8.64) shows a metagranite from the Western Gneiss Region of Norway. Sedimentary compaction and diagenesis contributed little to forming a shape preferred orientation (SPO) within the analysed samples. Protoliths can be igneous, sedimentary, or metamorphic rock of all sorts. Thus, for example, heat is always flowing from Earth’s hot interior to the cooler surface by conduction. If reactions cease before a rock has reached stable equilibrium, the rock is at metastable equilibrium. The pink color in this sample comes from hematite that may have been part of the cement that held the sandstone together. Finally, some metamorphic rocks form by retrograde reactions (metamorphism in response to temperature decrease). It includes all stable minerals and reactions. The table below lists the most common and important minerals in these rocks. The weathered outcrop in Figure 8.72 is a typical occurrence. Click on image to see enlarged photo. 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. Different metamorphic textures characterized different kinds of metamorphic rocks. 8.8 Shatter cones caused by a meterorite impact. I remember the order of the index minerals with the mnemonic "CBGSKS", which I read as "See Bigs kiss". (This sometimes leads to confusion because builders and others use the same word to describe any polished slab of rock.). xxxandalusite = sillimanite 8.23 Garnet-muscovite schist, Graeme Churchard, Wikimedia Commons 8.58 Hematite, ebay.com Petrologists use the term marble for all metamorphic carbonate rocks – rocks that form from limestone or dolostone – dominated by calcite or dolomite. which of the following statements about the metamorphism of shale is false? At the lower end of this range, diagenesis overlaps metamorphism. Minor minerals at all grades include many that are present in mafic igneous rocks. It contains serpentine and chlorite, both hydrous minerals, that formed during metamorphism of a mafic protolith. 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). Granulites form at the highest grades of metamorphism and can form from many sorts of protoliths. Some are present in low-pressure rocks but not in high-pressure rocks. It occurs because of sudden pressure exerted by faults or meteorite impacts. (of rock) changed into a new form and structure by very great heat and pressure: 2. The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade. Like schists, gneisses may also include large crystals of accessory minerals such as garnet. Directed stress, sometimes called differential pressure, is also a force applied to an object, but the force is not the same in all directions. Figure 8.66, below, shows a greenschist from the Homestake Gold Mine in Lead, South Dakota. Like slates, phyllites exhibit fissility. They mostly form at low pressures, too. 8.20 Gneiss, slideserve.com A mineral assemblage is at chemical equilibrium if no such changes are occurring. 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. And, at the highest grades, garnet and pyroxene become stable. Like omphacite, it incorporates its sodium component from albite. Several studies concluded that the Napier rocks were metamorphosed at pressures of 7 to 8 kbar and temperatures of more than 1,000 °C, perhaps as high as 1,075 °C. Share. The metamorphism of limestone or dolostone composed only of carbonate minerals produces few mineralogical changes. 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. More specific names abound – for example, pelitic gneisses form by metamorphism of originally clay-rich sedimentary rocks, granitic gneisses (such as the one shown in Figure 8.31) form by metamorphism of granites, and mafic gneisses form by metamorphism of mafic igneous rocks. Garnet, biotite, and light-colored amphiboles such as anthophyllite or cummingtonite may also be present. We use phase diagrams like the ones seen in Figure 8.39 to show the conditions at which particular minerals or mineral assemblages are stable. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Figure 8.35 shows an outcrop of greenstone in Italy. Above gneiss, when the rock actually starts to melt, it is called a migmatite. Thus, the zone names in this map. Retrograde metamorphism is, in many ways, just the opposite of prograde metamorphism. Accessory minerals such as garnet are common. This kind of pressure is equivalent to the pressure that swimmers feel on their ears when they go to the bottom of the deep end of a swimming pool. These minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in. slate, phyllite, schist, gneiss. For more information contact us at firstname.lastname@example.org or check out our status page at https://status.libretexts.org. Topics similar to or like Schist. Figure 8.34 shows a 9-cm wide sample of greenstone from Ely, Minnesota. Some common examples include the following: biotite schist, garnet-staurolite schist, sillimanite schist, granitic gneiss, mafic gneiss, etc. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock 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. At higher grade, biotite forms. See more ideas about Metamorphic rocks, Metamorphic, Rocks and minerals. Mineral grains may rotate, align, become distorted, or disintegrate. 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. Low-grade minerals are at the top of the table, and grade increases downward. Schist is similar to these topics: Slate, Metamorphic rock, Metamorphic facies and more. 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 diopside marble is the highest grade of the four. Unlike lithostatic pressure, high levels of directed stresses are not sustained for long because rocks deform to reduce the stress. Greenstones, which are a specific kind of hornfels, form by metamorphism of basalts. Metamorphism produced parallel layers of contrasting mineralogy (and color) and subsequent deformation caused the layers to deformed. Quartz usually dominates, and the amounts of other minerals depend on how much clay was in the protolith. 8.52 Marble, James St. John, Wikimedia Commons If the metamorphism is gradual and predictable, we call it progressive metamorphism. However, many granites contain mafic minerals, most commonly biotite and hornblende. Image by C. Geiss. The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade. The felsic light-colored layers typically contain quartz and feldspars, and the more mafic darker layers typically contain biotite, hornblende, or pyroxene. Phlogopite is typically one of the first minerals to form during carbonate metamorphism. Learn more. This facies represents the lowest grade of metamorphism; it is often hard to distinguish zeolite-facies metamorphism from diagenesis. The other asbestos minerals are amphiboles. Plagioclase and augite are stable at all grades but other minerals are not. The minerals in the left column may exist in low-grade rocks, those in the right column are exclusively in high-grade rocks, and the ones in the middle are generally in medium-grade rocks.