Metamorphic rocks, and the processes that create them, are key parts of the rock cycle that also includes igneous and sedimentary rocks and processes. 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. Figure 8.46 contains conspicuous blades of blue kyanite surrounded by quartz, some of which is stained reddish by hematite. Some are present in low-pressure rocks but not in high-pressure rocks. At the lowest grades of metamorphism, magnetite and hematite most commonly dominate. The photos in Figures 8.4 and 8.5 below show two outcrops of regional metamorphic rocks. A mineral assemblage is at chemical equilibrium if no such changes are occurring. Metamorphic mineral reactions and equilibria. In such cases, heat from the magma can cause contact metamorphism that affects shallow or surface rocks. Figure 8.20 shows how directed stress can change granite (igneous rock) into gneiss (metamorphic rock). 8.55 Forsterite marble, modified from gimpf, flickr Figure 8.47 contains centimeter-sized crystals of blue cordierite. Usually, the presence of foliation means the rock was under high pressure that deformed it so that the minerals grew in the direction that the rock was stretched. For example, a garnet gneiss is a gneiss that contains conspicuous garnet crystals. In mountain belts and other places where volcanic activity occurs, convective heat flow due to rising magmas contributes much more heat than normal conduction. Convective heat transfer, which is more efficient than conductive heat transfer, is the transfer of heat due to the flow of material, such as the flow of hot water or hot air. The table below lists the most important of these minerals, roughly in order of their appearance in response to increasing metamorphic grade. Rocks similar to the ones described by Barrow, are found worldwide. The specimen is 5.5 cm across, 8.82 Glaucophane with fuchsite from Brittany, France. At higher grades, greenalite, minnesotaite, and glauconite (all iron silicates) may form. 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. 8.7 Metamorphism along a fault zone. The next three fabric types are foliated. 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. Pyrophyllite itself breaks down at higher temperatures – around 400 ̊C, so pyrophyllite is only be stable over a limited range of temperature. Gneisses are often named based on their protoliths, and petrologists use the general terms orthogneiss for gneisses derived from igneous rocks, and paragneiss for gneisses derived from sedimentary rocks. The banding in the garnet gneiss (Figure 8.30) is not particularly well-developed but is present. Glaucophane, the inky blue mineral in the lower left photo (Figure 8.82) is an Na-rich amphibole. At low grades, metasandstones typically appear massive and homogeneous, containing light-colored quartz and feldspar grains. 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. Metamorphic reactions involve changes in mineralogy or in mineral composition. 8.72 Outcrop of a serpentinite containing primarily antigorite, 8.73 Lizardite with stichtite from Tasmania. Some investigators believe that a magmatic heat must have been involved. garnet, staurolite) Porphyroblast vs. granoblast Granoblastic-crystals that grow in interlocking equant (equidimensional) shapes; typical of monomineralic may grow into large crystals Amphibolites contain large grains of black hornblende and whitish plagioclase in subequal proportions. Reactions between kyanite, andalusite, and sillimanite are only three of many that involve minerals of Al2O3, SiO2, and H2O chemical system. Metamorphic grade is a general term we use to describe the temperature at which metamorphism occurs. A metamorphic rock is an igneous, sedimentary, or another metamorphic rock that has either been squeezed by incredible pressures deep underground and/or has been exposed to very high temperatures, altering its structure, mineral alignment, or chemical composition. Geologists generally call metamorphosed carbonate rocks marbles, although this term is used in different ways by building contractors and others.
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