These terms describe the rock; for specific minerals in a rock description, you would give measurements in mm or cm. Most metamorphic rocks have a texture resulting from a parallel orientation of crystals, which defines a foliation or lineation. PLAY. As with igneous and sedimentary rocks, metamorphic rocks are classified on the basis of texture (grain size, shape, orientation) and mineral composition. Some rocks may even appear to be non-foliated and banded. (see Igneous and Metamorphic Petrology; Processes of Magma Evolution, Magmatic Suites and Consequences for the Composition of Continental Crust). Rock microstructure includes the texture of a rock and the small scale rock structures. Metamorphic rocks were once igneous or sedimentary rocks, but have been changed (metamorphosed) as a result of intense heat and/or pressure within the Earth’s crust. Schistose. (Source: Belinda Madsen) 6.2.1 Foliation and Lineation. Exposure to these extreme conditions has altered the mineralogy, texture, and chemical composition of these rocks. Metamorphic texture is the description of the shape and orientation of mineral grains in a metamorphic rock. Metamorphic rocks are the product of “metamorphism”, which is the partial or complete recrystallization of rocks. Rock cycle means the process by which rocks are formed, degraded and reformed by the internal geological processes like plutonism, volcanism, uplift etc and/or by external geological process like erosion, weathering, … (i) Slaty structure: The slaty structure is also called slaty cleavage. Basalt is an example of extrusive igneous rocks. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance. Metamorphic rock is classified by texture and composition. However, the classifiction of metamorphic rocks is more difficult to get into than with igneous and sedimentary rocks, for a couple of reasons. Grain shapes 2. Schist is finely foliated whereas gneiss is organized in noticeable, wide bands of minerals. This is a set of photomicrographs illustrating metamorphic textures, arranged as a slide show. Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized. (Source: Belinda Madsen) 6.2.1: Foliation and Lineation. porphyroblastic. The original rock (called the “protolith”) is either an igneous or sedimentary rock. Metamorphic rock identification table. The dark bands are amphibole-rich, the light bands are feldspar-rich. Foliated rocks can be ordered in terms of increasing metamorphism. These rocks are formed by the settling down of sediments. The texture of a rock provides a clue whether the magma cooled fast or slowly and where the rock was formed. Throughout their cooling history all magmatic rocks try to achieve mechanical and chemical equilibrium, but rarely succeed. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith. The foliated rock types that form under these conditions are called schist or gneiss, depending on their texture. Metamorphic texture is the description of the shape and orientation of mineral grains in a metamorphic rock. Grain orientations 4. Others have a more isotropic arrangement of mineral grains. cryptocrystalline. Metamorphic rocks are the third great class of rocks. Metamorphic Microstructures. Metamorphic rock Texture and Structure of Metamorphic Rocks Texture Grain size from EASC 1402 at The University of Hong Kong Allothimorph —a constituent of a metamorphic rock that has not changed its original crystal outline. Learn vocabulary, terms, and more with flashcards, games, and other study tools. The following are textures, or terms used in relation to textures, of metamorphic rocks; * indicates an illustration in the cross-reference. even in the microscope, individual crystals cannot be discerned. Metamorphic rocks are classified as either foliated or non foliated depending on their mineral structure and pattern; Metamorphic rocks are rocks that have changed from one type of rock to another. • Texture – microscopic to small-scale mesoscopic features of the rock produced by: 1. METAMORHISM is the term used to express the process responsible for all the changes that take place in an original rock under the influence of changes in the surrounding conditions of temperature, pressure and chemically active fluids. Earth Sciences 2: Introduction to Metamorphic Rocks. The changed rock is called the metamorphic rock and it will be stable under the new set of conditions till there is a further change in those conditions. You can either follow the sequence through in order, or else scroll down the list below (which roughly follows the outline of lecture 3) and click on individual terms. Such features include folds, nappes, and faults with a wide variety of geometries. The texture of metamorphic rocks is foliated due to pressure. Slate. Metamorphic rocks typically have different mineral assemblages and different textures from their parent rocks (Figure 7.1) but they may have the same overall composition. Allotrioblast(ic) —see Xenoblast(ic) Atoll —see * Atoll garnet. Foliated metamorphic rocks. Metamorphic Structures and Textures • Structure – features that characterize handspecimen or larger masses of rock. aphanitic. grains too small to see without a microscope, but rock isn't glassy. These are generally grainy. Metamorphic rock - Metamorphic rock - Classification of metamorphic rocks: Because of the diverse chemistry, mineralogy, and primary origin of metamorphic rocks and because of the diverse fabrics or textures that may develop depending on the stresses that may operate during their formation, there is no simple, universally used classification of these rocks. Metamorphic rock textures are foliated, non-foliated, or lineated are described below. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. The rock possessing slaty cleavage has a unique property of splitting into thin sheets. Metamorphic rock textures are foliated, non-foliated, or lineated are described below. Non-foliated rocks occur when heat is high, but pressure is low or equal on all sides. Grain distributions 5. They are crystalline and often have a “squashed” (foliated or banded) texture. Structure and Texture Anastasia Neni Metamorphic structure • Foliation; planar rock fabric (e.g. Cleavage Planar Fabric of Preferred Fracture, sub-parallel orientation, non-penetrative. STUDY. If the transformation is incomplete we can observe a serpentine mesh structure composed of a lattice work of longitudinally divided cross-fibre veins enclosing olivine cores. The rate of crystal growth is another factor, and that's how quickly new constituents come to the surface of the growing crystal. Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels. Like water cycle, there exist rock cycle (geological cycle) in geology. Metamorphic rocks are rocks that have changed from igneous rocks or metamorphic rocks. The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. Sediments are the smaller particles / fragments that are formed by the breaking … Foliation is a term used that describes minerals lined up in planes. The Deccan Plateau is made up of basalt rocks. Grain sizes 3. Metamorphic. Relict igneous textures in metamorphic rocks are relatively common, mostly requiring that deformation has been limited enough to allow them to survive. It all comes down to the rate at which the rock cools. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The structures of the metamorphic rocks are: (i) Slaty structure, (ii) Schistose structure, (iii) Gneissose structure, and (iv) Granulose structure. Metamorphic rocks are rocks whose structure and appearance have been affected or changed by greater heat, pressure and chemical processes, usually while buried deep below the Earth’s surface or where tectonic plates meet. The texture of sedimentary rocks depends on their clast, age, and depositional setting. grains visible with the unaided eye. microcrystalline . Gneiss rocks exhibit a unique form of foliation known as gneissic banding, which are thicker bands of foliation than most metamorphic rocks display. burial, regional metamorphic rocks) – Slaty cleavage – Phyllitic – Schistose – Gneissose – Mylonitic • Non Foliation; equidimentional rock fabric (e.g. Textural features of metamorphic rocks have been discussed in the previous lecture. Foliated metamorphic rocks appeared banded or layered. The nomenclature of metamorphic rocks is classified according to various criteria, especially texture, composition, and P–T conditions of metamorphism. These rocks undergo a change, … While sedimentary rock is formed from sediments, and igneous rock is formed from molten magma, metamorphic rock is rock made from pre-existing rocks. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature. 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