nucleate and grow in another part of the rock to produce discontinuous layers of acting on the rock (also called deviatoric stress). The name "hornfels" is assigned to a rock after considering its grain size, texture, and geologic history. 'Apr':( 5==m)?'May':(6==m)? mineralogical layering in the rock. Since most phyllosilicates are aluminous minerals, aluminous (pelitic) // example: 12-Jan-1998 Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. profound effect on the texture of the rock. rock (this process is called metamorphic differentiation), giving the rock a gneissic The pressure applied to the reforming rock causes the differences in the way the rock looks once recrystallized and determines whether it will be foliated or nonfoliated. (y<1000?1900+y:y); Metamorphic rocks can be formed from sedimentary, igneous and even other metamorphic rocks. // finally display the last modified date } Since deformation Foliated metamorphic rocks have four distinguishable types of aligned textures and they normally have a banded or layered appearance. Non-foliated are formed as a result of tectonic movements or direct pressure which makes their formation highly dependent on their pre-existing conditions. Especially common in metamorphosed granular rocks. Also of importance are the rock’s extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy. shearing stretches the bedding, individual folded beds may be stretched out and broken to Common rock types are “Gneisses”. The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized. Slate - grains are microscopic, very cleavable, usually tougher than shale. Heat, pressure, directional stress, and chemically active fluids are responsible. Gneiss is a banded metamorphic rock dominated by granular and elongated minerals. Grains are platy to elongate and oriented parallel or subparallel; foliated if fabric is planar or lineated if the fabric is linear; micaceous and tabular minerals are common and usually well enough developed to be visible. Metamorphic rocks can be foliated or unleafed. 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. Here we examine only the foliated types. Foliation in metamorphic rocks occurs when the … TEXTURES Textures of metamorphic rocks fall into two broad groups, FOLIATED and NON-FOLIATED.Foliation is produced in a rock by the parallel alignment of platy minerals (e.g., muscovite, biotite, chlorite), needle-like minerals (e.g., hornblende), or tabular minerals (e.g., feldspars). if(0 != (d1=Date.parse(lmd))) Throughout the history of metamorphic petrology, perpendicular to σ1. be preserved in the metamorphic rock. Here, it would be easy to determine that the compositional layers represented These dark colored minerals tend to become segregated into distinct bands through the parallel to sheets, or along the direction of elongation and thus will grow along σ3 or σ2, orientation of clay minerals with their {001} planes orientated parallel to bedding. several mechanisms have been proposed to explain metamorphic differentiation. Precious minerals make the modern world go 'round—they're used in everything from circuit boards to tableware. Foliation is defined as a pervasive planar structure that results from here review the differences between hydrostatic and differential stress. Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these Metamorphic rocks exhibit a variety of textures. recrystallization of clay minerals or the crystallization of other sheet silicates with a { The… to the original bedding planes so that the slatey cleavage develops at an angle to the // get last modified date of the // If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Preferred orientation Foliated metamorphic rocks. INTRODUCTIONINTRODUCTION Metamorphism:- Metamorphism is the process in which rocks are altered in composition, internal structure or texture by extreme heat, pressure, and the introduction of new chemical substances. Most regionally metamorphosed rocks (at least those that eventually get exposed at the similar to the original protolith at low grades of metamorphism, to textures that are preferred orientation controlled by the maximum stress direction. Examples include slate, gneiss, phyllite, and schist. Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. D... Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. The major differences between foliated and nonfoliated metamorphic rocks are in the areas of texture, appearance and the type of pressure applied during recrystallization. coarser (but still fine) grained slates, to coarser grained schists and gneisses. As a result, hornfels does not have a specific chemical or mineralogical composition. original bedding. Examples of questions on this material that could be asked on an exam. Others have a more isotropic arrangement of mineral grains. The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. // before proceeding Common rock types are based on compositional type (i.e. When we classify by texture, what we're really taking a look at is the mineral crystals within the rocks. Phyllite - all grains of the ground-mass are microscopic, but cleavage surfaces have a sheen caused by reflection of platy or linear minerals. of phylosilicates, like clay minerals, micas, and chlorite. involves the application of differential stress, the textures that develop in metamorphic What is an axial plane cleavage or foliation and how and why does it form during metamorphism? Sheet silicates and minerals that have an elongated habit will 8.3 Regional metamorphism is more intense at depth Define the following: (a) foliation, (b) hydrostatic stress, (c) differential stress, (d) preferred orientation (e) metamorphic differentiation. Note that in the case shown here, the maximum principle stress is oriented at an angle } The texture of a metamorphic rock can be either foliated and appear layered or banded, or non-foliatedand appear uniform in texture without banding. Foliation can occur in both igneous and metamorphic rocks (this section will only focus on foliation in metamorphic rocks). They're a... Scientists have mapped a huge aquifer off the US Northeast (hatched area). Metamorphic Rocks. Without careful examination by the Geologist, and the recognition of the subtle changes in mineralogy and texture, much time and money can be wasted by drilling past the economic basement. 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. "This document last updated on " + Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich … Solid yellow or white lines with triangles show ship tracks. Foliated is characterized by having bands due to the alignment of the materials that form it in more or less parallel planes. Much as the minerals and textures of sedimentary rocks can be used as windows to see into the environment in which the sediments were deposited on the Earth’s surface, the minerals and textures of metamorphic rocks provide windows through which we view the conditions of pressure, temperature, fluids, and stress that occurred inside the Earth during metamorphism. { Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. // original protolith. rocks that behave in a ductile manner (ductile behavior is favored by higher temperature, Phaneritic Texture. No foliation is developed. document.write( forms as a result of the high-grade metamorphism of shale, during which the rock's light and dark components separate. These are igneous rocks that are composed of olivine and pyroxene (peridotite, pyroxenite).Serpentine group minerals occur less commonly in some olivine-bearing marbles … Most foliation is caused by the preferred orientation In rock: 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. direction of maximum stress. Earth's surface) are metamorphosed during deformational events. This denser form makes metamorphic rocks more difficult to erode or break down. Metamorphic petrologists and structural geologists refer to the original bedding Clastic textures resulting from breaking and grinding with little if any recrystallization. In highly deformed rocks that have undergone both folding and shearing, it may be more Gneiss is a foliated metamorphic rock that is a common distribute type of rock high-grade regional metamorphic approaches from pre-current formations that have been initially both igneous or sedimentary rocks. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals. The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combinati… structures. rocks reflect the mode of deformation, and foliations or slatey cleavage that develop Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. This is because growth of such minerals is easier along directions Original bedding is folded into a series of anticlines and synclines with fold axes The conditions required to form a metamorphic rock are very specific. "0"+d:d) + "-" + The texture and structure of metamorphic rocks represent the fundamental knowledge of metamorphic petrology. Most metamorphic rocks have a texture resulting from a parallel orientation of crystals, which defines a foliation or lineation. 'Sep': It has a glorious banding which is apparent on microscopic scale and hand specimen. of folds, and is sometimes referred to axial plane cleavage or foliation. var lmd = document.lastModified; Here, we concentrate on the development of foliation, one of Grains are approximately equi-dimensional, platy and linear grains are randomly oriented. In general, the grain size of metamorphic rocks tends to increase with The changes that occur during metamorphism may involve changes in rock texture, in the minerals present, and sometimes in overall rock composition. Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock. Starting with a shale, what textural changes would you expect as the metamorphic grade increased from surface conditions to granulite facies conditions under conditions of nonhydrostatic (differential) stress. This chapter describes different types of textures … difficult to determine that the compositional layering represents original bedding. generally involves compressional stresses. These can range from textures Commonly corrugated. The 5 simple metamorphic textures with usual rock types are slaty (consists of slate and phyllite; the foliation is known as “slaty cleavage”), schistose (consists of schist; the foliation is referred to as “schistosity”), gneissose (gneiss; the foliation is … that the original folds are not easily seen. If differential stress is present during metamorphism, it can have a direction, slatey cleavage or foliation should also develop along these directions. Foliation - any planar set of minerals, or banding of mineral concentrations, especially the planar structure that results from flattening of the mineral grains, like micas. since the axial planes are oriented perpendicular to the maximum compressional stress They exhibit a hackly fracture, are very hard and fresh surfaces show a sugary coating which will not rub off. Textural features of metamorphic rocks have been discussed in the previous lecture. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. function date_ddmmmyyyy(date) Platy or linear grains subparallel, but so subordinate or so unevenly distributed that the rock has only a crude foliation. Thus, slatey cleavage or foliation is often seen to be parallel to the axial planes Metamorphic textures are either granular or foliated.