Classification Of Metamorphic Rocks - Encyclopedia Britannica

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• Introduction
• Metamorphic variables
  • Temperature
  • Pressure
  • Rock composition
    • Classification into four chemical systems
    • Thermodynamics of metamorphic assemblages
• Metamorphic reactions
  • Reactions in a kaolinite-quartz system
  • Reactions of other mineral systems
  • Isograds
  • Principal types
  • Retrograde metamorphism
• Textural features
  • Major features
  • Lamination
• Structural features
• Metamorphic facies
  • Facies series
  • Pressure-temperature-time paths
• Origin of metamorphic rocks: types of metamorphism
  • Hydrothermal metamorphism
  • Dynamic metamorphism
  • Contact metamorphism
    • Sanidinite facies
    • Pyroxene-hornfels facies
    • Hornblende-hornfels facies
    • Albite-epidote-hornfels facies
  • Regional metamorphism
    • Zeolite facies
    • Prehnite-pumpellyite facies
    • Blueschist facies
    • Eclogite facies
    • Greenschist facies
    • Amphibolite facies
    • Granulite facies
• Distribution of metamorphic rocks
• Classification of metamorphic rocks
  • Schist
  • Slate
  • Gneiss
  • Hornfels
  • Marble
  • Mylonites and cataclastites
  • Other classes

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External Websites

• Tulane University - Metamorphism and Metamorphic Rocks (PDF)
• The Canadian Encyclopedia - Metamorphic Rock
• University of Saskatchewan Pressbooks - Physical Geology, First University of Saskatchewan Edition - Classification of Metamorphic Rocks
• King Saud University - Metamorphic Rocks
• Columbia University - Metamorphic Rocks
• PNAS - Shear heating reconciles thermal models with the metamorphic rock record of subduction
• Texas A and M International University - Identification of Metamorphic Rocks (PDF)
• Geosciences LibreTexts - Metamorphic Rocks
• Open Oregon Educational Resources - Principles of Earth Science - Introduction to Metamorphic Rocks
• Oregon State University - Volcano World - Metamorphic Rocks
• BCcampus Open Publishing - Classification of Metamorphic Rocks
• CUNY Pressbooks Network - Geology 101 for Lehman College (CUNY) - Rocks and Rock Cycle (II): Metamorphism and Characteristics of Metamorphic Rocks

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Classification of metamorphic rocks in metamorphic rock Ask Anything Homework Help Written by Jane Selverstone Professor of Earth & Planetary Sciences, University of New Mexico. Jane SelverstoneAll Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Feb. 3, 2026 •History Britannica AI Ask Anything Table of Contents Table of Contents Ask Anything

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. Any classification of metamorphic rocks tends to stress either their fabric, mineralogy, or primary origin. Some common metamorphic rock types are described here.

Schist

Rocks in which metamorphic minerals are easily seen by eye or hand lens and in which the mineral grains have a highly orientated fabric are called schists. Grains of acicular (needlelike) or platy minerals (e.g., amphiboles and micas) tend to lie with their long directions parallel or their planar directions parallel. Often the rocks show a pronounced mineralogical layering; quartz layers a few millimetres or centimetres in thickness may lie between mica layers, for example. Other words often qualify schist: as described above, greenschist is a schist rich in the green mineral chlorite; blueschist is rich in the blue amphibole, glaucophane; mica-schist is rich in mica; and a graphite-schist is rich in graphite. Schists that are rich in the amphibole hornblende and are often derived by metamorphism of common igneous rocks of the basalt-gabbro type are called amphibolites.

Slate

A very fine-grained metamorphic rock (usually developed from clay-rich sediments) exhibiting perfect planar layering and perfection of splitting into layers (slaty cleavage) is slate. Such rocks are normally rich in micas and chlorites. As the intensity of metamorphism increases, porphyroblasts may grow; such slates are sometimes called spotted slates. As metamorphism proceeds, the average crystal size increases, and mineral segregation develops; the rock then may be termed a phyllite.

Gneiss

A gneiss is produced by intense metamorphism, at high temperature and pressure. The grain size is coarser than that in schists, and layering is often well developed; mineral orientation is less perfect than in schists, however. Very common granitic gneisses of Precambrian areas have been derived from metamorphism of granitic igneous rocks.

Hornfels

The hornfels are formed by contact metamorphism and typically show little sign of the action of directed pressure. They are fine-grained rocks in which crystals display little orientation.

Marble

Rocks derived from the metamorphism of carbonate sediments containing calcite or dolomite are marbles. The main result of metamorphism is an increase in grain size. Because of the rather equidimensional habit of calcite and dolomite crystals, they rarely appear schistose unless they contain other minerals such as mica.

Mylonites and cataclastites

These are rocks in which the texture is the result of ductile shearing or mechanical shattering of grains. They often show only slight, if any, development of new minerals. They form on fault planes or in zones of intense shearing. If the crustal rocks have an appropriate composition, phyllonites may develop where new mica crystals grow parallel to the shearing direction. If shearing is extreme, melting may occur, locally producing a pseudo-tachylite. Tachylite is a term applied to certain types of glass formed by rapid cooling of molten rocks.

Other classes

Most of the above terms indicate structural or fabric classification of metamorphic rocks. Sometimes terms are used to indicate chemical features. Several types of schists, for example, include the following: pelitic schists contain much aluminum oxide and often are derivatives of clay-rich sediments; quartzofeldspathic schists are high in quartz and feldspars and often are derivatives of sandstones or quartz-rich igneous rocks; calcareous schists have a high content of lime (CaO) and often are derivatives of impure limestones, dolomites, or calcareous muds; and mafic schists contain the elements of mafic igneous rocks—namely, calcium, magnesium, and iron.

William S. Fyfe Jane Selverstone