Classification of Metamorphic Rocks

Metamorphic rocks can be classified based on several criteria, including origin (protolith), chemical composition, Texture, and mineral composition. Each classification provides different insights into the rock’s formation, the original rock type (protolith), and the environmental conditions under which metamorphism occurred.
 
Texture refers to the size, shape, and arrangement of minerals in the rock, while mineral composition depends on the specific minerals that form during metamorphism. These rocks originate from pre-existing igneous, sedimentary, or earlier metamorphic rocks that undergo changes due to heat, pressure, and chemically active fluids.

Classification of Metamorphic Rocks

Classification of metamorphic rocks depends on what is visible in the rock and its degree of metamorphism. Note that classification is generally loose and practical such that names can be adapted to describe the rock in the most satisfactory way that conveys the important characteristics. Three kinds of criteria are normally employed. These are:
  1. Mineralogical - The most distinguishing minerals are used as a prefix to a textural term. Thus, a schist containing biotite, garnet, quartz, and feldspar, would be called a biotite-garnet schist. A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called a hornblende-pyroxene gneiss. A schist containing porphyroblasts of K-feldspar would be called a K-spar porphyroblastic schist.
  2. Chemical - If the general chemical composition can be determined from the mineral assemblage, then a chemical name can be employed. For example a schist with a lot of quartz and feldspar and some garnet and muscovite would be called a garnet-muscovite quartzo-feldspathic schist. A schist consisting mostly of talc would be called a talc-magnesian schist.
  3. Protolithic -  If a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite.
Classification of Metamorphic Rocks
Classification of Metamorphic Rocks

 

Classification Based on Chemical Composition

Metamorphic rocks can also be classified according to their bulk chemical composition, which is inherited from the original rock. The chemical composition plays a major role in determining the types of minerals that will form during metamorphism. The main chemical components considered include silica (SiO₂), alumina (Al₂O₃), iron (Fe), magnesium (Mg), calcium (Ca), and carbonates.

A. Siliceous Metamorphic Rocks

Siliceous rocks are rich in silica (SiO₂) and are typically derived from siliceous sedimentary rocks (like quartz sandstone) or igneous rocks. Siliceous metamorphic rocks include:

Quartzite:

Formed from quartz-rich sandstone, quartzite is nearly pure silica. It is extremely hard and resistant to chemical weathering.

Gneiss:

Gneiss contains significant amounts of silica due to its quartz and feldspar content. It often originates from igneous rocks like granite or sedimentary rocks like arkose (quartz-rich sandstone).

B. Calcareous Metamorphic Rocks

Calcareous rocks are rich in calcium carbonate (CaCO₃) and form from limestone or dolostone. Common calcareous metamorphic rocks include:

Marble:

Composed mainly of calcite or dolomite, marble forms from the metamorphism of limestone or dolostone.

Skarn:

Skarn forms from the chemical interaction between limestone or dolostone and silica-rich fluids, typically associated with igneous intrusions. It contains a variety of calcium silicate minerals such as garnet, epidote, and diopside.

C. Mafic Metamorphic Rocks

Mafic rocks are rich in iron (Fe) and magnesium (Mg) and typically form from mafic igneous rocks such as basalt and gabbro. Mafic metamorphic rocks include:

Amphibolite:

Amphibolite contains amphibole (hornblende) and plagioclase, and it forms from the metamorphism of basaltic rocks.

Greenschist:

Greenschist is a low-grade mafic metamorphic rock characterized by the presence of green minerals like chlorite, actinolite, and epidote.

D. Pelitic Metamorphic Rocks

Pelitic rocks are rich in aluminum and potassium, often derived from clay-rich sedimentary rocks such as shale or mudstone. Examples of pelitic metamorphic rocks include:

Slate/Phyllite/Schist:

These rocks are rich in aluminum-bearing minerals like mica (muscovite, biotite), garnet, and kyanite, and they form from clay-rich protoliths like shale.

E. Carbonaceous Metamorphic Rocks

Carbonaceous rocks are rich in organic carbon, typically forming from coal or carbon-rich sediments. The most common carbonaceous metamorphic rocks are:

Anthracite:

A high-grade metamorphic form of coal, anthracite has a high carbon content and a metallic luster.

Based on Origin (Protolith)

The protolith refers to the original, pre-existing rock from which the metamorphic rock forms. The nature of the protolith largely influences the mineral composition and texture of the metamorphic rock. Based on protoliths, metamorphic rocks are classified into three broad categories:

A. Metamorphic Rocks from Igneous Protoliths

These rocks form from the metamorphism of igneous rocks (both extrusive and intrusive types).

Granite → Gneiss:

When granite undergoes high-grade metamorphism, it forms gneiss. Gneiss exhibits distinct banding due to the segregation of light and dark minerals (gneissic banding).

Basalt → Amphibolite:

Amphibolite is derived from the metamorphism of mafic igneous rocks like basalt and gabbro. It consists primarily of amphibole (hornblende) and plagioclase feldspar.

Peridotite → Serpentinite:

Serpentinite forms from the metamorphism of ultramafic igneous rocks like peridotite. The primary mineral in serpentinite is serpentine, which gives it a characteristic greenish color.

B. Metamorphic Rocks from Sedimentary Protoliths

These metamorphic rocks originate from sedimentary rocks such as limestone, sandstone, or shale.

Limestone/Dolostone → Marble:

When limestone or dolostone is metamorphosed, it forms marble. Marble consists mainly of calcite or dolomite and is characterized by its crystalline texture and ability to be polished.

Shale → Slate/Phyllite/Schist:

Shale, a fine-grained sedimentary rock, undergoes low-grade metamorphism to form slate, a foliated rock with slaty cleavage. With increasing metamorphism, slate can transition to phyllite and eventually to schist.

Sandstone → Quartzite:

Quartzite forms from the metamorphism of quartz-rich sandstone. It consists almost entirely of quartz, making it a very hard and durable rock.

Bituminous Coal → Anthracite Coal:

Anthracite is a high-grade metamorphic rock formed from bituminous coal. It is a hard, lustrous, and high-carbon rock that burns with a clean flame.

C. Metamorphic Rocks from Pre-existing Metamorphic Protoliths

In some cases, a metamorphic rock can undergo further metamorphism, resulting in another metamorphic rock.

Slate → Phyllite → Schist → Gneiss:

As slate is subjected to progressively higher temperatures and pressures, it can transform into phyllite, then schist, and eventually gneiss, reflecting increasing metamorphic grade.

Classification Based on Mineral Composition

The mineral assemblage that develops in a metamorphic rock is dependent on
  • The pressure and temperature reached during metamorphism
  • The composition of any fluid phase present during metamorphism, and
    •
  • The bulk chemical composition of the rock.

The mineral composition of metamorphic rocks reflects the chemical composition of the protolith and the metamorphic conditions (temperature, pressure, and fluids). Metamorphic rocks are often classified based on the specific minerals they contain, with each mineral stable under a particular set of conditions.

Some terms that describe this general bulk chemical composition are as follows:

  • Quartzo-Feldspathic.  Rocks that originally contained mostly quartz and feldspar like granitic rocks and arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since these minerals are stable over a wide range of temperature and pressure.  Those that exhibit mostly quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic. 
  • Pelitic. These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks.  Because of their high concentrations of alumina they are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.

  • Calcareous.  Calcareous rocks are calcium rich.  They are usually derivatives of carbonate rocks, although they contain other minerals that result from reaction of the carbonates with associated siliceous detrital minerals that were present in the rock.  At low grades of metamorphism calcareous rocks are recognized by their abundance of carbonate minerals like calcite and dolomite. With increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich biotite), chlorite, and tremolite.  At even higher grades anhydrous minerals like diopside, forsterite, wollastonite, grossularite, and calcic plagioclase.

  • Basic.  Just like in igneous rocks, the general term basic refers to low silica content.  Basic metamorphic rocks are generally derivatives of basic igneous rocks like basalts and gabbros.  They have an abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like plagioclase and epidote.

  • Magnesian. Rocks that are rich in Mg with relatively less Fe, are termed magnesian.  Such rocks would contain Mg-rich minerals like serpentine, brucite, talc, dolomite, and tremolite.  In general, such rocks usually have an ultrabasic protolith, like peridotite, dunite, or pyroxenite.

  • Ferriginous. Rocks that are rich in Fe with little Mg are termed ferriginous.  Such rocks could be derivatives of Fe-rich cherts or ironstones. They are characterized by an abundance of Fe-rich minerals like greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades.

  • Manganiferrous. Rocks that are characterized by the presence of Mn-rich minerals are termed manganiferrous.  They are characterized by such minerals as Stilpnomelane and spessartine.

    • Special Cases:

    • Migmatite: A rock at the boundary between metamorphic and igneous, showing partially melted characteristics.
    • Blueschist: Formed under high pressure but relatively low temperature, often associated with subduction zones, characterized by blue minerals like glaucophane.
    • Eclogite: High-pressure metamorphic rock containing garnet and pyroxene, another indicator of subduction zone metamorphism.

    Based on Texture 

    Metamorphic textures are categorized primarily as foliated or non-foliated.

    Foliated Metamorphic Rocks

    Foliation refers to the alignment of minerals within the rock, usually due to directional pressure during metamorphism. This leads to the formation of parallel layers or bands of minerals. Foliated rocks are classified into several types based on the size of mineral grains and the degree of metamorphism.

    Slate

    • Texture: Fine-grained
    • Metamorphic Grade: Low-grade
    • Description: Slate is derived from shale and other clay-rich rocks. It has a fine texture and tends to break into thin sheets along well-developed planes of foliation called slaty cleavage. The minerals in slate, such as mica and chlorite, are typically microscopic.

    Phyllite

    • Texture: Fine to medium-grained
    • Metamorphic Grade: Low to medium-grade
    • Description: Phyllite is similar to slate but has a slightly coarser grain size. It has a silky sheen due to the growth of mica minerals (like muscovite or biotite), giving it a more reflective surface. The foliated texture is more pronounced than in slate.

    Schist

    • Texture: Medium to coarse-grained
    • Metamorphic Grade: Medium-grade
    • Description: Schist is characterized by larger, visible minerals, particularly micas, that form parallel layers or schistosity. The mineral grains, such as biotite, muscovite, or garnet, are visible without magnification. Schists have a wavy, foliated appearance and can exhibit various mineral compositions.

    Gneiss

    • Texture: Coarse-grained
    • Metamorphic Grade: High-grade
    • Description: Gneiss shows well-defined banding of light and dark minerals (called gneissic banding). It forms under higher pressures and temperatures. The minerals like feldspar, quartz, and amphibole often segregate into distinct layers or bands, giving it a striped appearance.

    Non-foliated Metamorphic Rocks

    Non-foliated rocks do not show a layered or banded appearance, typically because they form under uniform pressure or from minerals that do not align easily. These rocks are classified primarily by their mineral composition:

    Amphibolites

    These are medium to coarse grained, dark colored rocks whose principal minerals are hornblende and plagioclase. They result from metamorphism of basic igneous rocks.  Foliation is highly variable, but when present the term schist can be appended to the name (i.e. amphibolite schist).

    Marbles

    These are rocks composed mostly of calcite, and less commonly of dolomite. They result from metamorphism of limestones and dolostones.  Some foliation may be present if the marble contains micas.

    Eclogites

    These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphacite, that result from high grade metamorphism of basic igneous rocks. Eclogites usually do not show foliation.

    Quartzites 

    Quartz arenites and chert both are composed mostly of SiO2.  Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz.   Such a rock is called a quartzite.

    Serpentinites

    Serpentinites are rocks that consist mostly of serpentine.  These form by hydrothermal metamorphism of ultrabasic igneous rocks.

    Soapstones

    Soapstones are rocks that contain an abundance of talc, which gives the rock a greasy feel, similar to that of soap.   Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.

    Skarns

    Skarns are rocks that originate from contact metamorphism of limestones or dolostones, and show evidence of having exchanged constituents with the intruding magma.  Thus, skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant calcium and magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma.  The chemical exchange is that takes place   is called metasomatism.

    Mylonites

    Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust.  They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear.

    Metamorphic rocks can be classified in different ways, depending on their origin (protolith), chemical composition, or mineral composition. Each classification provides insight into the rock’s formation, its parent material, and the specific conditions under which it was metamorphosed. Understanding these classifications helps geologists reconstruct the geologic history of an area and the processes that shaped it.

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