Classification of Metamorphic Rocks

by Russell B. Travis
Web pages adapted from Quarterly of the Colorado School of Mines, vol. 50, no. 1

Color Light Intermediate
(Includes Red or Brown)
Dark
(Includes Green)
Chief Minerals Quartz
Feldspar
Calcite
Dolomite
Talc
Muscovite
Sericite
About Equal Proportions of Light-colored and Dark-colored Minerals Quartz
Calcite
Dolomite
Feldspar
Chlorite
Hornblende
Serpentine
Biotite
Pyroxene
Actinolite
Epidote
Olivine
Magnetite
Characterizing
Accessory
Minerals
Muscovite, Sericite, Sillimanite, Kyanite, Cordierite, Tremolite, Wollastonite, Albite, Andalusite, Garnet, Phlogopite, Diopside, Enstatite, Staurolite, Glaucophane, Anthophyllite, Pyrophyllite, Chloritoid, Actinolite, Tourmaline, Epidote, Chiastolite, Olivine, Serpentine, Chlorite, Biotite, Graphite, Chondrodite, Scapolite
Non-directional
Structure

Massive or Granulose

Contact
Metamorphism*
Fine
Grained
(Aphanitic)
Metaquartzite
Marble
Brucite Marble
Soapstone: chiefly talc
Hornfels: any metamorphic rock with nondirectional structure
Metaquartzite
Marble
Skarn: pyroxene-garnet-carbonate hornfels
Soapstone: chiefly talc
Hornfels: any metamorphic rock with nondirectional structure
Serpentine *
Metaquartzite
Marble
Skarn: pyroxene-garnet-carbonate hornfels
Graphite Marble
Chlorite Marble
Serpentine Marble (Ophicalcite)
Hornfels: any metamorphic rock with nondirectional structure
Serpentine *
Medium or
Coarse Grained
(Phaneritic)
Metaquartzite
Marble
Brucite Marble
Tremolite Marble
Wollastonite Marble
Tremolite Hornfels
Wollastonite Hornfels
Calc-silicate Hornfels: chiefly calc-silicate minerals
Metaquartzite
Marble
Diopside Marble
Chondrodite Marble
Andalusite Hornfels
Skarn: pyroxene-garnet-carbonate hornfels
Garnet Hornfels
Kyanite Hornfels
Anthophyllite Hornfels
Calc-silicate Hornfels
Serpentine *
Cordierite-Anthophyllite Hornfels
Metaquartzite
Marble
Graphite Marble
Chlorite Marble
Olivine Marble
Skarn: pyroxene-garnet-carbonate hornfels
Actinolite Marble
Actinolite-Epidote Hornfels
Actinolite Hornfels
Pyroxene Hornfels
Epidote Hornfels
Andalusite-Biotite Hornfels
Serpentine *
Eclogite: pyrope-omphacite hornfels
Magnetite Rock
Cordierite Hornfels
Amphibolite: chiefly hornblende and/or plagioclase
Directional
Structure

Lineate or Foliate

Mechanical Metamorphism Cataclastic
These rocks are formed by crushing and shearing with only minor recrystallization. If there are no conspicuous directional features, the rock is called "crush breccia" if coarse grained and "cataclasite" if fine grained. Most of them, however, are foliate. Mylonite: finely ground, foliate
Flaser Granite, Flaser Diorite, Flaser Conglomerate, etc.: flaser structure
Augen gneiss: augen structure
Ultramylonite: partially fused mylonite
Rocks with only minor deformation may be called "schistose," for example "schistose sandstone," "schistose rhyolite," etc., but these are not properly metamorphic rocks.
Regional Metamorphism Slaty Most slates are dark colored Silty Slate
Green Slate
Black Slate
Spotted Slate
Andalusite Spotted Slate
Chiastolite Spotted Slate
Biotite Spotted Slate
Carbonaceous Slate
Calcareous Slate
Phyllitic

Phyllite is intermediate between slate and schist. It differs from slate in that crystallization of micaeous minerals imparts a sheen to the rock; it differs from schist in that grains are too small for megascopic identification.

Phyllonite: a phyllite owing its fine grain to mylonitization

Schistose Quartz-Mica Schist
Talc Schist
Sillimanite Schist
Albite-Mica Schist
Quartz-Sericite Schist
Kyanite Schist
Calcite Schist
(Schistose marble)
Mica Schist
Chiastolite Schist
Andalusite Schist
Staurolite Schist
Kyanite Schist
Pyrophyllite Schist
Garnet-Mica Schist
Serpentine
*
Tourmaline-Mica Schist
Anthophyllite Schist
Staurolite-Kyanite Schist
Sillimanite-Garnet Schist
Graphite Schist

Calcite Schist (Schistose marble)
Schistose Quartzite
Greenschist
Chlorite Schist
Chloritoid Schist
Amphibolite
(Hornblende Schist)
Actinolite Schist
Graphite Schist
Pyroxene Schist
Epidote-Chlorite Schist
Hornblende-Biotite Schist
Biotite-Chlorite Schist
Serpentine
*
Tourmaline Schist
Epidote Amphibolite
Garnet-Pyroxene Amphibolite
Garnet-Chlorite Schist
Gneissose

Rhyolite Gneiss
Quartz Porphyry Gneiss
Quartzite Gneiss
Sillimanite Gneiss
Granulite: banding due to elongated quartz or feldspar grains

Granite Gneiss
Syenite Gneiss
Monzonite Gneiss
Granodiorite Gneiss
Anorthosite Gneiss
Trachyte Gneiss
Conglomerate Gneiss
Arkose Gneiss
Sandstone Gneiss
Augen Gneiss: augen structure
Biotite Gneiss
Staurolite Gneiss
Plagioclase Gneiss
Garnet Gneiss
Muscovite-Biotite-Quartz Gneiss
Kyanite Gneiss
Granulite: banding due to elongated quartz or feldspar grains
Quartz Diorite Gneiss
Diorite Gneiss
Gabbro Gneiss
Peridotite Gneiss
Diabase Gneiss
Pyroxene Gneiss
Graywacke Gneiss
Epidote Gneiss
Garnet-Biotite Gneiss
Skarn Gneiss
Amphiolite Gneiss
Plutonic Metamorphism Migmatitic These rocks have a gneissose, streaked, or irregular structure produced by intimate mixing of metamorphic and magmatic materials. When they can be recognized as "mixed rock," they are called migmatite or migmatite gneiss. They may originate by injection (injection migmatite, injection gneiss, or lit-par-lit gneiss) or by differential fusion. Many so-called migmatites probably originate by partial granitization or by metamorphic differentiation. But at great depth these processes apparently do not differ substantially from the igneous processes forming migmatite, so the products are usually indistinguishable.

Migmatites are named by prefixing the rock name of the granitic material to the appropriate root as "granite migmatite," "monzonite injection migmatite," etc.

* Serpentine is a product of hydrothermal alteration which some authorities consider to be an igneous process and others a metamorphic process.
For this reason, serpentine appears both on this chart and on the igneous rock chart.

As can be noted from the chart, naming a metamorphic rock consists chiefly of prefixing the structural term with mineral names or an appropriate rock name. The rock name indicates either the original rock, if recognizable, or the new mineral composition. The prefix "meta," as in "metagabbro," "metasandstone," "metatuff," etc., is applied to rocks that have undergone considerable recrystallization but have largely retained their original fabric. Most of the minerals listed as accessories are genetically important and if present should be included in the rock name regardless of their quantity.