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Classification of Metamorphic Rocks

© Geoff Habiger

This generalized classification, from slate to phyllite to schist to gneiss, if often enough for quick field identification, but closer examination of the mineralogy will give you a more detailed and descriptive name. Often the major mineral component(s) of the rock will be added to the rock name. If the major mineral components of a schist were mica and garnet, then the rock would be called a mica garnet schist. A gneiss with large amounts of biotite would be called a biotite gneiss. Sometimes, if the metamorphic rock has a similar mineral composition of an igneous rock, then the rock is given an appropriate descriptor like granite gneiss for a gneiss with a mineral composition similar to that of a granite.

Non-foliated metamorphic rocks are usually named exclusively on the basis of mineral composition. A marble is a granoblastic metamorphic rock composed exclusively of calcite or dolomite. (Granoblastic is a term to describe a metamorphic rock with crystals that are all nearly the same size.) Similarly a quartzite is a granoblastic metamorphic rock composed of quartz. A few of the more common names applied to different metamorphic rocks:

Amphibolite: A medium- to coarse-grained metamorphic rock composed of an amphibole (like hornblende) and plagioclase.

Eclogite: A medium-grained metamorphic rock composed of pyroxene and pyrope-rich garnet. Eclogites share the same mineral composition as basalts.

Granulite: An even-grained metamorphic rock composed of granoblastic minerals like quartz and feldspar.

Hornfels: A fine-grained, thermally metamorphosed rock (no pressure changes were applied) composed of minerals with equidimensional grains in a random orientation.

Serpentinite: A rock composed almost exclusively of minerals from the serpentine group of minerals.

Skarn: An impure marble that contains calcium-silicate minerals such as garnet and epidote.

This list of metamorphic rocks is by no means complete. As with other metamorphic rocks the predominant one or two minerals in the rock can be added to the name as a descriptor, such as garnet hornfels.

The naming and classification of metamorphic rocks is dependent on the textural and structural features of the rock and the rocks mineralogy. The first step in naming a metamorphic rock is to always identify whether the rock is foliated or non-foliated. Once this determination has been made a close examination of the rock will reveal the dominant one or two minerals within the rock. These two steps can then be combined to generate the name for the metamorphic rock.

Glossary
Granoblastic: A descriptive term applied to a metamorphic rock with crystals that are all nearly the same size.