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Geology is the Way

Gneiss

Gneiss is a medium- to high-grade foliated metamorphic rock displaying a coarse-grained banding (also known as gneissose structure). ‘Gneiss’ derives, indeed from the German gneist, ‘spark’, likely a reference to the presence of large grains that reflect light. In structural terms, banding is a foliation with a spacing larger than 1 cm, consisting of coarse-grained minerals that we well visible with the naked eye. A rock which is more fine-grained and more pervasively foliated is a schist (representing also a lower grade metamorphic rock, compared to gneiss). In gneissic rocks, banding is usually an alternation of light-colored layers (typically containing quartz and feldspars) and dark-colored layers (containing biotite, amphibole, garnet etc.). This gneissose structure, or banding, is very rarely a reflection of original sedimentary bedding. Rather, it results from a complete reorganization of the original protolith into a metamorphic fabric through deformation and differentiation of minerals with different composition. The term gneiss has a purely structural connotation (i.e. does not give information about composition) and it is preferred to include the most important metamorphic minerals present as a suffix (e.g. garnet-staurolite gneiss).

Some gneisses show lens- or eye-shaped grains, called augens (from the German word for ‘eye’). The related rock term for a gneiss containing such grain is augen gneiss. A gneiss can also display a strong linear fabric, defined by elongated, coarse-grained mineral grans. In this case, the rock is a lineated gneiss (or pencil gneiss). It is possible to distinguish gneisses based on their protolith: a paragneiss derives from the metamorphism of sedimentary rocks, whereas an orthogneiss from igneous ones. The recognition of the protolith is based on the presence of diagnostic relic structures/grains, rock chemistry, and/or metamorphic minerals.

Since gneiss forms up to very high metamorphic grade, it can possibly experience partial melting. This leads to the formation of migmatites, rocks with relic metamorphic structures and crystallized melt. It is not possible to recognize migmatites based on field data alone, as they resemble many high-grade rocks, including gneisses. A gneiss that experienced partial melting can be called migmatitic gneiss.

Lewisian gneiss with plagioclase-rich white bands and amphibole-rich brown bands. Loch Bealach, Isle of Rum, Scotland. Photo by Robert Stalham via geograph.org.uk.

Folded gneiss with light-colored quartz-feldspar bands and black biotite-rich bands. Mt. Sullivan, Antarctica. Photo by euphro.

Gneiss with white quartz-feldspar bands and dark biotite-rich bands. Bedford, New York State, USA. Photo by James St. John.

Precambrian folded gneiss with white bands of quartz and feldspar, and dark bands with biotite, red garnet, and minor white mica. Width: about 7-8 cm. Joshimath Formation, Joshimath, Uttarakhand State, Himalayan Mountains, northern India. Photo by James St. John.

Augen gneiss with feldspar grains about 4 cm long. Leblon, Rio de Janeiro City, Brazil. Photo by Eurico Zimbres.

Augen gneiss with ‘augens’ of feldspar surrounded by quartz and biotite. La Morcuera gneiss, Miraflores de la Sierra, Madrid, Spain. Photo by PePeEfe.

An augen gneiss from Estonia (glacial erratic from the Finnish Bedrock) with nice pink crystals of K-feldspar. Width of sample 30 cm. Photo Siim Sepp via sandatlas.org.

Sample of lineated orthogneiss with typical pencil structure. Doubravčany, Zásmuky, Czech Republic. Sample from the Department of Geology and Paleontology Comenius University Bratislava. Approximately 10 cm long. Photo by Pelex.

Migmatitic orthogneiss. White bands represent crystallized melt. Limpopo, South Africa. Photo by Jean-François Moyen.

References
Passchier, C. W., Myers, J. S., & Kröner, A. (2012). Field geology of high-grade gneiss terrains. Springer Science & Business Media.

        

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