Augen gneiss

The word gneiss has been used in English since at least 1757. It is borrowed from the German word Gneis, formerly also spelled Gneiss, which is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters).

Gneiss is a metamorphic rock that has a distinct banding, which is apparent in hand specimen or on a microscopic scale. Gneiss usually is distinguished from schist by its foliation and schistosity; gneiss displays a well-developed foliation and a poorly developed schistosity and cleavage. Most of the mineral grains of gneissose rocks are visible to the naked eye. Banding in this rock is a result of mineral segregation into separate, typically light- and dark-colored layers; light-colored layers are usually composed of feldspars and quartz and dark-colored layers are usually composed of hornblende and biotite. Individual bands are usually 1-10 mm in thickness. Layers larger than that imply that partial melting or the introduction of new material have probably taken place. Such rocks are called migmatite. In some areas, gneiss grades laterally into granitic rocks. This feature is one of the important factors that have led some to call upon a metamorphic process (granitization) for the development of granitic plutons.

Gneiss can be classified based on minerals that are present, process of formation, chemical composition, and protolith:

Orthogneiss: orthogneiss is formed by the metamorphism of igneous rocks.
Paragneiss: paragneiss is formed by the metamorphism of original sedimentary rocks.
Augen gneiss: A medium- to coarse-grained, banded, regional metamorphic rock composed mainly of quartz and feldspar with hornblende and mica also present in variable quantities, and characterized by large ovoidal megacrysts of feldspar known as "augens" (derived from the German for "eyes").

gneiss2020(5).jpg

Augen-gneiss (feldspar size average 4 cm long). Leblon, Rio de Janeiro City, Brazil. From Eurico Zimbres.

Augen-gneiss with quartz, biotite, and lenses of feldspar. Mount Fairplay, Alaska. From Cochise college.

Gneiss with pink K-feldspar, gray quartz, and black biotite layers. Karelia, Russia. From Sand atlas.

Biotite gneiss with quartz, and black biotite layers. Evje, Norway. From Sand atlas.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by muscovite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen, borded by muscovite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by muscovite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by muscovite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen, borded by Biotite, in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by Biotite, in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by Biotite, in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)
Feldspar augen, borded by Biotite, in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by Biotite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)
Feldspar augen, borded by Muscovite, in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by Muscovite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen, borded by Muscovite, in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a deformed groundmass in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a deformed groundmass in a gneiss. XPL image, 2x (Field of view = 7mm)