Gneiss Rock: Definition, Uses & Formation

Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about gneiss, a foliated metamorphic rock found around the world. It is easily identified by its characteristic alternating layers of minerals known as gneissic banding.

What is Gneiss?

Gneiss (pronounced 'nice') is a globally common type of metamorphic rock that can easily be identified by its alternating layers of minerals known as gneissic banding. It forms as a result of high-grade, regional metamorphism. Metamorphism is the process by which all metamorphic rocks form. Metamorphic rocks form when pre-existing rocks (either igneous, metamorphic or sedimentary) are exposed to extreme temperature and pressure conditions, which causes the minerals in those rocks to recrystallize. When they recrystallize, it is often into new minerals and mineral structures, which creates some very beautiful and unique looking rocks. Gneiss's name is derived from gneist, the German word for spark or sparkle, because of the glittery appearance that many forms of gneiss exhibit. While not as widely recognized as granite or marble as a building material, gneiss can be used for many of the same applications, including countertops, gravestones, and flooring.

This sample of Acasta Gneiss from Canada exhibits a characteristic shiny surface.

Formation of Gneiss

All gneiss forms as a result of high-grade, regional metamorphic conditions. High grade means that the metamorphism occurs at high pressures and at temperatures at or above 320 degrees Celsius. Any water that is present in the minerals pre-metamorphism is frequently lost as the temperature increases, resulting in hard metamorphic rocks that are generally resistant to dissolution in water. Regional means that the metamorphic conditions occur over large geographic areas and include differential (or shearing) stresses, which help to form the layered structure known as foliation. Gneiss rocks exhibit a unique form of foliation known as gneissic banding, which are thicker bands of foliation than most metamorphic rocks display. It is one of the features that helps differentiate gneiss from other foliated rocks. Mineralogically, gneiss tends to include quartz, feldspar, mica, chlorite, and other clay minerals. Some also contain larger crystals imbedded in the rock matrix, most frequently garnet, topaz, and beryl minerals.

This sample of gneiss clearly shows gneissic banding, alternating layers of different colored minerals.

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