gneiss

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Coarse-grained metamorphic rock, formed under conditions of high temperature and pressure, and often occurring in association with schists and granites. It has a foliated, or layered, structure consisting of thin bands of micas and/or amphiboles dark in colour alternating with bands of granular quartz and feldspar that are light in colour. Gneisses are formed during regional metamorphism; paragneisses are derived from metamorphism of sedimentary rocks and orthogneisses from metamorphism of granite or similar igneous rocks.

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