gneiss

Gneiss is a high-grade metamorphic rock characterized by pronounced mineral banding or gneissic foliation, typically formed by regional metamorphism of preexisting rocks such as granite or sedimentary rocks.

The characteristic banding results from the segregation of light-colored felsic minerals (quartz and feldspar) and dark-colored mafic minerals (biotite, amphibole, sometimes garnet) into alternating layers. This gives a coarse-grained rock with visible layers.

Formation occurs under high temperatures and pressures, usually at granulite to amphibolite facies, during deep crustal burial and tectonic compression. Common protoliths include granitic rocks, and some gneisses can originate from sedimentary precursors such as quartz arenites or pelites that have been reconstituted.

Distribution and examples: gneiss is widespread in the continental crust, prevalent in ancient shield areas such as the Canadian Shield and Fennoscandian Shield, and in major mountain belts like the Alps and Himalayas. Gneiss typically lacks significant partial melting, which distinguishes it from migmatite.

Economic uses include its use as dimension stone and decorative stone; some varieties can host economically important minerals depending on their specific composition and history.

Overall, gneiss represents a class of coarse, banded metamorphic rocks formed under high-grade conditions, illustrating the deep crustal processes that rework preexisting rocks into new mineral assemblages and textures.