gneisses

Gneiss is a high-grade metamorphic rock characterized by pronounced foliation and distinct compositional banding, produced when minerals segregate into light and dark layers during metamorphism. The grains are typically coarse, reflecting prolonged recrystallization under elevated temperatures and pressures.

Most gneisses form from pre-existing rocks (the protoliths) such as granites, granodiorites, diorites, or sedimentary rocks that are subjected to regional metamorphism associated with mountain-building processes. If partial melting occurs, migmatite forms; otherwise, rock remains a gneiss.

Minerals commonly found include quartz, feldspars, biotite, muscovite and amphiboles; the light bands are quartz and feldspar-rich, the dark bands mafic minerals. In contrast to schist, gneiss typically shows coarser grains and a clear compositional banding rather than only foliation.

Gneisses are widespread in the continental crust and are major components of many ancient metamorphic belts, including parts of the Canadian Shield, the Scandinavian and Scottish Highlands, and other orogenic terranes. They are an important source of insights into the metamorphic history of continents and, in some regions, host ore deposits or mineral resources.