Gneiss is characterized by compositional banding of metamorphic origin. Feldspar and quartz are abundant, while muscovite, biotite, and hornblende are commonly present. Other minerals typical of high-grade regional metamorphism, such as pyroxene and garnet, may also occur.

TEXTURE A medium- to coarse-grained rock characterized by discontinuous, alternating light and dark bands. The presence of quartz and feldspar helps form the lighter bands, which usually have a granular texture. The darker bands of ferro-magnesian minerals tend to be foliated.

ORIGIN This rock forms from the high-grade regional metamorphism of any preexisting rock. The minerals are segregated into bands as a result of high temperatures and pressures. Gneisses may be either meta-sediments or meta-igneous rocks and occur in association with migmatites and granites. Gneiss is thought to comprise much of the lower continental crust.

Group: METAMORPHIC

Grain size: Coarse

Classification: Regional

Pressure: High

Temperature: High

Structure: Foliated, Crystalline