Gneiss

Gneiss is a deep metamorphic rock with a significant gneissic texture. It generally contains coarse-grained feldspar and quartz (over 50%). The flaky and columnar minerals that make up gneiss are mainly biotite, muscovite, hornblende and pyroxene. Gneiss sometimes contains marker metamorphic minerals such as andalusite, sillimanite, garnet and cordierite. Gneiss is generally named based on the presence of the flaky or columnar minerals plus the feldspar, such as biotite orthoclase gneiss and hornblende plagioclase gneiss. Gneiss can form from igneous rocks, which is known as ‘normal gneiss’, or from sedimentary rocks or metamorphic rocks, which is known as ‘secondary gneiss’. Gneiss can be formed by several processes, and the widely distributed gneiss is a product of regional metamorphism. Contact metamorphism and migmatization can also form gneiss.