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Summary:
Gneiss is a medium to coarse-grained banded rock formed as a result of high grade regional metamorphism. It is characterised by a layered appearance caused by the segregation of ferromagnesian minerals from quartzo-feldspathic minerals in discontinuous layers. The ferromagnesian minerals are commonly biotite and/or hornblende with pyroxene being less common. Garnet is often present as an accessory mineral.
Augen gneiss is characterised by very coarse-grained eye-shaped lenticles of quartz and feldspar.
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