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Gneiss

Gneiss is coarse grained and usually banded with alternating layers of light (felsic) and dark (mafic) minerals. These foliation bands are often folded. Gneiss is a product of high grade metamorphism of (above 450ºC) shale, schist, granite or many other rocks. Use the images below to sketch the outcrop and compile your rock description.


This image is a thin section of Lewisan gneiss, not the Achaean gneiss we see on the field trip. Although not the same gneiss, it is a good representation of what we might see.

Notice how the minerals in this thin section have no space between grains. The grains are interlocking, no pore space is available for water storage.