Gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than schist. It is formed by the metamorphosis of granite, or sedimentary rock. Gneiss displays distinct foliation, representing alternating layers composed of different minerals. However, unlike slate and schist, gneiss does not preferentially break along planes of foliation because less than 50% of the minerals formed during the metamorphism are aligned in thin layers. Because of the coarseness of the foliation, the layers are often sub-parallel, i.e. they do not have a constant thickness, and discontinuous.
Gneiss is typically associated with major mountain building episodes. During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes. Gneisses from western Greenland comprise the oldest crustal rocks known (more than 3.5 billion years old). Gneiss is an old German word meaning bright or sparkling.
Other specimens - Click the thumbnails to enlarge
Grain size -medium to coarse grained; can see crystals with the naked eye.
Hardness - hard.
Colour - variable - generally alternating lighter and darker sub-parallel discontinuous bands.
Mineralogy - felsic minerals such as feldspar (orthoclase, plagioclase) and quartz generally form the light coloured bands; mafic minerals such as biotite, pyroxene (augite) and amphibole (hornblende) generally form the dark coloured bands; garnet porphyroblasts common.
Other features - generally rough to touch.
Uses - dimension stone for building facings, paving etc.
New Zealand occurrences - Westland, Fiordland.