Gneiss is a medium- to coarse-grained, semischistose metamorphic
rock. It is characterized by alternating light and dark bands
differing in mineral composition (coarser grained than schist). The
lighter bands contain mostly quartz and feldspar, the darker often
contain biotite, hornblende, garnet or graphite. The minerals are
orientated due to schistosity.
In behalf of the origin of the source rock of gneiss has to be
devided between:
orthogneiss is formed by the metamorphism of igneous rocks (igneous
protolith), paragneiss results from the metamorphism of sedimentary
rocks (sedimentary protolith).
Gneiss usually has little tendency to split along its planes, some
para- or orthogneisses split easily to slabs (“Plattengneis”)
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