Gneiss
Gneiss is a coarse to medium grained banded
metamorphic rock formed from igneous or sedimentary rocks
during regional metamorphism. Rich in feldspars and quartz,
gneisses also contain mica minerals and aluminous or
ferromagnesian silicates. In some gneisses thin bands of
quartz feldspar minerals are separated by bands of micas;
in others the mica is evenly distributed throughout. Common
orthogneisses (gneisses formed from igneous rocks; those
formed from sedimentary rocks are called paragneisses) are
similar in composition to granite or granodiorite, and some
may have originally been lava flows. Augen gneiss is a
variety containing large eye shaped grains (augen) of
feldspar. Injection gneisses are formed by injection of
veinlets of granitic material into a schist or some other
foliated rock. Banded gneisses called migmatites are
composed of alternating light colored layers of granite or
quartz feldspar and dark layers rich in biotite. Some
migmatites were formed by injection, others by segregation
of quartz and feldspars.
The origin of a gneiss can usually
be determined by its chemical composition and mineral
content. A distinction between gneiss and schist is
difficult to draw, for many gneisses look far richer in
mica than they are, when mica rich parting plane is seen.