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Gneiss
Gneiss is a rock of metamorphic origin, resulting from the
deformation of arkose sediments or granites. Its composition is
of several minerals, more than 20% of potassium feldspar,
plagioclase, and also quartz and biotite, being therefore
considered essentially quartzfeldspathic. Its grain is often
between medium and coarse; the structure is very variable, from
massive, granitoid and with foliation (given by the flattening
of the grains) to banded, with generally millimetric to
centimetric bands alternating with more mafic ones, derived from
metamorphic segregation processes that culminate in magmatic
rocks.
Gneiss is a clad metamorphic rock identified by its variable
mineral bands and lenses and composition.
Some of these bands (or lenses) contain granular minerals that
come together in a woven texture. Other bands contain layered or
elongated minerals that show a preferred orientation parallel to
the general bands in the rock.
How are
bands formed in a gneiss?
Some evolved directly from the original texture of a rock. For
example, the metamorphism of a protolith made up of alternating
layers of sandstone and shale produces a gneiss made up of
alternating layers of quartzite and mica.
Gneiss bands can also form when the protolith undergoes extreme
shear under conditions where the rock can flow like soft
plastic. Such a flow stretches and bends the pre-existing
texture in the rock and turns them into aligned bands.
Origin and
training
Gneiss is usually formed by regional metamorphism at convergent
plate boundaries. It is a high-grade metamorphic rock in which
mineral grains recrystallize under intense heat and pressure.
This alteration increased the size of the mineral grains and
separated them into bands, a transformation that made the rock
and its minerals more stable in their metamorphic environment.
A gneiss is
described as a rock with the appearance of alternating layers
of different mineral compositions and colors, that is, a
foliated and striped texture (gneiss texture). Gneisses are
characterized by a gneiss texture that is generally
"augen-eyed" or porphyroblastic, and often grainy in texture.
Composition and minerals
Although gneiss is not defined by its composition, most
specimens show bands of feldspar and quartz grains in an
interwoven texture.
These bands are usually light in color and alternate with
bands of darker colored minerals with laminated or elongated
habits. Dark minerals sometimes exhibit an orientation
determined by metamorphic pressures.
Types
of gneiss
Foliated rocks can
be classified according to three types of texture,
corresponding to different degrees of metamorphism. Rocks with
ardosiferous cleavage (such as slate, corresponding to a low
degree of metamorphism); rocks with schistosity (such as
schist, corresponding to a medium degree of metamorphism) and
rocks with a gneiss band (such as gneiss, corresponding to a
high degree of metamorphism). These rocks are formed, in
general, from rocks made up of various minerals and which have
been subjected to conditions of directed stress and increasing
temperatures.
Orthogneiss
If the original rocks contain quartz and feldspar, the
resulting product will be an "orthogneiss" and this is the
case for quartz and clay minerals known as "paragneiss".
In other words, orthogneisses are derived from igneous rocks
and paragneisses originate from sedimentary rocks.
Orthogneisses are derived from an igneous rock.
Orthogneisses are medium to coarse-grained, largely
recrystallized, leaf rocks. The mineral composition of
orthogneiss is similar to that of granites.
The essential minerals are quartz and feldspar (orthoclase,
microcline, orthite and Na plagioclase), and with a
significant amount of biotite and/or muscovite.
 Paragneiss
Paragneiss results from the metamorphism of fine-grained
pelitic clastic sediments (mudstones, shales, siltstones and
other clayey rocks) that have recrystallized deep in the
Earth's crust in metamorphic amphibolite facies.
In other words, they are derived from a mostly pelitic
sedimentary rock. Paragneiss contains quartz and feldspar
and is widely mixed with other minerals typically
sillimanite, kyanite, cordierite, andalusite and staurolite.
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