What is Gneiss?
Gneiss is a common distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. It is often foliated (composed of layers of sheet-like planar structures). The foliations are characterised by alternating darker and lighter colored bands, called "gneissic banding". Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture. Other bands contain platy or elongate minerals with evidence of preferred orientation. It is this banded appearance and texture - rather than composition - that define a gneiss. Gneiss is a high grade metamorphic rock which has been subjected to high pressure and temperature formed from the pre-existing rocks either from the igneous or sedimentary rock. Gneiss is composed of foliation which is represented by bands of dark and light colour. In gneiss less than 50% of minerals are aligned in thin foliated layers and unlike slate and schist, it does not break along planes of foliation.
How does Gneiss forms?
Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallised under intense heat and pressure. This alteration increased the size of the mineral grains and segregated them into bands, a transformation which made the rock and its minerals more stable in their metamorphic environment.
Gneiss can form in several different ways. The most common path begins with shale, which is a sedimentary rock. Regional metamorphism can transform shale into slate, then phyllite, then schist, and finally into gneiss. During this transformation, clay particles in shale transform into micas and increase in size. Finally, the platy micas begin to recrystallise into granular minerals. The appearance of granular minerals is what marks the transition into gneiss.
Intense heat and pressure can also metamorphose granite into a banded rock known as "granite gneiss." This transformation is usually more of a structural change than a mineralogical transformation. Granite gneiss can also form through the metamorphism of sedimentary rocks. The end product of their metamorphism is a banded rock with a mineralogical composition like granite.
Gneiss is associated with mountain building (orogeny)
Gneiss is associated with major mountain building events. In these events sedimentary or felsic igneous rocks are subjected to high pressure and temperature from the great burial depths, igneous intrusions and tectonic forces generated in these events.
Composition of gneiss
Gneiss is composed of felsic minerals such as felspar (orthoclase and plagioclase) and quartz from the light coloured bands. From the dark coloured bands mafic minerals are present such as biotite, pyroxene and amphibole. Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.
Gneissic banding
Gneiss appears to be striped in bands, called gneissic banding. The banding is developed under high temperature and pressure conditions.
The minerals are arranged into layers that appear as bands in cross section. The appearance of layers, called compositional banding, occurs because the layers, or bands, are of different composition. The darker bands have relatively more mafic minerals (those containing more magnesium and iron). The lighter bands contain relatively more felsic (silicate minerals, containing more of the lighter elements, such as silicon, oxygen, aluminium, sodium, and potassium).
A common cause of the banding is the subjection of the protolith (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction. These forces stretch out the rock like a plastic, and the original material is spread out into sheets.
Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica.
Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.
Not all gneiss rocks have detectable banding. In kyanite gneiss, crystals of kyanite appear as random clumps in what is mainly a plagioclase (albite) matrix.
Grains of gneiss
Grains of gneiss are medium to coarse grained, crystals large enough to be seen with naked eye.
Types of Gneiss
Augen gneiss
Augen gneiss, from the German: Augen, meaning "eyes", is a coarse-grained gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.
Henderson Gneiss
Henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms. The second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest.
Lewisian gneiss
Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma.
Archean and Proterozoic gneiss
Gneisses of Archean and Proterozoic age occur in the Baltic Shield. In antiquity, gneisses were also utilised in architectural construction. They were used to erect the Sphinx of Taharqo in the Nile Valley.
Uses of gneiss
Gneiss is a hard metamorphic rock because of the forming from high pressure and temperature. It can be used as a dimension stone by making slabs and square shape. It can also be used in construction as pavement, crush in roads etc. Gneiss usually does not split along planes of weakness like most other metamorphic rocks. This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects.
Some gneiss is durable enough to perform well as a dimension stone. These rocks are sawn or sheared into blocks and slabs used in a variety of building, paving, and curbing projects.
Some gneiss accepts a bright polish and is attractive enough for use as an architectural stone. Beautiful floor tiles, facing stone, stair treads, window sills, countertops, and cemetery monuments are often made from polished gneiss.
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