The Earth's crust is rich in natural resources, of which mineral and organic minerals can be distinguished separately. People use them in a wide variety of fields - from fuel (oil, coal, gas) to construction (for example, facing with marble and granite) and the production of various items necessary in everyday life. One such resource is the gneiss rock.
Definition
Gneiss is usually called a metamorphic, that is, formed in the bowels of the Earth, rock. Metamorphism is understood as the transformation of sedimentary and igneous natural mineral formations as a result of changes in physico-chemical conditions (temperature, pressure, exposure to various gas and water solutions). Such processes occur due to fluctuations in the earth's crust and other processes occurring in them. As a result, various transformations occur and metamorphic rocks are formed. Gneiss is often characterized by a distinct parallel-schistose, often finely banded texture.
The grain size of the mineral is usually greater than 0.2 mm. Data granular-crystallineformations are rich in feldspar and are usually represented by quartz, muscovite, biotite and other minerals. Among the colors, light shades predominate (gray, red and others).
Gneiss is one of the most common metamorphic rocks, a very popular and practical finishing material in construction. It looks like a compacted rounded piece with a rough and uneven surface. Possesses the big durability, transfers big amplitudes of temperatures. These physical and mechanical properties determine long-term, reliable and aesthetic results in construction, building and pavement cladding, and interior design.
Terminology issue
In the scientific community, there was controversy over the question of which rocks gneiss belongs to. Some researchers (Levinson-Lessing, Polovinkina, Sudovikov) believed that quartz must certainly be present here. Other scientists (Saranchina, Shinkarev) put forward a different point of view, according to which the rock abounds in feldspars, and also includes quartz. That is, in the second option, the presence of quartz is not necessary.
However, the first interpretation is close to its original interpretation, when this term denoted only shales corresponding in mineral composition to granites. That is, quartz is still typomorphic, the defining mineral in the composition of gneisses.
Hypotheses about education
The origin of the gneiss rock is not fully understood even in our time, although it existsseveral dozen scientific assumptions, as well as many literary sources that touch on this topic. Nevertheless, all judgments converge in some basic opinions. For example, that the occurrence of gneisses is determined by the processes of deep metamorphism of various rocks.
Some petrologists consider gneiss as fragments of the primordial earth's crust, which covered the planet during cooling and changing the state of aggregation from fiery-liquid to solid. There is also an assumption that these are igneous rocks, which, as a result of metamorphism, have acquired layering. Still others consider gneisses to be a chemical sediment of the primordial ocean, which crystallized under high atmospheric pressure from superheated water. Still others see them as sedimentary rocks that have changed over the millennia under the influence of the heat of the earth, pressure and the activity of groundwater.
There is another hypothesis, according to which gneisses are sedimentary rocks that crystallized during or shortly after their deposition in the earth's crust. It is believed that the most impressive formation of gneiss in the history of the Earth occurred about 2.5-2.0 billion years ago.
Composition and structure
Gneiss is a rock that has a typical banded texture due to the alternating arrangement of light and dark minerals. The color is usually light. Main components: quartz, feldspar and others.
The chemical composition is close to granites and shale, varied. As a rule, this60-75% silicic acid, 10-15% alumina and a small amount of iron oxide, lime, Mg, K, Na and H2O.
Physical parameters are highly dependent on the structure and level of schistosity. The density characteristic is 2600-2900 kg / m3, the proportion of pore volume in the total volume is 0.5-3.0%.
Based on the mineral components, it is customary to distinguish between biotite, muscovite gneisses and so on. By structure, they are, for example, tree-like, spectacle, tape.
According to the type of primary rocks, there is a division into para- and orthogneisses. The first arise as a result of changes in sedimentary rocks; the second - due to the modification of igneous (usually volcanogenic) rocks.
A typical feature of gneiss rock is schistosity, which has different characteristics. It is either a remnant of primary bedding of sedimentary rocks, or is an intrusion.
Varieties
The division of gneisses into different types is due to the diversity of the mineralogical and elemental composition, the degree of grain size (structural features) and the arrangement of grains in the rock (textural characteristics).
As a result of the transformation of sedimentary rocks, alumina-rich gneisses are formed, often including garnet and andalusite (high alumina).
Rocks with a porphyroblastic texture, in which usually rounded or elliptical feldspar porphyroblasts (sometimes together with quartz) in cross section are visible in the formpeepholes are called spectacles.
Complex metamorphic formations of mixed structure, penetrated by granite material, including its veins, are called migmatites.
Gneiss can be composed of several minerals: biotite, muscovite, diopside and others. Some varieties of gneiss have their own names, such as charnockites and enderbites.
Besides, the division according to the type of initial breeds is widely used. Gneiss as an igneous rock is represented by orthogneisses that arose as a result of the transformation of igneous rocks (for example, granites). It is believed that their main initial source is volcanic eruptions. Paragneisses are the result of deep metamorphism of sedimentary rocks.
Relationship between gneiss and granite
Gneiss is a common rock, which is dominated by feldspar, quartz and mica. Similar components are also characteristic of granite, but there is a fundamental difference. It lies in the fact that in granite there is no clear distribution of its constituent components. In gneiss, all the minerals are parallel to each other, giving it layering. In addition, minerals often occur in the earth's crust in massive plates and layers.
However, there are frequent cases when the gneiss rock loses its layering and turns into granite. This circumstance indicates a close relationship between these natural formations.
Features of occurrence in the earth's crust
It is noteworthy that despite its wide distribution, gneiss is very diverse. As a result of variousprocesses, the way and direction of the mutual arrangement of its constituent parts changes, to which, among other things, new minerals can also join or partially replace them. As a result, new diverse types of gneiss are emerging.
Gneisses are very common, mainly among the rocks of the Precambrian period. Thus, the gray-gneiss deposits of the basement of the Canadian Shield are considered the oldest rocks on the planet: according to scientists, they are more than three billion years old. However, younger rocks of the Cenozoic era, formed as a result of high temperatures, are also common.
Distribution (distribution)
The gneiss rock comes out of the depths to the surface, mainly in countries where, due to various processes and factors, there was a failure in the horizontal arrangement of layers, or as a result of erosion of the newly formed and exposure of older ones.
Mainly significant deposits are related to outcrop of the crystalline basement. On the B altic Shield, this is the Republic of Karelia, the Leningrad and Murmansk regions, and abroad - Finland.
In the Russian Federation, gneisses are often found in the central strip of the Ural Range, in the southeast of the Siberian Platform (Aldan shield), the Caucasian Labino-Malkinskaya zone and in the axial zone of the uplift of the Main Range.
Also, abroad, deposits are concentrated in the Canadian Acasta complex, Scandinavia, on the Ukrainian Shield of the East European Platform.
Practical application (use) of gneiss
Rock predominantlyused for the production of building stone (crushed stone and rubble), as well as as a finish. This natural material is used to make butyl in the form of slabs for foundations, slabs for pedestrian zones; they are also used for lining canals and embankments. It is believed that the closer the texture of gneiss rocks to granites, the higher their quality.
This rock is used to build objects of social significance: buildings, temples, footpaths, squares, yards.
Gneiss is often used to create internal and external decoration of buildings and structures: facing walls, columns, stairs, floors and fireplaces.
