The earth's crust is rich in natural resources, from which mineral and organic minerals can be separately distinguished. People use them in a wide variety of fields - from fuel (oil, coal, gas) to construction (for example, marble and granite cladding) and the production of various items needed in everyday life. One of these resources is the gneiss rock.
Definition
Gneiss is called metamorphic, that is, a rock formed in the bowels of the Earth. By metamorphism is understood the transformation of sedimentary and magmatic natural mineral formations as a result of a change in physicochemical conditions (temperature, pressure, exposure to various gas and aqueous solutions). Such processes occur due to fluctuations in the earth's crust and other processes occurring in them. As a result, various transformations take place and metamorphic rocks are formed. Gneiss is often characterized by a clear manifestation of parallel-shale, often thin-striped texture.
The grain size of the mineral is usually greater than 0.2 mm. These granular crystalline formations are rich in feldspar and are usually represented by quartz, muscovite, biotite and other minerals. Among the colors, light shades prevail (gray, red and others).
Gneiss is one of the most common metamorphic rocks, a very popular and practical decoration material in construction. It looks like a compacted rounded piece with a rough and uneven surface. It has great strength, tolerates large temperature amplitudes. These physical and mechanical properties determine long-term, reliable and aesthetic results in construction, when facing buildings and sidewalks, and in interior decoration.
Terminology problem
A controversy arose in the scientific community over the question of which rocks gneiss belongs to. Some researchers (Levinson-Lessing, Polovinkina, Sudovikov) believed that quartz should certainly be present here. Other scientists (Saranchina, Shinkarev) put forward another point of view, according to which the breed is replete with feldspars, and also includes quartz. That is, in the second embodiment, the presence of quartz is not necessary.
However, the first interpretation is close to its initial interpretation, when this term denoted only shales corresponding to granites in mineral composition. That is, quartz is still typomorphic, the determining mineral in gneiss.
Education Hypotheses
The origin of the gneiss rock is still unclear in our time, although there are several dozen scientific assumptions, as well as many literary sources that touch on this topic. Nevertheless, all judgments agree 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 first-born earth's crust that covered the planet when it cools and the state of aggregation changes from fiery liquid to solid. There is also an assumption that these are igneous rocks, which, as a result of metamorphism, acquired stratification. Still others consider gneiss to be a chemical precipitate of the primeval ocean, which crystallized under high atmospheric pressure from superheated water. The fourth see sedimentary rocks in them, changing for thousands of years under the influence of the heat of the earth, pressure and groundwater.
There is another hypothesis according to which gneisses are sedimentary rocks that crystallized during or soon after their deposition in the earth's crust. It is believed that the most impressive gneiss formation in the history of the Earth took place about 2.5-2.0 billion years ago.
Composition and structure
Gneiss refers to rocks for which a banded texture is typical, arising due to the alternate arrangement of light and dark minerals. The color is usually light. The main components: quartz, feldspar and others.
The chemical composition is close to granites and shales, diverse. As a rule, these are 60-75% 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 shale. 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, treelike, spectacle, ribbon.
According to the type of primitive rocks, there is a division into para- and orthogneys. The former 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 rocks is shale, which has different characteristics. It represents either the remainder of the primary layering of sedimentary rocks, or is an intrusion.
Varieties
The division of gneisses into different species is due to the diversity of mineralogical and elemental composition, the degree of granularity (structural features) and the location of grains in the rock (texture characteristics).
As a result of the transformation of sedimentary rocks, gneisses rich in alumina are formed, often including garnets and andalusite (high alumina).
Rocks with a porphyroblastic texture, in which usually round or ellipsoidal porphyroblasts of feldspar (sometimes together with quartz) are visible in the cross-section in the form of ocelli, called eyeglasses.
Complex metamorphic formations of mixed structure penetrated by granite material, including its veins, are called migmatites.
Gneiss can consist of several minerals: biotite, muscovite, diopside and others. Some gneiss species have their own names, for example, charnockites and enderbits.
In addition, the separation according to the type of initial rocks is widely used. Gneiss as a igneous rock is represented by orthogneisses resulting from the transformation of igneous rocks (for example, granites). It is believed that their main initial source are volcanic eruptions. Paragneisses are the result of a deep metamorphism of sedimentary rocks.
The relationship of gneiss and granite
Gneiss is a frequently encountered rock whose composition is dominated by feldspar, quartz and mica. Similar components are characteristic of granite, however, 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 lie in the earth's crust with massive plates and strata.
However, there are frequent cases when the gneiss rock loses its lamination and passes into granite. This circumstance indicates a close relationship of these natural formations.
Features of occurrence in the earth's crust
It is noteworthy that with its widespread prevalence, gneiss is very diverse. As a result of various processes, the method and direction of the relative position of its constituent parts are changing, to which, among other things, new minerals can also join or partially replace them. As a result, various new types of gneiss arise.
Gneisses are very common, mainly among rocks of the Precambrian period. So, the gray-gneiss deposits of the Canadian Shield foundation are considered the oldest rocks on the planet: according to scientists, they are more than three billion years old. However, the younger rocks of the Cenozoic era, formed as a result of high temperatures, are also widespread.
Distribution (distribution)
Gneiss rocks emerge from the bowels to the surface, mainly in countries where, due to various processes and factors, a failure occurred in the horizontal arrangement of layers, or as a result of erosion of the newly formed and exposure of older ones.
Mostly, significant deposits are related to exposure of the crystalline basement. On the Baltic Shield, these are the Republic of Karelia, the Leningrad and Murmansk regions, and Finland abroad.
In the Russian Federation, gneisses are often found in the central strip of the Ural ridge, in the southeast of the Siberian platform (Aldan shield), the Caucasian Labino-Malkinsky zone, and in the axial zone of elevation of the Main ridge.
Also, abroad, deposits are concentrated in the Canadian complex of Acast, Scandinavia, on the Ukrainian shield of the East European platform.
