Gneiss is a coarse-grained rock of metamorphic origin with a characteristic structure in the form of alternating layers of various minerals. As a result of this arrangement, it has a striped appearance. The term "gneiss" is not associated with a specific mineral composition, since the latter varies greatly and depends on the protolith (precursor). This rock has many varieties.
What is gneiss
As noted above, the name "gneiss" is an indicator of texture, not component composition. This definition includes many metamorphic rocks with a banded structure, reflecting the separation of light and dark minerals. This type of location indicates the rigidity of the conditions for the formation of all gneisses.
Separation of minerals occurs with a sufficiently strong migration of ions, which is possible only at very high temperatures(600-700 °C). The second necessary condition is strong pressure, which leads to the appearance of stripes. Moreover, the latter can be both straight and curved and have different thicknesses.
A characteristic feature of the gneiss texture is also that its bands are not continuous sheets or plates, but layers with a granular structure. In most cases, mineral granules are visible to the naked eye.
Visually gneisses may look different. Each type of breed of this type has a unique pattern. Black and light mineral layers can be straight, wavy or have an irregular shape. In the latter case, their arrangement looks chaotic. In some stones, the bands are so thick that the gneiss structure is visible only on a sufficiently large piece of rock.
General information
Gneiss is a very common type of rock, most characteristic of the lower zones of the continental crust. However, in some places it is often found on the surface. This is found in those parts of the globe where crystalline rocks are not covered by sedimentary layers (Scandinavia, Canada, etc.).
The answer to the question, what is gneiss, was not always unambiguous. For the first time this term was used by Agricola in 1556 to refer to a rock with iron-bearing veins. The basis of the modern use of this name was supposedly laid in 1786 by Wegner. He defined gneiss as a feldspar rock with quartz mica andcoarse schist structure.
Features of metamorphic rocks
Metamorphic rocks are called, which are formed as a result of the transformation of precursors of igneous or sedimentary origin. The changes are mainly associated with tectonic processes, which lead to the fact that certain parts of the earth's crust fall into conditions of elevated temperature and pressure. This sets off a series of physical and chemical processes that result in:
- to recrystallization - a change in the orientation, location and structure of minerals;
- dehydration;
- migration of solutions;
- transformation of some chemical compounds into others;
- introduction of new components of the composition.
As a result, the original rock (sedimentary, igneous or metamorphic) acquires completely different properties. At the same time, the degree of change depends on the strength and duration of the influence of the factors causing the transformation.
Typical examples of metamorphic rocks are quartzite, marble, and shale, formed from sandstone, limestone, and clay, respectively. Igneous and sedimentary protoliths behave differently during transformation. Often metamorphism occurs in several stages.
Gneiss is an example of a high quality metamorphic rock. This means that it was formed under very harsh physical conditions.
Structure and composition of gneiss
As noted above, the component composition of gneiss is quite variable. However, in all breeds of this group it is possibleidentify a number of the most common minerals. Most gneisses are based on:
- feldspar (orthoclase, plagioclase);
- quartz;
- mica (biscovite, biotite, etc.).
A small amount may contain hornblende (augite), as well as various impurities.
The mineral spectrum may also include:
- graphite;
- staurolite;
- kyanite;
- garnet;
- sillimanite;
- amphiboles;
- porphyroblasts;
- epidote.
Generally, we can say that the structure of gneiss is formed by light and dark silicates, which form irregular subparallel strips with a thickness of 1 to 10 mm. However, sometimes they can be much thicker. This suggests that such gneiss underwent partial melting or the introduction of new material. Such changes occur during the transition to another type of rock - migmatite.
Despite the well-developed layering, the key property of gneiss is integrity. This is a fairly strong breed. Under the influence of loads, it does not split along the lamination planes, as, for example, slate does. This is explained by the fact that less than 50% of mineral grains receive the correct orientation in gneiss. As a result, a rather coarse layered structure is formed. The nature of splitting is one of the key parameters by which it is possible to determine which rock is gneiss and which is phyllite or shale.
Light streaks are usually formed by feldspar andquartz, and dark ones - mafic minerals (hornblende, pyroxene, biotite, etc.).
Breed formation
Gneiss is formed as a result of recrystallization of mineral grains under strong heat and pressure. This process occurs at the plate collision boundary and is called regional metamorphism. During these changes, the mineral grains increase in size and separate into bands, making the rock more stable.
Gneiss can form from a variety of precursors, including:
- clay and sand deposits;
- igneous rocks;
- silico-carbonate and carbonate deposits.
The most typical gneiss protolith is shale. Under the influence of temperature and pressure, it turns into phyllite, then into metamorphic schist and finally into gneiss. This process is accompanied by the transformation of the clay components of the original rock into micas, which, as a result of recrystallization, are transformed into granular minerals. The appearance of the latter is considered the boundary of the transition to gneiss.
Diarite is also a fairly common protolith. Granite can also serve as a precursor, which, as a result of exposure to high temperature and pressure, acquires a striped structure. Such gneiss is called granite. During its formation, mineralogical transformations practically do not occur. Changes are predominantly structural.
Granite gneiss is also formed as a result of the metamorphism of some sedimentary rocks. Final producttheir transformation has a banded structure and a mineralogical composition similar to granite.
Classification
Rock classification is based on four characteristics of gneiss:
- protolith type;
- protolith name;
- mineral composition;
- structure and texture.
A double term is usually used to designate a breed variety. For example, the presence of the word "granite" in the name indicates that such gneiss was formed from granite, and "diorite" - from diorite. In this case, the qualifying term corresponds to a particular protolith.
Classification according to the type of predecessor breed is wider. According to her, all gneisses are divided into two types:
- orthogneisses - formed from igneous rocks;
- paragneisses - originated from sedimentary rocks.
The following types of gneisses are distinguished by their mineral composition:
- pyroxene;
- alkaline;
- amphibole;
- biotite;
- two-mica;
- muscular;
- plagiogneisses.
If there is no qualifying term before the word "gneiss", then the component composition is conditionally considered classical (feldspar, quartz, biotite).
Structural classification characterizes the shape and arrangement of layers. Dark and light bands can form different textures, in connection with which tree-like, leaf, ribbon gneisses, etc. are distinguished.
Physical and mechanical properties
Within the gneiss group, the degree of shearing of different rocksvaries over a fairly wide range, and therefore the indicators of physical and mechanical properties fluctuate greatly. The following values were experimentally established for the main characteristics:
- density - 2650-2870 g/m3;
- water absorption - 0.2-2.3%;
- porosity - 0.5-3.0%.
In general, gneiss can be described as a heavy, hard and rough rock with a high density and a distinctly layered structure that is resistant to splitting. The hardness of this stone is comparable to steel.
Practical application
Gneiss is widely used in construction and landscape design. Most of this stone is used to make gravel and crushed stone, but this rock is also suitable:
- for laying foundations;
- for making tiles;
- for facing sidewalks, embankments;
- as rubble stone.
The advantages of gneiss as a building material are its strength and resistance to household acids. The aesthetic beauty of this stone makes it suitable for the production of facing tiles. Gneiss is often substituted for granite, as the latter is much more expensive to mine.
