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 formation has many varieties.
What is gneiss?
As noted above, the name "gneiss" is an indicator of texture, not component composition. Under this definition, many metamorphic rocks with a banded structure, reflecting the separation of light and dark minerals, fall. This type of arrangement indicates the severity of the formation conditions of all gneisses.
The separation of minerals occurs with a fairly strong migration of ions, which is possible only at a very high temperature (600-700 ° C). The second necessary condition is strong pressure, which leads to the appearance of strips. Moreover, the latter can be either straight or curved and have different thicknesses.
A characteristic feature of the gneiss texture is also that its stripes are not solid sheets or plates, but layers with a granular structure. In most cases, mineral granules are visible to the naked eye.
Visually, gneisses can look different. Each variety 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 location looks chaotic. In some stones, the stripes are so thick that the gneiss structure is visible only on a piece of rock of a sufficiently large size.
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 located on the surface. This is found in those parts of the world where crystalline rocks are not covered by a layer of sedimentary ones (Scandinavia, Canada, etc.).
The answer to the question of what is gneiss was not always unambiguous. For the first time this term was used by Agricola in 1556 to refer to a rock having iron-containing veins. The foundations of the modern use of this name were supposedly laid in 1786 by Wegner. He defined gneiss as a feldspar rock with quartz mica and a coarse schist structure.
Features of metamorphic rocks
Rocks called metamorphic 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 areas of the earth's crust fall into conditions of elevated temperature and pressure. This starts a number of physicochemical processes that lead to:
- to recrystallization - a change in the orientation, location and structure of minerals;
- dehydration;
- migration of solutions;
- the conversion of some chemical compounds into others;
- the introduction of new components of the composition.
As a result, the source rock (sedimentary, igneous, or metamorphic) acquires completely different properties. Moreover, the degree of change depends on the strength and duration of exposure to the factors causing the transformation.
Typical examples of metamorphic rocks are quartzite, marble and shale, formed from sandstone, limestone and clay, respectively. Magmatic and sedimentary protoliths behave differently during transformation. Often metamorphism is carried out in several stages.
Gneiss is an example of high-quality metamorphic rock. This means that it was formed in very harsh physical conditions.
The structure and composition of gneiss
As noted above, the component composition of gneiss is quite variable. However, in all the rocks of this group, a number of the most common minerals can be distinguished. The basis of most gneisses are:
- feldspar (orthoclase, plagioclase);
- quartz;
- mica (biscuit, biotite, etc.).
In a small amount, hornblende (augite), as well as various impurities, may be present.
The mineral spectrum may also include:
- graphite;
- staurolite;
- kyanite;
- Garnet;
- sillimanite;
- amphiboles;
- porphyroblasts;
- epidote.
In general, we can say that the gneiss structure is formed by light and dark silicates, which form irregular subparallel bands 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 stratification, the key property of gneiss is integrity. This is a fairly strong breed. Under the influence of loads, it does not crack along the foliation planes, such as, for example, slate. This is because less than 50% of the mineral grains receive the correct orientation in the gneiss. As a result, a rather rough layered structure is formed. The nature of the splitting is one of the key parameters by which it is possible to determine which rock is gneiss and which is phyllite or slate.
The light bands are usually formed by feldspar and quartz, and the dark ones by mafic minerals (hornblende, pyroxene, biotite, etc.).
Rock formation
Gneiss is formed as a result of recrystallization of mineral grains under strong heating and pressure. This process occurs at the border of plate collisions and is called regional metamorphism. During such changes, mineral grains increase in size and are divided into strips, which makes the rock more stable.
Gneiss can form from various predecessors, 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 slate, and finally into gneiss. This process is accompanied by the transformation of clay components of the source rock into mica, which, as a result of recrystallization, are converted into granular minerals. The appearance of the latter is considered the boundary of the transition to gneiss.
Diaritis 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 primarily structural in nature.
Granite gneiss is also formed as a result of metamorphism of some sedimentary rocks. The final product of their conversion has a striped structure and mineralogical composition similar to granite.
Classification
The breed classification is based on four gneiss characteristics:
- type of protolith;
- name of protolith;
- mineral composition;
- structure and texture.
A double term is usually used to refer to a breed variety. For example, the presence of the word "granite" in the name indicates that such gneiss was formed from granite, and "dioritic" from diorite. In this case, the qualifying term corresponds to a specific protolith.
Classification by type of precursor breed is wider. According to her, all gneisses are divided into two types:
- orthogneisses - formed from igneous rocks;
- paragneisses - originated from sedimentary rocks.
According to the mineral composition, the following types of gneisses are distinguished:
- pyroxene;
- alkaline;
- amphibole;
- biotite;
- two-mica;
- muscular
- plagiogneisses.
If the word “gneiss” does not have a qualifying term, then the component composition is conventionally considered classical (feldspar, quartz, biotite).
Structural classification characterizes the shape and location of the layers. Dark and light stripes can form various textures, and therefore distinguish between tree, leaf, tape gneiss, etc.
Physical and mechanical properties
Within the gneiss group, the degree of schistosity of different rocks varies over a fairly wide range, in connection with which the indicators of physical and mechanical properties fluctuate greatly. The following values were established experimentally for the main characteristics:
- density - 2650-2870 to g / m 3;
- water absorption - 0.2-2.3%;
- porosity - 0.5-3.0%.
In general, gneiss can be characterized as a heavy, strong and rough to the touch rock with a high density and distinctly layered structure, resistant to splitting. The hardness of this stone is comparable to steel.
