Formation of metamorphic rocks is a long and complex process. It includes the wide range of temperature amplitudes and pressure in the bowels of the Earth.
This is a set of endogenous modifications of the mineral composition and structure of the rocks, which is needed for their successful adaptation to the already-existing conditions in the deep layers under the Earth’s crust.
It plays an important role in the alteration of the mineral composition of the rocks. This reflects on the composition of the rock structure itself. With the increase of the depth, the temperature usually rises, but this also depends on:
• the heat, which is transferred from the deep fluids and solutions;
• the energy of the radioactive decay in the Earth’s crust;
• the thermal effect of chemical reactions in the crust of the planet
• the heat that is generated due to magma rising from beneath Earth’s mantle
There is hydrostatic pressure or stagnation pressure. Hydrostatic pressure is equal on all sides as a result of the weight of the top rock masses. Stagnation pressure is caused by tectonic movements and is dependent on their intensity.
Types of Metamorphism:
• Contact metamorphism;
• Dislocational (cataclastic) metamorphism;
• Regional (deep-crustal) metamorphism.
Minerals that make up the metamorphic rocks are:
• minerals that are often found in igneous and metamorphic rocks: quartz, feldspars, mica, amphibole, olivine, peridot, etc.;
• minerals that are often found in sedimentary and metamorphic rocks: dolomite, calcite, etc.;
• minerals that are primary in the composition of metamorphic rocks and secondary in the composition of igneous rocks: serpentine, sericit, chlorite, actinolite, , etc.;
• minerals which are found only in metamorphic rocks: wollastonite, distin, sillimanite, stavrolit, andalusite, staurolit, grenades, cordierit, and others.
Orto-metamorphic and Para-metamorphic Rocks:
• Clay schist rocks;
• Amphibolite rocks;
• Eclogite rocks
• Quartz rocks
• Cataclastic rocks
Gneiss is a metamorphic stone, distinguished by its exceptional strength and high durability during environmental changes; its surface is rough and non-slippery. Therefore, it provides stable, aesthetic and long-lasting results in the construction, tiling, flooring and decoration of interior and exterior spaces. The gneissic pavements and facings can give a natural and beautiful finishing touch to any space. Gneiss is available for usage both in its naturally occurring form and shaped (after processing). The different mineral composition of the gneissic stones cause variety of colors in its texture: white, yellow, gray, green, metallic, brown etc.
Structure of Gneiss
Metamorphic gneiss is also called shale gneiss. Its geological structure is similar to that of granite. Its hardness is great and thus, the stone can easily replace granite.
Metamorphic rocks such as gneiss are most likely to be found at sites, located deep beneath the Earth’s surface. The collision of plates in the lithosphere causes high pressure which in turn leads to the clustering of several minerals in a rock.
Useful Characteristics of gneiss rock
Shale gneiss has beautiful patterns and loops that make it perfect for polished marble countertops and tiling. Due to its great hardness, things created by gneiss can endure for centuries. It is also a good insulator, so houses or buildings lined with gneiss, besides being beautiful and cozy, remain cool in the summer and warm in the winter.
Structure and texture of gneiss stone
The structure can be divided into three basic types:
• crystal structure
• cataclastic structure
• relic rock structure
The texture of the gneiss can be:
• relic rock texture – when it retains part of the texture features, inherent to the source rock;
• massive marble texture – formed without direct pressure;
• oriented texture – formed under the impact of direct pressure and spread quite widely It varies: lenticular, linear, striped, etc.