Metamorphic rocks are formed from sedimentary or volcanic rocks as a result of pressure, heat, and chemical processes, which have led to changes in the texture and composition of the original rocks.
As to the structure of igneous rocks, it is subdivided into three types: cataclastic, crystalloblastic, and relict. Cataclastic structure is typical of rocks, formed as a result of destruction of rock in areas close to the surface. Sometimes beside the destruction of the rock, partial recrystallization of the minerals takes place as well. Crystalloblastic structure involves full recrystallization of the newly-formed metamorphic rock. Relict structure is characterized by recrystallization of the minerals, but at the same time there remain relicts of the structure of the original rock that have given rise to the new type of rock.
The texture of metamorphic rocks is also subdivided into three types: oriented, relict and massive metamorphic. Oriented texture is quite varied: it can be banded, linear, etc. Moreover, it is very common. Relict texture is characterized by the fact that it preserves some of the properties of the original rock. Massive metamorphic texture is not the result of directed pressure.
Metamorphic rock types
Amphibolites
Amphibolites are a type of metamorphic rock with dark green to black colour and are found in Germany, Austria, Canada, Greece, Turkey, etc. They are used for the production of building materials and stone castings.
Marble
Marble is created as a result of metamorphism of dolomites and limestones and its colour is usually white, but it can vary as well. Marble is extracted in France, Italy, Norway, Greece. In Bulgaria there are marble fields in the Rila Mountain, the Pirin Mountains, the Rhodope Mointains, and Strandzha. Marble is usually used for mosaics and cladding.
Gneiss
Gneiss is usually made of granites, mica, feldspar, quartz, and andalusite and is one of the natural cladding stones that are most frequently used both in the interior and in the exterior. The colour of gneiss is determined by the minerals that it is made of and it can vary from green to yellow, grey and white. In Bulgaria there are gneiss fields in the Western Rhodope Mountains and in the Eastern Rhodope Mountains. Ivaylovgrad gneiss is very popular because of its qualities – it does not crumble or break over time, since it is extracted at a great depth.
Schists
Schists are metamorphic rocks that contain different minerals: graphite, quartz, silicates, etc. Schists have foliated structure and are used as raw material in the chemical industry or as part of the composition of different building materials.
Quartzite
Quartzite is often used for pavement of railways. Moreover, it is found in various colours: yellow, orange, red, pink, as well as in shades from white to grey. This variety of colours makes it very attractive, so quartzite is frequently used for pavement and wall cladding.