The chemical composition and minerals

Each mineral has its own unique chemical composition and internal structure, with which the atoms are arranged within the crystal lattice. The crystal lattice determines the shape and fission of the crystals that make up the mineral. In some crystal lattices the atoms are closer together than in others, these minerals are denser. Minerals with the same chemical composition are called polymorphs. Diamond, for example, is a polymorph of graphite, both minerals consist of pure carbon, but diamond has a denser crystal structure. Under high pressure, graphite can convert to diamond, such a transition from one polymorphic to another is called a phase transition.

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Chemical composition
Which minerals are present in a rock depends in the first place on the chemical composition of the rock. A rock that contains enough silica (SiO2, the oxide of the element silicon) may contain quartz or another mineral consisting of pure silica. The rock-forming minerals consist almost exclusively of nine elements (silicon, sodium, potassium, aluminium, oxygen, hydrogen, iron, magnesium and calcium). Together, these elements make up 98% of the total mass of the continental earth’s crust.

Most rocks consist mainly of silicates, minerals that contain silica. Almost all coagulation rocks and most sedimentary and metamorphic rocks are included. An important exception is carbonate rock, which consists of carbonates (compounds of carbonate, CO32-). Examples of carbonate rocks are limestone, marble and dolostone. There are other exceptions, such as halite (rock salt) and gypsum are also not silicates.

This breccy is an example of a clastical rock in which the cliffs (dark) of the matrix can be clearly distinguished. Because the cliffs all consist of the same limestone, the distinction is sharp. Location: slope of the Devínska Kobyla in the Slovakian Small Carpathians. Age: Early Jurassic.
Two rocks with the same chemical composition do not have to contain the same minerals. Whether a certain mineral could be formed in a rock depends on the temperature and pressure that prevailed in the rock and the composition of any liquids that flowed through the rock.

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Texture
Some rocks consist exclusively of minerals, the crystals of which have grown together, they are crystalline substances. However, there are also rocks that consist of fragments of minerals or dead organisms. Such fragments are called claws and these rocks are called clastic rocks. Examples are sand (and sandstone), clay, conglomerate or breccy. Some clastic rocks consist of very different claws: pieces of different types of rocks or minerals, fragments of shells, plant remains or fossils. However, there are also clastical rocks in which all or most of the claws have the same composition.

An important concept when describing rocks is the texture of a rock. This refers to the external properties on a small scale, which are present everywhere in the rock. In the case of a crystalline rock, this includes, for example, the size and shape of the crystals; in the case of clastic rocks, this includes the size and shape of the cliffs. Also important is how much volume is taken up by claws and how much by the matrix (all the material in between the claws).

Some rocks have minerals or cliffs that are predominantly in a certain direction. Whether and how strong this is the case is called the “fabric” of the rock. The product is part of the texture of a rock.

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Rock destruction

A soil profile indicating soil horizons. The O-Horizont consists of organic material, which has become humus in the A-Horizont. Due to leaching from the upper O-horizont there is little left of the parent rock in the B-horizont; in the C-horizont the parent rock is still visibly present, although it is weathered. Under the C horizont is then the bare parent rock (R).

Weathering

The natural degradation of rocks is called weathering. Weathering occurs through chemical, physical and biological processes. Physical weathering includes, for example, rock fragmentation due to freezing of water (frost weathering).

It does not change the chemical composition of the rock, but causes it to disintegrate into smaller pieces. Chemical weathering occurs when rocks come into contact with the atmosphere or ground water so that chemical reactions take place and the chemical composition of the rock changes. Certain components of the rock can be dissolved and removed. Biological weathering is all weathering as if it were

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