Clay

Clay minerals referred to on the one hand minerals that occur predominantly very fine-grained (grain size < 2 microns ), but on the other hand, the layer silicates, which are named according to their layered crystal structure of silicon and oxygen, and hydrogen and usually magnesium and aluminum. Both definitions are not identical. Some very fine-grained predominantly occurring minerals, such as goethite or gibbsite, are not silicates. On the other hand, there are sheet silicates, such as kaolinite, often greater than two micrometers. Clay minerals therefore typically refers to those minerals that meet both criteria.

Formation

Clay minerals produced by weathering of other minerals or glasses or created from supersaturated soil solutions or hydrothermal waters. At diagenesis occurs order processes in the crystal lattice of the clay minerals that can be used as a measure for the ripening of a sediment.

Structure

Clay minerals consist of two distinct components:

• Tetrahedral layer: corner-sharing SiO4 tetrahedra, Si partially substituted by Al
• Octahedron: edge-sharing octahedra AlO6, partly substituted by Al Mg

Depending on the arrangement of these layers are distinguished:

By substituting (especially of tetravalent Si by trivalent Al in the tetrahedral layer or of trivalent Al by divalent Mg in the octahedral layer ) produces a negative layer charge, which is neutralized by the inclusion of cations in the interlayer. The layer charge of 1:1 clay minerals is always zero. The 2:1 clay minerals are classified according to their x layer charge:

• X ≈ 0: talc - pyrophyllite group
• X ≈ 0.25 to 0.6: smectite, for example, montmorillonite, beidellite, nontronite, saponite or hectorite
• X ≈ 0.6 to 0.9, to: vermiculite - illite group
• X ≈ 1 - 2: mica group

Clay minerals with non-integer layer charges have the ability to swell, that is for temporary and reversible water absorption in their interlayers.

Alternatively, the layer of charge can also be compensated in the octahedron layer, that only two out of three octahedra are occupied. Therefore, it may be:

• Dioctahedral clay minerals with two occupied octahedral positions as kaolinite
• Trioctahedral clay minerals with three occupied octahedral positions, for example chrysotile

Properties

Clay minerals are very soft ( Mohs hardness 1) and plastically respond to mechanical stress. They change when heated in harder and firmer minerals to (ceramic). Clay minerals, have a large specific surface area, can be adsorbed on and desorbed substances. With the large surface area, a high cation exchange capacity is connected. Clay minerals have a low water permeability. Suspensions of clay minerals react thixotropic mechanical stress.

Geological significance

The type and amount of clay minerals in soils largely determines their fertility. 2:1 clay minerals have a higher cation exchange capacity than 1:1 clay minerals and can therefore provide more nutrients such as potassium or ammonium ions to plants while they store the output from the roots of hydronium ions in their place in their interlayer. The crystallographic order degree of the clay mineral illite is used by mineralogists to determine the time that has elapsed since the deposition of sediment. It increases with progressive diagenesis.

Technical significance

Sound is the oldest and most important raw material for the production of ceramics. As an ingredient of clay it is needed for the production of bricks. In addition, it is used together with limestone to produce cement. In the visual arts it is used to produce plastics. Clay minerals are used as ion exchangers, for example in the cleaning of drinking water and for bleaching solutions. In particular, montmorillonite is used because of its water absorption capacity, for example in the cat litter. Kaolinite used in the paper industry as sizing agent, smoothes the surface and receives ink. Expanded clay ( highly porous fired clay ) is used as an insulating building material and for hydroponics. Other clays serve as a seal in landfills are filling, cutting, and additives in paints, food and pharmaceutical products or be used as catalysts.

For use as a filler, for example in plastics, clays are previously modified with organic modifiers to it (ie, hydrophobic) to make organophilic. This makes them lose their good water absorption ability, but can be mixed well with organic materials (eg polymer melts ). They may be used as nanofiller.