Clay minerals denote on the one hand minerals that are predominantly finely grained (grain size <2 µm), on the other hand the phyllosilicates , which are named after their layered crystal structure of silicon and oxygen, as well as hydrogen and mostly magnesium and aluminum. Both definitions are not the same. Some minerals that are predominantly finely grained, such as goethite or gibbsite , are not silicates . On the other hand, there are layered silicates, such as kaolinite , which are often larger than two micrometers. Clay minerals therefore generally designate those minerals that meet both criteria.
Clay minerals arise from the weathering of other minerals or glasses, or are newly formed from oversaturated soil solutions or hydrothermal waters. During diagenesis there are processes of order in the crystal structures of the clay minerals, which can be used as a measure of the maturity of a sediment .
Clay minerals consist of two characteristic components:
- Tetrahedron layer: corner linked SiO 4 - tetrahedra , Si partially substituted by Al
- Octahedron layer: edge-sharing Al (OH) 6 - octahedron , partly Al substituted by Mg
Depending on the arrangement of these layers, one differentiates:
- 1: 1 clay minerals (two-layer clay minerals): tetrahedron-octahedral layer (TO), for example kaolinite or chrysotile
- 2: 1 clay minerals (three-layer clay minerals): Tetrahedral layer-octahedral layer-tetrahedral layer (TOT), for example illite , smectite or vermiculite
- 2: 1: 1 clay minerals (four-layer clay minerals): Tetrahedron-octahedron-layer-tetrahedron-octahedron-layer (TOTO), for example chlorite
The substitution (especially of tetravalent Si by trivalent Al in the tetrahedron layer or of trivalent Al by divalent Mg in the octahedron layer) creates a negative layer charge which is neutralized by the incorporation of cations in the intermediate layer . The layer charge of the 1: 1 clay minerals is always zero. The 2: 1 clay minerals are classified according to their layer charge x:
- x ≈ 0: talc - pyrophyllite group
- x ≈ 0.25 - 0.6: smectite group , for example montmorillonite , beidellite , nontronite , saponite or hectorite
- x ≈ 0.6-0.9: vermiculite - illite group
- x ≈ 1 - 2: mica group
Clay minerals with non-integer layer charges have the ability to swell , i.e. to temporarily and reversibly absorb water in their intermediate layers.
Alternatively, the stratified charge in the octahedron layer can also be compensated for in that only two of three octahedra are occupied. Therefore one differentiates:
- dioctahedral clay minerals with two occupied octahedral positions, for example kaolinite
- trioctahedral clay minerals with three occupied octahedral positions, for example chrysotile
Clay minerals are very soft ( Mohs hardness 1) and react plastically to mechanical stress. When heated, they transform into harder and firmer minerals ( ceramics ). Clay minerals have a large specific surface on which substances can be adsorbed and desorbed. A high cation exchange capacity is associated with the large surface . Clay minerals have low water permeability. Suspensions of clay minerals react thixotropically to mechanical stress.
The type and proportion 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 release more nutrients such as potassium or ammonium ions to plants, while they store the hydronium ions released by the roots in their place in their intermediate layer. The crystallographic degree of order of the clay mineral illite is used by mineralogists to determine the time that has passed since a sediment was deposited. It increases as the diagenesis progresses .
Clay is the most important and oldest raw material for making ceramics . As a component of clay , it is required for the manufacture of bricks . It is also used together with limestone to produce cement . In the fine arts it is used to produce sculptures . Clay minerals are used as ion exchangers , for example to clean drinking water and to decolorize solutions. Montmorillonite in particular is used because of its water absorption capacity, for example in cat litter . Kaolinite is also used in the paper industry as a finishing agent, smoothes the surface and absorbs ink. Expanded clay (highly porous fired clay) is used as an insulating building material and for hydroponics . Other clays serve as sealing in landfills , are fillers, separating agents and additives in paint , food and pharmaceutical products or are used as catalysts .
For use as fillers , for example in plastics , clays are previously modified with organic modifiers to make them organophilic (i.e. hydrophobic ). As a result, they lose their good water absorption capacity, but can be mixed well with organic substances (e.g. polymer melts ). They can then be used as a nanofiller .
- Karl Jasmund , Gerhard Lagaly (ed.): Clay minerals and clays. Structure, properties, applications and use in industry and the environment. Steinkopff, Darmstadt 1993, ISBN 3-7985-0923-9 .
- University of Greifswald - clay minerals
- Clay extraction, processing and recultivation of clay pits , Bundesverband Keramische Rohstoffe und Industrieminerale eV