Smectite group
The smectites , or smectites (from the Greek smectos = 'painted, smeared'; smectis = ' fuller's earth ') are mineral mixtures of different swellable layered silicates (phyllosilicates), which have a three-layer structure and belong to the clay minerals . Smectites consist mainly of montmorillonite , but also contain quartz and calcite dust .
The three-layer structure consists of two tetrahedron layers that are electrostatically crosslinked via the cations of an octahedron intermediate layer . The layers are not rigidly connected to one another, but can swell due to the reversible storage of water and other substances.
Smectites are formed when basalt and gabbro are weathered .
structure
Like all sheet silicates, clay minerals are characterized by two-dimensional layers, the SiO 4 tetrahedron and AlO 4 octahedron of which have common corners. Each SiO 4 tetrahedron shares three of its oxygen atoms with other tetrahedra. The fourth oxygen atom remains free, so if it is not shared with other tetrahedra, all tetrahedra point in the same direction; d. H. all undivided vertices are on the same side of the plate.
In clays, the tetrahedral plates are always bound to the octahedral plates by small cations, such as aluminum or magnesium. The tetrahedron plate's free oxygen also forms part of one side of the octahedron plate, but an additional oxygen atom is located over the gap in the tetrahedron plate at the center of the six tetrahedra. This oxygen atom is connected to a hydrogen atom that forms as an OH group in the clay structure.
Tones can be categorized according to how the tetrahedron and octahedron are packaged. If only one tetrahedron is connected to one octahedron group in each, the tone is known as a 1: 1 tone . The alternative, known as 2: 1 tone , has two tetrahedral plates with their free vertices facing each other, thus each forming one side of the octahedral plate.
The binding between the tetrahedra and the octahedral plates requires that the tetrahedra be distorted or rotated, causing a ditrigonal distortion to the hexagonal row, while the octahedral plate becomes flat. This minimizes the overall distortion of the band valence of the crystallites.
Depending on the composition of the tetrahedron and octahedron plates, the layer is charge-free or has a negative net charge. When the layers are charged, this charge is balanced by an intermediate layer of cations such as Na + or K + . In any case, the intermediate layer can also contain water of crystallization. The crystal structure is formed by stacking different layers.
Smectites have a very high affinity for water and therefore often store it, expanding many times their original size. Very high pressures (up to 2–6 MPa) occur, which are stronger than in the physical weathering processes. By swelling and shrinking, the crystals change their size significantly. The large spaces between the smectites allow the incorporation of polar organic liquids instead of the inorganic hydrated cations. This allows organic toxins to be absorbed in such a way that their toxicity is canceled. Smectites are therefore used to remediate or secure contaminated sites.
See also
Web links
- Mineral Atlas: Smectite
- Smectite group. In: mindat.org. Hudson Institute of Mineralogy, accessed July 27, 2020 .
Individual evidence
- ^ Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 753 (first edition: 1891).