Clay minerals are aqueous phyllosilicates of aluminum, sometimes with various impurities of iron, magnesium, alkali and alkaline earth metals, as well as other cations found on or near some planetary surfaces.
They are formed in the presence of water, and once they were important for the emergence of life, because many theories of abiogenesis take into account their role in this process. They are important components of soils and have been beneficial to humans since ancient times in agriculture and production.
Education
Clays form flat hexagonal sheets similar to mica. Clay minerals are common weathering products (including weathering of feldspar) and low-temperature products of hydrothermal change. They are very common in soils, in fine-grained sedimentary rocks such as schists, mudstones and siltstones, as well as in fine-grained metamorphic schists and phyllites.
Characteristics
Clay minerals, as a rule (but not necessarily), are ultrafine-grained. It is generally believed that they have a size of less than 2 micrometers in the standard classification of particle sizes, so special analytical methods may be required to identify and study them. These include X-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, Raman spectroscopy and SEM-EDS, or automated mineralogy processes. These methods can be supplemented by polarized light microscopy, a traditional technique that establishes fundamental phenomena or petrological relationships.
Spread
Given the need for water, clay minerals are relatively rare in the solar system, although they are widespread on Earth, where water interacts with other minerals and organic matter. They have also been discovered in several places on Mars. Spectrography confirmed their presence on asteroids and planetoids, including the dwarf planet Ceres and Tempel 1, as well as the moon of Jupiter Europe.
Classification
The main clay minerals are included in the following clusters:
- The kaolin group, which includes the minerals kaolinite, dikkit, halloysite and nakrit (polymorphs Al2Si2O5 (OH) 4). Some sources include the kaolinite-serpentine group due to structural similarities (Bailey 1980).
- A smectite group that includes dioctahedral smectites, such as montmorillonite, nontronite and beidellite, and trioctahedral smectites, for example, saponite. In 2013, analytical tests by the Curiosity rover found results consistent with the presence of smectite clay minerals on the planet Mars.
- Illite group, which includes clay mica. Illit is the only common mineral of this group.
- The chlorite group includes a wide range of similar minerals with significant chemical variation.
Other species
There are other types of these minerals such as sepiolite or attapulgite, clays with long water channels, internal in structure. Mixed layer clay variations are relevant for most of the above groups. Ordering is described as random or regular ordering and is further described by the term “Reichweit”, which in German means “range” or “coverage”. Literary articles cite, for example, the ordered illite-smectite R1. This type is included in the ISISIS category. R0, on the other hand, describes random ordering. In addition to them, one can also find other extended types of ordering (R3, etc.). Clay mixed clay minerals, which are the perfect types of R1, often get their own names. R1-ordered chlorite-smectite is known as corrensite, R1-illite-smectite-rectorite.
Study history
Knowledge of the nature of clay became more understandable in the 1930s with the development of x-ray diffraction technologies necessary for the analysis of the molecular nature of clay particles. The standardization of terminology also emerged during this period with particular attention to similar words, which led to confusion, such as a sheet and a plane.
Like all phyllosilicates, clay minerals are characterized by two-dimensional strata of angled SiO4 tetrahedra and / or AlO4 octahedra. Sheet blocks have the chemical composition (Al, Si) 3O4. Each silicon tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra, forming a hexagonal lattice in two dimensions. The fourth vertex is not shared with another tetrahedron, and all tetrahedrons "point" in the same direction. All unseparated vertices are on one side of the sheet.
Structure
In clays, tetrahedral sheets are always bonded to octahedral sheets, formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshaped vertex of the tetrahedral sheet also forms part of one side of the octahedral, but an additional oxygen atom is located above the gap in the tetrahedral sheet in the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure.
Clays can be divided into categories depending on the method of packing tetrahedral and octahedral sheets in layers. If in each layer there is only one tetrahedral and one octahedral group, then it belongs to the 1: 1 category. An alternative, known as clay 2: 1, has two tetrahedral sheets with an undivided vertex of each of them, directed towards each other and forming each side of the octagonal sheet.
The connection between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted, causing ditrigonal distortion of the hexagonal matrix, and the octahedral sheet is aligned. This minimizes the overall valence distortion of the crystallite.
Depending on the composition of the tetrahedral and octahedral sheets, the layer will not have a charge or will have a negative. If the layers are charged, this charge is balanced by interlayer cations, such as Na + or K +. In each case, the intermediate layer may also contain water. The crystal structure is formed from a stack of layers located between other layers.
"Clay chemistry"
Since most clays are made from minerals, they have high biocompatibility and interesting biological properties. Due to the shape of the disk and charged surfaces, clay interacts with a number of macromolecules of substances such as protein, polymers, DNA, etc. Some of the clay's uses include drug delivery, tissue engineering, and bioprinting.
Clay chemistry is an applied discipline of chemistry that studies the chemical structures, properties and reactions of clay, as well as the structure and properties of clay minerals. This is an interdisciplinary field that includes concepts and knowledge from inorganic and structural chemistry, physical chemistry, material chemistry, analytical chemistry, organic chemistry, mineralogy, geology and others.
The study of the chemistry (and physics) of clays and the structure of clay minerals is of great academic and industrial importance, since they are among the most widely used industrial minerals used as raw materials (ceramics, etc.), adsorbents, catalysts, etc.
