Clay minerals are aqueous aluminum phyllosilicates, sometimes with various impurities of iron, magnesium, alkali and alkaline earth metals, and other cations found on or near some planetary surfaces.
They are formed in the presence of water, and were once important for the emergence of life, so many theories of abiogenesis include them in the role in this process. They are important constituents of soils and have been beneficial to humans since ancient times in agriculture and manufacturing.
Education
Clays form flat hexagonal sheets similar to micas. Clay minerals are common weathering products (including feldspar weathering) and low temperature products of hydrothermal alteration. They are very common in soils, in fine-grained sedimentary rocks such as shales, mudstones and siltstones, as well as in fine-grained metamorphic shales and phyllites.
Features
Clay minerals are typically (but not necessarily) ultrafine in size. They are generally considered to be less than 2 micrometers in the standard particle size classification, so special analytical techniques may be required to identify and study them. These include X-ray diffraction, electron diffraction techniques, 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.
Distribution
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 found in several places on Mars. Spectrography has confirmed their presence on asteroids and planetoids, including the dwarf planet Ceres and Tempel 1, and Jupiter's moon Europa.
Classification
The main clay minerals are included in the following clusters:
- The kaolin group, which includes the minerals kaolinite, dickite, halloysite and nakrite (polymorphs of Al2Si2O5 (OH) 4). Some sources include the kaolinite-serpentine group due to structural similarity (Bailey1980).
- Smectite group, which includes dioctahedral smectites such as montmorillonite, nontronite and beidellite and trioctahedral smectites such as saponite. In 2013, analytical testing by the Curiosity rover found results consistent with the presence of smectite clay minerals on the planet Mars.
- Illite group, which includes clay micas. Illite is the only common mineral in 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 aforementioned groups. The ordering is described as random or regular ordering and is further described by the term "Reichweit", which means "range" or "coverage" in German. Literature articles refer, for example, to ordered illite-smectite R1. This type is included in the ISISIS category. R0, on the other hand, describes a random ordering. In addition to these, you can also find other extended ordering types (R3, etc.). Mixed layer clay minerals, which are 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 understandablein the 1930s with the development of X-ray diffraction technologies needed to analyze the molecular nature of clay particles. Standardization of terminology emerged during this period as well, with particular attention to similar words that led to confusion such as leaf and plane.
Like all phyllosilicates, clay minerals are characterized by two-dimensional sheets of SiO4 corner tetrahedra and/or AlO4 octahedra. Sheet blocks have a 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 tetrahedra "point" in the same direction. All undivided vertices are on the same 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 lone vertex of the tetrahedral sheet also forms part of one side of the octahedral, but the extra oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to the hydrogen atom that forms the OH group in the clay structure.
Clays can be categorized according to how the tetrahedral and octahedral sheets are packed into layers. If each layer has only one tetrahedral and one octahedral group, then it belongs to the 1:1 category. An alternative known as 2:1 clay has two tetrahedral sheets withthe 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 the tetrahedral sheet to become corrugated or twisted, causing ditrigonal distortion of the hexagonal matrix, and the octahedral sheet to flatten. This minimizes the overall valence distortion of the crystallite.
Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge or will have a negative one. 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
Because most clays are made from minerals, they have high biocompatibility and interesting biological properties. Due to its disc shape and charged surfaces, the clay interacts with a wide range of macromolecules such as proteins, polymers, DNA, etc. Some of the applications for clays 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. It is an interdisciplinary field, incorporating concepts and knowledge from the inorganic and structuralchemistry, physical chemistry, chemistry of materials, 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.
The Importance of Science
The unique properties of soil clay minerals, such as the layered structure of the nanometer scale, the presence of fixed and interchangeable charges, the ability to adsorb and retain (intercalate) molecules, the ability to form stable colloidal dispersions, the possibility of individual surface modification and interlayer chemical modification, and others make the study of clay chemistry is a very important and extremely diverse field of study.
Many different fields of knowledge are influenced by the physicochemical behavior of clay minerals, from environmental sciences to chemical engineering, from ceramics to nuclear waste management.
Their cation exchange capacity (CEC) is of great importance in balancing the most abundant cations in the soil (Na+, K+, NH4+, Ca2+, Mg2+) and pH control, which directly affects soil fertility. The study of clays (and minerals) also plays an important role in dealing with Ca2+, which usually comes from land (river water) to the seas. The ability to change and control the composition and content of minerals offers a valuable tool in developmentselective adsorbents with various applications, such as, for example, the creation of chemical sensors or cleaning agents for contaminated water. This science also plays a huge role in the classification of clay mineral groups.
