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The secret of silicates: Why is the combination of silicon and oxygen so strong?
Silicates are important chemical compounds, mainly composed of silicon and oxygen, and compounds of these two elements are ubiquitous on Earth. They not only exist in nature in the form of minerals, but are also widely used in industry and daily life. Therefore, a deeper understanding of the structure and chemistry of silicates can help us understand their potential applications and why these compounds can withstand extreme environments.
The structure of silicates is based on an idealized tetrahedron, with a silicon atom at the center and four oxygen atoms at the corners, connected by single covalent bonds.
Structural principles
In most silicates, the silicon atom forms strong, stable bonds with four oxygen atoms. This structure produces some very strong materials that behave geologically like rocks. The classification of silicates depends mainly on the length and cross-linking of the silicate anions. Here are some of the main types of silicates:
Isolated silicates
The isolated tetrahedral silicate anion has the chemical formula SiO4−4 and is found in minerals such as olivine ((Mg,Fe)2 sub>SiO4). In this group, two or more silicon atoms can share oxygen atoms to form more complex anions, such as pyrosilicate Si2O6−7.
Chain silicate
Chain silicates are structures where each silicon atom shares two oxygen atoms, resulting in a straight chain or ring structure. For example, a common mineral in single-chain silicates is pyroxene, while a representative of double-chain silicates is hornblende.
Sheet silicate
In sheet silicates, each silicon atom shares three oxygen atoms, forming a two-dimensional structure. This structure gives them a strong cleavage plane. For example, micas such as muscovite and biotite fall into this category.
Framework Silicate
In framework silicates (i.e., structural silicates), each tetrahedron shares its four oxygen atoms to form a three-dimensional structure. Quartz and feldspar belong to this type.
Non-tetrahedral silicate
While tetrahedral is the common coordination geometry for silicon(IV) compounds, silicon can also exist in higher coordination numbers. For example, in hexafluorosilicate SiF6−2, the silicon atom is surrounded by six fluorine atoms in an octahedral arrangement.
At extremely high pressures, silicon's geometry changes, and even silicon dioxide in some high-pressure minerals such as garnet adopts a six-coordinated octahedral geometry.
Chemical properties
Silicates are chemically inert, which makes them relatively stable as common minerals. In certain industrial applications, silicates react with other metal oxides and water to form structural materials such as cement. On the other hand, the solubility of silicates varies depending on the nature of the anion, and some types of silicates are soluble in water and further used in a variety of wet chemical applications.
Reactivity and Detection
The reactions of silicates are generally stable and can be used to detect their forms in solution. When reacting with molybdate anions, yellow silicomolybdate complexes are produced. This provides a method to study the involvement of silicates in nature, and the reaction rates of silicates with different degrees of polymerization vary.
Geological and industrial importance
The solubility of silicates is crucial for understanding biomineralization and the synthesis of industrially important catalysts such as zeolites. In geology, silicates can provide key information for a variety of specific environments and show potential in reducing CO2 emissions in industrial applications such as cement and concrete.
Conclusion
As we gain a better understanding of silicates, we can better harness these abundant and important materials in nature. However, this also raises a question: How can we use silicates more effectively to promote sustainable development in the face of global environmental challenges?
