Silicates

Silicates are chemical compounds comprising silicon, oxygen, and a metal. Most of the earth’s crust is made up of silicates, as is about one-third of the world’s minerals. A silicate’s chemical base is a tetrahedron, a pyramid-shaped molecule with negatively charged ions. Silicates are considered the building blocks of the earth’s crust, appearing in a wide range of forms, including sand, quartz, and gemstone.

Basic Principles

Silicates are considered the building blocks of the planet, constituting more than 90 percent of the earth’s crust. While there are a wide range of types of silicates of varying size and degrees of solidity, each has a basic chemical composition consisting of a silicon and oxygen-based tetrahedron (a molecule in which the central atom forms four bonds). In this case, the basic formula is SiO4.

The chemical bond forming SiO4 is said to be mesodesmic: In the case of a silicate, the silicon and oxygen ions share an electrostatic charge, causing them to bond with one another. However, one of the oxygen ions is not shared with a silicon ion, leaving it capable of bonding with other ions. The result of a mesodesmic bond is that the molecule will be able to bond with ions from outside the group.

There are seven basic types of silicates, all of which are classified based on the degree of polymerization (a process by which single molecules bond to form larger compounds known as polymers) that occurs within the SiO4 tetrahedron. These seven types of silicates are nesosilicates (island silicates), sorosilicates (group silicates), cyclosilicates (ring silicates), two types of inosilicates (single- and double-chain silicates), phyllosilicates (sheet silicates), and tectosilicates (framework silicates).

Because of their mesodesmic nature, silicates appear in a broad spectrum of forms. For example, nearly all igneous rocks (rocks that have been formed from magma thrust from deep within the earth’s surface) are silicates, as are most metamorphic rocks (stones that change in structure when subjected to certain pressure, temperature, and chemical conditions). Furthermore, silicates are found in an overwhelming majority of sedimentary rocks—stones that are weathered from other rocks. Sand is one of the best-known of these types of sedimentary silicates.

Different Types of Silicates

There are seven general types of silicates. These categories are determined based on how the silicate tetrahedron’s four oxygen atoms have connected to the single silicon atom. Island silicates (nesosilicates), for example, are configured in such a way that the oxygen atoms surround the silicon atom at the corners of the tetrahedron. In this configuration, the remaining oxygen atom is shared with other charged ions, such as magnesium, calcium, and iron.

When two silicate tetrahedra share a single oxygen atom, the dual configuration is known as a double island silicate (sorosilicate). The unusual, hourglass-shaped configuration of the sorosilicate makes it somewhat rare when compared with other silicates. However, one type of double island silicate, the mineral epidote, is found in large volume in certain metamorphic rocks (rocks that, under intense heat and pressure, change in structure to adapt to the environment).

In some cases, two of the oxygen atoms are shared among tetrahedra. The resulting structure appears in the shape of a ring. One of the best-known examples of these cyclosilicates is the mineral beryl. Varieties of beryl include such gemstones as emeralds and aquamarine.

In other cases, two of the oxygen atoms are shared in such a way that the resulting configuration of linked tetrahedra appears in the shape of a single, linked chain. Single-chain silicates (inosilicates) typically make up igneous rocks (rocks that have been pushed from the earth’s interior through volcanic eruptions) known as clinopyroxenes. Other examples of single-chain silicates are minerals known as orthopyroxenes that are found in meteorites primarily. Inosilicates also appear in double-chain configurations (amphiboles). The mineral crocidolite, an example of such a silicate, is common in naturally formed asbestos.

The sixth type of silicate is the sheet silicate (phyllosilicate). This group is formed when three of the oxygen atoms in a silicate tetrahedron are shared. The sheet variety of silicate includes such examples as mica, clay, and talc. Phyllosilicates are easily cleaved, which means that they will break along certain planes where chemical bonds are weakest.

The seventh and final manifestation of silicates is formed when all of the corner oxygen atoms in the silicate tetrahedron are shared by another silicate tetrahedron. The resulting framework configuration is three-dimensional rather than linear, creating a complex and diverse mineral that may include other elements. Some examples of these tectosilicates or framework silicates are quartz and feldspar, the latter of which is one of the most prevalent minerals found in the earth’s crust.

Silicates and

The mesodesmic properties of silicates do not simply help form minerals and other compounds. They also deconstruct rocks and minerals through the process known as weathering.

In weathering, elements contained in the atmosphere and in water bond with the elements on the surface of igneous, metamorphic, and sedimentary rocks. The compounds that are sloughed off these origin rocks are carried to sedimentary basins. These sedimentary rocks vary in size, from granular sand to larger pieces.

The presence of silicates in most of the earth’s minerals means that, when weathering occurs, rocks and minerals are more likely to be broken down because of silicates’ bonding characteristics. Many sedimentary rocks are again bonded with other igneous, metamorphic, and sedimentary rocks to form clastic rocks while in these sedimentary basins. The ability of many silicates to exchange charges between different external elements plays a key role in the formation of such clastic rocks as well.

Silicates and the

In light of the way charges are exchanged within silicate tetrahedra, scientists believe that silicates play an integral role in many of the earth’s major systems. One such system is the carbon cycle.

Under the framework of the carbon cycle, carbon (which is essential to life and to Earth’s geochemical processes) is first released from within Earth’s interior through carbonate rocks and carbon dioxide. Carbon dioxide is absorbed from the soil by plants, which are in turn eaten by animals (including humans). Animals breathe carbon dioxide, sending it into the atmosphere, where it acts to block harmful radiation from permeating the atmosphere. Precipitation (rain and snow) collects the carbon from the air and returns it to the soil and to bodies of water. The carbon cycle also entails the degradation of dead plant and animal tissue and the weathering process, returning carbon to Earth’s crust.

Scientific evidence reveals that silicates play an important role in many facets of the carbon cycle. For example, when metamorphic rocks are formed under extreme conditions beneath glaciers and oceans (where vertical force pushes downward on the earth’s tectonic plates, creating intense pressure and temperatures), the silicates in the changing rock bond with the resulting carbon dioxide, bringing this compound back to the surface and into the atmosphere. Additionally, during chemical weathering (whereby water is the key factor in the degradation of rocks), the hydrogen ions that are released bond quickly with silicates, producing clays and other metal compounds in sedimentary basins. Carbon dioxide, freed from the weathered rocks, returns to the atmosphere.

Although scientists understand chemical weathering and its role in the carbon cycle, a missing element of the study of this framework is the extent to which weathering may be predicted. In a 2011 study, researchers analyzed 338 North American rivers and the chemical weathering that occurs therein. Scientists concluded in part that the manner and volume by which charged particles (ions) are transferred—a key characteristic of silicates—may potentially serve as a predictor for the rate of carbon breakdown in chemical weathering.

Silicates and Hydration

Just as they differ in configuration, so silicates vary widely in terms of their textures. Some silicates, like phyllosilicates, are easily broken, while others, like quartz, are extremely hard and difficult to cleave. The resulting minerals and compounds are typically different, too: Some are gemstones, while others (like sand) are granular.

Many silicates are used for industrial purposes. For example, superheated sand (which is largely made of the tectosilicate quartz) may be re-formed as the crystal glass. Silicates also play a critical role in the creation of cement. In this latter example, heated and ground limestone is mixed with clay to form Portland cement powder. Silicates (specifically, calcium silicates) are added to form nodules known as clinker.

These silicates are the major contributors to the strength of cement. When water is added (a process called hydration), the silicates in the clinker bond, creating the solid mass with which walls and building foundations are constructed. Recent studies show that repeated hydration (including reheating the cement mixture with the silicate additive) of Portland cement, one of the world’s most popular cements, enhances the strength of the cement. This improvement comes from the bonding caused by the silicate tetrahedron configuration.

Research Tools

Researchers employ a number of tools in the identification and study of silicates. One of these tools is X-ray spectroscopy, which radiates X-rays onto the subject, which in turn absorbs the X-rays, creating a visible profile for researchers to examine.

Spectroscopy can help scientists assess the shape of a given mineral’s silicate tetrahedron, enabling scientists to identify the mineral within a larger rock. Spectroscopy has evolved to include other types of radiation, such as ultraviolet light and infrared, all of which can further help profile silicate minerals and their compositions.

To study the processes in which silicates play a role (such as weathering) and the processes by which the silicates themselves form, scientists are increasingly using computer modeling software. Modeling systems help researchers deconstruct a silicate (down to the molecular and even submolecular levels), compiling what is commonly a large amount of data on each component. Additionally, these systems can provide a detailed profile of silicate crystallization as it occurs through time and under certain conditions. Computer modeling also has become useful for researching the development and application of artificial silicates (such as insulation and cement additives), helping to create more efficient products and to reduce environmental waste.

In addition to the use of computer modeling, scientists have another useful tool in studying silicates: the Internet. Because of the global reach provided by the Internet, scientists can compare data on similar silicate deposits and on environmental conditions in different parts of the world. Such collaboration, which can take place in real time, can greatly enhance the pursuit of silicate study.

Implications and Future Research

With nearly all of the planet’s crust and nearly one-third of all minerals derived from silicates, it is understandable that silicates also play a major role in so many of the planet’s processes. Silicates are major contributors, for example, to the carbon cycle, helping to break down rocks and release carbon back into the atmosphere through weathering.

In light of the ongoing scientific pursuit of greenhouse gas reduction, researchers are exploring chemical weathering as an area of interest. In this regard, scientists hope to prevent emissions in the weathering process by sequestering carbon dioxide.

Some studies are examining the silicate-induced dissolution of certain minerals. In one such case, researchers are using their knowledge of weathering to artificially enhance a silicate mineral like olivine. Using such an approach could lead to new methods of carbon sequestration and a reduction in the overall release of carbon dioxide into the atmosphere.

In a similar vein, researchers are examining the use of silicates in fertilizer to reduce greenhouse gases in agriculture. According to one study of rice growth, an increase in silicate-based fertilizers can lead to the suppression of methane, another greenhouse gas. The study concluded that silicate-based fertilizer can help reduce the volume of greenhouse gas emissions. As research continues on the reduction and sequestration of greenhouse gas emissions, scientists will likely continue to explore silicates as an important factor in this pursuit.

Principal Terms

glass: crystal formed from superheated quartz silicate (sand)

mesodesmic: molecular bond in which one of the molecule’s oxygen ions is capable of bonding with other ions

Portland cement: common form of cement used in industry and construction; comprises hydrated, ground, and heated limestone, clay, and calcium silicates

sand: granular sedimentary silicate

tetrahedron: a molecule in which the central atom forms four bonds

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