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A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate ([SiF6]2-) and other anions are also included. Silicates comprise the majority of the earth's crust, as well as the other terrestrial planets, rocky moons, and asteroids. Sand, Portland cement, and thousands of minerals are examples of
Silicate compounds, including the minerals, consist of silicate anions whose charge
is balanced by various cations. Myriad silicate anions can exist, and each can form
compounds with many different cations. Hence this class of compounds is very
large. Both minerals and synthetic materials fit in this class.
In the vast majority of silicates, including silicate minerals, the Si occupies a
tetrahedral environment, being surrounded by 4 oxygen centres. In these structures,
the chemical bonds to silicon conform to the octet rule. These tetrahedra
sometimes occur as isolated ??SiO?_4?^(4-)centres,but most commonly, the tetrahedral are joined together in various ways, such as pairs ?(?Si?_2 O_7)?^(6-) and rings ?(?Si?_6 O_18)?^(12-) .
Commonly the silicate anions are chains, double chains, sheets, and threedimensional
frameworks. All such species have negligible solubility in water at
Silicates with non-tetrahedral silicon
Although the tetrahedron is the common coordination geometry for silicon compounds, silicon is well known to also adopt higher coordination numbers. A well known example of such a high coordination number is hexafluorosilicate?(SiF_6)?^(2-). Octahedral coordination by 6 oxygen centres is observed. At very high pressure, even SiO2 adopts this geometry in the mineral stishovite, a dense polymorph of silica found in the lower mantle of the Earth. This structure is also formed by shock during meteorite impacts. Octahedral Si in the form of hexahydroxysilicate ([Si(OH)6]2-) is observed in thaumasite a mineral occurring rarely in nature but sometimes observed amongst other calcium silicate hydrate artificially formed in cement and concrete submitted to a severe sulfate attack.
Silicate Rocks and Minerals
In geology and astronomy, the term silicate is used to denote types of rock that consist predominantly of silicate minerals. On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that form and rework the crust.
These processes include partial melting, crystallization, fractionation, metamorphism, weathering and diagenesis. Living things also contribute to the silicate cycle near the Earth's surface. A type of plankton known as diatoms construct their exoskeletons, known as tests, from silica. The tests of dead diatoms are a major constituent of deep ocean sediment. Silica, or silicon dioxide, SiO2, is sometimes considered a silicate, although it is the special case with no negative charge and no need for counterions. Silica is found in nature as the mineral quartz, and its polymorphs.
Properties of silicon:
1. Good electrical insulation. Because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide
range of electrical applications.
2. Thermal stability (constancy of properties over a wide operating range of -100 to 250 ï¿½C).
3. Though not a hydrophobe, the ability to repel water and form watertight seals.
4. Excellent resistance to oxygen, ozone and UV light (sunlight). This has led to widespread use in the construction industry (e.g.
coatings, fire protection, glazing seals), and automotive industry (external gaskets, external trim).
5. Does not stick.
6. Low chemical reactivity.
7. Low toxicity, but does not support microbiological growth.
How to classify silicates? ^([1,2])
For the study of many silicate based minerals, a classification scheme is required. Otherwise, the huge amount of information becomes unmanageable. We use a simple classification scheme based on the number of SiO44- units that are connected together by sharing the oxygen atoms.
Orthosilicates are minerals consisting of only single SiO44- units. The cations are some other metals. For example, the following minerals are orthosilicates:
The Be and Zn ions are tetrahedrally bonded to the oxygen of the silicate in these two minerals: phenacite, Be2SiO4 and willemite, Zn2SiO4.
In olivine, (Fe, Mg)2SiO4, the cations are either Fe2+ or Mg2+. This formula suggests that this mineral is a mixed salt of iron and magnesium silicates. These cations are octahedrally coordinated to the oxygen atoms of the silicate. Pure salt Fe2SiO4 is called fayalite, and Mg2SiO4 is called forsterite.
When two SiO44- units are linked together, they form the pyrosilicate group, Si2O76-. For example, thortveitite, Sc2Si2O7 is a pyrosilicate.
Ring and chain silicates
When two oxygen of SiO44- units share with other SiO44- units, the silicates form a ring or an infinite chain. The stoichiometry of the silicates becomes (SiO3)n2n-.
Benitoite BaTi(SiO3)3 contain three silica rings, but these are relaxed 6-atom rings
The precious stone beryl Be3Al2(SiO3)6 contain six-silica rings.
Single chain silica are called pyroxenes. Some synthetic metasilicates Na2(SiO3) have been shown to contain the simple chain silicates (SiO3)n, in which the Si-O bonds of the type Si-O-Si are 168 nm, with the Si-O-Si angles of 137o. The Si=O bonds are shorter, 1.57 nm. The natural pyroxenes include enstatite, MgSiO3, diopside, CaMg(SiO3)2, and jadeite, NaAl(SiO3)2.
Double chain silicates
Double chain silicates are called amphiboles, part of the double chain is shown here, same as the double chain shown in Inorganic Chemistry by Swaddle. These chains have a stoichiometry of (Si4O11)n6n. You can easily identify one such unit in the diagram. Tremolite, Ca2Mg5(Si4O11)2(OH)2, is such a mineral.
The true asbestoses such as crocidolite or blue asbestos consist of double chain silicates. Asbestoses have been identified as carcinogens, and its application has since been limited due to a ban to limit its exposure to the public. Federal Health Canada has given some strict guidelines for the applications of crocidolite.
Most comercial asbestoses are chrysotile, which contain layers of silicate sheet as we shall below.
Silicates with sheet structures
Sheet slilicates are called phyllosilicates (phyllo means leaflike). These silicates are easy to cleave (as does graphite). Talc is a typical sheet silicate, Mg3(OH)2(Si2O5. Talc is a main ingredient of the soapstone (steatite).
The diagram below shows the arrangement of sheets in brucite, Mg(OH)2, in which the sheets consist of corner sharing octahedrons of Mg(OH)6. In chlorite, there are two types of sheets. Half of the sheets are the same as those of brucite, but half of the brucite-sheets are sandwiched between sheets of silicates. The talc consists of only the sandwiched sheets.
The diagram came from a polysome series which discusses sheet silicates. Serpentine, Mg3(OH)4Si2O5, has curved sheets.
The comercial asbestos chrysotile is a sheet silicate, but the sheets are rolled up like a tube. These tubes appear as fibers, and they are usually known as asbestos.
Silicates with 3-dimensional framework
As mentioned earlier, the SiO44- units can share every oxygen with other units to form a three dimensional network, and quartz has such a structure. A portion of such a framework is shown here. In this arrangement, the stoichiometry is reduced to SiO2, which is often called silica. A collection of small pieces of quartz is sand.
Silicates in industry
In the industry Silicates of many metals are used. These compounds have many and broad uses. Some of the compounds and their uses are listed below:
Aluminium silicate- Aluminium silicate (or aluminum silicate) has the chemical formula Al2(SiO3)3. It has a density of 2.8 to 2.9 g/cmï¿½, a vitreous lustre, a refractive index of 1.56, a Mohs hardness of 1-2, and an orthorhombic crystallography. It is insoluble and used as a refractory in glassmaking.
Magnesium Silicate- Also known as talc, which is hydrated magnesium silicate having chemical formula Mg3Si4O10(OH)2 . It has a perfect basal cleavage, and the folia are non-elastic, although slightly flexible. It is very soft and sectile (can be cut with a knife); as the defining mineral for the value of 1 on Mohs hardness, it can be easily scratched by a fingernail. It has a specific gravity of 2.5ï¿½2.8, a clear or dusty lustre, and is translucent to opaque.
Uses of Silicates
Glass aquarium manufacturers have used 100% silicone sealant exclusively from its inception in order to join glass plates, making aquariums of every size and shape. Glass joints made with silicone sealant can withstand a great deal of pressure, making obsolete the original aquarium construction method using angleiron and putty. This same silicone is also used to make hinges in aquarium lids or even for minor repairs. Not all commercial silicones are safe or aquarium manufacture, nor is silicone used for the manufacture of acrylic aquariums as silicones do not have longterm adhesion to plastics.
In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not watersoluble
and is far less likely than other lubricants to foul. Automotive spark plug wires are often insulated by multiple layers of silicone to prevent sparks from jumping to adjacent wires, causing misfires.
Silicone tubing is sometimes used in automotive intake systems (especially for engines with forcedinduction).
Sheet silicone is used to manufacture gaskets used in automotive engines, transmissions and other applications.
Automotive body manufacturing plants and paint shops must avoid the presence of all silicones, as they may cause "fish eyes," small, circular craters that appear in the finish.
Additionally, silicone compounds such as silicone rubber are used in the automotive industry for airbags as coatings and sealants. The high strength of silicone rubber makes it an optimal adhesive/sealant for high impact airbags.
Silicone films can be applied to silicabased substrates like glass to form a covalently bonded hydrophobic coating. Fabrics may be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.
As a low taint, nontoxic material, silicone can be used where contact with food is required. Silicone is becoming an important product in the cookware industry, particularly bakeware and kitchen utensils.
It is used as an insulator in heat resistant potholders and similar, however it is more onductive of heat than the less dense fiberbased ones. Silicone oven mitts are able to withstand temperatures up to 357 ï¿½C (675 ï¿½F), and allow reaching into boiling water. Molds for chocolate, ice, cookies, muffins, etc. Some novel designs are steamer, egg boiler, vegetables cooker, cooking lids, pot handle, kitchen mats, etc.
Silicones are used as active compound in defoamers due to the low water solubility and good spreading properties.
Liquid silicone can be used as a dry cleaning solvent. Touted as an "environmentally friendly" alternative to the traditional perchloroethylene (or perc) solvent, the decamethylpentacyclosiloxane (D5) process has been patented by the company GreenEarth Cleaning.
The solvent degrades into silica and trace amounts of water and carbon dioxide; and while the silica waste produced is hazardous inhaled, currently, there is no evidence that it is environmentally worse than the silica from regular beach sand. This significantly reduces
the environmental impact of a typically highpolluting industry.
Additionally, liquid silicone is chemically inert, meaning it does not react with fabrics or dyes during the cleaning process. This reduces the amount of fading and shrinking that most drycleaned garments experience.
Electronic components are sometimes encased in silicone to increase stability against mechanical and electrical shock, radiation and vibration. This is often called "potting".
Silicones are used where durability and high performance are demanded of components under hard conditions, such as in space (satellite technology). They are selected over polyurethane or epoxy encapsulation when a wide operating temperature range is required (-65 to 315ï¿½C). Silicones also have the advantage of little exothermic heat rise during cure, low toxicity, good electrical properties and high purity. The use of silicones in electronics is not without problems, however. Silicones are relatively expensive and can be attacked by solvents.^()
Silicone easily migrates as either a liquid or vapor onto other components.
Silicone contamination of electrical switch contacts can lead to failures by causing an increase in contact resistance, often late in the life of the contact, well after any testing is Completed.^() Use of siliconebased spray products in electronic devices during maintenance or repairs can cause later failures.
Silicone foams have been used in North American buildings in an attempt to firestop openings within fireresistancerated wall and floor assemblies to prevent the spread of flames and smokefrom one room to another. Silicone foam firestops have been the subject of controversy and press attention due to smoke development from pyrolysis of combustible
components within the foam, hydrogen gas escape, shrinkage and cracking. These problems have been exposed by whistleblower Gerald W. Brown and have led to a large number of reportable events among licensees (operators of nuclear power plants) of the Nuclear Regulatory Commission (NRC). When properly installed, siliconefoam firestops can be fabricated for building code compliance. Advantages include flexibility and high
dielectric strength. Disadvantages include combustibility (hard to extinguish) and significant smoke development. Silicone can also be found in aircraft technology.
Silicone greases are used for many purposes, such as bicycle chains. A dryset lubricant is delivered with a solvent carrier to penetrate the chain. The solvent evaporates, leaving a clear film that lubricates but does not attract dirt and grit as much as a traditional "wet" lubricant.
Silicone personal lubricants are also available, for use in medical procedures or sexual activity.
Silicone, particularly the gel form, is used in bandages and dressings, in breast implants and a variety of other medical uses. Polydimethylsiloxane (PDMS) has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersomes.
Twopart silicone systems are used to create rubber molds which can be used for production casting of resins, foams, rubber and lowtemp alloys.
A mold made of silicone generally requires little or no mold release or surface preparation as most materials do not adhere to moldmaking silicone.
For experimental uses, ordinary onepart silicone can also be used, either to make molds, or to mold into shapes. Common vegetable cooking oils and petroleum jelly can be used on mating surfaces as a mold release agent.^()
Silicones are ingredients in many hair conditioner, shampoo, and hair gel products. Some silicones, notably the amine functionalized
amodimethicones, are excellent conditioners. They improve combability, feel, and softness, and also lessen frizz. Another silicone family, the phenyltrimethicones, are used in veflectionenhancing and colorcorrecting hair products, where they increase shine and glossiness (and possibly effect subtle color changes). Phenyltrimethicones, unlike the conditioning amodimethicones, have refractive indices (typically 1.46) close to that of human hair (1.54). Achieving both highshine and excellent conditioning in one hair care product is much more difficult than simply adding two different silicones to the formulation, because amodimethicone and phenyltrimethicone interact with and dilute each other. This is one reason why modern hair care products, and cosmetics generally, are among the most highly engineered consumer cosmetic products.
Silicones are also used in some shaving products and personal lubricants. Menstrual cups are often made of medical grade silicone for its durability, reusability, and biocompatibility. Silicone is also material of choice for soft sex toys, due to its durability, cleanability and lack of phthalates, chemicals suspected of having carcinogenic and mutagenic effects on the skin and mucous membranes. ^() Specific grades of silicone rubber are used widely in the production of baby bottle teats due to their cleanliness, aesthetic appearance, and
low extractable content.
Plumbing and building construction
The strength and reliability of silicone rubber is widely acknowledged in the construction industry. Onepart silicone sealants and caulks are in common use to seal gaps, joints and crevices in buildings. Onepart silicones cure by absorbing atmospheric moisture, which helps in the professional installation. In plumbing, silicone grease is typically applied to Orings
in faucets and valves. Whilst the film is extant it prevents lime from sticking to
Silicone balls have become a juggler's favorite due to the high bounce back, and are used as a response system in many low response yoyos. ^( )Silicone has the potential of replacing plastic in creating many forms of toys.
Silicates are the most abundant class of minerals on Earth. Sand and quartz crystals are silicates, and silicon is its central part. Because of their unique properties, silicates have many cool technological uses.
Unique properties of silicates include the ability to conduct electricity, produce a high-frequency vibration and provide thermal insulation. Silicon is the central component of silicates. It is a very hard crystal that can be cut to minuscule sizes, and it conducts electricity. It is because of these characteristics that silicon is the perfect material to make microchips, which run every computer, cell phone and gaming device.
Microchips are silicon cut down to micro proportions with governing instructions imprinted on them through photographic processes. Silicon's ability to conduct electricity powers the microchip to operate. The governing instructions tell the computer how to operate similarly to how the human brain tells the body to operate.
Quartz crystals are another silicate with a unique ability to produce a rhythmic high-frequency vibration. For this reason, these crystals are used to make oscillators used in watches, radios and pressure gages. The crystal emits a rhythmic beat that when hooked up to an electronic source and movement piece can be logged, generally as a megahertz (MHz), which is 1 million cycles per second. In the instance of a clock, the crystal's rhythmic beat moves the second hand. Quartz crystals are also used in mysticism.
Silicates are also used to make glass and ceramics. To do so, hard, formless material like sand or ceramic clay is heated to high temperatures, turning it into malleable material that can be formed to make drinking glasses, for example, or when lead is added to the molten liquid--crystal glass.
Silicate ceramics have clearly defined thermal properties and, as such, are particularly useful for advanced engineering and technology. For example, silicate ceramic tiles are used on the space shuttle to shield it from the extreme temperatures of the outer atmosphere.
Silicates are the most abundant mineral class on Earth. In general, they are hard and cut easily to micro pieces, produce a rhythmic high-frequency vibration, and provide thermal insulation properties. These unique characteristics make them highly useful for myriad high- and low-tech products, from microchips to watches.
Silicates have a very wide application in our day to day life. Millions of uses have been discovered and research work is in progress to find innumerable other. The abundance of silicate compounds and their ability to dis-integrated into considerably small pieces in a regular fashion has made way for vast variety of uses.
Above all, the semiconductor properties also makes way for possibilities in discovery of new compounds and their uses in the electronics industry.