Silicates – Classifications, Reactions and Uses
When we say silicates most people mistake it for silicon, but these two are different. Although they are not so different from each other but they can’t be used interchangeably.
A Silicate is a compound that contains anion (positive charge) of silicon and oxygen. In other words, silicate is the combination of the positive charge gotten from Silicon and oxygen. It is a salt – like mineral.
The Silicate Formula
The formula for silicate is (SiO.4-x)n, and it’s minerals are made up of what is known as silicate groups.
The Silicate Structure
Silica is known as a crystalline polymer, it is also considered to be a giant molecule, which contains tetrahedral SiO44_ monomer. It’s units polymerizes through what is known as the Si-O-Si bonds. Also when studying the structure you will discover that it’s oxygen atom has been surrounded by 2 silicon atoms. One thing you need to take note of is that the hybridization of silicon in silica is known as sp3.
The Classification Of Silicates
Silicates have it’s own classification, we shall be discussing each one of them. The classifications of silicates includes the following
The Nesosilicates (Orthosilicate) – This is a silicate ion that is a conjugate base of weak orthosilic acid. They are very rare and only found with cation (negative charge) for highly insoluble salts.
The Pryosilicate (disilicate) – This is the joining of two tetrahedra units by sharing one oxygen, that is to say this contains two tetrahedra units which are supposed to have separate oxygen each but instead they are sharing one oxygen.
The Cyclic or Ring silicate – Just by hearing the name of this silicate you should already know what it will look like when constructing it’s chemical structure. Cyclic silicate are combination of tetrahedron that have two oxygen atom per tetra, so instead of sharing one like the pryosilicate they actually have two separate oxygen atom for each tetra. When this happens they form a close ring which in turn is used for name identification.
The Chain silicate – This is similar to the cyclic silicate when it comes to sharing of oxygen but unlike the cyclic where it from close rings, here it forms a chain like pattern.
There’s also the presence of double chain silicate, this is much easier because it’s just the joining of two chain silicate to form one. They are called amphiboles.
The Sheet Silicate – These type of silicates are also refered to as the “Phyllosilicates”. In order to form this silicate, the 3 bridging has to be shared per silicon atom. some of the basic examples of the sheet silicates are clay, muscovite, and mica.
The 3 – dimensional silicates – let’s talk about how this 3 – dimensional silicates are being formed. In order to form a three – dimensional silicate, we need to share all four corners of the oxygen atom of tetrahedra. The 3 – dimensional silicates can be seen represented as (SiO2)n.
Do you also know that silicate contains Rock and they are known as silica rocks ?
So before we explain this, I want us all to understand what silica rocks are. So, what is the silicate rock all about ???
The Silicate Rocks
Silicate rocks are rocks which contains a large portion of silica dioxide. These rocks has the ability to exist in a form known as quartz, amorphous silicate, or cristobalite.
The Uses Of Silicates
Silicate like all other elements have it’s uses both industrially and domestically.
We have taken time to list a few uses of silicate below:
1. They are used in making concrete materials like glass. Our regular glasses with we use in making windows and wine cups are gotten from silicate.
2. They are useful when making experimental (lab) prisms. Those prisms used in laboratory for conducting physics experiments are gotten from the breaking down of silicate.
3. They are used for eyeglass making. The lenses on glasses are gotten from silicate.
4. Domestic use for silicate are our everyday graphite and gravel. In most house/ building constructions you’ll see then use graphite and gravel, those are gotten from silicate.
The Different Types of Silicates
Silicates comprise a large portion of Earth’s crust. The crystal structures of crystalline silicate minerals were determined early in the 20th century by x-ray diffraction.
A wide variety of silicate mineral structures exist. They are classified into groups depending on how the corner oxygens of the SiO4-4 tetrahedra are linked together.
We shall now examine another type of classification of silicates.
Orthosilicates
Orthosilicates are silicate minerals with a general chemical formula SiO2. They have tetrahedral structures and a basic nature. They react with weak acids readily. They have a low melting point and a low boiling point. They can be formed from silica or alkali materials. They are usually found in metamorphic rocks and igneous rocks. They are very common in the Earth’s crust. They are also common in many different types of rock forming processes.
The basic structure of orthosilicates is two oxygen atoms sharing one silicon atom in tetrahedra. These tetrahedra are connected in chains. This arrangement is called the monoclinic system. Monoclinic silicates have a very high degree of crystallinity and a very high specific gravity. They are the most common silicate minerals and they occur in a wide variety of environments and conditions.
They are also commonly found in the Earth’s mantle. The most famous example of a monoclinic silicate mineral is olivine, which is found in volcanic lava. Olivine is a green mineral with a specific gravity of 4.11.
When a single oxygen atom in a tetrahedron shares itself with other tetrahedra, it forms a disilicate. The resulting ion is less basic than the orthosilicate ion and it can only be found in minerals with an odd number of tetrahedra. Disilicates are quite common in nature and they can form a wide variety of minerals.
Another type of disilicate is the aluminosilicate. This mineral is formed from tetrahedral and monoclinic tetrahedra. It has a stoichiometry of SiO4. It is a highly brittle mineral with a low melting point and a high boiling point. It is very useful for making ceramics.
It is also found in many clays. The aluminosilicates are mainly used for building and construction material, but they are also used in pottery and glass manufacturing. They are very strong, non-combustible and water resistant. They are also very durable and can withstand extreme heat.
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Metasilicates
The metasilicates are a group of sodium silicate salts which form crystals with a honeycomb porous structure. They are often used as an emulsifier, humectant, deflocculant, or binder for other products. They are nontoxic and have purifying, emulsifying, moistening, dispersing, permeating, and PH buffering properties. Sodium metasilicate is available as an anhydrous, pentahydrate or non-ahydrate, and in bulk or fine powdered form.
Sodium metasilicate is used in cement, passive fire protection mixtures, refractories, textile and lumber processing, and automobiles. It is also found in many industrial cleaning applications including laundry, dairy, metal, and floor cleaning; deinking paper; washing carbonated drink bottles; insecticides, fungicides, and antimicrobial compounds; and as a chemical intermediate for silica gel catalysts. It is a popular ingredient in household and commercial cleaning and detergent products, and in laboratory chemicals, adhesives, sealants, finger paints, non-metal surface treatment products, polishes, waxes, and polymers.
Because of its ability to react with aluminum oxide and other metal oxides, it is an important component of protective coatings for a number of different industrial processes such as electrolytic galvanizing and hot dip coating. The salt also provides a physical barrier against alkali attacks by forming a silica film on the surface of the affected material which reduces its susceptibility to corrosion.
As an aqueous solution, it has excellent deflocculating and suspending properties. It is also a good lubricant, with a low pour point and high viscosity. Unlike caustic soda, it is not toxic and does not degrade plastics. It is also non-corrosive to steel, copper, zinc, and tin.
Oral repeated dose toxicity studies on rats, mice, and turkeys have shown that disodium metasilicate does not induce any toxic effects (Ito et al, 1975; Saiwai et al, 1980; Kayongo-Male and Jia, 1999). Using the OECD QSAR toolbox v.2.3 (2012), it is estimated that it is unlikely that disodium metasilicate will be metabolised by the cytochrome P450 enzyme system in-vivo. This is confirmed by the lack of any significant changes in blood plasma calcium, magnesium, or liver Zn observed after 90 days administration of disodium metasilicate pentahydrate to experimental animals.
Inosilicates
Silicon is the second most abundant element on Earth. It occurs in a wide variety of mineral forms. Its pure crystalline form is found as quartz, jasper and opal and in the brown amorphous powder known as “dirty beach sand.” Pure silicon has a valence shell of four oxygen ions. It is therefore an insulator, but it can be made to act as a semiconductor by the addition of a very small number of impurities that allow some charge movement. This is the process of doping.
The sharing of one, two, three or all of the oxygen atoms between silicate tetrahedra by various metal cations produces the characteristic structures that distinguish the different types of rock-forming silicate minerals. These are classified according to their structure into cyclosilicates, inosilicates, nesosilicates, phyllosilicates and sorosilicates.
Inosilicates are those with infinite chains of linked tetrahedrons. They are further subdivided into single chain pyroxenes (ex: spodumene) and double chain amphiboles (ex: actinolite). These groups are important in both igneous and metamorphic rocks.
Cyclosilicates are closed ring-like silicates with the sixfold composition of SiO6O18. Nesosilicates have isolated groups of tetrahedrons with the unit composition of SiO2. Phyllosilicates are infinite flat sheets of tetrahedrons that can be split into pairs, and have a unit composition of Si2O5. Sorosilicates are silicates with pores in the tetrahedral crystal lattice.
Silica perovskites are not stable at the surface but are found in the lower part of Earth’s mantle. They are composed of spinel-type olivine mineralogy with ferropericlase.
In polarized light thin section orthorhombic pyroxenes (ex: augite) will display parallel extinction when looking down the c-axis. In contrast, clinopyroxenes will show inclined extinction.
An interesting inosilicate with a triclinic structure is nepheline syenites (ex: spodumene). It is found in granulated and fibrous crystalline masses and has the Mohs hardness of 5 to 5. It shows green to blue pleochroism in hand specimen and in thin section, and its cleavage is usually elongated. It is a common constituent in igneous and metamorphic basalts, diabases and serpentinites. It is also associated with zeolites, datolite, prehnite and calcite.
Tectosilicates
The tectosilicate group, or polysilicates as they were once known, are silicate minerals that contain tetrahedral SiO4-units linked together in a three-dimensional framework. Each SiO4-unit has four oxygen atoms at its corners which link it to the other three in an arrangement that resembles a honeycomb. This structure gives tectosilicates their strength and allows them to occupy an enormous range of positions within the Earth’s crust.
The members of the feldspar family are classified as tectosilicates, and their structure makes them excellent rock-forming minerals. Plagioclase and orthoclase are common in igneous rocks, while sanidine and microcline are found in metamorphic rocks. The silicate minerals nepheline and sodalite are also members of the feldspar family, and their structures provide them with excellent insulation properties. A member of the feldspar group is quartz, which has a high melting point that allows it to be used in thermocouples and in glass making.
Other tectosilicates include zircon, corundum and titanite. These have a similar crystal structure to quartz and are also important rock-forming minerals. Ilmenite, a major ore of Ti, is a member of this group as well and is common in igneous volcanic and plutonic rocks, and in metamorphic clastic sedimentary rocks. It forms a series with magnetite and is usually acicular or isometric. It is opaque which makes it difficult to distinguish from other oxide minerals by hand specimen, but when thin sectioned it has a distinctive octahedral habit.
Cyclic silicates, also known as amphiboles, contain (SiO3)n2n- ions which are formed by linking a number of tetrahedral SiO44- units cyclically. Each tetrahedral shares two oxygen atoms with adjacent tetrahedra. Beryl, a gem mineral, is an example of a cyclic silicate.
Chain silicates, or pyroxenes, are similar to amphiboles, and they are often considered to be a subclass of tectosilicates. They contain (SiO3)n2n-ions which are formed by linking a number, called n, of tetrahedral SiO44-units linearly. Each tetrahedron has one oxygen atom shared with the adjacent tetrahedra.
Petalite, leucite and calcite are examples of chain silicates. They are basic and react with weak acids to form silicates.
Silicates are a family of minerals which are found in the Earth’s crust. They contain silicon and oxygen, two of the most abundant elements in the planet. This means they make up a large proportion of the rocks that are found on our planet and form the basis of almost all sedimentary, metamorphic and igneous rock types. This makes them the most important raw material in natural geology and it is no wonder that silicates are also a key part of many industrial products.
Most silicate minerals consist of tetrahedral silicon-oxygen units (SiO44-). The structure of these minerals can vary greatly, however. Some have a layered structure, others have a ring-like structure and some even have double chains. This variation gives rise to the many different minerals that can be classified as silicates.
In the simplest structure, which is common in the mineral olivine, a group of silicon-oxygen tetrahedra are linked together to form rings. These tetrahedra are then bonded to divalent iron or magnesium cations. These minerals are called cyclosilicates, or ring silicates. Three-member rings are common, as in olivine; four-member rings, as in axinite; and six-member rings, as in beryl, can also occur. The general chemical formula for these minerals is SiOx2n3-x32-yn34.
A second type of silicate contains two oxygen atoms per silicon-oxygen tetrahedron. This is known as the inosilicate group. It includes minerals such as feldspar, micas and talc. Inosilicates have a relatively high degree of crystallinity and are very hard. They have a good cleavage, which is the ability to split along definite smooth planar surfaces.
Inosilicates are the most common silicates and are found in a wide variety of rock types. In addition, the inosilicate group has several minerals that are known as pyrosilicates, or polysilicates. Pyrosilicates are characterized by the presence of large tetrahedra that are arranged in a hexahedron. This allows for the substitution of aluminium ions, or alkali metals, for the silicon atoms, and this changes the structure and chemical composition of the mineral.
The third type of silicate is the tectosilicate group, which contains a large number of minerals. These are characterized by the presence of aluminium and magnesium atoms in the structure, either replacing silicon atoms or joining with them to form a polymer. These minerals are extremely tough and are used in a wide variety of industrial applications, for example as catalysts and in metallurgy.
Other types of silicates include quartz, alumina and tridymite. Quartz and alumina are very pure forms of silica and therefore are the main source of silica for glass, enamels and refractory materials. Silica is also present in sand, clay and some sedimentary rocks such as limestone and chert. It is also a very important constituent of igneous rocks such as granite and basalt. Other uses of silicates include inorganic binders like cement and water glasses, synthetic zeolites and organosilicates such as tetraethyl orthosilicate (TEOS), which is the most widely used industrial silicic acid precursor.
