Silicate

A Silicate is member of compounds consisting of silicate ([SiO]) anions, basically. However, it is usually used to refer to a member of family of anions which consist of silicon and oxygen with a general formula of [SiO_{4-x}^{(4-2x)}]_n, where x ∈ [0,2]. Hence, [SiO] is the basic one with x = 0 and called as orthosilicate. 
The parent elements of silicate anions, oxygen and silicon, make up the most of Earth's crust. They form the basis of silicates. Silicates make up more than 87% of the earth’s crust.
==History==
The earliest uses of silicates extend back to 5000 B.C. where the Egyptians industrialized pottery and ceramics. Silicates were profoundly used in making durable primitive tools, smooth containers, and even strong roofs to survive storms and keep warm.
Glass making arrived in the third century B.C. Today, glass making has been largely industrialized in numerous fashions. However, all methods of glass making use silica as a major component which is heated up to over 1700° Celsius. Interestingly enough, the types of silicates used influence the type of glass created as an end product.
==Bonding Chemistry==
In a silicate anion, the Si atom is covalently bonded to 4 oxygen atoms, Each oxygen atom possesses a formal negative charge. Hence, each tetrahedral unit has a formal charge of -4. The covalent Si-O bond has a bond enthalpy of 466 kJ/mol. The linkage between Si-O is very stable and instead of existing in discrete form, the silicates tend to form chains, sheets or networks.
The radius ratio of to requires that be coordinated by 4 ions in tetrahedral coordination.
In order to neutralise the +4 charge on the Si cation, one negative charge from each of the Oxygen ions will reach the Si cation. Thus, each Oxygen will be left with a net charge of -1, resulting in a SiO tetrahedral group that can be bonded to other cations. It is this [SiO] tetrahedron that forms the basis of the silicate minerals. Since is a highly charged cation, Pauling's rules state that it should be separated a far as possible from other ions. Thus, when these tetrahedrons are linked together, only corner oxygens will be shared with other groups. Several possibilities exist and give rise to the different silicate groups.
==Structures==
Silicate minerals are salt-like crystalline materials with metal cations and various types of silicate anions. Each silicon atom in a silicate is tetrahedrally coordinated by oxygen atoms.
===Orthosilicates===
The basic unit of a silicate, stated earlier, is the [SiO_4]^{4-} oxyanion. Minerals with isolated silicate anions are called orthosilicates. Examples include olivine (Mg,Fe)_2SiO_4, phenacite Be_2SiO_4, zircon ZrSiO_4, and garnets M_3M'_2(SiO_4)_3 where M = , , or and M' = , , or . 
===Pyrosilicates===
If two silicate tetrahedra share one vertex (Si_2O_2^{6-}), the anion is called a pyrosilicate. Pyrosilicate minerals include thorveitite Sc_2SiO7, and barysilite MnPb_8(SiO_7)_3.
===Metasilicates===
Metasilicates contain cyclic [SiO_3]_n^{2n-} anions. Beryl Be_3Al_2Si_6O_{18} is a metasilicate. If two of the oxygens are shared and the structure is arranged in a ring, such as that shown here, we get the basic structural unit of the cyclosilcates (Metasilicates) or ring silicates.
===Pyroxenes===
Each silicon tetrahedra in pyroxenes share a vertex with two tetrahedra forming a single chains of silicate [SiO_3]_n^{2n-}.
Examples include enstatite MgSiO_3, and diopside CaMg(SiO_3)_2. 
===Amphiboles===
When two single chains are connected side-to-side through shared vertices, the double chain structure is called an amphibole.
Amphiboles and pyroxenes are structurally similar.
Two examples of amphiboles are blue asbestos Na_2Fe_5(OH)_2(Si_4O_{11})_2, and gray-brown asbestos (Mg,Fe)_7(OH)_2(Si_4O_{11})_2.
===Phyllosilicates===
When three oxygen vertices are shared among silicates, a sheet structure is formed.
The fourth oxygen is coordinated to a metal cation. Talc, Mg_3(OH)_2[Si_4O_{10}], is a typical phyllosilicate. It has a slippery feel because the layers can slide over one another. Chrysotile or white asbestos, Mg_3(OH)_4[Si_4O_{10}], is a commonly used insulator. This materials always curves in one direction creating tubular fibers.
===Silicon Dioxide===
When all four oxygen of a silicate are shared with other silicon atoms, the three-dimensional structure of SiO forms. Beach sand and quartz crystals are both examples of silicon dioxide.
===Summary of Silicate Structures===
Category:Chemistry