Silicates
Silicates are compounds of silicon, oxygen and other elements. Silica (silicon dioxide), silicate, and aluminosilicate minerals make up over 90% of the Earth's crust. Naturally occurring silicates are important building materials and make up the clays upon which ceramics and brickmaking are based. Silica is the basis of glass technology, and synthetic silicates are important in detergent and adhesive applications.
Like carbon, the silicon atom has a marked tendency to form four covalent bonds. Unlike carbon, however, it has little or no tendency to form multiple bonds. In silica, each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicons (Figure 1). Energetic considerations prohibit two silicon atoms from sharing more than one bonding oxygen. One can visualize the structures of silica by thinking of the SiO4 units as tetrahedra sharing corners. A little experimentation with models reveals that numerous different silica structures are possible.
Silica exists in several crystalline forms, in a large number of colloidal forms, and as an amorphous solid. At atmospheric pressure silica exists in three basic crystalline forms. Quartz exists from low temperatures up to 1,598°F (870°C), undergoing a phase transition from alpha quartz to beta quartz at 1,063°F (573°C). Tridymite is stable up to 2,678°F (1,470°C) and crystoballite up to the melting point at 3,110°F (1,710°C). Because there is a significant difference in crystal structure among the three forms, cooling tridymite or crystoballite below 572°F (300°C) results in metastable forms that retain the high temperature structure. Cooling molten silica below the melting point results in fused silica, a rigid, transparent substance, chemically unreactive to the vast majority of substances. By melting silica mixed with sodium hydroxide or calcium oxide one is able to interrupt the long chains of silicon-oxygen bonds to form more easily melted glasses.
Soluble silicates can be obtained by heating alkali metal carbonates and silica. Aqueous solutions of sodium silicate are called "water glass." As the pH of water glass is lowered, colloidal particles of amorphous silica are formed. At still lower pH these particles join together to form silica gel, a rigid material with water trapped between chains of small particles. Partial dehydration to about 4% water by weight produces the commercial silica gel product, a highly porous material often used as a drying agent or adsorbent for chromatography. Replacing the water with alcohol and evaporating the alcohol above its critical temperature results in a silica aerogel, in which the original arrangement of silica particles is retained. Aerogels can have densities as low as 0.02 grams per cubic centimeter.
There are over 600 known silicate minerals. The minerals can be grouped on the basis of the ways in which silica tetrahedra can be connected together. In a few materials, called orthosilicates, the tetrahedra share no oxygen atoms but exist as quadruply charged negative ions, with the compensating charge provided by doubly or triply charged cations. Orthosilicate minerals include willemite, in which the cations are zinc ions, phenacite, with beryllium cations, and forsterite, with magnesium cations. Zircon is a zirconium orthosilicate, sometimes used as a substitute for diamond in jewelry.
When the silica tetrahedra share one corner a disilicate ion is formed and the compounds are called pyrosilicates or sorosilicates. Pyrosilicates are relatively rare, although they are formed by a number of the lanthanide elements. By sharing two oxygen atoms, silicates can form either cyclic or chain structures. The most common ring structure involves six silicon atoms and eighteen oxygen atoms. This structure occurs in the mineral beryl, a beryllium aluminum silicate. Chain silicates are called pyroxenes or inosilicates. When parallel single chains are bound together by shared oxygen to form a double chain, the structure is called an amphibole. This structure is found in some of the asbestos minerals.
Silicates in which the tetrahedra share three corners are called phyllosilicates or sheet silicates, because they tend to cleave into thin sheets. A sheet of silica tetrahedra will have three oxygen atoms in the sheet and the fourth oxygen protruding to one side. The spacing between the protruding oxygen atoms is comparable to the spacing between certain of the hydroxyl groups in layers of magnesium hydroxide (brucite) or aluminum hydroxide (gibbsite). The silica oxygens can thus replace these hydroxyl groups to form a bilayer or trilayer structure. A large number of sheet silicates are known. These include clay minerals such as kaolinite and talc, micas, montmorillonites, and the remainder of the asbestos minerals including chrysotile (white asbestos).
When all four corners of the tetrahedra are shared, the mineral is called a tectosilicate. Silicate minerals can be derived from the quartz, tridymite, and crystoballite structures by replacing a subset of the silicon ions with ions of a lower charge, together with placing other cations in places between tetrahedra to achieve overall neutrality. This group of minerals includes feldspars, which are aluminosilicates with additional mono- or divalent cations, and the ultramarines, which incorporate bimolecular anions. Perhaps the most useful group of tectosilicate minerals, at least from a chemical standpoint, are the zeolites, aluminosilicates with alkali metal or alkaline earth ions and usually including tightly bound water molecules. These structures incorporate large channels, large enough to accommodate ions and small molecules. Zeolites are used for ion exchange, for example removing calcium ions from "hard" water, replacing them with sodium ions, and as molecular sieves, to separate smaller molecules, which fit into the channels, from larger ones that do not. Zeolite-based catalysts have also come to play a major role in petroleum refining.
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