Simple compounds of silicon with oxygen

Key point: The Si-O-Si link is present in silica, a wide range of metal silicate minerals, and silicone polymers. The complicated silicate structures are often easier to comprehend if the tetrahedral SiO₄ unit from which they are built is drawn as a tetrahedron with the Si atom at the centre and O atoms at the vertices. The representation is often cut to the bone by drawing the SiO₄ unit as a simple tetrahedron with the atoms omitted. Each terminal O atom contributes -1 to the charge of the SiO₄ unit, but each shared O atom contributes 0. Thus, orthosilicate is [SiO₄]^-4 (1), disilicate is [O₃SiOSiO₃ ]^6- (2), and the SiO₂ unit of silica has no net charge because all the O atoms are shared. With these principles of charge balance in mind, it should be clear that an endless single-stranded chain or a ring of SiO4 units, which has two shared O atoms for each Si atom, will have the formula and charge [(SiO₃)^-2]_n . An example of a com pound containing such a cyclic metasilicate ion is the mineral beryl, Be₃ Al₂Si₆O₁₈, which contains the [Si₆O₁₈ ]^12- ion (16). Beryl is a major source of beryllium. The gemstone emerald is beryl in which Cr³⁺ ions are substituted for some Al³⁺ ions. A chain metasilicate (17) is present in the mineral jadeite, NaAl(SiO₃)², one of two different minerals sold as jade, the green colour arising from traces of iron impurities. In addition to other config-urations for the single chain, there are double-chain silicates, which include the family of minerals known commercially as asbestos (Box 14.5).

A brief illustration. The cyclic silicate anion [Si₃O₉] ^n- is a six-membered ring with alternating Si and O atoms and six terminal O atoms, two on each Si atom. Because each terminal O atom contributes -1 to the charge, the overall charge is -6. From another perspective, the conventional oxidation numbers of silicon and oxygen, +4 and -2, respectively, also indicate a charge of -6 for the anion.The composition of silicate glasses has a strong influence on their physical properties. For example, fused quartz (amorph-ous SiO₂) softens at about 1300°C, borosilicate glass (which contains boron oxide, Section 13.8) softens at about 800°C, and soda-lime glass softens at even lower temperatures. The variation in softening point can be understood by appreciating that the Si O Si links in silicate glasses form the frame work that imparts rigidity. When basic oxides such as Na₂O and CaO are incorporated (as in soda-lime glass), they react with the SiO₂ melt and convert Si-O-Si links into terminal SiO groups and hence lower its softening temperature.

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