Silicate Classification

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[SiO 4 ] tetrahedra: ~50% ionic/~50% covalent • may polymerize (share O between Si) to form silicate “skeleton” with cations to provide charge balance and hold it all together Si +4 O 4 -2 : -4 charge Pauling Bond Strength Analysis: SiO 4 Si-O Si +4 # bonds: 4 P.B.S. : +4/4 = +1 (i.e., only 1/2 the O -2 charge, mesodesmic, perfect for polymerization)
Rules for [SiO 4 ] polymerization: (1) Tetrahedra only link by corners, not edges or faces (i.e., 2 Si never share more than one O), a consequence of Pauling’s 3 rd Rule, highly charged, tetravalent Si cations can’t get that close together (2) Formal charge (Si +4 , O -2 ) only used to keep track of charge balance (3) Al +3 may substitute for Si +4 (but messes up charge balance, and therefore requires coupled substitution) • When Al subs in, there is no sharing of O with other Al’s “aluminum avoidance principal” O O O Si O NO Si Si O O O Si O YES Si
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Silicate Classification

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