Solid State Chemistry and its Applications. Anthony R. West

formulae and structures. The purpose of this section is not to give a review of their structures but simply to show that a considerable amount of structural information may be obtained from their chemical formulae. Using certain guidelines, one can appreciate, without the necessity of remembering a large number of complex formulae, whether a particular silicate is a 3D framework structure, sheet‐like, chain‐like, etc.

      It is common practice to regard silicate structures as composed of cations and silicate anions. Various silicate anions are possible, ranging from the extremes of isolated tetrahedra in orthosilicates such as olivine (Mg₂SiO₄), to infinite 3D frameworks, as in silica itself (SiO₂). The structures of silicate anions are based on certain principles:

      1 Almost all silicate structures are built of SiO4 tetrahedra.

      2 The tetrahedra may link by sharing corners to form larger polymeric units.

      3 No more than two SiO4 tetrahedra may share a common corner (i.e. oxygen).

      4 SiO4 tetrahedra never share edges or faces with each other.

      Exceptions to (1) are structures in which Si is octahedrally coordinated to O as in one polymorph of SiP₂O₇ and in high‐pressure polymorphs of SiO₂ (coesite, stishovite). The number of these exceptions is very small, however, and we can regard SiO₄ tetrahedra as the normal building block in silicate structures. Guidelines (3) and (4) are concerned, respectively, with maintaining local electroneutrality and with ensuring that highly charged cations, such as Si⁴⁺, are not too close together.

      The important factor in relating the formula to structure type is the Si:O ratio. This ratio is variable since two types of O may be distinguished in the silicate anions: bridging oxygens and non‐bridging oxygens. Bridging oxygens are those that link two tetrahedra, Fig. 1.52. Effectively, they belong half to one Si and half to another Si. In evaluating the net Si:O ratio, bridging oxygens count as . Non‐bridging oxygens are linked to only one Si or silicate tetrahedron as in (b). They are also called terminal oxygens. In order to maintain charge balance, non‐bridging oxygens must of course also be linked to other cations in the crystal structure. In evaluating the overall Si:O ratio, non‐bridging oxygens count as 1.

      The overall Si:O ratio in a silicate structure depends on the relative number of bridging and non‐bridging oxygens. Some examples are given in Table 1.27; they are all straightforward and one may deduce the type of silicate anion directly from the chemical formula.

      Many more complex examples could be given. In these, although the detailed structure cannot be deduced from the formula, one can at least get an approximate idea of the type of silicate anion. For example, in Na₂Si₃O₇, the Si:O ratio is 1:2.33. This corresponds to a structure in which, on average, two‐thirds of an O per SiO₄ is non‐bridging. Clearly, therefore, some SiO₄ tetrahedra must be composed entirely of bridging oxygens whereas others contain one non‐bridging oxygen. The structure of the silicate anion would therefore be expected to be something between an infinite sheet and a 3D framework. In fact, it is an infinite, double‐sheet silicate anion in which two‐thirds of the silicate tetrahedra have one non‐bridging O.

      The relation between formula and anion structure is more complex when Al is present. In some cases, Al substitutes for Si, in the tetrahedra; in others, it occupies octahedral sites. In the plagioclase feldspars typified by albite, NaAlSi₃O₈, and anorthite, CaAl₂Si₂O₈, Al partly replaces Si in the silicate anion. It is therefore appropriate to consider the overall ratio (Si + Al):O. In both cases, this ratio is 1:2 and, therefore, a 3D framework structure is expected, as in SiO₂ itself, Fig 1.52(c). Framework structures also occur in orthoclase, KAlSi₃O₈, kalsilite, KAlSiO₄, eucryptite, LiAlSiO₄, and spodumene, LiAlSi₂O₆.

       Figure 1.52 Silicate anions with (a) bridging and (b) non‐bridging oxygens. (c) The quartz structure formed by a 3D network of corner‐sharing silicate tetrahedra, within which, 6‐membered rings can be identified. These rings enclose cavities that can accommodate interstitial cations such as Li+, Section 2.3.3.1. (d, e, f) Building blocks, schematically, of clay mineral structures.

       Adapted from W. M. Carty, Bull. Amer. Ceram. Soc. 72 (1999).

       Table 1.27 Relation between chemical formula and silicate anion structure

	Number of oxygens per Si
Si:O ratio	Bridging	Non‐bridging	Type of silicate anion	Examples
1:4	0	4	Isolated	Mg2SiO4 olivine, Li4SiO4
1:3.5	1	3	Dimer	Ca3Si2O7 rankinite, Sc2Si2O7 thortveite
1:3	2	2	Chains	Na2SiO3, MgSiO3 pyroxene
			Rings, e.g.	CaSiO3 ^a, BaTiSi3O9 benitoite
				Be3Al2Si6O18 beryl
1:2.5	3	1	Sheets Скачать книгу