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Silicates, especially many minerals, often have complex 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	Na2Si2O5
1:2	4	0	3D framework	SiO2 b

^a CaSiO₃ is dimorphic. One polymorph has rings and the other has infinite chains.

^b The three main polymorphs of silica, quartz, tridymite, and cristobalite, each have a different kind of 3D framework structure.

Substitution of Al for Si occurs in many sheet structures such as micas and clay minerals. Talc has the formula Mg₃(OH)₂Si₄O₁₀ and, as expected for an Si:O ratio of 1:2.5, the structure contains infinite silicate sheets. In the mica phlogopite, one‐quarter of the Si in talc is effectively replaced by Al and extra K is added to preserve electroneutrality. Hence phlogopite has the formula KMg₃(OH)₂(Si₃Al)O₁₀. In talc and phlogopite, Mg occupies octahedral sites between silicate sheets; K occupies 12‐coordinate sites.

The mica muscovite, KAl₂(OH)₂(Si₃Al)O₁₀, is more complex; it is structurally similar to phlogopite, with infinite sheets, (Si₃Al)O₁₀. However, two other Al³⁺ ions replace the three Mg²⁺ ions of phlogopite and occupy octahedral sites. By convention, only ions that replace Si in tetrahedral sites are included as part of the complex anion. Hence octahedral Al³⁺ ions are formally regarded as cations in much the same way as alkali and alkaline earth cations, Table 1.27.

With a few exceptions, silicate structures cannot be described as cp. However, this disadvantage is offset by the clear identification of the silicate anion component which facilitates classification and description of a very wide range of structures. In addition, the Si–O bond is strong and partially covalent and the consequent stability of the silicate anion is responsible for many of the properties of silicates.

Many examples of silicate crystal structures are given in the minerals section of the CrystalViewer Companion website. To facilitate viewing of the silicate anion, the remaining cations in the structures can be hidden; you can check out the numbers of bridging and non‐bridging oxygens for consistency with the Si–O ratio in the mineral formulae.

The structural building blocks of clay minerals are illustrated in Fig. 1.52(d, e, f). They consist of layers of silicate or aluminosilicate tetrahedra which all have the same orientation; their apices are connected to layers of Al, Mg octahedra to form either (d) double- or (e,f) triple-layered sheets. Clay minerals form in nature by the decomposition of feldspars such as microcline or orthoclase to give kaolinite:

Kaolinite has a two layer, 1:1, structure (d) whose opposite surfaces are the basal planes of Si₂O₅ ²⁻ layers and Al₂(OH)₄ ²⁺ layers. These double layers are stacked together and separated by weak van der Waals bonds, that give rise to the softness and easy cleavage of such clay minerals. Other layered minerals such as muscovite (e), montmorillonite (f), talc, bentonite and pyrophyllite have a three layer, 2:1, structure in which the layer of octahedra is sandwiched between two layers of tetrahedra. If the overall structure is electrically neutral, then the interlayer, van der Waals space may contain only water molecules (f), but if the layers have a net negative charge then cations such as K⁺ also occupy the interlayer space, as in micas (e).

Kaolinite clays have long‐standing industrial uses as the fundamental components of whitewares and traditional ceramics. The interlayer bonding forces control the rheological properties and viscosity of slips during processing and formation of the green bodies. In a kaolin particle, the opposite faces of the sheets have different chemical properties and respond differently in an aqueous environment. The bases of the silicate tetrahedra are weakly acidic whereas those of the hydroxide octahedra are weakly basic. Further, the effective surface charges change sign at a different pH for the two particle surfaces. Thus, the silicate surface has an isoelectric point, IEP at pH ~ 3 at which the surface charge, or zeta potential, changes sign, whereas the IEP for the hydroxide surface is at pH ~ 10. This means that at neutral pH, for instance, the two surfaces are oppositely charged. Control of surface charges is vital to the dispersion/agglomeration of particles during slip casting and therefore, for the quality of the ceramic product.

There is increasing interest in the synthesis of new functional materials by starting with layered structures such as clay minerals and modifying the components – ionic or molecular – that occupy the interstitial space between the layers. A wide range of materials and properties can be synthesised by chimie douce methods, such as pillared clays that have novel porous structures and magnetic hybrid materials. Such materials are kinetically stable, although thermodynamically metastable and cannot be prepared by the usual high temperature solid state reaction methods (Section 4.3.7.2).