Anthony R. West - Solid State Chemistry and its Applications

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 SiP 2O 7and in high‐pressure polymorphs of SiO 2(coesite, stishovite). The number of these exceptions is very small, however, and we can regard SiO 4tetrahedra 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 Si 4+, 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 Na 2Si 3O 7, the Si:O ratio is 1:2.33. This corresponds to a structure in which, on average, two‐thirds of an O per SiO 4is non‐bridging. Clearly, therefore, some SiO 4tetrahedra 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, NaAlSi 3O 8, and anorthite, CaAl 2Si 2O 8, 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 SiO 2itself, Fig 1.52(c). Framework structures also occur in orthoclase, KAlSi 3O 8, kalsilite, KAlSiO 4, eucryptite, LiAlSiO 4, and spodumene, LiAlSi 2O 6.

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	Mg 2SiO 4olivine, Li 4SiO 4
1:3.5	1	3	Dimer	Ca 3Si 2O 7rankinite, Sc 2Si 2O 7thortveite
1:3	2	2	Chains	Na 2SiO 3, MgSiO 3pyroxene
			Rings, e.g.	CaSiO 3 a , BaTiSi 3O 9benitoite
				Be 3Al 2Si 6O 18beryl
1:2.5	3	1	Sheets	Na 2Si 2O 5
1:2	4	0	3D framework	SiO 2 b

a CaSiO 3is 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 Mg 3(OH) 2Si 4O 10and, 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 KMg 3(OH) 2(Si 3Al)O 10. In talc and phlogopite, Mg occupies octahedral sites between silicate sheets; K occupies 12‐coordinate sites.

The mica muscovite, KAl 2(OH) 2(Si 3Al)O 10, is more complex; it is structurally similar to phlogopite, with infinite sheets, (Si 3Al)O 10. However, two other Al 3+ions replace the three Mg 2+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 Al 3+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.