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
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
