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An introduction to minerals and rocks under the microscope
An introduction to minerals and rocks under the microscope

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3.2 Silicate mineral structures

Silicate minerals all share the same basic building block: a silicon atom bonded to four oxygen atoms at the corners of a tetrahedron, called the silicate group (or SiO4 group) (Figure 41).

Figure 41 (a) The SiO4 tetrahedral unit with atoms of oxygen (red) and silicon (grey) shown at true relative scale. (b) The atomic structure of the SiO4 unit, showing how the silicon and oxygen atoms are linked. (c) The simplified SiO4 tetrahedron, the basic shape of the SiO4 unit.

The wide range of silicate minerals that exists in nature owes much to a property of the SiO4 group, known as polymerisation. Polymerisation generally leads to increasingly complex mineral structures. At the simplest level, SiO4 groups bond with metal cations, forming isolated groups (Figure 42) as in olivine minerals (Section 3.3.1). With increasing polymerisation, the SiO4 group can also build two- and three-dimensional structures, such as chains, sheets and frameworks, by linking up SiO4 tetrahedra. With the exception of the silica minerals, where all the oxygens are shared, these structures are also linked by a variety of cations to make more complex three-dimensional structures.

Figure 42 Structural classification of common silicate minerals, based on polymerisation of the silicate tetrahedron. The ratio of the tetrahedral (T) sites to oxygen (O) sites increases with polymerisation as more oxygen atoms are shared between tetrahedral sites. The structures shown are only very small parts of what are effectively infinite structures. The right-hand column of the diagram illustrates how the atomic structures affect mineral properties.

The first stage of polymerisation of SiO4 groups involves the formation of chain structures, as in pyroxene minerals (Section 3.4.1), produced by corner-sharing of the oxygen atoms (Figure 42). The ratio of silicon to oxygen atoms increases from 1 : 4 in structures with isolated SiO4 groups, to 1 : 3 in chain structures. The degree of polymerisation increases further in more complex chains (Figure 42), as in amphibole minerals (Section 3.4.2), and the ratio of silicon to oxygen increases to 4 : 11. Further polymerisation produces sheet structures, whereby tetrahedra are linked in two dimensions to form sheets (Figure 42). These sheets are bonded together by a variety of chemical groups, such as hydroxyl (OH) or metal cations, to form sandwich structures and the Si : O ratio rises to 2 : 5. In fully polymerised three-dimensional framework structures, as in quartz (Section 3.6.1), the Si : O ratio reaches 1 : 2.

In some minerals, aluminium replaces silicon in some of the SiO4 tetrahedra producing AlO4 groups. It is therefore more appropriate to think of oxygen atoms being shared between tetrahedral (T) sites, which may contain either silicon or aluminium. So, more generally, increasing polymerisation results in an increase in the ratio of tetrahedral sites to oxygen - the T : O ratio (Figure 42). The net charge on the SiO4 group is −4, but on the AlO4 group it is −5, so additional positive charges are required to compensate for the excess negative charge on the tetrahedral groups. These may be provided by cations, commonly metals, residing in cavities or interstices in the tetrahedral structure (Section 1.4.2). Figure 42 summarises the structural styles of the major silicate mineral groups. In the following sections, you will look at each of these groups in more detail.