4.2 Ionic structures and bonding
About 90 per cent of all minerals are essentially ionic compounds. Ionic bonding is generated by the transfer of one or more electrons from one atom to another. This creates two ions of opposite charge, which are attracted to each other (Figure 14). For example, in halite (sodium chloride, NaCl) there is a positively charged sodium cation, Na+, and a negatively charged chlorine anion, Cl−.
As with metallic bonding, ionic bonds are non-directional, so ionic crystals tend to have fairly dense, close-packed structures. However, ionic bonds tend to be stronger than metallic bonds, so crystals containing ionic bonds tend to be unmalleable and much more brittle than metal crystals.
Even though a close-packed structure looks densely packed, there are actually lots of spaces between the atoms. These spaces are called interstices and are important in metallic structures because they provide sites for smaller atoms to reside. Interstices also provide a basis for many ionic structures: they provide locations for smaller ions, in the presence of large ions. There are two kinds of interstices:
- a tetrahedral interstice, surrounded by four atoms, one at each of the corners of an imaginary tetrahedron (Figure 15a)
- an octahedral interstice, surrounded by six atoms, arranged at the corners of an imaginary octahedron (Figure 15b).
The mineral halite (Figure 16) is an example of a structure with octahedral interstices (as in Figure 15b). The chlorine ions are arranged a bit like the atoms in a metal - although they do not quite touch each other. The sodium ions, which are much smaller, fit snugly between the large chlorine ions, as illustrated in a space-filling model (Figure 16a).
The structure of sphalerite (zinc sulfide, ZnS) (Figure 17) has a close-packed arrangement of sulfur ions, a structure in which zinc ions fill half of the tetrahedral interstices (as in Figure 15a).