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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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1.4.2 Ionic structures and bonding

About 90% of all minerals are essentially ionic compounds. An ionic bond 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 12b). 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 13a); and an octahedral interstice, surrounded by six atoms, arranged at the corners of an imaginary octahedron (Figure 13b).

Figure 13 Interstices (vacant sites) between two close-packed planes of spheres. (a) A tetrahedral interstice is formed between four close-packed atoms (three in the lower layer and one in the upper layer). The atoms are arranged at the corners of a tetrahedron (schematic figure on the right), with the interstice at its centre. (b) An octahedral interstice is formed between six close-packed atoms (three in each layer). The atoms are arranged at the corners of an octahedron (schematic figure), with the interstice at its centre.

The mineral halite (Figure 14) is an example of a structure with octahedral interstices (as in Figure 13b). 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 14a).

Figure 14 The structure of halite (sodium chloride, NaCl): (a) a space-filling model (sodium and chlorine ions shown at their correct relative sizes); (b) a ball-and-stick model. (c) Cubic crystals of halite (field of view 10 cm across).

The structure of sphalerite (zinc sulfide, ZnS) (Figure 15) has a close-packed arrangement of sulfur ions, a structure in which zinc ions fill half of the tetrahedral interstices (as in Figure 13a).

Figure 15 Structure of sphalerite (zinc sulfide, ZnS): (a) a space-filling model; (b) ball-and-stick model; the unit cell (see Section 1.6) is shown by black lines in both (a) and (b). (c) Sphalerite (with white quartz) (2 cm across).