Box 46 Isomorphous substitution

Isomorphism describes substances which have very similar structure. The carbonate mineral system is a good example, where some minerals differ only on the basis of the cation, for example, CaCO3 (calcite), MgCO3 (magnesite), FeCO3 (siderite). The basic similarity of structure allows interchangeability of cations between end-member minerals. For example, most natural calcite has a measurable amount of both Mg2+ and Fe2+ substituted for some of the Ca2+. The amount of isomorphous substitution is shown by the following notation, (Ca0.85Mg0.iFe0.o5)CO3. In other words, 85% of the Ca2+ sites are occupied by Ca2+, 10% of the Ca2+ sites are occupied by Mg2+, and 5% of the Ca2+ sites are occupied by Fe2+. The complex chemistry of freshwater and seawater means that natural minerals incorporate many trace elements and rarely conform to their ideal formulae.

The radius ratio rule (Section 4.2.1) predicts that divalent Ca2+, Mg2+ and Fe2+ will have six-fold coordination because of their similar ionic radii (0.106 nm Ca2+, 0.078nm Mg2+, 0.082 nm Fe2+). They are therefore interchangeable without upsetting either the physical packing or the electrical stability of an ionic compound.

In compounds where bonding has covalent character, isomorphous substitution is prevented. This is because the need for electron sharing in the bond modifies structures away from the simple packing geometries predicted by the radius ratio rule.

Fig. 4.11 The structure of muscovite mica.

12-fold coordination with oxygen (Section 4.2.1), but this does not occur, due to slight distortion in the illite structure.

It is important to note that bonding between 2 : 1 illite units cannot be fulfilled by hydrogen bonds associated with OH groups (as in kaolinite), since each

2 : 1 unit presents only basal tetrahedral oxygens on its outer surfaces. Moreover, the ionic bonding between K+ in the interlayer site and the tetrahedral oxygens is a relatively strong bond, making illitic clays stable minerals. This accounts for their abundance as weathering products, particularly in temperate and colder climates.

The smectite group of clay minerals are structurally similar to illites (Fig. 4.12). In octahedral sites, substitution of Al3+ by Mg2+ or Fe3+ is common and some substitution of Si4+ by Al3+ in the tetrahedral sites also occurs, resulting in a net negative layer charge. This charge is, however, only about one-third the strength of the illite layer charge. Consequently, smectite is not able to bond interlayer cations effectively and the 2 : 1 units are not tightly bonded together. This allows water and other polar solvents to penetrate interlayer sites, causing the mineral to swell. Cations, principally H+, Na+, Ca2+ and Mg2+, also enter the interlayer site with water and neutralize the negative charge. Bonding between the 2 : 1 units is effected by the hydrated cation interlayer, by a combination of hydrogen bonds and van der Waals' forces (Box 4.7). This weak bonding holds cations loosely in the interlayer sites, making them prone to replacement by other cations. As a consequence, smectites have high cation exchange capacity (Section 4.8).

The similar structure of illite and smectite allows mixing or interstratification of 2:1 units to form mixed-layer clays. Most illites and smectites are interstrati-fied to a small degree, but they are not classified as such until detectable by X-ray diffraction. As one might expect, illite-smectite mixed-layer clays have intermediate cation exchange capacity between the end-member compositions.

Fig. 4.12 The structure of a 2:1 clay mineral (smectite).
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