Two to one clay mineral structure

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, CaC03 (calcite), 

MgC03 (magnesite), FeC03 (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, (Ca0.85Mgo.iFe0.o5)C03. 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 

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 (0.106nm 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. 

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

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