The Chemistry of Continental Solids 91Box 4.6 Isomorphous substitutionIsomorphism 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, CaCO 3 (calcite),
MgCO 3 (magnesite), FeCO 3 (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
Mg^2 +and Fe^2 +substituted for some of the
Ca^2 +. The amount of isomorphous substitution
is shown by the following notation,
(Ca0.85Mg0.1Fe0.05)CO 3. In other words, 85% of
the Ca^2 +sites are occupied by Ca^2 +, 10% of
the Ca^2 +sites are occupied by Mg^2 +, and 5%
of the Ca^2 +sites are occupied by Fe^2 +. 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 Ca^2 +, Mg^2 +and Fe^2 +will
have six-fold coordination because of their
similar ionic radii (0.106 nm Ca^2 +, 0.078 nm
Mg^2 +, 0.082 nm Fe^2 +). 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.2 K+
6 O6 O3 Si + 1 Al3 Si + 1 Al4 O + 2[OH]4 O + 2[OH]4 Al1.0 nm OHOHOHOHFig. 4.11The 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 ful-
filled by hydrogen bonds associated with OH groups (as in kaolinite), since each