The radius ratio rule is only applicable to ionic compounds. In silicate min-
erals, however, it is the bonds between oxygen and silicon and between oxygen
and aluminium (Al) which are structurally important. These bonds are almost
equally ionic and covalent in character and the radius ratio rule predicts the coor-
dination of these ions adequately.
4.2.2 The construction of silicate minerals
The SiO 4 tetrahedron has a net 4-charge, since silicon has a valency of 4+, and
each oxygen is divalent (2-). This means that the silicon ion (Si^4 +) can satisfy only
half of the bonding capacity of its four oxygen neighbours. The remaining bonds
are used in one of two ways as silicates crystallize from magma:
1 Some magmas are rich in elements which are attracted to the electronegative
tetrahedral oxygen (Box 4.2). The bonds between these elements (e.g. magne-
sium (Mg)) and oxygen have ionic character (Box 4.2) and result in simple crystal
structures, for example olivine (the magnesium-rich form is called forsterite)
(Section 4.2.3). The cohesion of forsterite relies on the Mg^2 +–SiO 4 ionic bond.
Bonding withinthe SiO 4 tetrahedron has a more covalent character. During
weathering, water, a polar solvent (Box 4.1), severs the weaker metal–SiO 4 tetra-
hedron ionic bond, rather than bonds within the tetrahedron itself. This releases
metals and free SiO 44 - as silicic acid (H 4 SiO 4 ).
2 In some magmas, electropositive elements (opposite behaviour to electroneg-
ative elements) like magnesium are scarce. In these magmas each oxygen ion is
likely to bond to two silicon ions, forming bonds of covalent character. The for-
mation of extended networks of silicon–oxygen is called polymerization, and is
used to classify structural organization in silicate minerals (Section 4.2.3).
4.2.3 Structural organization in silicate minerals
Silicate minerals are classified by the degree to which silicon–oxygen bonded net-
works (polymers) form (Fig. 4.4). The degree of polymerization is measured by
the number of non-bridging oxygens (i.e. those bonded to just one Si^4 +).
Monomer silicatesThese are built of isolated SiO 4 tetrahedra, bonded to metal cations as in olivine
(Fig. 4.4b) and garnet. The basic unit of the polymer, the SiO 4 tetrahedra, is
uncombined or single (mono), giving rise to the term monomer. These minerals
have four non-bridging oxygens and are also known as orthosilicates.
Chain silicatesIf each SiO 4 tetrahedron shares two of its oxygens, chains of linked tetrahedra
form (Fig. 4.4c). Chain silicates have two non-bridging oxygens and an overall
Si : O ratio of 1 : 3, giving the general formula SiO 3. The pyroxene group of
minerals provides the most important chain silicates—for example, enstatite
The Chemistry of Continental Solids 73