4.5 Clay minerals
The reactions to illustrate weathering of complex silicates during acid hydroly-
sis (eqns. 4.13 & 4.14) predict that clay minerals will be an important solid
product and this is confirmed by looking at soils. Clay minerals are important
constituents in most soils. These sheet silicates that are less than 2mm (Section
4.2.3) are constructed of layers of atoms in tetrahedral and octahedral coordina-
tion, known as tetrahedral and octahedral sheets.
The tetrahedral sheets are layers of SiO 4 tetrahedra which share three oxygens
with neighbouring tetrahedra. These basal oxygens form a hexagonal pattern
(Section 4.2.3). The fourth tetrahedral (apical) oxygen of each tetrahedron is
arranged perpendicular to the basal sheet (Fig. 4.7a). The sheet carries a net
negative charge.
The octahedral sheet is composed of cations, usually aluminium, iron or mag-
nesium, arranged equidistant from six oxygen (or OH) anions (Fig. 4.7b). Alu-
minium is the common cation and the ideal octahedral sheet has the composition
of the aluminium hydroxide mineral, gibbsite (Al(OH) 3 ). Where octahedral sites
are filled by trivalent aluminium, only two of every three sites are occupied to
The Chemistry of Continental Solids 87OHOHOHOHOHOHOHOHOHOHOHOH(a)(b)i ii iii ivSilicon
Aluminium
OxygenFig. 4.7(a) A sheet of SiO 4 tetrahedra linked via basal oxygens, with apical oxygens pointing
upward. (b) Octahedra and the octahedral sheet: (i) the atoms packed together; (ii) the
octahedron expanded; (iii) conventional representation of an octahedron; (iv) conventional
representation of an octahedral sheet, showing aluminium equidistant between six hydroxyls—
forming the mineral gibbsite.