Clay
minerals form an important group of the phyllosilicate or sheet silicate family
of minerals, which are distinguished by layered structures composed of polymeric
sheets of SiO4 tetrahedra linked to sheets of (Al, Mg, Fe)(O,OH)6
octahedra. The geochemical importance of clay minerals stems from their ubiquity
in soils and sediments, high specific surface area, and ion exchange capacities.
Clay minerals tends to dominate the surface chemistry of soils and sediments.
Furthermore, these properties give rise to a wide range of industrial
applications throughout the history of mankind. The use of clay for mainly clay
figures, pottery and ceramics was already known by primitive people about 25000
years ago (Shaikh and Wik, 1986). Today clay is an important material with a
large variety of applications in ceramics, oil drilling, liners for waste
disposal, and the metal and paper industry. Clay is furthermore used as
adsorbent, decoloration agents, ion exchanger, and molecular sieve catalyst (Fowden
et al., 1984; Murray, 1991).
Smectites
are phyllosilicates or layer silicates having a layer lattice structure in which
two-dimensional oxoanions are separated by layers of hydrated cations. The
oxygen atoms define upper and lower sheets enclosing tetrahedral sites, and a
central sheet having the brucite or gibbsite structure enclosing octahedral
sites (Fig. 1). Smectites having two tetrahedral sheets around the
central octahedral sheet relation are known as 2:1 phyllosilicates. Kaolinite,
on the other hand, has one tetrahedral and one octahedral sheet (1:1).
A further designation can be
made based on the type and location of the cations in the oxygen framework. In
one unit cell composed of twenty oxygen atoms and four hydroxyl groups, there
are eight tetrahedral and six octahedral sites. A smectite is dioctahedral if
two-thirds of the octahedral sites are occupied by trivalent cations, and
trioctahedral if all octahedral sites are filled with bivalent cations. Table
1 summarizes the most common smectites and their idealized structural
formulas. For comparison kaolinite, a dioctahedral 1:1 clay, chrysotile, a
trioctahedral 1:1 clay, and the micas phlogopite and paragonite are included.
Micas, though no smectites, have identical 2:1 phyllosilicate oxygen frameworks.
In talc and pyrophyllite all
tetrahedral sites are filled with Si4+ and the octahedral sites
either completely with Mg2+ or for 2/3 with Al3+,
respectively. The electrically neutral sheets are bonded together by relatively
weak dipolar and van der Waals forces (Giese, 1975). In contrast, smectite
layers have a positive charge deficiency resulting from the isomorphous
substitutions, viz., (i) Si4+ by Al3+ at tetrahedral
sites, and (ii) Al3+ by Mg2+, or (iii) Mg2+ by
Li+ (or a vacancy) at octahedral sites. The charge deficiency is
balanced by hydrated interlayer cations, such as, Na+, K+,
or Ca2+.
The charge deficiency of
smectites is intermediate between that of micas and that of pyrophyllite and
talc. Differences the charge of the layers, the origin of the charge deficiency,
and interlayer cations results in different physical and chemical properties,
such as, thermal stability and swelling behavior (Fig. 2). The layer
charge of an octahedrally substituted smectite (e.g. montmorillonite) is
distributed over the complete oxygen framework, whereas tetrahedral substitution
(as e.g. in beidellite) leads to a more localized charge distribution and the
last smectites tend to a higher three-dimensional order (Brindley, 1980; Suquet
et al., 1975).