Clay minerals are essentially crystalline in nature though some clay minerals do contain
material which is non-crystalline (for example allophane). Two fundamental building blocks
are involved in the formation of clay mineral structures. They are:
- Tetrahedral unit.
- Octahedral unit.
The tetrahedral unit consists of four oxygen atoms (or hydroxyls, if needed to balance
the structure) placed at the apices of a tetrahedron enclosing a silicon atom which combines
together to form a shell-like structure with all the tips pointing in the same direction. The
oxygen at the bases of all the units lie in a common plane.
Each of the oxygen ions at the base is common to two units. The arrangement is shown in
Fig. 2.2. The oxygen atoms are negatively charged with two negative charges each and the silicon
with four positive charges. Each of the three oxygen ions at the base shares its charges with the
adjacent tetrahedral unit. The sharing of charges leaves three negative charges at the base per
tetrahedral unit and this along with two negative charges at the apex makes a total of 5 negative
charges to balance the 4 positive charges of the silicon ion. The process of sharing the oxygen ions
at the base with neighboring units leaves a net charge of -1 per unit.
The second building block is an octahedral unit with six hydroxyl ions at apices of an octahedral
enclosing an aluminum ion at the center. Iron or magnesium ions may replace aluminum ions in some
units. These octahedral units are bound together in a sheet structure with each hydroxyl ion common to
three octahedral units. This sheet is sometimes called as gibbsite sheet. The Al ion has 3 positive charges
and each hydroxyl ion divides its -1 charge with two other neighboring units. This sharing of negative
charge with other units leaves a total of 2 negative charges per unit [(1/3) x 6]. The net charge of a unit
with an aluminum ion at the center is +1. Fig. 2.3 gives the structural arrangements of the units.
Sometimes, magnesium replaces the aluminum atoms in the octahedral units in this case, the
octahedral sheet is called a brucite sheet.
Formation of Minerals
The combination of two sheets of silica and gibbsite in different arrangements and conditions lead
to the formation of different clay minerals as given in Table 2.3. In the actual formation of the sheet
silicate minerals, the phenomenon of isomorphous substitution frequently occurs. Isomorphous
(meaning same form) substitution consists of the substitution of one kind of atom for another.
This is the most common mineral of the kaolin group. The building blocks of gibbsite and
silica sheets are arranged as shown in Fig. 2.4 to give the structure of the kaolinite layer. The
structure is composed of a single tetrahedral sheet and a single alumina octahedral sheet
combined in units so that the tips of the silica tetrahedrons and one of the layers of the
octahedral sheet form a common layer. All the tips of the silica tetrahedrons point in the same
direction and towards the center of the unit made of the silica and octahedral sheets. This gives
rise to strong ionic bonds between the silica and gibbsite sheets. The thickness of the layer is
about 7 A (one angstrom = 10~8 cm) thick. The kaolinite mineral is formed by stacking the
layers one above the other with the base of the silica sheet bonding to hydroxyls of the gibbsite
sheet by hydrogen bonding. Since hydrogen bonds are comparatively strong, the kaolinite
crystals consist of many sheet stackings that are difficult to dislodge. The mineral is therefore,
stable, and water cannot enter between the sheets to expand the unit cells. The lateral
dimensions of kaolinite particles range from 1000 to 20,000 A and the thickness varies from
100 to 1000 A. In the kaolinite mineral there is a very small amount of isomorphous substitution.
Halloysite minerals are made up of successive layers with the same structural composition as those
composing kaolinite. In this case, however, the successive units are randomly packed and may be
separated by a single molecular layer of water. The dehydration of the interlayers by the removal of
the water molecules leads to changes in the properties of the mineral. An important structural
feature of halloysite is that the particles appear to take tubular forms as opposed to the platy shape
Montmorillonite is the most common mineral of the montmorillonite group. The structural
arrangement of this mineral is composed of two silica tetrahedral sheets with a central alumina
octahedral sheet. All the tips of the tetrahedra point in the same direction and toward the center of the
unit. The silica and gibbsite sheets are combined in such a way that the tips of the tetrahedrons of each
silica sheet and one of the hydroxyl layers of the octahedral sheet form a common layer. The atoms
common to both the silica and gibbsite layer become oxygen instead of hydroxyls. The thickness of
the silica-gibbsite-silica unit is about 10 A (Fig. 2.5). In stacking these combined units one above the
other, oxygen layers of each unit are adjacent to oxygen of the neighboring units with a
consequence that there is a very weak bond and an excellent cleavage between them. Water can
enter between the sheets, causing them to expand significantly and thus the structure can break into
10 A thick structural units. Soils containing a considerable amount of montmorillonite minerals
will exhibit high swelling and shrinkage characteristics. The lateral dimensions of montmorillonite
particles range from 1000 to 5000 A with thickness varying from 10 to 50 A. Bentonite clay
belongs to the montmorillonite group. In montmorillonite, there is isomorphous substitution of
magnesium and iron for aluminum.
The basic structural unit of illite is similar to that of montmorillonite except that some of the
silicons are always replaced by aluminum atoms and the resultant charge deficiency is balanced by
potassium ions. The potassium ions occur between unit layers. The bonds with the
nonexchangeable K+ ions are weaker than the hydrogen bonds, but stronger than the water bond of
montmorillonite. Illite, therefore, does not swell as much in the presence of water as does
montmorillonite. The lateral dimensions of illite clay particles are about the same as those of
montmorillonite, 1000 to 5000 A, but the thickness of illite particles is greater than that of
montmorillonite particles, 50 to 500 A. The arrangement of silica and gibbsite sheets are as shown
in Fig. 2.6.