Ceramic and Glass Materials

7 Clays 115

composed of alternating octahedral and tetrahedral sheets. Bonding within each repeat
unit is covalent, making the layers strong. In contrast, the bonding between repeat
units is relatively weak, allowing the layers to separate when placed in an excess of
water or under a mechanical load. The chemical formula for kaolinite, as determined
by site occupancy and charge neutrality requirements, is Al 2 Si 2 O 5 (OH) 4 , which is
commonly expressed as the mineral formula Al 2 O 3 ⋅2SiO 2 ⋅2H 2 O. The structure and
properties of kaolinite are summarized in Table 2. The repeat units for clay minerals
other than kaolinite are produced by altering the stacking order of the octahedral and
tetrahedral sheets or by isomorphous substitution of cations such as Mg2+ and Fe3+ into
the octahedral sheets [17].
Conceptually, the next simplest clay mineral is pyrophyllite, which is produced by
attaching tetrahedral sheets above and below an octahedral layer (Fig. 6), compared with
just one octahedral sheet for kaolin [15]. The resulting chemical composition of pyro-
phyllite is Al 2 Si 4 O 10 (OH) 2 , which is equivalent to the mineral formula Al 2 O 3 .4SiO 2 .H 2 O.
The structure and properties of pyrophyllite are summarized in Table 2.

Table 2Composition and crystallography of common clay minerals

Kaolinite Pyrophyllite Mica (Muscovite)

Chemical formula Al 2 Si 2 O 5 (OH) 4 Al 2 Si 4 O 10 (OH) 2 KAl 3 Si 3 O 10 (OH) 2
Mineral formula Al 2 O 3 .2SiO 2 .2H 2 OAl 2 O 3 .4SiO 2 .H 2 OK 2 O.3Al 2 O 3 .6SiO 2 .2H 2 O
Crystal class Triclinic Monoclinic Monoclinic
Space group P 1 ̄ C2/c C2/c
Density 2.6 g cm−3 2.8 g cm−3 2.8 g cm−3
c-Lattice parameter 7.2 Å 18.6 Å 20.1 Å

Kaolinite

7.2 Å

Silicon Aluminum Oxygen Hydroxyl

18.5 Å

Pyrophyllite

Potassium Al, Fe, or Mg

20.1 Å

Mica

Si or Al

Fig. 6 Schematic representation of the structure of koalinite, pyrophyllite, and mica (muscovite)

after Brindley [13]