Callister - An introduction - 8th edition

12.3 Silicate Ceramics • 467
Figure 12.13 Schematic representation of the
two-dimensional silicate sheet structure having a
repeat unit formula of (Si 2 O 5 ) 2 .
Si 4+ O 2–
Figure 12.14, which is exploded in the vertical direction to provide a better perspective
of the ion positions; the two distinct layers are indicated in the figure. The
midplane of anions consists of O 2 ions from the (Si 2 O 5 ) 2 layer, as well as OH
ions that are a part of the Al 2 1OH2 2
4 layer. Whereas the bonding within this twolayered
sheet is strong and intermediate ionic-covalent, adjacent sheets are only
loosely bound to one another by weak van der Waals forces.
A crystal of kaolinite is made of a series of these double layers or sheets stacked
parallel to each other, which form small flat plates typically less than 1 m in
diameter and nearly hexagonal.
These silicate sheet structures are not confined to the clays; other minerals
also in this group are talc [Mg 3 (Si 2 O 5 ) 2 (OH) 2 ] and the micas [e.g., muscovite,
KAl 3 Si 3 O 10 (OH) 2 ], which are important ceramic raw materials.As might be deduced
Al 2 (OH) 4
2+ Layer
Anion midplane
Figure 12.14 The structure of kaolinite
clay. (Adapted from W. E. Hauth,
“Crystal Chemistry of Ceramics,”
American Ceramic Society Bulletin,
Vol. 30, No. 4, 1951, p. 140.)
(Si 2 O 5 ) 2– Layer
Si 4+
Al 3+
OH –
O 2–