Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33

56 • Chapter 3 / Structures of Metals and Ceramics

Si4+ O2–

Figure 3.13 Schematic representation of the

two-dimensional silicate sheet structure having a

repeat unit formula of (Si 2 O 5 )^2 −.

associated with the unbonded oxygen atoms projecting out of the plane of the page.

Electroneutrality is ordinarily established by a second planar sheet structure having

an excess of cations, which bond to these unbonded oxygen atoms from the Si 2 O 5

sheet. Such materials are called the sheet or layered silicates, and their basic structure

is characteristic of the clays and other minerals.

One of the most common clay minerals, kaolinite, has a relatively simple two-

layer silicate sheet structure. Kaolinite clay has the formula Al 2 (Si 2 O 5 )(OH) 4 in

which the silica tetrahedral layer, represented by (Si 2 O 5 )^2 −, is made electrically neu-

tral by an adjacent Al 2 (OH)^24 +layer. A single sheet of this structure is shown in Figure

3.14, which is exploded in the vertical direction to provide a better perspective of

OH–

O2–

Al3+

Si4+

Anion midplane

Al 2 (OH) 4 2+ Layer

(Si 2 O 5 )2– Layer

Figure 3.14 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.)