GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33
3.8 Silicate Ceramics • 57Figure 3.15
Electron micrograph
of kaolinite crystals.
They are in the form
of hexagonal plates,
some of which are
stacked on top of one
another. 15,000×.
(Photograph
courtesy of Georgia
Kaolin Co., Inc.)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 (OH)^24 +layer. Whereas the bonding within this two-layered 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 1μm in diameter
and nearly hexagonal. Figure 3.15 is an electron micrograph of kaolinite crystals at
a high magnification, showing the hexagonal crystal plates some of which are piled
one on top of the other.
These silicate sheet structures are not confined to the clays; other miner-
als also in this group are talc [Mg 3 (Si 2 O 5 ) 2 (OH) 2 ] and the micas [e.g., mus-
covite, KAl 3 Si 3 O 10 (OH) 2 ], which are important ceramic raw materials. As might be