Callister - An introduction - 8th edition

466 • Chapter 12 / Structures and Properties of Ceramics
SiO 4
4–
(a)
Si 2 O 7
6–
(b)
Si 3 O 9
6–
(c)
Figure 12.12 Five
silicate ion structures
formed from SiO 4
4
tetrahedra.
12–
Si 6 O 18
(d)
Si 4+ O 2–
2n–
(SiO 3 ) n
(e)
The Silicates
For the various silicate minerals, one, two, or three of the corner oxygen atoms of
the SiO 4
4 tetrahedra are shared by other tetrahedra to form some rather complex
structures. Some of these, represented in Figure 12.12, have formulas SiO 4
Si 3 O 6
4 , Si 2 O 6
7 ,
9 and so on; single-chain structures are also possible, as in Figure 12.12e.Positively
charged cations such as Ca 2 ,Mg 2 , and Al 3 serve two roles. First, they compensate
the negative charges from the SiO 4
4 units so that charge neutrality is
achieved; second, these cations ionically bond the tetrahedra together.
SiO 4
4
Simple Silicates
Of these silicates, the most structurally simple ones involve isolated tetrahedra (Figure
12.12a). For example, forsterite (Mg 2 SiO 4 ) has the equivalent of two Mg 2 ions
associated with each tetrahedron in such a way that every Mg 2 ion has six oxygen
nearest neighbors.
The Si 2 O 6
7 ion is formed when two tetrahedra share a common oxygen atom
(Figure 12.12b). Akermanite (Ca 2 MgSi 2 O 7 ) is a mineral having the equivalent of
two Ca 2 ions and one Mg 2 ion bonded to each Si 2 O 6
7 unit.
Layered Silicates
A two-dimensional sheet or layered structure can also be produced by the sharing
of three oxygen ions in each of the tetrahedra (Figure 12.13); for this structure the
repeating unit formula may be represented by (Si 2 O 5 ) 2 . The net negative charge is
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 twolayer
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 neutral
by an adjacent Al 2 1OH2 2
4 layer. A single sheet of this structure is shown in