GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33
3.8 Silicate Ceramics • 55SiO 44 –
(a)
Si 2 O 76 –
(b)Si 3 O 96 –
(c)Si 6 O 1812 –Si4+ O^2 –(d)(SiO 3 )n
(e)2n–Figure 3.12 Five silicate
ion structures formed
from SiO^44 −tetrahedra.complicated, and comparatively open; that is, the atoms are not closely packed to-
gether. As a consequence, these crystalline silicas have relatively low densities; for
example, at room temperature quartz has a density of only 2.65 g/cm^3. The strength
of the Si–O interatomic bonds is reflected in its relatively high melting temperature,
1710 ◦C (3110◦F).
Silica can also be made to exist as a noncrystalline solid or glass; its structure is
discussed in Section 3.21.The Silicates
For the various silicate minerals, one, two, or three of the corner oxygen atoms
of the SiO^44 −tetrahedra are shared by other tetrahedra to form some rather com-
plex structures. Some of these, represented in Figure 3.12, have formulas SiO^44 −,
Si 2 O^67 −,Si 3 O^69 −and so on; single-chain structures are also possible, as in Figure
3.12e. Positively charged cations such as Ca^2 +,Mg^2 +, and Al^3 +serve two roles.
First, they compensate the negative charges from the SiO^44 −units so that charge
neutrality is achieved; and second, these cations ionically bond the SiO^44 −tetrahedra
together.Simple Silicates
Of these silicates, the most structurally simple ones involve isolated tetrahedra (Fig-
ure 3.12a). 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^67 −ion is formed when two tetrahedra share a common oxygen atom
(Figure 3.12b). 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^67 −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 3.13); for this structure the
repeating unit formula may be represented by (Si 2 O 5 )^2 −. The net negative charge is