GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33
54 • Chapter 3 / Structures of Metals and Ceramics
- Si4+ O2–
Figure 3.10 A
silicon–oxygen
(SiO^44 −) tetrahedron.3.8 SILICATE CERAMICS
Silicates are materials composed primarily of silicon and oxygen, the two most abun-
dant elements in the earth’s crust; consequently, the bulk of soils, rocks, clays, and
sand come under the silicate classification. Rather than characterizing the crystal
structures of these materials in terms of unit cells, it is more convenient to use var-
ious arrangements of an SiO^44 −tetrahedron (Figure 3.10). Each atom of silicon is
bonded to four oxygen atoms, which are situated at the corners of the tetrahedron;
the silicon atom is positioned at the center. Since this is the basic unit of the silicates,
it is often treated as a negatively charged entity.
Often the silicates are not considered to be ionic because there is a significant
covalent character to the interatomic Si–O bonds (Table 3.2), which are directional
and relatively strong. Regardless of the character of the Si–O bond, there is a formal
charge of –4 associated with every SiO^44 −tetrahedron, since each of the four oxygen
atoms requires an extra electron to achieve a stable electronic structure. Various
silicate structures arise from the different ways in which the SiO^44 −units can be
combined into one-, two-, and three-dimensional arrangements.Silica
Chemically, the most simple silicate material is silicon dioxide, or silica (SiO 2 ). Struc-
turally, it is a three-dimensional network that is generated when the corner oxygen
atoms in each tetrahedron are shared by adjacent tetrahedra. Thus, the material
is electrically neutral and all atoms have stable electronic structures. Under these
circumstances the ratio of Si to O atoms is 1:2, as indicated by the chemical formula.
If these tetrahedra are arrayed in a regular and ordered manner, a crystalline
structure is formed. There are three primary polymorphic crystalline forms of sil-
ica: quartz, cristobalite (Figure 3.11), and tridymite. Their structures are relativelySi4+ O2–Figure 3.11 The arrangement of silicon and oxygen
atoms in a unit cell of cristobalite, a polymorph of SiO 2.