15.4.The Science Behind Pottery 297 times permeated with materials such as feldspar [(K,Na)2O Al2O36SiO2]or mica [KAl3Si3O10(OH)2].In other words,the clay minerals belong to a larger family of silicates which are common among an extended number of ceramic materials.It is therefore necessary to digress for a moment from our theme and promul- gate a few concepts on the physics and crystallography of silicates. Silica,or silicon dioxide,is composed of the two most abun- dant elements of the earth's crust,which consists of 59 mass% SiO2.Silica is the major component (at least 95%)of rocks,soils clays,and sands.Silica exists in several allotropic forms upon raising the temperature,having slightly different crystal struc- tures.Specifically,the room temperature a-quartz transforms at 573C into B-quartz by a displacive transformation (similar to that found in martensite;see Section 8.3)involving a rapid but slight distortion of the crystal lattice.A further transformation (of the reconstructive type)takes place at 870C from B-quartz to B-tridymite,during which the bonds between atoms are broken and a new crystal structure is formed by nucleation and growth. A third allotropic transformation occurs at 1470C from B-tridymite into B-cristobalite,which is again of the reconstruc- tive type.The melting point of pure SiOz is finally reached at 1723C but can be reduced by additional constituents as shown in Figure 15.5. It is common to represent silicates as a series of(SiO4)4-tetra- hedra,3 which means that four oxygen atoms tetrahedrally sur- round one silicon atom;Figure 15.2.This basic tetrahedral unit of silicates is fourfold negatively charged.The bonding between the silicon and the oxygen atoms is mostly covalent.Thus,each bond is strong and directional (see Section 3.2 and Figure 3.4). The melting temperature of silica is therefore high(1723C).Bulk silica can be represented by a three-dimensional network of the just-discussed tetrahedral units whereby each corner oxygen atom is shared by an adjacent tetrahedron(Figure 15.2).The sil- ica tetrahedra can combine,for example,to chains [Figure 15.3 (a)]or to rings [Figure 15.3 (b)].To satisfy the charge balance, each oxygen ion (or group of oxygen ions)can combine with, say,metal ions.For example,two Mg2+ions may combine with one (SiO4)4-tetrahedral unit,thus forming Mg2SiO4,called foresterite.Compounds of this type are termed orthosilicates (or olivines). Of particular interest in the present context are the clays.In this case,the silicon combines with oxygen to yield(Si2Os)2-; see Figure 15.3(c),which forms a sheet-type structure.Specifi- cally,in kaolinite the silicate sheets are ionic-covalently bound BTetraetros (Greek)=four-faced.times permeated with materials such as feldspar [(K, Na)2O Al2O36SiO2] or mica [KAl3Si3O10(OH)2]. In other words, the clay minerals belong to a larger family of silicates which are common among an extended number of ceramic materials. It is therefore necessary to digress for a moment from our theme and promul￾gate a few concepts on the physics and crystallography of silicates. Silica, or silicon dioxide, is composed of the two most abun￾dant elements of the earth’s crust, which consists of 59 mass% SiO2. Silica is the major component (at least 95%) of rocks, soils, clays, and sands. Silica exists in several allotropic forms upon raising the temperature, having slightly different crystal struc￾tures. Specifically, the room temperature -quartz transforms at 573°C into -quartz by a displacive transformation (similar to that found in martensite; see Section 8.3) involving a rapid but slight distortion of the crystal lattice. A further transformation (of the reconstructive type) takes place at 870°C from -quartz to -tridymite, during which the bonds between atoms are broken and a new crystal structure is formed by nucleation and growth. A third allotropic transformation occurs at 1470°C from -tridymite into -cristobalite, which is again of the reconstruc￾tive type. The melting point of pure SiO2 is finally reached at 1723°C but can be reduced by additional constituents as shown in Figure 15.5. It is common to represent silicates as a series of (SiO4)4 tetra￾hedra,3 which means that four oxygen atoms tetrahedrally sur￾round one silicon atom; Figure 15.2. This basic tetrahedral unit of silicates is fourfold negatively charged. The bonding between the silicon and the oxygen atoms is mostly covalent. Thus, each bond is strong and directional (see Section 3.2 and Figure 3.4). The melting temperature of silica is therefore high (1723°C). Bulk silica can be represented by a three-dimensional network of the just-discussed tetrahedral units whereby each corner oxygen atom is shared by an adjacent tetrahedron (Figure 15.2). The sil￾ica tetrahedra can combine, for example, to chains [Figure 15.3 (a)] or to rings [Figure 15.3 (b)]. To satisfy the charge balance, each oxygen ion (or group of oxygen ions) can combine with, say, metal ions. For example, two Mg2 ions may combine with one (SiO4)4 tetrahedral unit, thus forming Mg2SiO4, called foresterite. Compounds of this type are termed orthosilicates (or olivines). Of particular interest in the present context are the clays. In this case, the silicon combines with oxygen to yield (Si2O5)2; see Figure 15.3 (c), which forms a sheet-type structure. Specifi￾cally, in kaolinite the silicate sheets are ionic-covalently bound 15.4 • The Science Behind Pottery 297 3Tetraetros (Greek)
four-faced