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 promulgate a few concepts on the physics and crystallography of silicates. Silica, or silicon dioxide, is composed of the two most abundant 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 structures. 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 reconstructive 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 tetrahedra,3 which means that four oxygen atoms tetrahedrally surround 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 silica 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. Specifically, in kaolinite the silicate sheets are ionic-covalently bound 15.4 • The Science Behind Pottery 297 3Tetraetros (Greek) four-faced
298 15·No Ceramics Age? 02 Si4+ (a) (b) FiGURE 15.2.Two schematic representations of an(SiO4)4-tetrahedron. (a)Spacial arrangement of the oxygen atoms with respect to a silicon atom.(b)The atoms touch each other when assuming a hard-sphere- model.Note:The ionic radii are not drawn to scale.The silicon atom (black)is barely visible in the center between the four oxygen atoms. to Al2(OH)42+-layers to yield Al2(Si2O)(OH)4;see Figure 15.4. Now,it is important to know that adjacent Al2(Si2Os)(OH)4 sheets are quite weakly bound to one another involving van der Waals forces (Section 3.2).It is this weak van der Waals force that al- lows for the easy gliding of the individual sheets or platelets past each other,rendering the above-mentioned ductility (plasticity) of clay,particularly when water is present between the sheets. Interestingly enough,the silicate sheet structure is not re- stricted to clays.It is also found in other minerals,such as mica (KAl3Si3010(OH)2).Moreover,graphite,one of the polymorphic forms of carbon,is likewise composed of layers whereby the car- bon atoms assume the corners of a hexagon.Each atom is bonded to its three coplanar neighbors by strong covalent bonds.The fourth bond to the next layer is,however,of the van der Waals type.For this reason,graphite sheets slide easily past each other and can therefore be used as a low-temperature lubricant.(For lubrications at higher temperatures,another substance with a hexagonal-layered structure is used,namely,boron nitride,which is also called white graphite. The crystallography of clay minerals is certainly one of the most complex among inorganic materials.Kaolinite is only one group (but the most common)of these minerals which are clas- sified into allophanes,halloysites,smectides,vermiculites,etc., to mention just a few.They all have different crystal structures and compositions.Moreover,clay minerals are able to adsorb on the outside of their structural unit various impurity elements
to Al2(OH)4 2 -layers to yield Al2(Si2O5)(OH)4; see Figure 15.4. Now, it is important to know that adjacent Al2(Si2O5)(OH)4 sheets are quite weakly bound to one another involving van der Waals forces (Section 3.2). It is this weak van der Waals force that allows for the easy gliding of the individual sheets or platelets past each other, rendering the above-mentioned ductility (plasticity) of clay, particularly when water is present between the sheets. Interestingly enough, the silicate sheet structure is not restricted to clays. It is also found in other minerals, such as mica (KAl3Si3O10(OH)2). Moreover, graphite, one of the polymorphic forms of carbon, is likewise composed of layers whereby the carbon atoms assume the corners of a hexagon. Each atom is bonded to its three coplanar neighbors by strong covalent bonds. The fourth bond to the next layer is, however, of the van der Waals type. For this reason, graphite sheets slide easily past each other and can therefore be used as a low-temperature lubricant. (For lubrications at higher temperatures, another substance with a hexagonal-layered structure is used, namely, boron nitride, which is also called white graphite.) The crystallography of clay minerals is certainly one of the most complex among inorganic materials. Kaolinite is only one group (but the most common) of these minerals which are classified into allophanes, halloysites, smectides, vermiculites, etc., to mention just a few. They all have different crystal structures and compositions. Moreover, clay minerals are able to adsorb on the outside of their structural unit various impurity elements FIGURE 15.2. Two schematic representations of an (SiO4)4 tetrahedron. (a) Spacial arrangement of the oxygen atoms with respect to a silicon atom. (b) The atoms touch each other when assuming a hard-spheremodel. Note: The ionic radii are not drawn to scale. The silicon atom (black) is barely visible in the center between the four oxygen atoms. 298 15 • No Ceramics Age? (a) O2– Si4+ (b)
15.4.The Science Behind Pottery 299 (a) (b) (c) FIGURE 15.3.Schematic representation of (a)a silica chain (SiO3)n2n-, (b)a silica ring (Si30)6-,and (c)a silica sheet(Si2Os)2-.[Note:The fourth Si bond in (c)points into the paper plane.]See also Figure 15.4. such as calcium,iron,or sodium,which can be easily mutually exchanged.As an example,for a characteristic composition of kaolin,the one found in North-Central Florida may serve: 47% SiO2 37.9% A1203 0.45% Fe203 0.18% TiO2 0.08% Cao 0.3% MgO 0.2% K20 0.24% Na2O 13.5% Loss,i.e.,mainly H2O
such as calcium, iron, or sodium, which can be easily mutually exchanged. As an example, for a characteristic composition of kaolin, the one found in North-Central Florida may serve: 47% SiO2 37.9% Al2O3 0.45% Fe2O3 0.18% TiO2 0.08% CaO 0.3% MgO 0.2% K2O 0.24% Na2O 13.5% Loss, i.e., mainly H2O. 15.4 • The Science Behind Pottery 299 FIGURE 15.3. Schematic representation of (a) a silica chain (SiO3)n 2n, (b) a silica ring (Si3O9)6, and (c) a silica sheet (Si2O5)2. [Note: The fourth Si bond in (c) points into the paper plane.] See also Figure 15.4. (a) (b) (c)