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 elementsto 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 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 (KAl3Si3O10(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 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. 298 15 • No Ceramics Age? (a) O2– Si4+ (b)