Phyllosilicates, in other words, layer silicates, occupy one of six subclasses under the silicates class (Table 21.1). Phyllo-is aer the Greek word describing the state of leaves, like a bed or heap of leaves. ¤us in science, oen the word is used as a prex to express thin layer or paper sheet like in a book. ¤is re¥ects general crystal habits and morphological features of the phyllosilicates. ¤e silicates are abundant on the earth and of greater importance than any other minerals, which is easily understood from the fact that oxygen and silicon constitute nearly 75% of the elements in the earth’s crust (Figure 21.1). ¤e fundamental unit in all silicate structures is the silicon-oxygen tetrahedron (Figure 21.2). As Bragg (1950) stated, the bonds between silicon and oxygen are so strong that the four oxygen atoms are always found at the corners of a tetrahedron of nearly constant dimensions and regular shape whatever the rest of the structure may be like. ¤e Si-O distance is about 0.16 nm, and the O-O distance 0.26 nm. Besides the crustal abundance of these two elements, the ionic radius of Si4+, 0.042 nm, is conveniently accommodated in surroundings with four oxygens to form the fundamental structural unit of all silicates, the tetrahedron having the chemical formula (SiO4)4−. ¤is unit can occur independently with compensating cations like Mg2+ and Fe2+. And also these SiO4 tetrahedra may be linked together by having one or more oxygen atoms shared in common by neighboring tetrahedra to form indenitely extended and more complicated structural units to cover the existing silicate minerals. ¤is
mechanism of linking together is known as polymerization, and the classied structural units with their compositions are given in Table 21.2.