chapter  21
50 Pages

Phyllosilicates

Phyllosilicates, in other words, layer silicates, occupy one of six subclasses under the silicates class (Table 21.1). Phyllo-is aer the Greek word describing the state of leaves, like a bed or heap of leaves. ¤us in science, oen the word is used as a prex 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 indenitely extended and more complicated structural units to cover the existing silicate minerals. ¤is

mechanism of linking together is known as polymerization, and the classied structural units with their compositions are given in Table 21.2.