Structural features

The next group of silicate minerals that we discuss are sheet silicates, also called phyllosilicates (after the Greek φυλλον (phyllon, meaning leaf)). Most of the minerals in this group have a flaky habit and an excellent single cleavage. The habit and cleavage are due to planar units in the crystal structure. All sheet silicates contain, in addition to O2-, OH- groups as anions.

We mentioned in Chapter 26 that silicon tetrahedra (SiO4-4) in sheet silicates are polymerized to form an infinite two-dimensional net with six-membered rings and hexagonal symmetry (Figure 27.1). Contrary to the tetrahedral nets of tridymite and cristobalite, where alternating apices point in opposite directions (and are linked to another net) (Figure 19.3a), in sheet silicates all free tetrahedral apices point in the same direction. Within a sheet silicate net, three corners of each tetrahedron are shared with another, resulting in a structural base of the tetrahedral network Si2O52-. Such a layer has a negative charge that is neutralized by interstitial layers of cations and anions.

The distance between free oxygen apices in a net is close to 1.3 Å, and this value is very similar to the O–O distance of a coordination octahedron (1.4 Å). Therefore the tetrahedral layer fits almost perfectly on top of a layer of octahedra that lie on triangular sides and share edges (Figure 27.2a). As the tetrahedral layer is attached to the octahedral layer, corresponding oxygen atoms are shared.