3.5 Sheet silicate minerals

The sheet silicates comprise a large number of different minerals, including micas. They all share the same basic building blocks: a tetrahedral sheet (Figure 48a), and one of two kinds of octahedral sheet.

Figure 48 Building blocks of sheet silicate minerals. (a) Part of a sheet made up of SiO₄ tetrahedra. (b) Part of a Mg(OH)₂trioctahedral sheet, where every oxygen atom is shared between three octahedra. (c) Part of an Al(OH)₃dioctahedral sheet, where every oxygen atom is shared between two octahedra. (d) An oblique view of a sheet sandwich, formed by two tetrahedral sheets bonded to an octahedral sheet. (e) A schematic representation of the sheet sandwich in (d).

The octahedral sheets can be thought of as layers of metal hydroxides. One kind of octahedral sheet is effectively a layer of magnesium hydroxide, Mg(OH)₂, where each magnesium atom is bonded to six OH groups at the corners of an octahedron. The octahedra are arranged in a plane, sharing edges, to form a trioctahedral sheet as each oxygen atom is shared between three octahedra (Figure 48b).

The other kind of octahedral sheet contains aluminium instead of magnesium, and is effectively a layer of aluminium hydroxide, Al(OH)₃. In this case each oxygen atom is shared between two octahedra, hence this is a dioctahedral sheet (Figure 48c), and it leaves distinct holes in the structure. In sheet silicates, the sheets are combined to form composite structures: for example an octahedral sheet sandwiched between two tetrahedral sheets as shown in Figure 48d, and more schematically in Figure 48e.

A property common to all sheet silicate minerals is their tendency to be soft, with near-perfect cleavage parallel to the sheets. They form distinctive, platy or flake-like crystals. With (OH) groups as part of their structure, they are all hydrous minerals.