Bentonite Mineralogy Part 1: Methods of Investigation - Posiva

2 STRUCTURE AND CHEMICAL COMPOSITION OF CLAY MINERALS
2.1 Structure of clay minerals
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In this chapter, the structure of clay minerals is described. Without knowing the
fundamental clay mineral structure it is not possible to understand many properties of
the different clay mineral species.
Clay minerals are phyllosilicates, and their structure consists of two types of sheets:
- Tetrahedral sheets (Fig. 2-1) consist of corner-linked tetrahedra with oxygens in the
corners and cations (T) in the centre. Dominant cation is Si 4 +, but Ae+ is substituted
for it frequently, up to half of the Si. Fe 3 + is substituted for Si 4 + occasionally; T/0
ratio is T 20 5. The tetrahedra rest on a triangular face and share all three oxygens with
three other tetrahedra. The sheet of linked tetrahedra has hexagonal symmetry. The
fourth, apical, oxygen points in the figure downward from the base.
- Octahedral sheets (Fig. 2-2) consist of edge-linked octahedra with OH in the corners
and cations (R) in the centre. The cations are usually Al 3 +, Mg 2 +, Fe 2 + or Fe 3 +, but all
other transition elements and Li are possible. The octahedral sheets can be described
as composed of two planes of closest-packed hydroxyls with cations occupying the
octahedral sites between the two planes.
Figure 2-1. Tetrahedral sheet. The open circles are oxygens, the black circles
tetrahedrally coordinated cations.
Figure 2-2. Octahedral sheet. The filled black circles are OH:s and the filled grey
circles octahedrally coordinated cations.