Structural units of clay minerals

Basic structural units

Clay minerals are made from two basic building blocks:

1. Silica tetrahedron: a silicon atom at the centre surrounded by four oxygen atoms (or hydroxyls).
2. Alumina octahedron: an aluminium (or magnesium) atom at the centre surrounded by six hydroxyls (or oxygens).

Their repeated units form sheets, which are stacked to form clay mineral layers.

Silica sheet

Many silica tetrahedra are joined by sharing the three oxygens at the base, forming a flat sheet with hexagonal pattern; the apexes point in one direction. Formula $(\text{Si}_4\text{O}_{10})^{4-}$ or $\text{Si}_4\text{O}_6(\text{OH})_4$. Thickness about 4.93 Å.

Gibbsite sheet

Octahedral units with aluminium in the centre, joined by shared hydroxyls. Only two out of three possible positions of Al are filled (dioctahedral). Formula $\text{Al}_2(\text{OH})_6$ (Al$^{3+}$). Thickness about 5.05 Å.

Brucite sheet

Similar octahedral sheet with magnesium (Mg$^{2+}$) at the centre, in which all three positions are filled (trioctahedral). Formula $\text{Mg}_3(\text{OH})_6$.

 Tetrahedron      Octahedron
     Si              Al / Mg
    /|\            /| |\
   O O O          OH OH OH
     O            OH OH OH