Moore and Reynold,pages 104-137
Brindley and Brown, pages 102-144
The graphic below shows various views of tetrahedral sheets. The top three illustrate a sideview using three different motifs (space-filling spheres, ball and sticks, and polyhedra). The bottom row shows a top view perspective illustrating the slightly distorted hexagonal linking pattern that results when the basal oxygen share corners.
Octahedral sheets are composed of individual octahedrons that share edges composed of oxygen and hydroxyl anion groups with Al, Mg, Fe3+ and Fe2+ typically serving as the coordinating cation. These octahedrons too, are arranged in a hexagonal pattern.
Other cations include Li, Ti, V, Cr.... and
also vacancies. Note that in the top view a slightly distorted
hexagonal patter appears with dimensions very similar to the
The minerals gibbsite Al(OH)3 and brucite Mg(OH)2 are very similar in structure to the octahedral sheets found in many clay mineral structures. The difference being that all the coordinating anions are hydroxyls in gibbsite and brucite.
When trivalent cations (3+) occupy the edge sharing hexagonal sheet then the cation to oxygen ratio is 1:3 (in order to maintain electric neutrality). This leaves every third site empty, meaning only 2 out of 3 sites are occupied. This arrangement is referred to as a dioctahedral structure and sometimes called a gibbsite-like or -type sheet.
The tetrahedral, dioctahedral, and trioctahedral sheets are the fundamental building blocks for phyllosilicates. The principle criteria used for classification of phyllosilicates is based the sheet types in the structure.
Let's look at the criteria for classification of phyllosilicates and this will show us how these basic units can be put together to form clay minerals.
The type of tetrahedral-octahedral sheet combinations.
Examples of correct references are therefore:
Examples of incorrect references include:
||# of site occupied|
||3 out of 3|
||2 out of 3|
Sheets may be electrically neutral or
they may bear a net negative
charge. Charge imbalances usually
come about by isomorphous substitution or vacancies.
Most commonly :
Also possible are vacancies:
All 2:1 layer structures can be defined
in terms of their unit cell composition. The tetrahedral sheet
of most unit cells is composed of eight (8) tetrahedral cations
and twenty (20) oxygens (T8O20). It is common practice to report the formula
unit composition in terms of the half-unit cell (T4O10). With
silica as the tetrahedral cation (Si4O10), the
sheet is electrically neutral. By substitution
of Al for Si we get AlSi 3O10. In this case the amount of charge (x)
needed to balance the net negative charge is equal to one (i.e.,
x = 1).
4. The interlayer composition.
Neutrality is restore by a compensating
cation or ionic group in the interlayer space. Things that can
go into the interlayer space include:
Turbostratic = random pile of playing cards.
Examples from the true mica lepidolite are shown below. Click here for more polytype examples.
Isomorphous subsitution allows for solid
solutions chemical compositions. If a particular mineral
an element with higher than average concentrations (e.g., Cr)..
enough to constitute a new mineral name, than the mineral name
by that element (e.g., Cr-muscovite).
Type of component layers and nature of stacking for mixed-layer
Nomeclature for clay is formally address by international committees. See examples of discussions: 2002, 2006, 2009