Tetrahedral Sheets of Clay Minerals

(Continuation of Basic structural Elements)

will almost always be found in geochemical solutions, because silicate minerals are so ubiquitous. The solubility of quartz in pure water is about 0.1 mmol/L (3 ppm) and that of amorphous silica is about ten times higher, with up to 60 ppm commonly reported. Below pH 9 or 10, most soluble silicon is found in the form of silicic acid, Si(OH)4:


Above concentrations of a few ppm, soluble silica can polymerize, beginning with the dimer:


Note that, unlike octahedral cations such as Al and Fe, the tetrahedral Si monomers only share corners when they polymerize. The Si-O-Si bond that is thus created is one of the most flexible known, with Si-O-Si angles in minerals ranging nearly from 90 to 180 degrees.

Soluble silicic acid monomers and small polymers interacts with other soluble species, with organic matter, and with mineral surfaces. Metal oxides are good adsorbents of soluble silicon and are thought to act as templates for the condensation of soluble silica. Thus, if some gibbsite-like material is present:

Then soluble silica can build a new layer on the aluminum template to form a sheet silicate. There is no pure silica mineral that forms sheets, so the metal oxide template is apparently necessary to maintain the stability of the silicon sheet. When viewed separately (even though it does not exist separately), each Si shared O atoms with three other Si:

The Si tetrahedra are thereby arranged in a hexagonal pattern. The resultant sheet composition is T
2O5 where T is a general designation for the tetrahedral cation. While Si4+ is dominant, Al3+ and sometimes Fe3+ can also precipitate as impurities in the tetrahedral sheet.

Consider side-views of the tetrahedral sheet:

There are two planes of O atoms in this tetrahedral sheet:

Basal oxygens which link pairs of tetrahedra together are all (more or less) in one plane.

Apical oxygens, which provide linkage between the tetrahedral sheet and the octahedral sheet, are all in a separate plane. Note that these oxygens will be shared by both the octahedral and tetrahedral sheets. Since there is only one apical O per tetrahedron, each tetrahedron shares a corner with an octahedron in the octahedral sheet.

Once enough apical oxygens have linked an octahedral sheet to one or more tetrahedral sheets, we have a structure we can call a phyllosilicate clay mineral. For example one tetrahedral sheet and one trioctahedral sheet could be joined:

Once we put two or more sheets together, we have created a layer. There is an interlayer between each pair of layers.

So remember these definitions:

The correct nomenclature is therefore: "a plane of atoms" or "a tetrahedral or octahedral sheet" or "a 1:1 or 2:1 layer type".