Hydrogen bonding, layer silicates

Clays consist of parallel silicate layers in kaolinite, two unsymmetrical types of layers are linked by hydrogen bonds. One consists of aluminium ions and hydroxyl groups, the other of silicon and oxygen ions. Cairns-Smith does not postulate a detailed mechanism, but only describes the main thrust of his argument. Critics complain that clear experimental results are not available (however, other proponents of new hypotheses often provide no evidence to back up their suggestions ). 

Fig. 9.8 Formation of hydrogen bonds between PBS and MMT, which leads to the flocculation of the dispersed silicate layers. Reprinted from [30], 2003 American Chemical Society.

When fewer than three hydrogen bonds exist to satisfy the oxygen valence of a siloxy group, as for example in some hydrous layer silicates mentioned above, then they need to be stronger (yielding a low-field shift in proton NMR) and/or additional interaction partners should be present. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

As the temperature continues to rise, this jumping between the two configurations, similar to melting, leads to the peak in Cp with a maximum at about 270 K (Figure 3). In summary, the model suggests that the water in direct contact with the mineral surface (hole water) is strongly bonded to the silicate layer. The second layer of water (associated water) behaves very differently because it has few if any hydrogen bonds directly to the silicate layer. 

The ideal layered silicate is the mineral montmorrillonite, which is main component of the clay bentonite (fig 6). Montmorrillonite is a so called 2 1 layered silicate. Each layer consists of two sheets of silica tetrahedrae which are sandwiched by an alumina octahedrae sheet. The layers are only weakly bound, often by hydrogen bonding from water. The structure has been extensively studied by transmission electron diffraction especially by Z5wagin and coworkers [4]. 

The block iron silicate Si2 xFex Fe3ijFe 05(0H)4, cronstedtite, is representative of several mixed-valent minerals. Its structure consists of alternate layers of comer-shared tetrahedra and edge-shared octahedra. The tetrahedra Si " and Fe ions are coordinated by four oxide ions, the octahedral Fe " and Fe " ions share three common oxide ions with tetrahedral-site layers on one side and on the other are coordinated by three hydroxyl ions that hydrogen bond to the next tetrahedral-site layer. Mossbauer data at 35 K are able to resolve distinguishable Fe " and Fe " ions on the octahedral sites but in the range 300 < T < 500 K a rapid (t), < 10 s) charge transfer was found ... 

Figure 3.7. Schematic of the kaolin structure showing one silicate and one gibbsite sheet in each layer, which has been expanded along the c-axis to show bonding. The basic unit is repeated along the two horizontal axes to form layers. Adjacent layers are held together by hydrogen bonding (from Taylor and Ashcroft, 1972, with permission).

Hydrogen bonding occurs between H+ and ions of high electronegativity, such as F, O2-, and N3-. The hydrogen bond is essentially a weak electrostatic bond and is important in crystal structures of oxy compounds, such as the layer silicates. Summed over many atoms, the individually weak hydrogen bonds can strongly bond adjacent structures. 

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