Layered Structures

Clays and Lubricants

Index © M. Hewat 1998 Help 5 Jan 1999

We have see that the common structure of carbon, ¶graphite is a layer structure, with only very weak forces between the layers; because these layers can easily slip over each other, graphite is an important lubricant. We will now see that there are many similar structures, and in each case their properties are largely determined by this layer structure.

Apparently it should be quite easy to interleave other atoms between the layers of graphite, and in practice this is true. Ammonium, small hydrocarbon molecules and other materials can be 'intercalated' into graphite, with some interesting properties. For example, if we intercalate cations such as potassium or lithium we can obtain materials such as ¶KC8. Some of these materials have potential for new batteries and fuel cells.

Another important lubricant, which is added to engine oil for automobiles, is ¶moly-disulphide (MoS2). Molybdenite consists of molybdenum sandwiched between two layers of sulpher, these MoO6 layers are bound together by 6 strong Mo-S bonds, but they are only bound between themselves by weak S-S "van de Waals" forces, and can easily slide over each other. Note that the sulpher atoms within the MoS6 layers are not close packed, another indication of the strong Mo-S forces.

In contrast, the sulpher atom layers are indeed close-packed in ¶berndtite (SnS2), which is isomorphous with the simplest CdI structure. Here the Sn atoms occupy the octahedral holes within a bi-layer of close-packed S-atoms, with again only weak forces between the layers. In fact instead of the simple AB.AB... stacking of the large anions as shown here, more complex sequences are also possible such as ABC.ABC... or AB.AC... leading to many different polytype structures for CdI.

It is possible to construct materials similar to intercalated graphite from silicon as well as carbon. For example, ¶BaSi2 can be regarded as buckled graphite-type layers of silicon intercalated with barium atoms. However, these buckled layers of silicon do not slip as readily as the smooth layers of carbon in graphite. In nature layered silicate structures are very important as clay minerals.

One of the simplest layered silicates consists of sheets of Si2O5 corner connected silica tetrahedrae, as found in ¶Li2Si2O5, where the green alki metal ions are intercalated between the blue silica layers. (For simplicity, the oxygen atoms at the corners of the silica tetrahedrae have not been drawn). These Si2O5 structures are isomorphous with one form of P2O5.

Clay minerals, or phyllosilicates, consist of strongly bound sheets of silica tetrahedrae and alumina octahedrae which are held together by only weak interatomic forces between the layers, often hydrogen bonding from water. In ¶kaolinite, one of the most important clay minerals, a single sheet of corner connected Silica tetrahedrae is connected by common apex oxygen atoms to a single sheet of edge-connected alumina octahedrae (it is called a 1:1 phyllosilicate). Kaolinite is widely used in papermaking and rubber production. Neutron diffraction has been used to locate the water and study the hydrogen bonding (white atoms and sticks) between the layers of purple alumina octahedrae and blue silica tetrahedrae.

Common ¶talc or Mg3Si4O10.(OH)2, is another phyllosilicate usually found in the bathroom. It consists of double layers of silica tetrahedrae sandwiching a single layer of MgO octahedrae (and is called a 2:1 phyllosilicate). The hydrogen atoms appear to play little or no role in binding these layers together, which explains why talcum powder is so slippery ! Pyrophyllite Al2Si4O10.(OH)2 is another well known mineral with a similar structure.

Replacement of one quarter of the Si++++ by Al+++ in talc or pyrophyllite results in a net negative charge of one per formula unit on the layers, which requires the insertion of positive cations such as K+ between layers to give the mica structures biotite KFeMg2(AlSi3O10).(OH)2 and ¶muscovite KAl2Si3AlO10.(OH)2. The potassium ions occupy large holes between 12 oxygen atoms, 6 from the layer above and 6 from the layer below; the resulting K-O ionic bonds are rather weak and easily broken. This explains why mica can easily be cleaved to give uniformly thin layers.

Mica is anhydrous (contains no water), but in other layered minerals hydrated layers can be intercalated. This is facilitated when the charge on the layers is reduced to less than that of the micas by replacing, for example, part of the Al+++ in the octahedrae by Mg++ as in the "smectite" ¶montmorillonite [(Mg0.33Al1.67)Si4O10(OH)2]Na0.33. (The illustration is a numerical simulation of the hydrated layer by the G. Sposito group at the Lawrence Berkley Laboratory). The spacing between layers can expand and contract depending on the amount of water, even though the layers themselves remain intact. This expansion causes structural damage to buildings on soils with a high smectite clay content. With good drainage, Mg will be leached out and kaolinite will be formed instead of the treacherous montmorillonite.

Montmorillonite or "Fuller's earth" is a natural bleach, and was originally used by Fullers to "full" or remove grease from cloth. It is soft, expanding and turning into a paste when water is added, and is used as an absorbant, especially in "kitty litter" !

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