Chp 02b - aluminium cation with two oxygen ions and four...

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aluminium cation with two oxygen ions and four hydroxyl ions forms an octahedron, i.e. AlO 2 (OH) 4 . Thus, in each layer, a sheet of SiO 4 4 " tetrahedra lies parallel to a sheet of AlO 2 (OH) 4 octahedra, with the two sheets sharing common O 2- anions. Strong ionic and covalent bonding exists within each layer and each layer is electrically neutral. However, the uneven dis- tribution of ionic charge across the five sub-layers has a polarizing effect, causing opposed changes to develop on the two faces of the layer. The weak van der Waals bonding between layers is thus explicable. This asym- metry of ionic structure also unbalances the bonding forces and encourages cleavage within the layer itself. In general terms, one can understand the softness, easy cleavage and mouldability (after moistening) of this mineral. The ionic radii of oxygen and hydroxyl ions are virtually identical. The much smaller Al 3+ cations are shown located outside the SiO 4 4 " tetrahedra. How- ever, the radii ratio for aluminium and oxygen ions is very close to the geometrical boundary value of 0.414 and it is possible in other aluminosilicates for Al 3+ cations to replace Si 4+ cations at the centres of oxygen tetrahedra. In such structures, ions of different valency enter the structure in order to counterbalance the local decreases in positive charge. To summarize, the coor- dination of aluminium in layered aluminosilicates can be either four- or sixfold. Many variations in layer structure are possible in silicates. Thus, talc (French chalk), Mg 3 Si 4 OiQ(OH) 2 , has similar physical characteristics to kaolinite and finds use as a solid lubricant. In talc, each layer con- sists of alternating Mg 2+ and OH" ions interspersed between the inwardly-pointing vertices of two sheets of SiO 4 4 " tetrahedra. This tetrahedral-tetrahedral layering thus contrasts with the tetrahedral-octahedral layering of kaolinite crystals. Finally, in our brief survey of silicates, we come to the framework structures in which the SiO 4 4 " tetrahe- dra share all four corners and form an extended and regular three-dimensional network. Feldspars, which are major constituents in igneous rocks, are fairly com- pact but other framework silicates, such as the zeolites and ultramarine, have unusually 'open' structures with tunnels and/or polyhedral cavities. Natural and syn- thetic zeolites form a large and versatile family of compounds. As in other framework silicates, many of the central sites of the oxygen tetrahedra are occupied by Al 3+ cations. The negatively charged framework of (Si, Al)O 4 tetrahedra is balanced by associated cations; being cross-braced in three dimensions, the structure is rigid and stable. The overall (Al 3+ + Si 4+ ):O 2 ~ ratio is always 1:2 for zeolites. In their formulae, (H 2 O) appears as a separate term, indicating that these water molecules are loosely bound. In fact, they can be read- ily removed by heating without affecting the structure and can also be re-absorbed. Alternatively, dehydrated
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Chp 02b - aluminium cation with two oxygen ions and four...

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