Bonding studies of minerals

Applications of Surface Techniques to Chemical Bonding Studies of Minerals... 

Keeping these problems in mind, several of the more widely used techniques are discussed below, with an emphasis placed on their application to bonding studies of minerals and species adsorbed on them during mineral/water interface reactions. As with any experimental techniques, many of these problems can be minimized or even eliminated if due caution is taken in performing the studies. 

Structural studies of minerals are important both from mineralogical and commercial viewpoints. Some widely used commercial materials are difficult to structurally characterize fully from a purely experimental approach so that computational approaches can be extremely usefiil. Often quite large unit cells must be used in these studies so that empirical approaches have been extensively employed. However empirical models have well known limitations which can only be overcome through the use of first principles calculations. In addition, let us note that many minerals (and zeolites) are extensively used in catalytic processes where bonds are broken and created. Under such conditions the use of non-empirical methods is really important. Here we discuss some recent works which have used the SIESTA approach to study quite different problems (mostly structural) concerning minerals and zeolites. 

The use of magnetism in studies of minerals and transition metal complexes is alluded to above. Individual metal atoms reveal their presence and bonding environment (planar, tetrahedral, etc.. 

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. 

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

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