Minerals showing isomorphism

The best-known composition of olivine, the light green gemstone peridote, is (Mgo.9Feo.i)2Si04. The olivines are a group of minerals showing isomorphous replacement. 

For minerals showing isomorphism, the content of a particular component changes with the composition. The contents of components in this kind of mineral can be determined based on the mass loss of the components. For example, the amounts of COj evolved from carbonate differ with change in the isomorphic composition. Hence the composition and the amounts of the particular components can be estimated. 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Since adsorption at a mineral surface is a replacement process, we would expect mineral surfaces with weak affinity for water to have the strongest affinity for hydrophobic solutes. Infrared spectroscopy shows that siloxane surfaces on clays with little isomorphic substitution form weaker hydrogen bonds than water forms with itself (64), which corresponds to one of the definitions of a hydrophobic surface offered by Texter et al. (65) Therefore,... 

Figure 3A. Schematic of a brucite sheet showing the relative locations of all Mg2+ by removing the top layer of oxygen. Minerals may contain any combination of Al3+ and Mg2+, while Fe3+ or Fe2+ may also substitute isomorphously (from Taylor and Ashcroft, 1972, with...

The ideal constitution of the kaolin layer represents an electrically neutral unit, with rarely any isomorphous substitution of cations of different charges within the lattice. Consequently, kaolinite and related minerals would not be expected to show a large cation exchange capacity, and indeed this is usually the case. That a small but varying exchange capacity does occur may be attributed to two principal causes. 

The mineral hornblende provides an illustrative example of isomorphous replacement. The ideal composition of this silicate is Ca2Mg2-(Si40ii)2 (0H)2. a typical analysis of a naturally occurring sample might well show that up to a quarter of the silicon is replaced by aluminium most of the Mg " " replaced by Fe " ", together with smaller amounts of Fe " ", Mn " and Ti, and about a third of the Ca " " replaced by a mixture of Na and K+. 

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