Importance of Clay Minerals

Potter, R.M. Rossman, G.R. (1977) Desert varnish the importance of clay minerals. Science 196, 1446-1448. 

Certain organic compounds, especially polyphenols, stimulate mobilization of clay by neutralizing positive charges at the edges of clay minerals. As iron-saturated organic complexes are insoluble, this process might be of little importance in Fe-rich Luvisols (particularly common in the subtropics). 

The surfaces of clay minerals can catalyze the polymerization of organic compounds through a free radical-cationic initiation process. This type of reaction is believed to be initiated by the abstraction of an electron by Lewis acid sites on mineral surfaces however, Bronsted acidity has also been shown to be important in certain cases (see Chapter 22). 

The observed paragenesis contains a significant amount of feldspars. Although feldspars are not considered to crystallize in the numerical simulations (Jacquol 2000 Durst Vuataz 2001), our experimental results suggest that formation of clay minerals is controlled by previously crystallized feldspars. Therefore, the presence of feldspars is of importance for the crystallization of secondary minerals and the evolution of porosity in the stimulated reservoir of the HFR site. 

It should be possible, through a unification of chemical and mineral structure data and the results of experimental studies on silicate phase equilibria, to develop a general picture of clay mineralogy based upon the known chemical behavior of phyllosilicates under various physical conditions. The major elements for such a study are presently available in a rough outline. It is fact the purpose of this essay to summarize the available information and create a general outline of clay mineral petrology. It is hoped that such an attempt meets with some success and, more important in the long run, that such an attempt will interest others in similar exercises, especially those of precision and revision. 

CO, SC ), or the occurrence of the minerals is sufficiently rare to represent a special case—the various halide salts, for example. However, dolomite presents a special problem in that the existence of Mg is important to silicate equilibria under consideration. The main trouble here is that the conditions of crystallization and stability of dolomite in sediments and sedimentary rocks is imperfectly known, thus leaving a question as to its influence on silicates or the influence of silicates on its presence. One is forced more or less to ignore the importance of dolomite at present. This does not mean that it can be ultimately excluded from a complete discussion of clay mineral stability. 

From the above observations, it is certain that corrensite is a mineral which will form in normal sedimentary rocks and weathering environments. Thus it will be stable throughout the full range of clay mineral physical environments. It forms in sedimentary rocks which contain important quantities of iron (divalent) or magnesium but is not necessarily related to evaporite deposits and thus alkaline conditions of formation. 

Like sediments, colloids are often important in sorbing and transporting arsenic in soils (Sadiq, 1997 Waychunas, Kim and Banheld, 2005). Colloids may consist of clay minerals, organic matter, calcium carbonate, and various aluminum, manganese, and iron (oxy)(hydr)oxides (Sadiq, 1997). Important iron (oxy)(hydr)oxides include goethite, akaganeite (/J-FeO(OH)), hematite, ferrihydrites, and schwertman-... 

The computer-simulated images presented above and in Guthrie and Veblen (12) have shown that one-dimensional images of clay minerals and sheet silicates can be interpreted and can provide important information. The simulations have also shown, however, that even these simple images must be interpreted carefully. 

The experimental studies of water interactions with clay minerals are very extensive. The structure, dynamics and interactions of interlayer water with the surface of clay minerals were reviewed in several papers [32, 33] and described in a number of books [15, 34, 35]. Therefore, we will review only the most important studies concerning experimental investigations of the structure and interactions of water molecules on clays. 

The effect of montmorillonite and kaolin, saturated with calcium, aluminium, or cupric ions, as well as quartz, on humic-like substances formed from glucose-tyrosine was examined by Arfaioli et al.541 All systems promoted their formation, the effectiveness being strictly related to the amount of added cation. Humification appeared to be due more to the cations than to the type of clay mineral. The clayey systems gave more complex (aromatic) substances than the quartz ones. The cations seemed more effective when free, i.e., associated with quartz rather than with the clays. The nature of the cation was also important, cupric being the most active here. In the end, all systems took on a deep dark colour. 

It is generally appreciated that the mineral matter associated with some coals may act as a catalyst for liquefaction. A common observation is that among bituminous coals from the eastern United States, those with a relatively high mineral matter content also provide relatively high liquefaction yields. Also, addition of coal-derived mineral matter increases the liquefaction yields from those coals with low mineral matter content. The involvement of pyrite in these effects has been fairly well established. The importance of clay and/or other minerals is less well defined. 

As a result of the interfacial processes on rocks and soils, the structure and chemical bonds of the sorbed compounds can be changed. For this reason, different chemical reactions can be initiated in which the components of rocks or soils act as catalysts. The most important mineral catalysts are zeolites and clay minerals. Naturally, the different oxides also have catalytic effects, and nowadays some of them are being artificially produced for catalytic purposes such as framework silicates (zeolites), the most effective and selective catalysts in organic syntheses. The catalytic applications of zeolites are too wide to summarize in this book, so we deal with the catalytic effects of clay minerals. 

Clay minerals are effective catalysts of many organic reactions, frequently showing product, regio, or shape selectivity. The other important advantage of clay minerals as catalysts is that the reaction conditions are frequently milder than in traditional procedures. The catalyst can be eliminated by filtration, and the use of hazardous solvents can be avoided, which reduces the quantity of waste. Consequently, clay minerals are often employed as catalysts in the so-called green chemistry. 

Reactions below about 1300°C, of which the most important are (a) the decomposition of calcite (calcining), (b) the decomposition of clay minerals, and (c) reaction of calcite or lime formed from it with quartz and clay mineral decomposition products to give belite, aluminate and ferrite. Liquid is formed only to a minor extent at this stage, but may have an important effect in promoting the reactions. At the end of this stage, the major phases present are belite, lime, aluminate and ferrite. The last two may not be identical with the corresponding phases in the final product. 

Because of their large surface-to-volume ratio and high metabolic activity, microorganisms are important vectors in the introduction of heavy metal and radionuclide pollutants into food webs. As discussed in Chapter 5, heavy metals in soils and sediments tend to be immobilized by precipitation at neutral to alkaline pH and/or adsorption to cation exchange sites of clay minerals. Microbial production of acid and chelating agents can reverse this adsorption and mobilize toxic metals. Microbial metabolism products that can chelate metals include... 

It is clear from the preceding description of clay minerals that two kinds of binding can occur, on the surface and, in the case of montmorillonite, within the lattice. The latter would probably be more important in reducing pesticide activity. 

Smectites are one of the most important soil clay minerals as regards cation exchange. The reversible exchange reactions of these minerals with metal cations may be pictured as kinetically-favored (i.e., rapid) precipitation-dissolution reactions (44). From this point of view, it is meaningful to write the "exchange half-reaction" ... 

---