GEOLOGY OF CLAY

Clay is found everywhere on earth, usually as part of the outer layer of the earth s crust, the lithosphere. In a few places clay is also found and mined below the surface. Clay is an erosion product of magma or sedimentary rock. Before entering into the phenomenon erosion , first some information about the various kinds of rocks of which the earth s surface is built up. By now we know that the elements O, Si, Al, Fe, Ca, Na, K and Mg mainly occur in rocks (see Geology /Mineralogy). 

Rocks are subdivided into igneous, sedimentary and metamorphic rocks according to the manner in which they arose. Only igneous and sedimentary rocks are of importance for the formation of clay. 

Igneous rocks are also called solidification, eruption or magma rocks. They are formed from the solidification of magma, a liquid rock mass in which gases can also be present. When this solidification process takes place in the earth s crust, we speak of effusive or volcanic 

In figure 8.2 the rocks with a composition as indicated by the shaded part are called alkali granite . The comers of this shaded area represent four mixtures whose compositions can be found in table 8.1. 

Sedimentary rocks are produces by the weathering of preexisting rocks. In this way large deposits of unconsolidated particles (sediment) are 

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Millot, G. (1970). Geology of Clays Weathering, Sedimentology, Geochemistry. Chapman and Hall, London. 

G. Millot, Geology of Clays Weathering, Sedimentation, Geochemistry, Springer, New York, 1970. 

Millot, G. 1970. Geology of Clays. New York Springer-Veilag. 

Figure 9.4. Classification of elements into four groups on the basis of ionic charge (valence) and radius. (Adapted from G. Miliot. 1970. Geology of Clays. New York Springer-Verlag.)...

Shirozu, H. (1974) Clay minerals in altered wall rocks of the Kuroko-type deposits. In Ishihara, S. (ed.). Geology of Kuroko Deposits, Mining Geology Special Issue, 6, 303-310. 

The primary characteristic necessary for a liner, cover, or cutoff wall is low permeability, which essentially enables them to slow down the seepage or diffusion of chemicals. Clay is therefore the main material used to construct these containment systems. The thickness and chemical compatibility of containment systems are of concern in assessing the performance of a system. For example, clay liners are constructed as a simple liner that is 2 to 5 ft thick. In composite and double liners, the compacted clay layers are usually between 2 and 5 ft thick, depending on the characteristics of the underlying geology and the type of liner to be installed. Regulations specify that the clay used can only allow water to penetrate at a rate of less than 1.2 in./yr. However, the effectiveness of clay liners can be reduced by fractures induced by freeze-thaw cycles, drying out, and the presence of some chemicals. 

It is most likely then that the effective (although metastable) SiO equilibria in most geological environments of low temperature and pressure, weathering, sedimentation and the early stages of compaction as well as surface hydrothermal alterations, are governed by the solubility and precipitation of amorphous silica in aqueous solution. As a result, the existence of quartz in an assemblage of clay minerals in these environments does not necessarily represent a compositional limit or saturation with respect to SiC and, therefore, such an assemblage cannot be considered, a priori, as a system with silica as an effective component in excess. 

Frequently two or more species are found together in the same geologic sample. As pressure-temperature conditions become more severe, the mineralogy becomes more simple, feldspar appears and finally within the limits of clay mineral stabilities only calcic zeolites are found. However, the calcic minerals are generally confined to rocks of basic... 

In the discussion thus far, the application of systems with completely mobile components has been restricted to bed-rock weathering, sedimentation and free-flowing aquifer environments. As a first approximation in other geological situations, clay mineral suites can be adequately described using "inert" chemical components, i.e., those which are extensive variables of the system, and by using pressure-temperature as intensive variables. Hydrothermal alteration is, in contrast, an environment where many chemical components can be treated as being completely... 

Bassett, W.A., 1963. The geology of vermiculite occurrences. Clays Clay Miner., Proc., 10 61-69. 

Harrington, J.F. and Horseman, S.T. (1999) Gas transport properties of clays and mudrocks. In Geological Society, Special Publications, London, pp. 107-124... 

The process of sedimentation in which the settling of suspended particles results in a dense compaction, or coagulation, of particles in which liquid is squeezed out. Geologically, significant compaction of clay layers caused by lowering of the water table (dewatering). 

Among activated forms of amino acids, mixed anhydrides with inorganic phosphate or phosphate esters require a special discussion because they are universally involved in peptide biosynthesis through the ribosomal and non-ribosomal pathways. These mixed anhydrides have stimulated studies in prebiotic chemistry very early in the history of this field. Amino acyl adenylates 18c have been shown to polymerize in solution [159,160] and in the presence of clays [139]. However, their participation as major activated amino acid species to the prebiotic formation of peptides from amino acids is unlikely for at least two reasons. Firstly, amino acid adenylates that have a significant lifetime in aqueous solution become very unstable as soon as either CO2 or bicarbonate is present at millimolar concentration [137]. Lacey and coworkers [161] were therefore conduced to consider that CO2 was absent in the primitive atmosphere for aminoacyl adenylate to have a sufficient lifetime and then to allow for the emergence of the modern process of amino acid activation and of the translation apparatus. But this proposition is unlikely, as shown by the analysis of geological records in favor of CO2 contents in the atmosphere higher than present levels [128]. It is also in contradiction with most studies of the evolution of the atmosphere of telluric planets [30,32], Secondly, there is no prebiotic pathway available for adenylate formation and ATP proved to be inefficient in this reaction [162]. 

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