THE FORMATION OF CLAY

Clay is an erosion product of igneous and sedimentary rocks. The weathering proceeds via mechanical and chemical processes. Its result depends on many factors, among which climate, vegetation and composition and texture of the rocks. 

Mechanical erosion is influenced by temperature differences, plant roots, wind, water and glaciers. A well-known example is frost which not only affects nature but can also have annoying consequences inside, just think of frozen water pipes. In both cases the fact that water increases in volume when it freezes is the cause of all problems. 

Chemical erosion on the other hand is a complex process which is influenced by many things, among which are the transport of -mostly dissolved - substances, the acidity of the water and the crystal structure of the minerals in the rocks. When dissolved in water, sulphuric 

Four kinds of chemical erosion exist the ending lysis in the name means separation . The components of a rock are separated and new components can be formed. 

acidolysis under the influence of an acid. H30+ ions play an important part in the erosion of minerals. Because of their minute dimensions they can easily penetrate a crystal lattice. In those cases a relatively large K+ ion is replaced by a much smaller H30+ ion, owing to which part of the stability of the lattice is lost and that is the start of the erosion process in an acid environment. 

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Clays released by chemical breakdown of rock, which can lead not only to the release but also to the formation of clays. 

Another characteristic of 2 1 clays is isomorphous substitution, where iso means same and morphous means shape. During the formation of clay, cations other than aluminum and silicon become incorporated into the structure. In order for this to work and still ensure a stable clay, the cation must be about the same size as either aluminum or silicon, hence the term isomorphous. There are a limited number of cations that satisfy this requirement. For silicon, aluminum as Al3+ and iron as Fe3+ will tit without causing too much distortion of the clay structure. For aluminum, iron as Fe3+, magnesium as Mg2+, zinc as Zn2+, and iron as Fe2+ will fit without causing too much structural distortion (see Figure 3.4). 

Early on, water existed at least periodically on the Martian surface. The formation of clay minerals in the Noachian and precipitation of sulfates and chlorides in the Hesperian were consequences of this water. Some fraction of the water evaporated and was lost to space, as indicated by the high D/H ratios in SNCs. Other water was apparently sequestered at the poles or underground as permafrost. The surface of Mars is effectively dry now, and chemical weathering of crustal rocks is minimal. 

Mineral formation as a result of weathering will be discussed in chapter 8, which deals with the formation of clay. 

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. 

Hedges, J. I. (1978). The formation of clay mineral reactions of melanoidins. Geochim. Cos-mochim. Acta 42, 69-79. 

Figure 1. Computer simulation of the formation of clay tactoid by a process of aggregation of lCr particles. Ration length to thickness 9/1. In the three topmost diagrams each rectangle endorses a portion of the tactoid shown enlarged below, df is represented by the thickness of the line enclosing the sheets in contact. Adapted from ref. 6. N.B. With respect to the next section, the molar fraction xi consists of the water in the interlamellar space and in the closed spaces within the tactoid. The molar fraction xb is in a 10 A thick layer on the external contour of the tactoid.

The mechanism of the formation of clay smears and their trapping capacity appears to be reasonably well documented and relatively uncontroversial. This is not the case for shear bands. 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

Generalized in Fig. 9.4 from Brady (1974) are the conditions, including extent of weathering, that lead to the formation of clay minerals and secondary metal oxyhydroxides in soils. 

Ronov (1976) estimated the average CaO content in sedimentary layer of 15.91 %, and in granite layer, of 2.71 %. Accordingly, the calcium reservoir in sedimentary shell is 272.8 X 10 - tons, and in the granite pool is 222.8 x lO tons. The weathering and metamorphosis of deep-layer silicates is accompanied by the formation of clay minerals with release of calcium available for plant and microbial uptake. 

Several methods may be applied to prevent sedimentation and the formation of clays or cakes in a suspension [42, 43] ... 

Table 8 compares [106,107] the various parameters thus evaluated for the formation of clay-polyether nano composites from [H3N(CH2)5COOH]+ and [H3N(CH2)n COOH]+ montmorillonite. 

Layer-silicates Recent studies have also demonstrated the potential microbial influence on clay mineral (layer silicates) formation at hydrothermal vents. Bacterial cells covered (or completely replaced) with a Fe-rich silicate mineral (putative nontronite), in some cases oriented in extracellular polymers (as revealed by TEM analysis), were found in deep-sea sediments of Iheya Basin, Okinawa Trough (Ueshima Tazaki, 2001), and in soft sediments, and on mineral surfaces in low-temperature (2-50°C) waters near vents at Southern Explorer Ridge in the northeast Pacific (Fortin etal., 1998 Fig. 8.6). The Fe-silicate is believed to form as a result of the binding and concentration of soluble Si and Fe species to reactive sites (e.g. carboxyl, phosphoryl) on EPS (Ueshima Tazaki, 2001). Formation of Fe-silicate may also involve complex binding mechanisms, whereas metal ions such as Fe possibly bridge reactive sites within cell walls to silicate anions to initiate silicate nucleation (Fortin etal., 1998). Alt (1988) also reported the presence of nontronite associated with Mn- and Fe-oxide-rich deposits from seamounts on the EPR. The presence of bacteria-like filaments within one nontronite sample was taken to indicate that bacterial activity may have been associated with nontronite formation. Although the formation of clay minerals at deep-sea hydrothermal vents has not received much attention, it seems probable that based on these studies, biomineralisation of clay minerals is ubiquitous in these environments. 

Pusch, R. 1973b. Influence of salinity and organic matter on the formation of clay microstructure. Proceedings of the International Symposium on SoU Structure, Gothenburg, Sweden. Swedish Geotechnical Society, Stockholm, Sweden, pp. 165-175. 

The formation of clay minerals in stony meteorites exposed to solar radiation on the ice fields of Antarctica is attributable to the episodic melting of ice and snow in contact with meteorite specimens that are warmed by sunlight. However, meteorites that were still embedded in the ice when they were collected also contained aluminosilicate weathering products (Gooding 1986b Gow and Cassidy 1989 Gow 1990). Harvey and Score (1991) suggested that meteorites that were weathered while they were stiU embedded in the ice could be... 

More complicated are hydrolysis reactions of silicates with two or more metals in their formulas. Such a mineral commonly dissolves incongraently, which means that at first it only partially dissolves. A new mineral is then left behind, and the solution has constituents with a mole ratio different from that in the original mineral. A good example is the hydrolysis of aluminosilicate minerals such as the feldspars, which are particularly important since these reactions lead to the formation of clay minerals ... 

In nature, generally three mechanisms are in operation for the formation of clays. They are inheritance, neoformation and transformation. 

Nesbitt and Jambor (1998) have shown the fundamental role of mafic minerals in neutralization of the Waite-Amulet tailings. As in the weathering of feldspar, the weathering of felsic minerals leads to the formation of clay minerals. Muscovite, pyroxene and amphibole alter to chlorite. By decreasing pH, chlorite alters to sericite, kaolinite or Mg-montmorillonite. The products of biotite alteration are hydrobiotite, a regularly interstratified biotite-vermicuhte phase, vermiculite, and kaolinite (Acker and Bricker, 1992 Mahnstrom and Banwart, 1997). Direct conversion of biotite to kaolinite has also been described (Acker and Bricker, 1992). 

As proposed in Figure 3.19, ion-exchange reactions might result in the formation of clay anions and metallocene cations (soft ions), and sodium and chloride ions (hard ions). The ion exchange of metallocenium cations with clay surfaces is also discussed by Mariott et al. [83, 84]. 

According to Erhart (142) plants play a role in the formation of clays. In soils poor in Ca and Mg, the plants contain A1 and Si in the proportions of kaolin, and dead tissues rde e prekaolinite, which then crystallizes to clay. This appears to be substantiated by Pdnemanh and Ferreiro (143), who reported that in upper soil levels the fine amorphous.silica and clay fractions are generated by plants by forma tion of phytoliths and prekaolin.. . "... 

Deformation of material by shearing requires, as a mle, the application of a lower stress than for other modes of applying load. Such a deformation can be achieved at many points of the clay-water mix, if tangent stress is applied to its surface. The discovery of this behaviour of the clay-water mix is believed to have been the reason behind an innovation introduced around 5,000 years ago in Sumer, namely, the formation of clay products with a potter s wheel. 

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