Smectite formation

Awwiller D. N. (1993) Illite/smectite formation and potassium transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene. J. Sedim. Petrol. 63, 501-512. 

The conditions necessary for artificial synthesis of smectites suggest the kind of natural environment in which these minerals will be found. This environment is alkaline as a result of restricted drainage and/or evaporative salt accumulation (see Chapter 8), so that the normally mobile alkaline and alkaline earth ions (Na, K, Ca, Mg) accumulate, along with silica. Neoformation of smectites from Na-rich saline water, and possibly illites from more K-rich water, is favored under these conditions. Other very silica-rich silicate minerals, such as attapulgites, sepiolites, and zeolites, are also known to fomi under these conditions. Less alkaline conditions may be necessary for Fe and Al-rich (dioctahedral) smectite formation, and a reducing environment may assist crystallization. 

Isotope studies are frequently carried out to investigate the conditions of smectite formation (Yeh and Savin, 1977). In this study 6 0 and 6D isotopic compositions of smectite were used to characterize the nature of the fluids and the temperature during the transformation of pyroclastic rocks to a large bentonite deposit. It is supposed, that argilitization was related to meteoric water warmed up by hot volcanic rocks in the Ishirini area. However, bentonites could represent a multiphase alteration product of meteoric water as well as hydrothermal fluids. 

According to the geothermometric equations knowledge of the isotope composition of the smectites as well as the alteration fluids is required before the temperature of smectite formation can be calculated. 

Zeolite minerals (wairakite, laumontite etc.), mixed-layer clay minerals and sme-cite occur in the upper part of the propylitically altered rocks (e.g., Seigoshi, Fuke, Kushikino), but they are sometimes poor in amounts. Generally carbonates are more abundant in the mine area as in the Toyoha district. Temporal relationship between the formation of high temperature propylitic alteration minerals (epidote, actinolite, prehnite) and low temperature propylitic alteration minerals) (wairakite, laumontite, chlorite/smectite, smectite) in these areas (Seigoshi, Fuke, Kushikino) is uncertain. 

The Mg content of hydrothermally altered volcanic rocks is reflected by the extent of seawater-volcanic rock interaction at elevated temperatures, because it has been experimentally and thermodynamically determined that nearly all of the Mg in seawater transfer to volcanic rocks, owing to the reaction of the cycled seawater with volcanic rocks at elevated temperatures (Bischoff and Dickson, 1975 Mottl and Holland, 1978 Wolery, 1979 Hajash and Chandler, 1981 Reed, 1983 Seyfried, 1987). It has been shown that the CaO content of hydrothermally altered midoceanic ridge basalt is inversely correlated with the MgO content with a slope of approximately — 1 on a molar basis (Mottl, 1983). This indicates that Ca of basalt is removed to seawater and Mg is taken up from seawater by the formation of chlorite and smectite during the seawater-basalt interaction. This type of reaction is simply written as ... 

Gonzales and Laird145 have shown that smectites abiotically catalyze dehydration of glucose to form furfural under conditions similar to those found in soils. Four smectite clay minerals were used (saturated with Na, Ca, Fe, or Al), and the formation of HMF and furfural was detected by high-pressure liquid chromatography. The polymerization of furfural may thus be a pathway to the formation of new humic materials in soils. 

In terms of soil development and the development of soil horizons, the smectites and fine-grained micas are found in younger, less weathered soils. Kaolinite and amorphous clays are found in highly weathered soils. Considering a time sequence, at the beginning of formation, soil will contain more complex clays that weather to simpler forms over time. However, it is convenient to start with a description of the simpler layer silicate clays and then describe the more complex clays. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

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