Weathering montmorillonite

Finally, the montmorillonite weathering reactions also contribute in a kinetic fashion to the pH buffering capacity of the bentonite barrier according to the following... 

Bentonite is an impure clay that is formed by weathering of volcanic tuffs. It contains a high content of montmorillonite. Bentonites exhibit properties such as ability to swell, ion exchange, and thixotropy. Properties can be modified by ion exchange, for example, exchange of earth alkali metals to alkali metals. The specific surface can be modified with acid treatment. Organophilic properties can be increased by treatment with quaternary ammonia compounds. 

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. 

Selectivity decreased through the weathering sequence mica > i 11 ite = vermicul ite > montmorillonite. Selectivity for K over Ca has been ascribed to the low hydration number and polarizability of K (19), to wedge sites at the weathered edge of clay... 

The production of illite from chemical weathering occurs at all latitudes. It dominates the clay mineral assemblage in the North Atlantic and North Pacific Ocean, particularly at 40° reflecting aeolian transport by the westerlies (Figure 14.11). In the southern hemisphere, the input of illite by the westerlies is diluted by a large input of authigenic montmorillonite in the South Pacific and Indian Oceans and in the South Atlantic by a large input of kaolinite. 

The palagonite is thermodynamically unstable and, hence, reacts with seawater to form various clay minerals, including smectites (montmorillonite, nontronite, and saponite), micas (celadonite), and zeolites (phillipsite). This chemical weathering involves uptake of Si, Al, Mg, Ca, Na, and K and the release of water, reversing to some extent, the elemental effect of palagonitization. These mineral alterations tend to proceed progressively from the outer margin of the pillow basalts to their interior. 

Because of the relative scarcity of lithogenous particles and fast seafloor spreading rates, metalliferous sediments are common around the East Pacific Rise and very high densities of manganese nodules are present on the abyssal plains, especially in the Southern Hemisphere. In these locations, the weathering products of volcanic detritus, such as montmorillonite, phillipsite, nontronite, and celadonite, are also found in great abimdance. 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

Si, Fe and Fe is variable. Illite also appears to be the early product of weathering in cycles of intense alteration or one of the stable products under intermediate conditions (Jackson, 1959). It is apparently stable, or unaffected by transport in rivers for relatively short periods of time (Hurley, et al., 1961) but does change somewhat in the laboratory when in contact with sea water (Carroll and Starkey, 1960) it has been reported to be converted to chlorite or expandable minerals upon marine sedimentation (Powers, 1959). However, Weaver (1959) claims that much sedimentary illite is "reconstituted" mica which was degraded to montmorillonite by weathering processes. It is evident that a certain and usually minor portion of illite found in sedimentary rocks is of detrital origin (Velde and Hower, 1963) whether reconstituted or not. 

In general, a weathering profile which produces abundant montmorillonites indicates that the weathering process is advanced. It would appear... 

Tomita, et al., 1970 Meilhac and Tardy, 1970.) The prevalence of montmorillonites, in river sediments and those studied as deep-sea cores in the numerous JOIDES reports leads one to believe that montmorillonite is a very common weathering product. Certainly a portion of it is derived from degraded micas but if one considers that the next most common sedimentary mineral is illite, one is forced to conclude that either continental rocks are for the major part made of micas or that many other minerals are transformed into montmorillonite during the weathering process. 

A large variety of zeolite species crystallize from volcanic glasses under weathering conditions and several can be associated in the same sample. The process seems most pronounced for acid eruptive rocks (Hay, 1963 Iijima and Harada, 1969 Harada, t al., 1967) but does occur in basalts also (Hay and Iijima, 1970). In some African soil profiles, zeolites (analcite) are found to develop at the expense of montmorillonite and sodium carbonate their appearance is apparently a function of local drainage. They grow in more stagnant situations at the expense of montmorillonite (Frankart and Herbillon, 1970). 

Before attempting to document the formation of montmorillonite, a few remarks on weathering rates are of interest. Roughly one-half the initial C02 seems to be expended in the soil zone, and most of the rest alters rock minerals at deeper levels. The total reacting capacity is about... 

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