Expandable clay minerals

An understanding of much of aqueous geochemistry requires an accurate description of the water-mineral interface. Water molecules in contact with> or close to, the silicate surface are in a different environment than molecules in bulk water, and it is generally agreed that these adsorbed water molecules have different properties than bulk water. Because this interfacial contact is so important, the adsorbed water has been extensively studied. Specifically, two major questions have been examined 1) how do the properties of surface adsorbed water differ from bulk water, and 2) to what distance is water perturbed by the silicate surface These are difficult questions to answer because the interfacial region normally is a very small portion of the water-mineral system. To increase the proportion of surface to bulk, the expanding clay minerals, with their large specific surface areas, have proved to be useful experimental materials. 

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

In zones of hydrothermal alteration it is apparent that the formation of dioctahedral montmorillonites is limited by temperature. They almost never occur in the innermost zone of alteration, typically that of sericitization (hydro-mica or illite), but are the most frequent phase in the argillic-prophylitic zones which succeed one another outward from the zone where the hydrothermal fluid is introduced in the rock. Typically, the fully expandable mineral is preceded by a mixed layered phase (Schoen and White, 1965 Lowell and Guilbert, 1970 Fournier, 1965 Tomita, et al., 1969 Sudo, 1963 Meyer and Hemley, 1959 Bundy and Murray, 1959 Bonorino, 1959). However, temperature is possibly not the only control of expandable clay mineral occurrence, the composition of the solutions and the rock upon which they act might also be important. It is possible that high magnesium concentrations could form chlorite, for example, instead of expandable minerals. 

Weaver, C. E., 1958. The effects and geologic significance of potassium fixation" by expandable clay minerals derived from muscovite, biotite, chlorite, and volcanic material. Am. Mineralogist, 43 839-861. 

As mentioned previously, the total surface of the rock and soils is the sum of the external and internal surfaces. Both external and internal surfaces have charges, and so, when the internal surface is significant (such as in the case of humic substances and expandable clay minerals), that extent of the interfacial layer is quite great. All system has external surfaces, while internal surfaces are significant only in the case of certain minerals and organic matter therefore, we will discuss first the properties of external surfaces. 

Vertisols Old weathered soils with uniform, thick, clay-rich profiles. Deeply cracked and hummocky topography produced by intense seasonal drying of expandable clay minerals such as smectites. Soil profile poorly developed because of vertical mixing caused by the seasonal cracking. High in exchangeable cations (base-rich). 

Cations based on substituted silsesquioxanes have also been intercalated [7]. On heating the ion-exchanged clay minerals water is evolved, but oxycation or oxide pillars keep the siliceous layers apart. These materials have enhanced thermal stability compared with clay minerals expanded with organic cations. Expanded clay minerals cover at least as wide a range of accessibilities to the interlamellar micropore spaces as the zeolites, but the pore characteristics of clay minerals with inorganic pillars need more detailed investigation. 

Studies by Mortland et al. (34) suggested that residual water on the interlayer surfaces of expanding clay minerals is dissociated more... 

X-ray diffraction studies showed that the s-triazines and amitrole are adsorbed in the interlayer spacings of expanding clay minerals (53, 80, 106). The compounds are exchangeable with other cations (62, 95) and are not biologically unavailable (46,116), as were the cationic herbicides, diquat and paraquat. 

Kaolinite 1) A non-expandable clay mineral in the kaolin group with the formula Al2Si205 (0H>4, which does not exchange iron or magnesium. 2) Any kaolin clay mineral. 

On some substrates, such as expanding clay minerals, the adsorption of different molecules may occur according to different mechanisms. For example, the adsorption of nonpolar molecules can occur only on external surfaces, whereas polar molecules can also enter interlayer spaces. Consequently, application of Equation (6.5) to polar and nonpolar molecules may lead to different results [16,21]. 

Recently, Takahama et al. (18) have reported that a montmorillonite expanded with SiOj TiOj sol particles (4), when dried with a supercritical fluid, can generate an expanded clay mineral with a surface area and pore volume more typical of silicas than of pillared clays (18,12). It is the purpose of this chapter to examine the physicochemical properties of two smectite (montmorillonite and saponite) samples expanded with Si02 Ti02 clusters and dried using a CO2 fluid at supercritical conditions. 

Pillared rectorites are expanded clay minerals with a surface area in the 150-220 mVg range, and thermal and hydrothermal stability similar to that of zeolites with the faujasite structure (1-4). After steaming at 760°C/5h (100% steam, 1 atm), these materials retain their pillared structure, and at microactivity test conditions (MAT) they are as active as commercial fluid cracking catalysts (FCC) for gas oil conversion... 

Siebert, R. M. Lahann, R. W. Determination of expandable clay minerals at well sites. US Patent 4495292, 1985 Chem. Abstr. 1985, 102, 116400. 

In an effort to combine the possibilities and advantages of both types of tests, a novel test method was developed in which the compost matrix is replaced hy a mineral, inert medium [8,9]. In this novel test procedure the carrier matrix consists of vermiculite, which is a type of expanded clay mineral with an overall physical structure and waterholding capacity behaviour very similar to compost. This medium is inoculated with a compost extract and mineral medium is added. The inert matrix should permit the extraction and determination of metabolites and biomass. 

However, until relatively recently, most workers concerned with waste management have tended to consider chemical processes primarily because they may affect the physical containment properties of engineered barrier systems. Several texts have examined these physical aspects of containment in considerable detail (e.g. Bentley 1996). Implicitly, there has been a tendency to view chemical containment as an aspect of physical containment. For example, any collapse of expandable clay minerals, such as may be caused by interactions involving polar organic molecules, will affect the physical integrity of clay barriers (e.g. Bowders Daniele 1987 Hettirachi et al. 1988). However, this view of containment is simplistic. In reality, chemical and physical processes must be considered holistically. For example, where clay is used to confine a waste, it should be considered as a physico-chemical barrier to contaminant migration (Horseman et al. 1996). 

These results are confirmed by the work of Schlichting and Blume, who additionally demonstrate that with increasing podzolization of loessial soils, clay formation results from disintegration of the coarse clay and fine silt fraction (0.5 to 5 (iia). From Figure 6(a) and (b) it can be seen to what extent clay formation and clay migration contribute to the distribution of mica in a podzolized loessial soil. As shown by the decrease of mica content in the surface soil, transformation of mica into expandable clay minerals occurs due to weathering. 

Sayegh, A. H., 1964. Changes in lattice spacing of expanding clay minerals. Ph.D. thesis, Oregon State University. 

The effects and geologic significance of potassium fixation by expandable clay minerals... 

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