Montmorillonite layer charge

Wyoming-type montmorillonite [layer charge x = 0.75, with 1/3 being tetrahedral charge sites (4)] bearing one monolayer of adsorbed water has a water/counterion molar ratio of 5 1/3, equivalent to a 10.4 m solution. For two or three water monolayers, the H20/counterion ratio increases proportionately and the equivalent solution molalities are 5.2 m or 3.46 m, respectively. Thus, from the perspective of counterion solvation, interlayer water on montmorillonite should be similar to a very concentrated aqueous solution. 

Figure 5. Percentage illite layers versus layer charge for K-smectites subjected to 100 WD cycles in water at 60°C and 1 Sr-exchange. Numbers in parentheses refer to percentage of octahedral charge. Best fit line is for montmorillonites having 69% or more octahedral charge. Data from Tables III and IV.

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

The dioctahedral subgroup is by far the most abundant. The layer charge on the expanded clays ranges from 0.3 to 0.8 per Oi0 (OH)2 unit of structure. The low-charged (0.3-0.6), expanded minerals are called montmorillonite, montmorillonids, and smectites, among others. Subdivision of the expanded clay group is still a problem. 

Ross and Hendricks (1945) redefined beidellite as the aluminum-rich end member of the dioctahedral montmorillonites. Many of their samples were later found to be mixtures and for some time the concept of a high-aluminum montmorillonite was in considerable disrepute. Recently, Weir and Greene-Kelly (1962) made a careful analysis of purified material from the Black Jack Mine from Beidell, Colorado, and definitely established that it is monomineralic and an ideal Al-rich end member (Table XXVIII). They suggest that beidellites and montmorillonites should be divided at the composition at which the lattice charges from octahedral and tetrahedral substitution equal one another . The layer charge for beidellite does not appear to be any larger than for montmorillonites. 

In contrast, a study of the Tertiary bentonite beds of Texas (Roberson,1964) revealed considerable variation in chemical composition. Although no chemical analyses were made, the ability of the various montmorillonites to contract to 10 A when treated with K indicates a wide variation in layer charge. These bentonites are believed to have altered in a lacustrine environment and some are believed to be detrital, possibly having been formed in soils. 

Table XXXVIII). Brindley (1955) has suggested that stevensite is a mixed-layer talc-saponite however, Faust et al. (1959) considered it to be a defect structure with a random distribution of vacant sites in the octahedral sheets. A small proportion of domains with few or no vacancies would then be present having characteristics of talc. The layer charge in stevensite is due to an incompletely filled octahedral sheet (Faust and Murata, 1953). This deficiency is minor (0.05—0.10) and the resulting cation exchange capacity is only about one-third that of the dioctahedral montmorillonites (100 mequiv./lOO g.).

Many of these weathered micas and illites are completely leached of K and have the swelling characteristics of a montmorillonite. This material is usually leached to such an extent that the layer charge of some of the layers is sufficiently lowered so that they will no longer contract to 10 A. When these clays are exposed to sea water or K from any source, only a portion of their layers will contract and a mixed-layer clay is formed. 

K is obtained from associated K-feldspars and micas. The layer charge is increased by the reduction of iron in the octahedral sheet and incorporation of Al, entering through the ditrigonal holes in the basal oxygen plane, into the tetrahedral sheets (Weaver and Beck, 1971a Pollard, 1971). Weaver and Beck have presented evidence that indicates mixed-layer clays formed in this manner contain 20—30% chloritic layers and are actually mixed-layer illite-chlorite-montmorillonite clays. 

These diagrams indicate that when the total layer charge is less than 0.7, A1 will be the dominant cation when the seat of the charge is largely in the octahedral sheet as the predominant charge shifts to the tetrahedral sheet the larger Fe ion substitutes for A1 in the octahedral sheet. Radoslovich (1962) found that montmorillonite was the only layer silicate in which tetrahedral A1 caused the layer to increase in size in the b direction. He explains this by suggesting ... 

As indicated in Table 1, the three 2 1 groups differ from one another in two principal ways. The layer charge decreases in the order illite > vermiculite > smectite, and the vermiculite group is further distinguished from the smectite group by the extent of isomorphic substitution in the tetrahedral sheets. Among the smectites, those in which substitution of Al for Si exceeds that of Fe2+ or Mg for Al are called beidellite, and those in which the reverse is true are called montmorillonite. The sample chemical formula in Table 1 for smectite thus represents montmorillonite. In any of these 2 1 clay... 

Oxidation kinetics, 292-293 Reduction kinetics, 288 Removal from solution, 443-445 pe-pH diagram, 256,441 Manganese carbonate, 59,433 Manganese oxides, 131 Methane, 257-258, 324 Mica, 102-108 Layer charge, 113 Structure, 115 Molecular Weight, 13,14 Mole fraction, 202 Equivalent fraction, 202 Montmorillonite., 102,104, 109, 123 C-axis spacings, 171 Layer charge, 120 Structure, 171 Composition, 104 Physical properties, 123-124 Chemical properties, 123-124 Muscovite, 104, 123 Structure, 108 Composition, 104... 

Chiou, C. T., and D. W. Rutherford. 1997. Effect of exchanged cation and layer charge on the sorption of water and EGME vapors on montmorillonite clays. Clays Clay Miner. 45 867-880. 

The negative layer charge is mostly neutralized by the hydrated cations in the interlayer space. These cations are bonded to the internal surfaces by electrostatic forces, and they are exchangeable with other cations. The interaction strength between the hydrated cation and the layers (the internal surface) increases when the charge of the cation increases, and the hydrated ionic radius decreases. Cations with hydrate shell can be considered as outer-sphere complexes. Cation exchange is the determining interfacial process of the internal surfaces of montmorillonite. 

In montmorillonite, similar to other minerals, when the size of the exchanged cation is similar to the pore sizes in the crystal lattice, cations can build into the crystal lattice and, consequently, they reduce the negative layer charge (Chapter 1, Section 1.3.3.2). Other neutral molecules or cationic substances (Chapter 1, Sections 1.3.3.1 and 1.3.3.2) can also be sorbed in the interlayer space and on the external surfaces as well. They play an important role in defining the internal and total surface area and catalytic properties, and they may have an effect on the hydrophobicity of the mineral, as well as playing an important role in the production of pillared materials, etc. 

As mentioned in Chapter 1, Section 1.3.3.2, potassium and lithium ions can fit into the cavities of the silicate crystal lattice and decrease the layer charge. It is also the case for montmorillonite (Gast 1972 Maes and Cremers 1977 Eberl 1980 Goulding and Talibudeen 1980 Bouabid et al. 1991 Komadel et al. 2003), so the affinity order is modified. 

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