Crystal Structure and Layer Charge of Montmorillonite

Montmorillonite has some important characteristics that justify its use as a model substance for the study of the interfacial processes of rocks and soils. It is a dioctahedral three-layer clay (2 1 clays, TOT) an A10(0H) octahedral sheet is between two tetrahedral Si04 layers (Chapter 1, Table 1.2). The distance between the layers is not fixed (—O—O-bonds) the layers can be expanded. Because of the layered structure, it has two surface types external and internal surfaces. The external surface is the surface of the particles (edge surface), and its size depends on particle size distribution. Its area can be measured by the BET method, usually by the adsorption of nitrogen gas at the temperature of liquid nitrogen (Chapter 1, Section 1.1.3). The internal surface is the surface between the layers (interlayer surface), and its size can be determined by introducing substances into the interlayer space (e.g., water) (Chapter 1, Section 1.1.3). The internal surface area is independent of particle size distribution. 

As usual for clay minerals, the internal surface area of montmorillonite is about 80%-95% of the total surface area. For different montmorillonite samples, 

Basal (d00i) Spacing, Internal and Total Specific Surface Area, and Cation-Exchange Capacity of Natural Bentonite-Montmorillonite Samples 

The crystal lattice of montmorillonite, similar to other 2 1 phyllosilicates, may have isomorphic substitutions both in the tetrahedral and octahedral positions. In the tetrahedral positions, the central tetravalent silicon can be substituted by trivalent aluminum ions in the octahedral positions, the trivalent aluminum ions can be substituted by bivalent (usually magnesium and iron(II)) cations of similar 

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. 

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