Chapter 6: The concept of layer charge of smectites and its implications for important smectite-water properties
George E. Christidis
Layer charge is an important intrinsic property of smectites which stems from substitutions in the octahedral and/or tetrahedral sheet or from vacancies in the octahedral sheet. The layer charge is balanced by the interlayer cations which are exchangeable. There are three methods for determining layer charge: the structural formula method (SFM); the alkylammonium method (AMM); and the potassium-saturation method (KSM) which has been calibrated with the SFM. The layer charge affects important physical water-rock properties of smectites. Smectites with small layer charge have smaller cation exchange capacities than their high-charge counterparts. In contrast, during crystalline swelling, the activity of water at which the transition in hydration and dehydration occurs, increases with increasing layer charge of the smectites, suggesting that low-charge smectites have greater swelling capacity. In contrast, data on the influence of layer charge on double layer swelling are inconclusive, although in general low-charge smectites have greater swelling capacity than high-charge smectites. Similarly, the limited existing data suggest that, in general, low-charge smectites form more viscous suspensions than their high-charge counterparts. The influence of layer charge on the double layer swelling and the viscosity is attributed to the formation of quasicrystals, i.e. small stacks of smectite layers, which may breakup by hydrodynamic forces under shearing. Layer charge is a property of the unit cell, i.e. it refers to the atomic not the macroscopic level and it does not reflect the charge of the smectite particles. Smectite-water properties can be better explained by the concept of fundamental particle charge, i.e. the electric charge of the single 10 Å thick smectite particles.
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