Gerhard Lagaly, 1994. "Layer Charge Determination by Alkylammonium Ions", Layer Charge Characteristics of 2:1 Silicate Clay Minerals, A. R. Mermut, S. A. Boyd, W. J. Farmer, W. F. Jaynes, G. Lagaly, D. A. Laird, A. R. Mermut
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The Layer Charge (I.c.) of a 2:1 clay mineral is defined as the number (X+Y) of permanent charges per formula unit:
(Table 1)*.The elementary (unit) cell contains two formula units. The permanent charges result from isomorphic substitutions and defects in the octahedral and tetrahedral sheet. A further contribution may arise from protons which are adsorbed by oxygen ions or desorbed from hydroxide ions. The formula given above considers the layers to be very large so that the chemistry at the edges may be ignored.
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Layer charge is recognized as the single most important characteristic of 2:1 phyllosilicates. It indicates a mineral's capacity to retain cations and to adsorb water and various polar organic molecules. The affinity of organic-clays for sorption of organic contaminants in aqueous systems is now attracting considerable attention. The layer charge is also considered an important criterion for the classification of 2:1 silicate clay minerals. It is generally agreed that the magnitude of the layer charge can be taken into account to resolve the classification problem. Studies on structural chemistry also confirm the importance of layer charge for characterization of silicate clays.”