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Determination of layer-charge characteristics of smectites

Christidis Georgios, D. D. Eberl

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Summary

A new method for calculation of layer charge and charge distribution of smectites is proposed.The method is based on comparisons between X-ray diffraction (XRD) patterns of K-saturated, ethyleneglycol-solvated, oriented samples and calculated XRD patterns for three-component, mixed-layer systems.For the calculated patterns it is assumed that the measured patterns can be modeled as randominterstratifications of fully expanding 17.1 A Ê layers, partially expanding 13.5 A Ê layers and non-expanding9.98 AÊ layers. The technique was tested using 29 well characterized smectites. According to their XRDpatterns, smectites were classified as group 1 (low-charge smectites) and group 2 (high-charge smectites).The boundary between the two groups is at a layer charge of –0.46 equivalents per half unit-cell. Lowchargesmectites are dominated by 17.1 AÊ layers, whereas high-charge smectites contain only 20% fullyexpandable layers on average. Smectite properties and industrial applications may be dictated by theproportion of 17.1 AÊ layers present. Non-expanding layers may control the behavior of smectites duringweathering, facilitating the formation of illite layers after subsequent cycles of wetting and drying. Theprecision of the method is better than 3.5% at a layer charge of –0.50; therefore the method should beuseful for basic research and for industrial purposes.

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