New laboratory procedure for distinguishing smectites with varying permanent layer charge based on the combination of Cu-trien and ethylene glycol solvation methods and X-ray diffraction

Abstract

Permanent layer charge of smectites (or smectite layers in mixed-layer minerals) is important for their properties and materials' applicationss. However, distinguishing low- and high-charge smectites by traditional laboratory methods is time-consuming and instrumentally demanding and it can also produce equivocal results. In addition, the use of structural formulae based on chemical analyses combined with knowledge on crystallochemistry is challenging in arbitrary mineral mixtures. The aim of this work was to test a novel laboratory procedure for distinguishing actual multi-mineral specimens of smectites according to their permanent layer charge. This is conducted on interlayer cation exchange by the Cu-trien complex and subsequent solvation with ethylene glycol. This procedure returned more straightforward results than the traditional routine of ethylene glycol-saturated K+-exchanged specimens by producing better-resolved X-ray diffraction (XRD) patterns. The practical application was demonstrated by analysing sauconite samples from Skorpion in Namibia, which was shown to contain more sauconite phases of distinct crystallographic properties present in variable proportions. XRD of the oriented ethylene glycol-saturated specimens was insufficient for this purpose. The procedure was tested by reference clay minerals of the smectite group with varying structural parameters. It was distinguished more smectite phases with distinct layer charges and estimated the layer charge by combining interlayer cation exchange with the Cu-trien complex and ethylene glycol solvation. This was followed by XRD analyses and full-profile diffraction pattern modeling. Further investigation is required for comparison with other methods. Possible reasons for the formation of two populations of sauconites were discussed.