Effects of Ethylene Glycol Saturation Protocols on XRD Patterns: A Critical Review and Discussion

Abstract

AbstractStudy of the transformation of smectite to illite, chlorite or vermiculite via interstratified clay minerals needs precise qualitative and quantitative determinations of the different layers in the mixed-layer clays and is generally based on X-ray diffraction (XRD) patterns after specific treatments of the clay samples. Saturation with K or Mg followed by ethylene glycol (EG) solvation are classical methods used to identify high-charge smectite and vermiculite. These procedures have been applied to two experimental clays, one composed of smectite layers and the second, a mixture of vermiculite and smectite layers. Different methods of glycolation (EG vapor or liquid EG) produce significant differences in the XRD patterns. Comparison with literature data indicates that K-saturated, high-charge smectite (≈0.8 < total charge <1/unit-cell) and Mg-vermiculite (whatever its charge) do not expand in ethylene glycol vapor (d values ≈14–15 Å). Expansion to 17 Å in liquid ethylene glycol occurs for Mg-vermiculite with a total charge of <~1.2/unit-cell and for K-saturated, high-charge smectite, when the tetrahedral charge is <≈0.7/unit-cell. This study shows that: (1) glycolation procedures need to be standardized; (2) the use of saturation protocols using both liquid ethylene glycol and ethylene glycol vapor yields useful additional information about the distribution of charges in clay minerals.