Notes Transmission X-ray Diffraction Technique for Measuring Crystalline Swelling of Smectites in Electrolyte Solutions*

The crystalline swelling of smectites in electrolyte solutions can be controlled by varying the salt concentration of the solutions (Norrish and Quirk 1954, Norrish 1954a, Slade et al 1991). Such osmotic control offers a convenient means of studying the energetics of crystalline swelling. Several X-ray diffraction (XRD) techniques have been developed for measuring the basal spacing of smectites in equilibrium with salt solutions (Norrish 1954b, Posner and Quirk 1964). Norrish (1954b) placed an oriented flake of Na-saturated montmoril lonite (Wyoming bentonite) (about 0.03 mm thick) in a plastic capillary, saturated the clay with a desired cation by passing a concentrated salt solution through the tube, and then equilibrated the clay by passing a solution with the desired concentration of the same salt through the capillary. The capillary then was sealed and a modified X-ray camera technique was used to measure basal spacings up to 400 /~. With this technique, Norrish observed both the initial stepwise swelling of smectites up to 20 A and an increase in basal spacings > 40 ~ that varied linearly with the inverse square root of the electrolyte concentration. This technique requires a powder camera, a device that is not widely available with modern X-ray diffraction instruments. Posner and Quirk (1964) separated a clay paste from equilibrated clay suspensions by centrifugation and by sedimentation. Then they spread a layer of the paste on a ceramic tile, and covered the oriented paste with a thin, polyethylene film to prevent drying which could cause a change in concentration of the external solution. The oriented specimens were analyzed by XRD using the normal symmetrical reflection geometry. The technique used by Posner and Quirk (1964) is simple; however, measured basal spacings of the clay may be affected by the phase separation procedure, sample drying, and inaccurate positioning of the swollen clay at the reference plane of the goniometer. In this note, we describe a technique for measuring