Delamination of smectite in river-borne suspensions at the fluvial/marine interface – An experimental study

Fine fractions (<2 μm) from smectite-rich flood-plain sediments from the Elbe and Weser rivers in Germany were treated in the laboratory with synthetic sea salt solutions at different concentrations and for different periods of time to document sheet delamination of smectite particles at the fluvial/marine interface and thus simulate changes in clay minerals at the mouth of a river. The project consisted of two steps: (1) Pilot tests were made in 0.5% salt solutions (brackish environment) showing a nearly 2-min delay of the clearing of the clayey suspensions, when the suspension was redispersed 15 and 30 min later after the first measurement. This was due to an obviously increased number of fine laminae from disintegrated smectite particles. (2) X-ray diffraction studies (XRD) of the clayey material from suspensions in 0.0, 0.5, 1.0, and 3.0% salt solutions after standing 10 min, and one and three weeks were made to document alteration reactions of smectite. An aliquot was pipetted periodically from each series, and oriented clay-mineral specimens were analyzed by XRD in air-dried, glycol- and glycerin-solvated states. The peak intensities in the XRD patterns of the clay minerals smectite, random-ordered mixed-layer illite/smectite (I–S), kaolinite, chlorite, and illite did not change in the salt-free series during the experiments. In contrast the peak intensities of smectite and I–S mineral in the air-dried and glycerin-solvated states after suspension in saline solutions decreased significantly. The d-values of the smectitic materials in the air-dried state also decreased after contact with the saline solutions, and in the glycerin-solvated state the full-width-at-half-maximum (FWHM) of the 001 smectite reflection increased by about 20%, indicating a reduction in grain size. riations in FWHM, d-values and peak intensity of the smectitic material are assumed to be caused by an exchange of Na for Ca in the interlayer spaces and subsequent delamination of the particles along the Na-activated interlayers. Only those smectite particles in which adjacent sheets are still stabilized by the remaining Ca are preserved after contact with seawater. periments show that delamination of smectite and I–S mixed-layer minerals is likely to occur in the brackish environment at the mouth of a river. However, formation of these fine laminae is expected to be a transitional stage in the development of clay particle size, owing to the formation of flocs and aggregates in the higher salt concentrations of seawater.