The effect of iron concentration, hindered settling, saturating cation and aggregate density of clays on the size distribution determined by gravitation X‐ray sedimentometry

Abstract

Summary X‐ray sedimentation is potentially attractive for estimating the particle‐size distribution of soil, but has been thought to overestimate clay fractions because of the iron they contain. We have examined X‐ray sedimentation for estimating the particle‐size distribution of fully dispersed and of aggregated clay. We obtained good agreement between the pipette and X‐ray sedimentation methods using two different kaolinite samples with a small iron concentration. We also studied montmorillonite saturated with Li, Na, K, Mg or Ca to investigate possible effects of hindered settling. For the Ca montmorillonite, we obtained good agreement between X‐ray sedimentation and the pipette method at concentrations up to 40 g l −1 . We also found that X‐ray sedimentation could be used at concentrations less than those recommended by the manufacturer. Hindered settling was observed at concentrations > 20 g l −1 for Li‐ and Na‐saturated montmorillonite. Our data suggest that conducting experiments at different mass concentrations is a sensitive test to detect such problems. Finally we used a fractal model to investigate the possible errors that might result from the size dependence of aggregate density and show that the error caused by assuming a single value for particle density of solids is small.