Clay alignment takes place during early stages of sedimentation

The alignment of clay minerals in sediments is of high importance for their mechanical and physical properties. The development of this alignment starts with the deposition of clay, its strength is measured by the crystallographic preferred orientation. So far, the early stages of sedimentation have been restricted to post-mortem observations. Here we present particle settling experiments in four dimensions (time and orientation, as a function of overburden and composition) observed in situ using synchrotron diffraction, in which kaolinite and kaolinite-illite mixtures were sedimented in water columns. The alignment strength in freshly settled sediments increases with overburden, but is higher in deionized water than in seawater. Alignment strength increases within the first few millimetres of overburden and stagnates afterwards. With illite added, the resulting alignment strength is substantially decreased. Our results demonstrate that electrostatic interactions between particles are overcome by gravitational forces already within the upper millimetres of sediment.