Artificial clay mineral alignment during sedimentation and early compaction

The formation of crystallographic preferred orientations (CPO/texture) in sediments is often attributed to rigid grain rotation of minerals and aggregates, plastic-brittle deformation and dissolution-precipitation processes. Especially, clay minerals have a large shape anisotropy due to their platy habit. Here, we present an experimental approach in order to quantitatively explore the influence of particle settling and subsequent compaction in an undisturbed, ideal environment.

A powder of idiomorphic kaolinite grains was mixed with a fine-ground, illite aggregates in mass proportions of 0, 30, 50, 70 and 100 % in artificial seawater. The sludges settled in 80 cm high tubes. For each composition three samples were produced: Sedimentation-only and two drained compaction experiments (30 and 60 vol-%), which were carried out in a mechanical press with uniaxial load up to 0.4 MPa and 4 - 8 MPa. The CPO of clay minerals was measured using high energy X-ray diffraction at beamline P07b at Deutsches Elektronen-Synchrotron (DESY) and pole figure data was directly extracted using single peak evaluation.

The results indicate that sedimentation alone, can yield a strong texture of the clay minerals. The increase in texture strength (TS) is decreased at higher applied loads. TS is linearly related to shortening and porosity reduction. The kaolinite TS is inversely correlated with texture-inhibiting illite aggregate content which hampers further particle rotation. It is interpreted that the initial stages of settling and early rigid body rotation during compaction are thus the most important processes in the formation of a CPO in clay rich sediments.