Particle size distribution and crystallinity as indicators of kaolinite genesis

摘要

The possibility of using the particle size distribution curve obtained by the laser diffraction method to determine the kaolinite genesis is considered in the paper. The full width at half maximum (FWHM) and the peak shape index (PSI) are proposed to characterize the distribution curve shape. The normal distribution of microcrystals with a perfect structure and clear faceting is associated with a single mechanism of the rock transformation into kaolinite. Most likely, such a mechanism is the solid-phase transformation of the parent rock minerals into kaolinite. The lognormal distribution, reflected by the deformation of the descending branch of the distribution curve peak, is associated with two mechanisms of kaolinite formation. In addition to solid-phase transformations, kaolinite crystallization from the degradation products of the parent rock occurs. Secondary kaolinites are characterized by a broad particle size distribution, a wide FWHM, and low crystal perfection in comparison with primary kaolinites. It is associated with a change in the kaolinite properties as a result of their transportation from the primary weathering crust to the sedimentation zone. It is proposed to use the data on the particle size distribution together with X-ray diffractometry (XRD) and scanning electron microscopy (SEM) for the first time to determine the kaolinite genesis.