Crystallinity variations in kaolinite induced by grinding and pressure treatments

Kaolinite samples treated with uniaxial pressures higher than 0.1 GPa or grinding times greater than 0.3 h show changes in their crystallinity. These changes are easily detected by the X-ray diffraction (XRD) technique and can be quantified from Hinckley (HI), Lietard (R₂) and also reference intensity ratio (RIR) index studies. The sensitivity of these indices to the Crystallinity changes is; HI>RIR>R₂. In the range of pressures and grinding times considered, these values can be diminished by 50%. The variations in these indices with either the pressure or grinding time follow logarithmic laws whose correlation coefficients are closed to unity. Infrared (IR) spectroscopy and thermal analysis studies show low sensitivity to the changes introduced by grinding or pressure. Nevertheless, thermal gravimetric curves confirm the decrease in the dehydroxylation starting point of about 100°C for the pressed and ground samples. Transmission electron microscopy (TEM) reveals important changes in kaolinite particle morphologies after pressure and grinding treatments. The samples compressed at 0.1, 0.32, 0.85, 1.0 and 2.0 GPa show a large number of defects (fractures, bending, deformations and rolling of layers, glide and rotation of the shell). The ground samples show grain boundaries, dislocations, twins and rounded voids. These defects are responsible for the decrease in crystallinity of the kaolinite samples shown in the XRD and IR studies.