Mineralogy, surface properties and electrokinetic behaviour of kaolin clays derived from naturally occurring pegmatite and granite deposits

This paper presents research on the surface characteristics, specific surface area (SSA), cation exchange capacity (CEC) and the electrophoretic mobility (EPM) of kaolin clays obtained from different naturally occurring pegmatite and granite deposits worldwide. Particular emphasis was placed on studying the influence of ancillary minerals on these properties, as well as the diversity in morphological and structural characteristics of kaolinite in samples. Min­ eral composition and the morphology of the kaolin samples were investigated using X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM), respectively. The SSA was determined by the BET method, CEC by ammonium selective electrode and surface charge by EPM measurements. Results showed that the surface properties of the kaolin samples were strongly influenced by the presence of ancillary constituents, particularly smec­ tite minerals. Their occurrence, even in trace amounts, significantly increased the SSA and CEC values of the inves­ tigated samples. Most importantly, EPM measurements revealed that the character of the amphoteric surfaces of the dominant kaolinite mineral in the samples was rescinded by the permanent pH-independent siloxane basal charge of smectite. The presence of ancillary smectites, characterized by higher surface reactivity, hindered the surface proper­ ties of the prevailing kaolinite mineral in the kaolin clay deposits.